This is guile.info, produced by makeinfo version 4.7 from guile.texi. INFO-DIR-SECTION The Algorithmic Language Scheme START-INFO-DIR-ENTRY * Guile Reference: (guile). The Guile reference manual. END-INFO-DIR-ENTRY Guile Reference Manual Copyright (C) 1996 Free Software Foundation Copyright (C) 1997 Free Software Foundation Copyright (C) 2000 Free Software Foundation Copyright (C) 2001 Free Software Foundation Copyright (C) 2002 Free Software Foundation Copyright (C) 2004 Free Software Foundation Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that this permission notice may be stated in a translation approved by the Free Software Foundation.  File: guile.info, Node: Calling Scheme procedures from C, Next: Mixing gh and scm APIs, Prev: Memory allocation and garbage collection, Up: GH 19.12 Calling Scheme procedures from C ====================================== Many of the Scheme primitives are available in the `gh_' interface; they take and return objects of type SCM, and one could basically use them to write C code that mimics Scheme code. I will list these routines here without much explanation, since what they do is the same as documented in *Note R5RS: (r5rs)Standard procedures. But I will point out that when a procedure takes a variable number of arguments (such as `gh_list'), you should pass the constant SCM_UNDEFINED from C to signify the end of the list. -- Function: SCM gh_define (char *NAME, SCM VAL) Corresponds to the Scheme `(define name val)': it binds a value to the given name (which is a C string). Returns the new object. Pairs and lists =============== -- Function: SCM gh_cons (SCM A, SCM B) -- Function: SCM gh_list (SCM l0, SCM l1, ... , SCM_UNDEFINED) These correspond to the Scheme `(cons a b)' and `(list l0 l1 ...)' procedures. Note that `gh_list()' is a C macro that invokes `scm_listify()'. -- Function: SCM gh_car (SCM OBJ) -- Function: SCM gh_cdr (SCM OBJ) ... -- Function: SCM gh_c[ad][ad][ad][ad]r (SCM OBJ) These correspond to the Scheme `(caadar ls)' procedures etc ... -- Function: SCM gh_set_car_x (SCM PAIR, SCM VALUE) Modifies the CAR of PAIR to be VALUE. This is equivalent to the Scheme procedure `(set-car! ...)'. -- Function: SCM gh_set_cdr_x (SCM PAIR, SCM VALUE) Modifies the CDR of PAIR to be VALUE. This is equivalent to the Scheme procedure `(set-cdr! ...)'. -- Function: unsigned long gh_length (SCM LS) Returns the length of the list. -- Function: SCM gh_append (SCM ARGS) -- Function: SCM gh_append2 (SCM L1, SCM L2) -- Function: SCM gh_append3 (SCM L1, SCM L2, L3) -- Function: SCM gh_append4 (SCM L1, SCM L2, L3, L4) `gh_append()' takes ARGS, which is a list of lists `(list1 list2 ...)', and returns a list containing all the elements of the individual lists. A typical invocation of `gh_append()' to append 5 lists together would be gh_append(gh_list(l1, l2, l3, l4, l5, SCM_UNDEFINED)); The functions `gh_append2()', `gh_append2()', `gh_append3()' and `gh_append4()' are convenience routines to make it easier for C programs to form the list of lists that goes as an argument to `gh_append()'. -- Function: SCM gh_reverse (SCM LS) Returns a new list that has the same elements as LS but in the reverse order. Note that this is implemented as a macro which calls `scm_reverse()'. -- Function: SCM gh_list_tail (SCM LS, SCM K) Returns the sublist of LS with the last K elements. -- Function: SCM gh_list_ref (SCM LS, SCM K) Returns the Kth element of the list LS. -- Function: SCM gh_memq (SCM X, SCM LS) -- Function: SCM gh_memv (SCM X, SCM LS) -- Function: SCM gh_member (SCM X, SCM LS) These functions return the first sublist of LS whose CAR is X. They correspond to `(memq x ls)', `(memv x ls)' and `(member x ls)', and hence use (respectively) `eq?', `eqv?' and `equal?' to do comparisons. If X does not appear in LS, the value `SCM_BOOL_F' (not the empty list) is returned. Note that these functions are implemented as macros which call `scm_memq()', `scm_memv()' and `scm_member()' respectively. -- Function: SCM gh_assq (SCM X, SCM ALIST) -- Function: SCM gh_assv (SCM X, SCM ALIST) -- Function: SCM gh_assoc (SCM X, SCM ALIST) These functions search an "association list" (list of pairs) ALIST for the first pair whose CAR is X, and they return that pair. If no pair in ALIST has X as its CAR, the value `SCM_BOOL_F' (not the empty list) is returned. Note that these functions are implemented as macros which call `scm_assq()', `scm_assv()' and `scm_assoc()' respectively. Symbols ======= Vectors ======= -- Function: SCM gh_make_vector (SCM N, SCM FILL) -- Function: SCM gh_vector (SCM LS) -- Function: SCM gh_vector_ref (SCM V, SCM I) -- Function: SCM gh_vector_set (SCM V, SCM I, SCM VAL) -- Function: unsigned long gh_vector_length (SCM V) -- Function: SCM gh_list_to_vector (SCM LS) These correspond to the Scheme `(make-vector n fill)', `(vector a b c ...)' `(vector-ref v i)' `(vector-set v i value)' `(vector-length v)' `(list->vector ls)' procedures. The correspondence is not perfect for `gh_vector': this routine takes a list LS instead of the individual list elements, thus making it identical to `gh_list_to_vector'. There is also a difference in gh_vector_length: the value returned is a C `unsigned long' instead of an SCM object. Procedures ========== -- Function: SCM gh_apply (SCM proc, SCM args) Call the Scheme procedure PROC, with the elements of ARGS as arguments. ARGS must be a proper list. -- Function: SCM gh_call0 (SCM proc) -- Function: SCM gh_call1 (SCM proc, SCM arg) -- Function: SCM gh_call2 (SCM proc, SCM arg1, SCM arg2) -- Function: SCM gh_call3 (SCM proc, SCM arg1, SCM arg2, SCM arg3) Call the Scheme procedure PROC with no arguments (`gh_call0'), one argument (`gh_call1'), and so on. You can get the same effect by wrapping the arguments up into a list, and calling `gh_apply'; Guile provides these functions for convenience. -- Function: SCM gh_catch (SCM key, SCM thunk, SCM handler) -- Function: SCM gh_throw (SCM key, SCM args) Corresponds to the Scheme `catch' and `throw' procedures, which in Guile are provided as primitives. -- Function: SCM gh_is_eq (SCM a, SCM b) -- Function: SCM gh_is_eqv (SCM a, SCM b) -- Function: SCM gh_is_equal (SCM a, SCM b) These correspond to the Scheme `eq?', `eqv?' and `equal?' predicates. -- Function: int gh_obj_length (SCM OBJ) Returns the raw object length. Data lookup =========== For now I just include Tim Pierce's comments from the `gh_data.c' file; it should be organized into a documentation of the two functions here. /* Data lookups between C and Scheme Look up a symbol with a given name, and return the object to which it is bound. gh_lookup examines the Guile top level, and gh_module_lookup checks the module name space specified by the `vec' argument. The return value is the Scheme object to which SNAME is bound, or SCM_UNDEFINED if SNAME is not bound in the given context. [FIXME: should this be SCM_UNSPECIFIED? Can a symbol ever legitimately be bound to SCM_UNDEFINED or SCM_UNSPECIFIED? What is the difference? -twp] */  File: guile.info, Node: Mixing gh and scm APIs, Next: scm transition summary, Prev: Calling Scheme procedures from C, Up: GH 19.13 Mixing gh and scm APIs ============================  File: guile.info, Node: scm transition summary, Prev: Mixing gh and scm APIs, Up: GH 19.14 Transitioning to the scm Interface ======================================== The following table summarizes the available information on how to transition from the GH to the scm interface. Where transitioning is not completely straightforward, the table includes a reference to more detailed documentation in the preceding sections. Header file Use `#include ' instead of `#include '. Compiling and Linking Use `guile-config' to pick up the flags required to compile C or C++ code that uses `libguile', like so $(CC) -o prog.o -c prog.c `guile-config compile` If you are using libtool to link your executables, just use `-lguile' in your link command. Libtool will expand this into the needed linker options automatically. If you are not using libtool, use the `guile-config' program to query the needed options explicitly. A linker command like $(CC) -o prog prog.o `guile-config link` should be all that is needed. To link shared libraries that will be used as Guile Extensions, use libtool to control both the compilation and the link stage. The `SCM' type No change: the scm interface also uses this type to represent an arbitrary Scheme value. `SCM_BOOL_F' and `SCM_BOOL_T' No change. `SCM_UNSPECIFIED' and `SCM_UNDEFINED' No change. `gh_enter' Use `scm_boot_guile' instead, but note that `scm_boot_guile' has a slightly different calling convention from `gh_enter': `scm_boot_guile', and the main program function that you specify for `scm_boot_guile' to call, both take an additional CLOSURE parameter. *Note Guile Initialization Functions:: for more details. `gh_repl' Use `scm_shell' instead. `gh_init' Use `scm_init_guile' instead. `gh_eval_str' Use `scm_c_eval_string' instead. `gh_eval_file' or `gh_load' Use `scm_c_primitive_load' instead. `gh_new_procedure' Use `scm_c_define_gsubr' instead, but note that the arguments are in a different order: for `scm_c_define_gsubr' the C function pointer is the last argument. *Note A Sample Guile Extension:: for an example. `gh_defer_ints' and `gh_allow_ints' Use `SCM_DEFER_INTS' and `SCM_ALLOW_INTS' instead. Note that these macros are used without parentheses, as in `SCM_DEFER_INTS;'. `gh_bool2scm' Use `SCM_BOOL' instead. `gh_ulong2scm' Use `scm_ulong2num' instead. `gh_long2scm' Use `scm_long2num' instead. `gh_double2scm' Use `scm_make_real' instead. `gh_char2scm' Use `SCM_MAKE_CHAR' instead. `gh_str2scm' Use `scm_mem2string' instead. `gh_str02scm' Use `scm_makfrom0str' instead. `gh_set_substr' No direct scm equivalent. [FIXME] `gh_symbol2scm' Use `scm_str2symbol' instead. [FIXME: inconsistent naming, should be `scm_str02symbol'.] `gh_ints2scm' and `gh_doubles2scm' No direct scm equivalent. [FIXME] `gh_chars2byvect' and `gh_shorts2svect' No direct scm equivalent. [FIXME] `gh_longs2ivect' and `gh_ulongs2uvect' No direct scm equivalent. [FIXME] `gh_floats2fvect' and `gh_doubles2dvect' No direct scm equivalent. [FIXME] `gh_scm2bool' Use `SCM_NFALSEP' instead. `gh_scm2int' Replace `gh_scm2int (OBJ)' by scm_num2int (OBJ, SCM_ARG1, STR) where STR is a C string that describes the context of the call. `gh_scm2ulong' Replace `gh_scm2ulong (OBJ)' by scm_num2ulong (OBJ, SCM_ARG1, STR) where STR is a C string that describes the context of the call. `gh_scm2long' Replace `gh_scm2long (OBJ)' by scm_num2long (OBJ, SCM_ARG1, STR) where STR is a C string that describes the context of the call. `gh_scm2double' Replace `gh_scm2double (OBJ)' by scm_num2dbl (OBJ, STR) where STR is a C string that describes the context of the call. `gh_scm2char' Use the `SCM_CHAR' macro instead, but note that `SCM_CHAR' does not check that its argument is actually a character. To check that a `SCM' value is a character before using `SCM_CHAR' to extract the character value, use the `SCM_VALIDATE_CHAR' macro. `gh_scm2newstr' No direct scm equivalent. [FIXME] `gh_get_substr' No direct scm equivalent. [FIXME] `gh_symbol2newstr' No direct scm equivalent. [FIXME] `gh_scm2chars' No direct scm equivalent. [FIXME] `gh_scm2shorts' and `gh_scm2longs' No direct scm equivalent. [FIXME] `gh_scm2floats' and `gh_scm2doubles' No direct scm equivalent. [FIXME] `gh_boolean_p' Use the `SCM_BOOLP' macro instead, or replace `gh_boolean_p (OBJ)' by SCM_NFALSEP (scm_boolean_p (OBJ)) `gh_symbol_p' Use the `SCM_SYMBOLP' macro instead, or replace `gh_symbol_p (OBJ)' by SCM_NFALSEP (scm_symbol_p (OBJ)) `gh_char_p' Use the `SCM_CHARP' macro instead, or replace `gh_char_p (OBJ)' by SCM_NFALSEP (scm_char_p (OBJ)) `gh_vector_p' Use the `SCM_VECTORP' macro instead, or replace `gh_vector_p (OBJ)' by SCM_NFALSEP (scm_vector_p (OBJ)) `gh_pair_p' Use the `SCM_CONSP' macro instead, or replace `gh_pair_p (OBJ)' by SCM_NFALSEP (scm_pair_p (OBJ)) `gh_number_p' Use the `SCM_NUMBERP' macro instead, or replace `gh_number_p (OBJ)' by SCM_NFALSEP (scm_number_p (OBJ)) `gh_string_p' Use the `SCM_STRINGP' macro instead, or replace `gh_string_p (OBJ)' by SCM_NFALSEP (scm_string_p (OBJ)) `gh_procedure_p' Replace `gh_procedure_p (OBJ)' by SCM_NFALSEP (scm_procedure_p (OBJ)) `gh_list_p' Replace `gh_list_p (OBJ)' by SCM_NFALSEP (scm_list_p (OBJ)) `gh_inexact_p' Use the `SCM_INEXACTP' macro instead, or replace `gh_inexact_p (OBJ)' by SCM_NFALSEP (scm_inexact_p (OBJ)) `gh_exact_p' Replace `gh_exact_p (OBJ)' by SCM_NFALSEP (scm_exact_p (OBJ)) `gh_eq_p' Use the `SCM_EQ_P' macro instead, or replace `gh_eq_p (X, Y)' by SCM_NFALSEP (scm_eq_p (X, Y)) `gh_eqv_p' Replace `gh_eqv_p (X, Y)' by SCM_NFALSEP (scm_eqv_p (X, Y)) `gh_equal_p' Replace `gh_equal_p (X, Y)' by SCM_NFALSEP (scm_equal_p (X, Y)) `gh_string_equal_p' Replace `gh_string_equal_p (X, Y)' by SCM_NFALSEP (scm_string_equal_p (X, Y)) `gh_null_p' Use the `SCM_NULLP' macro instead, or replace `gh_null_p (OBJ)' by SCM_NFALSEP (scm_null_p (OBJ)) `gh_cons' Use `scm_cons' instead. `gh_car' and `gh_cdr' Use the `SCM_CAR' and `SCM_CDR' macros instead. `gh_cxxr' and `gh_cxxxr' (Where each x is either `a' or `d'.) Use the corresponding `SCM_CXXR' or `SCM_CXXXR' macro instead. `gh_set_car_x' and `gh_set_cdr_x' Use `scm_set_car_x' and `scm_set_cdr_x' instead. `gh_list' Use `scm_listify' instead. `gh_length' Replace `gh_length (LST)' by scm_num2ulong (scm_length (LST), SCM_ARG1, STR) where STR is a C string that describes the context of the call. `gh_append' Use `scm_append' instead. `gh_append2', `gh_append3', `gh_append4' Replace `gh_appendN (L1, ..., LN)' by scm_append (scm_listify (L1, ..., LN, SCM_UNDEFINED)) `gh_reverse' Use `scm_reverse' instead. `gh_list_tail' and `gh_list_ref' Use `scm_list_tail' and `scm_list_ref' instead. `gh_memq', `gh_memv' and `gh_member' Use `scm_memq', `scm_memv' and `scm_member' instead. `gh_assq', `gh_assv' and `gh_assoc' Use `scm_assq', `scm_assv' and `scm_assoc' instead. `gh_make_vector' Use `scm_make_vector' instead. `gh_vector' or `gh_list_to_vector' Use `scm_vector' instead. `gh_vector_ref' and `gh_vector_set_x' Use `scm_vector_ref' and `scm_vector_set_x' instead. `gh_vector_length' Use the `SCM_VECTOR_LENGTH' macro instead. `gh_apply' Use `scm_apply' instead, but note that `scm_apply' takes an additional third argument that you should set to `SCM_EOL'.  File: guile.info, Node: Reference Intro, Next: API Overview, Prev: GH, Up: Top Part IV: Guile API Reference **************************** Guile provides an application programming interface ("API") to developers in two core languages: Scheme and C. This part of the manual contains reference documentation for all of the functionality that is available through both Scheme and C interfaces.  File: guile.info, Node: API Overview, Next: Simple Data Types, Prev: Reference Intro, Up: Top 20 Overview of the Guile API **************************** Guile's application programming interface ("API") makes functionality available that an application developer can use in either C or Scheme programming. The interface consists of "elements" that may be macros, functions or variables in C, and procedures, variables, syntax or other types of object in Scheme. Broadly speaking, the interface as a whole can be divided into three groups. 1. Elements that are available equivalently as C functions or Scheme procedures. 2. Elements that are only available as macros, functions or variables for C programming. 3. Elements that are only available as procedures or other objects in Scheme. Functions/procedures in the first group are often known as "primitives", "subrs" or "builtins". An example is the `assq' Scheme procedure, which is also available as `scm_assq' in C. Elements in the second and third groups exist because they provide additional language-specific benefits in either Scheme or C. Examples are the C macro `SCM_CONSP', which is faster and more convenient in C programming than the primitive `scm_pair_p', and the procedure-with-setter `make-object-property', which provides a more convenient property handling interface in Scheme than the primitives on which it is based. * Menu: * Primitives:: Identical function for Scheme and C. * C Only:: Elements only available in C. * Scheme Only:: Elements only available in Scheme. * Reference Layout:: The layout of this part of the manual.  File: guile.info, Node: Primitives, Next: C Only, Up: API Overview 20.1 Identical Function in both Scheme and C ============================================ They form the majority of the API, and allow both C and Scheme programmers to perform identical operations. Scheme procedures marked "primitive functions" have a regular interface when calling from C, reflected in two areas: the name of a C function, and the convention for passing non-required arguments to this function. * Menu: * Transforming Scheme name to C name:: * Structuring argument lists for C functions::  File: guile.info, Node: Transforming Scheme name to C name, Next: Structuring argument lists for C functions, Up: Primitives 20.1.1 Transforming Scheme name to C name ----------------------------------------- Normally, the name of a C function can be derived given its Scheme name, using some simple textual transformations: * Replace `-' (hyphen) with `_' (underscore). * Replace `?' (question mark) with "_p". * Replace `!' (exclamation point) with "_x". * Replace internal `->' with "_to_". * Replace `<=' (less than or equal) with "_leq". * Replace `>=' (greater than or equal) with "_geq". * Replace `<' (less than) with "_less". * Replace `>' (greater than) with "_gr". * Replace `@' with "at". [Omit?] * Prefix with "gh_" (or "scm_" if you are ignoring the gh interface). * [Anything else? -ttn, 2000/01/16 15:17:28] Here is an Emacs Lisp command that prompts for a Scheme function name and inserts the corresponding C function name into the buffer. (defun insert-scheme-to-C (name &optional use-gh) "Transforms Scheme NAME, a string, to its C counterpart, and inserts it. Prefix arg non-nil means use \"gh_\" prefix, otherwise use \"scm_\" prefix." (interactive "sScheme name: \nP") (let ((transforms '(("-" . "_") ("?" . "_p") ("!" . "_x") ("->" . "_to_") ("<=" . "_leq") (">=" . "_geq") ("<" . "_less") (">" . "_gr") ("@" . "at")))) (while transforms (let ((trigger (concat "\\(.*\\)" (regexp-quote (caar transforms)) "\\(.*\\)")) (sub (cdar transforms)) (m nil)) (while (setq m (string-match trigger name)) (setq name (concat (match-string 1 name) sub (match-string 2 name))))) (setq transforms (cdr transforms)))) (insert (if use-gh "gh_" "scm_") name))  File: guile.info, Node: Structuring argument lists for C functions, Prev: Transforming Scheme name to C name, Up: Primitives 20.1.2 Structuring argument lists for C functions ------------------------------------------------- The C function's arguments will be all of the Scheme procedure's arguments, both required and optional; if the Scheme procedure takes a "rest" argument, that will be a final argument to the C function. The C function's arguments, as well as its return type, will be `SCM'.  File: guile.info, Node: C Only, Next: Scheme Only, Prev: Primitives, Up: API Overview 20.2 Elements Available Only in C ================================= For C this is usually a matter of better performance (e.g. the `SCM_CONSP' macro) or of accepting C language types rather than the generic `SCM'.  File: guile.info, Node: Scheme Only, Next: Reference Layout, Prev: C Only, Up: API Overview 20.3 Elements Available Only in Scheme ======================================  File: guile.info, Node: Reference Layout, Prev: Scheme Only, Up: API Overview 20.4 Reference Material Layout ============================== This part of the reference manual documents all of Guile's core Scheme-level language and features in functionally-related groups. Where a particular section of the manual includes both R5RS-compliant parts and Guile-specific extensions, the text indicates which parts of the documentation describe R5RS behaviour and which parts describe Guile extensions. For a quick way of identifying the parts of Guile that implement R5RS-compliant features, see the R5RS index: *Note R5RS Index::.  File: guile.info, Node: Simple Data Types, Next: Compound Data Types, Prev: API Overview, Up: Top 21 Simple Generic Data Types **************************** This chapter describes those of Guile's simple data types which are primarily used for their role as items of generic data. By "simple" we mean data types that are not primarily used as containers to hold other data -- i.e. pairs, lists, vectors and so on. For the documentation of such "compound" data types, see *Note Compound Data Types::. One of the great strengths of Scheme is that there is no straightforward distinction between "data" and "functionality". For example, Guile's support for dynamic linking could be described * either in a "data-centric" way, as the behaviour and properties of the "dynamically linked object" data type, and the operations that may be applied to instances of this type * or in a "functionality-centric" way, as the set of procedures that constitute Guile's support for dynamic linking, in the context of the module system. The contents of this chapter are, therefore, a matter of judgment. By "generic", we mean to select those data types whose typical use as _data_ in a wide variety of programming contexts is more important than their use in the implementation of a particular piece of _functionality_. The last section of this chapter provides references for all the data types that are documented not here but in a "functionality-centric" way elsewhere in the manual. * Menu: * Booleans:: True/false values. * Numbers:: Numerical data types. * Characters:: New character names. * Strings:: Special things about strings. * Regular Expressions:: Pattern matching and substitution. * Symbols:: Symbols. * Keywords:: Self-quoting, customizable display keywords. * Other Types:: "Functionality-centric" data types.  File: guile.info, Node: Booleans, Next: Numbers, Up: Simple Data Types 21.1 Booleans ============= The two boolean values are `#t' for true and `#f' for false. Boolean values are returned by predicate procedures, such as the general equality predicates `eq?', `eqv?' and `equal?' (*note Equality::) and numerical and string comparison operators like `string=?' (*note String Comparison::) and `<=' (*note Comparison::). (<= 3 8) => #t (<= 3 -3) => #f (equal? "house" "houses") => #f (eq? #f #f) => #t In test condition contexts like `if' and `cond' (*note if cond case::), where a group of subexpressions will be evaluated only if a CONDITION expression evaluates to "true", "true" means any value at all except `#f'. (if #t "yes" "no") => "yes" (if 0 "yes" "no") => "yes" (if #f "yes" "no") => "no" A result of this asymmetry is that typical Scheme source code more often uses `#f' explicitly than `#t': `#f' is necessary to represent an `if' or `cond' false value, whereas `#t' is not necessary to represent an `if' or `cond' true value. It is important to note that `#f' is *not* equivalent to any other Scheme value. In particular, `#f' is not the same as the number 0 (like in C and C++), and not the same as the "empty list" (like in some Lisp dialects). The `not' procedure returns the boolean inverse of its argument: -- Scheme Procedure: not x -- C Function: scm_not (x) Return `#t' iff X is `#f', else return `#f'. The `boolean?' procedure is a predicate that returns `#t' if its argument is one of the boolean values, otherwise `#f'. -- Scheme Procedure: boolean? obj -- C Function: scm_boolean_p (obj) Return `#t' iff OBJ is either `#t' or `#f'.  File: guile.info, Node: Numbers, Next: Characters, Prev: Booleans, Up: Simple Data Types 21.2 Numerical data types ========================= Guile supports a rich "tower" of numerical types -- integer, rational, real and complex -- and provides an extensive set of mathematical and scientific functions for operating on numerical data. This section of the manual documents those types and functions. You may also find it illuminating to read R5RS's presentation of numbers in Scheme, which is particularly clear and accessible: see *Note Numbers: (r5rs)Numbers. * Menu: * Numerical Tower:: Scheme's numerical "tower". * Integers:: Whole numbers. * Reals and Rationals:: Real and rational numbers. * Complex Numbers:: Complex numbers. * Exactness:: Exactness and inexactness. * Number Syntax:: Read syntax for numerical data. * Integer Operations:: Operations on integer values. * Comparison:: Comparison predicates. * Conversion:: Converting numbers to and from strings. * Complex:: Complex number operations. * Arithmetic:: Arithmetic functions. * Scientific:: Scientific functions. * Primitive Numerics:: Primitive numeric functions. * Bitwise Operations:: Logical AND, OR, NOT, and so on. * Random:: Random number generation.  File: guile.info, Node: Numerical Tower, Next: Integers, Up: Numbers 21.2.1 Scheme's Numerical "Tower" --------------------------------- Scheme's numerical "tower" consists of the following categories of numbers: * integers (whole numbers) * rationals (the set of numbers that can be expressed as P/Q where P and Q are integers) * real numbers (the set of numbers that describes all possible positions along a one dimensional line) * complex numbers (the set of numbers that describes all possible positions in a two dimensional space) It is called a tower because each category "sits on" the one that follows it, in the sense that every integer is also a rational, every rational is also real, and every real number is also a complex number (but with zero imaginary part). Of these, Guile implements integers, reals and complex numbers as distinct types. Rationals are implemented as regards the read syntax for rational numbers that is specified by R5RS, but are immediately converted by Guile to the corresponding real number. The `number?' predicate may be applied to any Scheme value to discover whether the value is any of the supported numerical types. -- Scheme Procedure: number? obj -- C Function: scm_number_p (obj) Return `#t' if OBJ is any kind of number, else `#f'. For example: (number? 3) => #t (number? "hello there!") => #f (define pi 3.141592654) (number? pi) => #t The next few subsections document each of Guile's numerical data types in detail.  File: guile.info, Node: Integers, Next: Reals and Rationals, Prev: Numerical Tower, Up: Numbers 21.2.2 Integers --------------- Integers are whole numbers, that is numbers with no fractional part, such as 2, 83 and -3789. Integers in Guile can be arbitrarily big, as shown by the following example. (define (factorial n) (let loop ((n n) (product 1)) (if (= n 0) product (loop (- n 1) (* product n))))) (factorial 3) => 6 (factorial 20) => 2432902008176640000 (- (factorial 45)) => -119622220865480194561963161495657715064383733760000000000 Readers whose background is in programming languages where integers are limited by the need to fit into just 4 or 8 bytes of memory may find this surprising, or suspect that Guile's representation of integers is inefficient. In fact, Guile achieves a near optimal balance of convenience and efficiency by using the host computer's native representation of integers where possible, and a more general representation where the required number does not fit in the native form. Conversion between these two representations is automatic and completely invisible to the Scheme level programmer. -- Scheme Procedure: integer? x -- C Function: scm_integer_p (x) Return `#t' if X is an integer number, else `#f'. (integer? 487) => #t (integer? -3.4) => #f  File: guile.info, Node: Reals and Rationals, Next: Complex Numbers, Prev: Integers, Up: Numbers 21.2.3 Real and Rational Numbers -------------------------------- Mathematically, the real numbers are the set of numbers that describe all possible points along a continuous, infinite, one-dimensional line. The rational numbers are the set of all numbers that can be written as fractions P/Q, where P and Q are integers. All rational numbers are also real, but there are real numbers that are not rational, for example the square root of 2, and pi. Guile represents both real and rational numbers approximately using a floating point encoding with limited precision. Even though the actual encoding is in binary, it may be helpful to think of it as a decimal number with a limited number of significant figures and a decimal point somewhere, since this corresponds to the standard notation for non-whole numbers. For example: 0.34 -0.00000142857931198 -5648394822220000000000.0 4.0 The limited precision of Guile's encoding means that any "real" number in Guile can be written in a rational form, by multiplying and then dividing by sufficient powers of 10 (or in fact, 2). For example, `-0.00000142857931198' is the same as `142857931198' divided by `100000000000000000'. In Guile's current incarnation, therefore, the `rational?' and `real?' predicates are equivalent. Another aspect of this equivalence is that Guile currently does not preserve the exactness that is possible with rational arithmetic. If such exactness is needed, it is of course possible to implement exact rational arithmetic at the Scheme level using Guile's arbitrary size integers. A planned future revision of Guile's numerical tower will make it possible to implement exact representations and arithmetic for both rational numbers and real irrational numbers such as square roots, and in such a way that the new kinds of number integrate seamlessly with those that are already implemented. -- Scheme Procedure: real? obj -- C Function: scm_real_p (obj) Return `#t' if OBJ is a real number, else `#f'. Note that the sets of integer and rational values form subsets of the set of real numbers, so the predicate will also be fulfilled if OBJ is an integer number or a rational number. -- Scheme Procedure: rational? x -- C Function: scm_real_p (x) Return `#t' if X is a rational number, `#f' otherwise. Note that the set of integer values forms a subset of the set of rational numbers, i. e. the predicate will also be fulfilled if X is an integer number. Real numbers will also satisfy this predicate, because of their limited precision.  File: guile.info, Node: Complex Numbers, Next: Exactness, Prev: Reals and Rationals, Up: Numbers 21.2.4 Complex Numbers ---------------------- Complex numbers are the set of numbers that describe all possible points in a two-dimensional space. The two coordinates of a particular point in this space are known as the "real" and "imaginary" parts of the complex number that describes that point. In Guile, complex numbers are written in rectangular form as the sum of their real and imaginary parts, using the symbol `i' to indicate the imaginary part. 3+4i => 3.0+4.0i (* 3-8i 2.3+0.3i) => 9.3-17.5i Guile represents a complex number as a pair of numbers both of which are real, so the real and imaginary parts of a complex number have the same properties of inexactness and limited precision as single real numbers. -- Scheme Procedure: complex? x -- C Function: scm_number_p (x) Return `#t' if X is a complex number, `#f' otherwise. Note that the sets of real, rational and integer values form subsets of the set of complex numbers, i. e. the predicate will also be fulfilled if X is a real, rational or integer number.  File: guile.info, Node: Exactness, Next: Number Syntax, Prev: Complex Numbers, Up: Numbers 21.2.5 Exact and Inexact Numbers -------------------------------- R5RS requires that a calculation involving inexact numbers always produces an inexact result. To meet this requirement, Guile distinguishes between an exact integer value such as `5' and the corresponding inexact real value which, to the limited precision available, has no fractional part, and is printed as `5.0'. Guile will only convert the latter value to the former when forced to do so by an invocation of the `inexact->exact' procedure. -- Scheme Procedure: exact? x -- C Function: scm_exact_p (x) Return `#t' if X is an exact number, `#f' otherwise. -- Scheme Procedure: inexact? x -- C Function: scm_inexact_p (x) Return `#t' if X is an inexact number, `#f' else. -- Scheme Procedure: inexact->exact z -- C Function: scm_inexact_to_exact (z) Return an exact number that is numerically closest to Z. -- Scheme Procedure: exact->inexact z Convert the number Z to its inexact representation.  File: guile.info, Node: Number Syntax, Next: Integer Operations, Prev: Exactness, Up: Numbers 21.2.6 Read Syntax for Numerical Data ------------------------------------- The read syntax for integers is a string of digits, optionally preceded by a minus or plus character, a code indicating the base in which the integer is encoded, and a code indicating whether the number is exact or inexact. The supported base codes are: * `#b', `#B' -- the integer is written in binary (base 2) * `#o', `#O' -- the integer is written in octal (base 8) * `#d', `#D' -- the integer is written in decimal (base 10) * `#x', `#X' -- the integer is written in hexadecimal (base 16). If the base code is omitted, the integer is assumed to be decimal. The following examples show how these base codes are used. -13 => -13 #d-13 => -13 #x-13 => -19 #b+1101 => 13 #o377 => 255 The codes for indicating exactness (which can, incidentally, be applied to all numerical values) are: * `#e', `#E' -- the number is exact * `#i', `#I' -- the number is inexact. If the exactness indicator is omitted, the integer is assumed to be exact, since Guile's internal representation for integers is always exact. Real numbers have limited precision similar to the precision of the `double' type in C. A consequence of the limited precision is that all real numbers in Guile are also rational, since any number R with a limited number of decimal places, say N, can be made into an integer by multiplying by 10^N.  File: guile.info, Node: Integer Operations, Next: Comparison, Prev: Number Syntax, Up: Numbers 21.2.7 Operations on Integer Values ----------------------------------- -- Scheme Procedure: odd? n -- C Function: scm_odd_p (n) Return `#t' if N is an odd number, `#f' otherwise. -- Scheme Procedure: even? n -- C Function: scm_even_p (n) Return `#t' if N is an even number, `#f' otherwise. -- Scheme Procedure: quotient Return the quotient of the numbers X and Y. -- Scheme Procedure: remainder Return the remainder of the numbers X and Y. (remainder 13 4) => 1 (remainder -13 4) => -1 -- Scheme Procedure: modulo Return the modulo of the numbers X and Y. (modulo 13 4) => 1 (modulo -13 4) => 3 -- Scheme Procedure: gcd Return the greatest common divisor of all arguments. If called without arguments, 0 is returned. -- Scheme Procedure: lcm Return the least common multiple of the arguments. If called without arguments, 1 is returned.  File: guile.info, Node: Comparison, Next: Conversion, Prev: Integer Operations, Up: Numbers 21.2.8 Comparison Predicates ---------------------------- -- Scheme Procedure: = Return `#t' if all parameters are numerically equal. -- Scheme Procedure: < Return `#t' if the list of parameters is monotonically increasing. -- Scheme Procedure: > Return `#t' if the list of parameters is monotonically decreasing. -- Scheme Procedure: <= Return `#t' if the list of parameters is monotonically non-decreasing. -- Scheme Procedure: >= Return `#t' if the list of parameters is monotonically non-increasing. -- Scheme Procedure: zero? Return `#t' if Z is an exact or inexact number equal to zero. -- Scheme Procedure: positive? Return `#t' if X is an exact or inexact number greater than zero. -- Scheme Procedure: negative? Return `#t' if X is an exact or inexact number less than zero.  File: guile.info, Node: Conversion, Next: Complex, Prev: Comparison, Up: Numbers 21.2.9 Converting Numbers To and From Strings --------------------------------------------- -- Scheme Procedure: number->string n [radix] -- C Function: scm_number_to_string (n, radix) Return a string holding the external representation of the number N in the given RADIX. If N is inexact, a radix of 10 will be used. -- Scheme Procedure: string->number string [radix] -- C Function: scm_string_to_number (string, radix) Return a number of the maximally precise representation expressed by the given STRING. RADIX must be an exact integer, either 2, 8, 10, or 16. If supplied, RADIX is a default radix that may be overridden by an explicit radix prefix in STRING (e.g. "#o177"). If RADIX is not supplied, then the default radix is 10. If string is not a syntactically valid notation for a number, then `string->number' returns `#f'.  File: guile.info, Node: Complex, Next: Arithmetic, Prev: Conversion, Up: Numbers 21.2.10 Complex Number Operations --------------------------------- -- Scheme Procedure: make-rectangular real imaginary -- C Function: scm_make_rectangular (real, imaginary) Return a complex number constructed of the given REAL and IMAGINARY parts. -- Scheme Procedure: make-polar x y -- C Function: scm_make_polar (x, y) Return the complex number X * e^(i * Y). -- Scheme Procedure: real-part Return the real part of the number Z. -- Scheme Procedure: imag-part Return the imaginary part of the number Z. -- Scheme Procedure: magnitude Return the magnitude of the number Z. This is the same as `abs' for real arguments, but also allows complex numbers. -- Scheme Procedure: angle Return the angle of the complex number Z.  File: guile.info, Node: Arithmetic, Next: Scientific, Prev: Complex, Up: Numbers 21.2.11 Arithmetic Functions ---------------------------- -- Scheme Procedure: + z1 ... Return the sum of all parameter values. Return 0 if called without any parameters. -- Scheme Procedure: - z1 z2 ... If called with one argument Z1, -Z1 is returned. Otherwise the sum of all but the first argument are subtracted from the first argument. -- Scheme Procedure: * z1 ... Return the product of all arguments. If called without arguments, 1 is returned. -- Scheme Procedure: / z1 z2 ... Divide the first argument by the product of the remaining arguments. If called with one argument Z1, 1/Z1 is returned. -- Scheme Procedure: abs x -- C Function: scm_abs (x) Return the absolute value of X. X must be a number with zero imaginary part. To calculate the magnitude of a complex number, use `magnitude' instead. -- Scheme Procedure: max x1 x2 ... Return the maximum of all parameter values. -- Scheme Procedure: min x1 x2 ... Return the minimum of all parameter values. -- Scheme Procedure: truncate Round the inexact number X towards zero. -- Scheme Procedure: round x Round the inexact number X to the nearest integer. When exactly halfway between two integers, round to the even one. -- Scheme Procedure: floor x Round the number X towards minus infinity. -- Scheme Procedure: ceiling x Round the number X towards infinity. For the `truncate' and `round' procedures, the Guile library exports equivalent C functions, but taking and returning arguments of type `double' rather than the usual `SCM'. -- C Function: double scm_truncate (double x) -- C Function: double scm_round (double x) For `floor' and `ceiling', the equivalent C functions are `floor' and `ceil' from the standard mathematics library (which also take and return `double' arguments).  File: guile.info, Node: Scientific, Next: Primitive Numerics, Prev: Arithmetic, Up: Numbers 21.2.12 Scientific Functions ---------------------------- The following procedures accept any kind of number as arguments, including complex numbers. -- Scheme Procedure: sqrt z Return the square root of Z. -- Scheme Procedure: expt z1 z2 Return Z1 raised to the power of Z2. -- Scheme Procedure: sin z Return the sine of Z. -- Scheme Procedure: cos z Return the cosine of Z. -- Scheme Procedure: tan z Return the tangent of Z. -- Scheme Procedure: asin z Return the arcsine of Z. -- Scheme Procedure: acos z Return the arccosine of Z. -- Scheme Procedure: atan z Return the arctangent of Z. -- Scheme Procedure: exp z Return e to the power of Z, where e is the base of natural logarithms (2.71828...). -- Scheme Procedure: log z Return the natural logarithm of Z. -- Scheme Procedure: log10 z Return the base 10 logarithm of Z. -- Scheme Procedure: sinh z Return the hyperbolic sine of Z. -- Scheme Procedure: cosh z Return the hyperbolic cosine of Z. -- Scheme Procedure: tanh z Return the hyperbolic tangent of Z. -- Scheme Procedure: asinh z Return the hyperbolic arcsine of Z. -- Scheme Procedure: acosh z Return the hyperbolic arccosine of Z. -- Scheme Procedure: atanh z Return the hyperbolic arctangent of Z.  File: guile.info, Node: Primitive Numerics, Next: Bitwise Operations, Prev: Scientific, Up: Numbers 21.2.13 Primitive Numeric Functions ----------------------------------- Many of Guile's numeric procedures which accept any kind of numbers as arguments, including complex numbers, are implemented as Scheme procedures that use the following real number-based primitives. These primitives signal an error if they are called with complex arguments. -- Scheme Procedure: $abs x Return the absolute value of X. -- Scheme Procedure: $sqrt x Return the square root of X. -- Scheme Procedure: $expt x y -- C Function: scm_sys_expt (x, y) Return X raised to the power of Y. This procedure does not accept complex arguments. -- Scheme Procedure: $sin x Return the sine of X. -- Scheme Procedure: $cos x Return the cosine of X. -- Scheme Procedure: $tan x Return the tangent of X. -- Scheme Procedure: $asin x Return the arcsine of X. -- Scheme Procedure: $acos x Return the arccosine of X. -- Scheme Procedure: $atan x Return the arctangent of X in the range -PI/2 to PI/2. -- Scheme Procedure: $atan2 x y -- C Function: scm_sys_atan2 (x, y) Return the arc tangent of the two arguments X and Y. This is similar to calculating the arc tangent of X / Y, except that the signs of both arguments are used to determine the quadrant of the result. This procedure does not accept complex arguments. -- Scheme Procedure: $exp x Return e to the power of X, where e is the base of natural logarithms (2.71828...). -- Scheme Procedure: $log x Return the natural logarithm of X. -- Scheme Procedure: $sinh x Return the hyperbolic sine of X. -- Scheme Procedure: $cosh x Return the hyperbolic cosine of X. -- Scheme Procedure: $tanh x Return the hyperbolic tangent of X. -- Scheme Procedure: $asinh x Return the hyperbolic arcsine of X. -- Scheme Procedure: $acosh x Return the hyperbolic arccosine of X. -- Scheme Procedure: $atanh x Return the hyperbolic arctangent of X. For the hyperbolic arc-functions, the Guile library exports C functions corresponding to these Scheme procedures, but taking and returning arguments of type `double' rather than the usual `SCM'. -- C Function: double scm_asinh (double x) -- C Function: double scm_acosh (double x) -- C Function: double scm_atanh (double x) Return the hyperbolic arcsine, arccosine or arctangent of X respectively. For all the other Scheme procedures above, except `expt' and `atan2' (whose entries specifically mention an equivalent C function), the equivalent C functions are those provided by the standard mathematics library. The mapping is as follows. Scheme Procedure C Function `$abs' `fabs' `$sqrt' `sqrt' `$sin' `sin' `$cos' `cos' `$tan' `tan' `$asin' `asin' `$acos' `acos' `$atan' `atan' `$exp' `exp' `$log' `log' `$sinh' `sinh' `$cosh' `cosh' `$tanh' `tanh' Naturally, these C functions expect and return `double' arguments.  File: guile.info, Node: Bitwise Operations, Next: Random, Prev: Primitive Numerics, Up: Numbers 21.2.14 Bitwise Operations -------------------------- -- Scheme Procedure: logand n1 n2 Return the bitwise AND of the integer arguments. (logand) => -1 (logand 7) => 7 (logand #b111 #b011 #b001) => 1 -- Scheme Procedure: logior n1 n2 Return the bitwise OR of the integer arguments. (logior) => 0 (logior 7) => 7 (logior #b000 #b001 #b011) => 3 -- Scheme Procedure: logxor n1 n2 Return the bitwise XOR of the integer arguments. A bit is set in the result if it is set in an odd number of arguments. (logxor) => 0 (logxor 7) => 7 (logxor #b000 #b001 #b011) => 2 (logxor #b000 #b001 #b011 #b011) => 1 -- Scheme Procedure: lognot n -- C Function: scm_lognot (n) Return the integer which is the ones-complement of the integer argument, ie. each 0 bit is changed to 1 and each 1 bit to 0. (number->string (lognot #b10000000) 2) => "-10000001" (number->string (lognot #b0) 2) => "-1" -- Scheme Procedure: logtest j k -- C Function: scm_logtest (j, k) (logtest j k) == (not (zero? (logand j k))) (logtest #b0100 #b1011) => #f (logtest #b0100 #b0111) => #t -- Scheme Procedure: logbit? index j -- C Function: scm_logbit_p (index, j) (logbit? index j) == (logtest (integer-expt 2 index) j) (logbit? 0 #b1101) => #t (logbit? 1 #b1101) => #f (logbit? 2 #b1101) => #t (logbit? 3 #b1101) => #t (logbit? 4 #b1101) => #f -- Scheme Procedure: ash n cnt -- C Function: scm_ash (n, cnt) The function ash performs an arithmetic shift left by CNT bits (or shift right, if CNT is negative). 'Arithmetic' means, that the function does not guarantee to keep the bit structure of N, but rather guarantees that the result will always be rounded towards minus infinity. Therefore, the results of ash and a corresponding bitwise shift will differ if N is negative. Formally, the function returns an integer equivalent to `(inexact->exact (floor (* N (expt 2 CNT))))'. (number->string (ash #b1 3) 2) => "1000" (number->string (ash #b1010 -1) 2) => "101" -- Scheme Procedure: logcount n -- C Function: scm_logcount (n) Return the number of bits in integer N. If integer is positive, the 1-bits in its binary representation are counted. If negative, the 0-bits in its two's-complement binary representation are counted. If 0, 0 is returned. (logcount #b10101010) => 4 (logcount 0) => 0 (logcount -2) => 1 -- Scheme Procedure: integer-length n -- C Function: scm_integer_length (n) Return the number of bits necessary to represent N. (integer-length #b10101010) => 8 (integer-length 0) => 0 (integer-length #b1111) => 4 -- Scheme Procedure: integer-expt n k -- C Function: scm_integer_expt (n, k) Return N raised to the non-negative integer exponent K. (integer-expt 2 5) => 32 (integer-expt -3 3) => -27 -- Scheme Procedure: bit-extract n start end -- C Function: scm_bit_extract (n, start, end) Return the integer composed of the START (inclusive) through END (exclusive) bits of N. The STARTth bit becomes the 0-th bit in the result. (number->string (bit-extract #b1101101010 0 4) 2) => "1010" (number->string (bit-extract #b1101101010 4 9) 2) => "10110"  File: guile.info, Node: Random, Prev: Bitwise Operations, Up: Numbers 21.2.15 Random Number Generation -------------------------------- -- Scheme Procedure: copy-random-state [state] -- C Function: scm_copy_random_state (state) Return a copy of the random state STATE. -- Scheme Procedure: random n [state] -- C Function: scm_random (n, state) Return a number in [0, N). Accepts a positive integer or real n and returns a number of the same type between zero (inclusive) and N (exclusive). The values returned have a uniform distribution. The optional argument STATE must be of the type produced by `seed->random-state'. It defaults to the value of the variable `*random-state*'. This object is used to maintain the state of the pseudo-random-number generator and is altered as a side effect of the random operation. -- Scheme Procedure: random:exp [state] -- C Function: scm_random_exp (state) Return an inexact real in an exponential distribution with mean 1. For an exponential distribution with mean u use (* u (random:exp)). -- Scheme Procedure: random:hollow-sphere! v [state] -- C Function: scm_random_hollow_sphere_x (v, state) Fills vect with inexact real random numbers the sum of whose squares is equal to 1.0. Thinking of vect as coordinates in space of dimension n = (vector-length vect), the coordinates are uniformly distributed over the surface of the unit n-sphere. -- Scheme Procedure: random:normal [state] -- C Function: scm_random_normal (state) Return an inexact real in a normal distribution. The distribution used has mean 0 and standard deviation 1. For a normal distribution with mean m and standard deviation d use `(+ m (* d (random:normal)))'. -- Scheme Procedure: random:normal-vector! v [state] -- C Function: scm_random_normal_vector_x (v, state) Fills vect with inexact real random numbers that are independent and standard normally distributed (i.e., with mean 0 and variance 1). -- Scheme Procedure: random:solid-sphere! v [state] -- C Function: scm_random_solid_sphere_x (v, state) Fills vect with inexact real random numbers the sum of whose squares is less than 1.0. Thinking of vect as coordinates in space of dimension n = (vector-length vect), the coordinates are uniformly distributed within the unit n-sphere. The sum of the squares of the numbers is returned. -- Scheme Procedure: random:uniform [state] -- C Function: scm_random_uniform (state) Return a uniformly distributed inexact real random number in [0,1). -- Scheme Procedure: seed->random-state seed -- C Function: scm_seed_to_random_state (seed) Return a new random state using SEED.  File: guile.info, Node: Characters, Next: Strings, Prev: Numbers, Up: Simple Data Types 21.3 Characters =============== Most of the characters in the ASCII character set may be referred to by name: for example, `#\tab', `#\esc', `#\stx', and so on. The following table describes the ASCII names for each character. 0 = `#\nul' 1 = `#\soh' 2 = `#\stx' 3 = `#\etx' 4 = `#\eot' 5 = `#\enq' 6 = `#\ack' 7 = `#\bel' 8 = `#\bs' 9 = `#\ht' 10 = `#\nl' 11 = `#\vt' 12 = `#\np' 13 = `#\cr' 14 = `#\so' 15 = `#\si' 16 = `#\dle' 17 = `#\dc1' 18 = `#\dc2' 19 = `#\dc3' 20 = `#\dc4' 21 = `#\nak' 22 = `#\syn' 23 = `#\etb' 24 = `#\can' 25 = `#\em' 26 = `#\sub' 27 = `#\esc' 28 = `#\fs' 29 = `#\gs' 30 = `#\rs' 31 = `#\us' 32 = `#\sp' The `delete' character (octal 177) may be referred to with the name `#\del'. Several characters have more than one name: * `#\space', `#\sp' * `#\newline', `#\nl' * `#\tab', `#\ht' * `#\backspace', `#\bs' * `#\return', `#\cr' * `#\page', `#\np' * `#\null', `#\nul' -- Scheme Procedure: char? x -- C Function: scm_char_p (x) Return `#t' iff X is a character, else `#f'. -- Scheme Procedure: char=? x y Return `#t' iff X is the same character as Y, else `#f'. -- Scheme Procedure: char? x y Return `#t' iff X is greater than Y in the ASCII sequence, else `#f'. -- Scheme Procedure: char>=? x y Return `#t' iff X is greater than or equal to Y in the ASCII sequence, else `#f'. -- Scheme Procedure: char-ci=? x y Return `#t' iff X is the same character as Y ignoring case, else `#f'. -- Scheme Procedure: char-ci? x y Return `#t' iff X is greater than Y in the ASCII sequence ignoring case, else `#f'. -- Scheme Procedure: char-ci>=? x y Return `#t' iff X is greater than or equal to Y in the ASCII sequence ignoring case, else `#f'. -- Scheme Procedure: char-alphabetic? chr -- C Function: scm_char_alphabetic_p (chr) Return `#t' iff CHR is alphabetic, else `#f'. Alphabetic means the same thing as the isalpha C library function. -- Scheme Procedure: char-numeric? chr -- C Function: scm_char_numeric_p (chr) Return `#t' iff CHR is numeric, else `#f'. Numeric means the same thing as the isdigit C library function. -- Scheme Procedure: char-whitespace? chr -- C Function: scm_char_whitespace_p (chr) Return `#t' iff CHR is whitespace, else `#f'. Whitespace means the same thing as the isspace C library function. -- Scheme Procedure: char-upper-case? chr -- C Function: scm_char_upper_case_p (chr) Return `#t' iff CHR is uppercase, else `#f'. Uppercase means the same thing as the isupper C library function. -- Scheme Procedure: char-lower-case? chr -- C Function: scm_char_lower_case_p (chr) Return `#t' iff CHR is lowercase, else `#f'. Lowercase means the same thing as the islower C library function. -- Scheme Procedure: char-is-both? chr -- C Function: scm_char_is_both_p (chr) Return `#t' iff CHR is either uppercase or lowercase, else `#f'. Uppercase and lowercase are as defined by the isupper and islower C library functions. -- Scheme Procedure: char->integer chr -- C Function: scm_char_to_integer (chr) Return the number corresponding to ordinal position of CHR in the ASCII sequence. -- Scheme Procedure: integer->char n -- C Function: scm_integer_to_char (n) Return the character at position N in the ASCII sequence. -- Scheme Procedure: char-upcase chr -- C Function: scm_char_upcase (chr) Return the uppercase character version of CHR. -- Scheme Procedure: char-downcase chr -- C Function: scm_char_downcase (chr) Return the lowercase character version of CHR.  File: guile.info, Node: Strings, Next: Regular Expressions, Prev: Characters, Up: Simple Data Types 21.4 Strings ============ Strings are fixed-length sequences of characters. They can be created by calling constructor procedures, but they can also literally get entered at the REPL or in Scheme source files. Guile provides a rich set of string processing procedures, because text handling is very important when Guile is used as a scripting language. Strings always carry the information about how many characters they are composed of with them, so there is no special end-of-string character, like in C. That means that Scheme strings can contain any character, even the NUL character `'\0''. But note: Since most operating system calls dealing with strings (such as for file operations) expect strings to be zero-terminated, they might do unexpected things when called with string containing unusual characters. * Menu: * String Syntax:: Read syntax for strings. * String Predicates:: Testing strings for certain properties. * String Constructors:: Creating new string objects. * List/String Conversion:: Converting from/to lists of characters. * String Selection:: Select portions from strings. * String Modification:: Modify parts or whole strings. * String Comparison:: Lexicographic ordering predicates. * String Searching:: Searching in strings. * Alphabetic Case Mapping:: Convert the alphabetic case of strings. * Appending Strings:: Appending strings to form a new string.  File: guile.info, Node: String Syntax, Next: String Predicates, Up: Strings 21.4.1 String Read Syntax ------------------------- The read syntax for strings is an arbitrarily long sequence of characters enclosed in double quotes (`"'). (1) If you want to insert a double quote character into a string literal, it must be prefixed with a backslash `\' character (called an "escape character"). The following are examples of string literals: "foo" "bar plonk" "Hello World" "\"Hi\", he said." ---------- Footnotes ---------- (1) Actually, the current implementation restricts strings to a length of 2^24 characters.  File: guile.info, Node: String Predicates, Next: String Constructors, Prev: String Syntax, Up: Strings 21.4.2 String Predicates ------------------------ The following procedures can be used to check whether a given string fulfills some specified property. -- Scheme Procedure: string? obj -- C Function: scm_string_p (obj) Return `#t' if OBJ is a string, else `#f'. -- Scheme Procedure: string-null? str -- C Function: scm_string_null_p (str) Return `#t' if STR's length is zero, and `#f' otherwise. (string-null? "") => #t y => "foo" (string-null? y) => #f  File: guile.info, Node: String Constructors, Next: List/String Conversion, Prev: String Predicates, Up: Strings 21.4.3 String Constructors -------------------------- The string constructor procedures create new string objects, possibly initializing them with some specified character data. -- Scheme Procedure: string . chrs -- Scheme Procedure: list->string chrs -- C Function: scm_string (chrs) Return a newly allocated string composed of the arguments, CHRS. -- Scheme Procedure: make-string k [chr] -- C Function: scm_make_string (k, chr) Return a newly allocated string of length K. If CHR is given, then all elements of the string are initialized to CHR, otherwise the contents of the STRING are unspecified.  File: guile.info, Node: List/String Conversion, Next: String Selection, Prev: String Constructors, Up: Strings 21.4.4 List/String conversion ----------------------------- When processing strings, it is often convenient to first convert them into a list representation by using the procedure `string->list', work with the resulting list, and then convert it back into a string. These procedures are useful for similar tasks. -- Scheme Procedure: string->list str -- C Function: scm_string_to_list (str) Return a newly allocated list of the characters that make up the given string STR. `string->list' and `list->string' are inverses as far as `equal?' is concerned. -- Scheme Procedure: string-split str chr -- C Function: scm_string_split (str, chr) Split the string STR into the a list of the substrings delimited by appearances of the character CHR. Note that an empty substring between separator characters will result in an empty string in the result list. (string-split "root:x:0:0:root:/root:/bin/bash" #\:) => ("root" "x" "0" "0" "root" "/root" "/bin/bash") (string-split "::" #\:) => ("" "" "") (string-split "" #\:) => ("")  File: guile.info, Node: String Selection, Next: String Modification, Prev: List/String Conversion, Up: Strings 21.4.5 String Selection ----------------------- Portions of strings can be extracted by these procedures. `string-ref' delivers individual characters whereas `substring' can be used to extract substrings from longer strings. -- Scheme Procedure: string-length string -- C Function: scm_string_length (string) Return the number of characters in STRING. -- Scheme Procedure: string-ref str k -- C Function: scm_string_ref (str, k) Return character K of STR using zero-origin indexing. K must be a valid index of STR. -- Scheme Procedure: string-copy str -- C Function: scm_string_copy (str) Return a newly allocated copy of the given STRING. -- Scheme Procedure: substring str start [end] -- C Function: scm_substring (str, start, end) Return a newly allocated string formed from the characters of STR beginning with index START (inclusive) and ending with index END (exclusive). STR must be a string, START and END must be exact integers satisfying: 0 <= START <= END <= (string-length STR).  File: guile.info, Node: String Modification, Next: String Comparison, Prev: String Selection, Up: Strings 21.4.6 String Modification -------------------------- These procedures are for modifying strings in-place. This means that the result of the operation is not a new string; instead, the original string's memory representation is modified. -- Scheme Procedure: string-set! str k chr -- C Function: scm_string_set_x (str, k, chr) Store CHR in element K of STR and return an unspecified value. K must be a valid index of STR. -- Scheme Procedure: string-fill! str chr -- C Function: scm_string_fill_x (str, chr) Store CHAR in every element of the given STRING and return an unspecified value. -- Scheme Procedure: substring-fill! str start end fill -- C Function: scm_substring_fill_x (str, start, end, fill) Change every character in STR between START and END to FILL. (define y "abcdefg") (substring-fill! y 1 3 #\r) y => "arrdefg" -- Scheme Procedure: substring-move! str1 start1 end1 str2 start2 -- C Function: scm_substring_move_x (str1, start1, end1, str2, start2) Copy the substring of STR1 bounded by START1 and END1 into STR2 beginning at position START2. STR1 and STR2 can be the same string.  File: guile.info, Node: String Comparison, Next: String Searching, Prev: String Modification, Up: Strings 21.4.7 String Comparison ------------------------ The procedures in this section are similar to the character ordering predicates (*note Characters::), but are defined on character sequences. They all return `#t' on success and `#f' on failure. The predicates ending in `-ci' ignore the character case when comparing strings. -- Scheme Procedure: string=? s1 s2 Lexicographic equality predicate; return `#t' if the two strings are the same length and contain the same characters in the same positions, otherwise return `#f'. The procedure `string-ci=?' treats upper and lower case letters as though they were the same character, but `string=?' treats upper and lower case as distinct characters. -- Scheme Procedure: string? s1 s2 Lexicographic ordering predicate; return `#t' if S1 is lexicographically greater than S2. -- Scheme Procedure: string>=? s1 s2 Lexicographic ordering predicate; return `#t' if S1 is lexicographically greater than or equal to S2. -- Scheme Procedure: string-ci=? s1 s2 Case-insensitive string equality predicate; return `#t' if the two strings are the same length and their component characters match (ignoring case) at each position; otherwise return `#f'. -- Scheme Procedure: string-ci? s1 s2 Case insensitive lexicographic ordering predicate; return `#t' if S1 is lexicographically greater than S2 regardless of case. -- Scheme Procedure: string-ci>=? s1 s2 Case insensitive lexicographic ordering predicate; return `#t' if S1 is lexicographically greater than or equal to S2 regardless of case.  File: guile.info, Node: String Searching, Next: Alphabetic Case Mapping, Prev: String Comparison, Up: Strings 21.4.8 String Searching ----------------------- When searching for the index of a character in a string, these procedures can be used. -- Scheme Procedure: string-index str chr [frm [to]] -- C Function: scm_string_index (str, chr, frm, to) Return the index of the first occurrence of CHR in STR. The optional integer arguments FRM and TO limit the search to a portion of the string. This procedure essentially implements the `index' or `strchr' functions from the C library. (string-index "weiner" #\e) => 1 (string-index "weiner" #\e 2) => 4 (string-index "weiner" #\e 2 4) => #f -- Scheme Procedure: string-rindex str chr [frm [to]] -- C Function: scm_string_rindex (str, chr, frm, to) Like `string-index', but search from the right of the string rather than from the left. This procedure essentially implements the `rindex' or `strrchr' functions from the C library. (string-rindex "weiner" #\e) => 4 (string-rindex "weiner" #\e 2 4) => #f (string-rindex "weiner" #\e 2 5) => 4  File: guile.info, Node: Alphabetic Case Mapping, Next: Appending Strings, Prev: String Searching, Up: Strings 21.4.9 Alphabetic Case Mapping ------------------------------ These are procedures for mapping strings to their upper- or lower-case equivalents, respectively, or for capitalizing strings. -- Scheme Procedure: string-upcase str -- C Function: scm_string_upcase (str) Return a freshly allocated string containing the characters of STR in upper case. -- Scheme Procedure: string-upcase! str -- C Function: scm_string_upcase_x (str) Destructively upcase every character in STR and return STR. y => "arrdefg" (string-upcase! y) => "ARRDEFG" y => "ARRDEFG" -- Scheme Procedure: string-downcase str -- C Function: scm_string_downcase (str) Return a freshly allocation string containing the characters in STR in lower case. -- Scheme Procedure: string-downcase! str -- C Function: scm_string_downcase_x (str) Destructively downcase every character in STR and return STR. y => "ARRDEFG" (string-downcase! y) => "arrdefg" y => "arrdefg" -- Scheme Procedure: string-capitalize str -- C Function: scm_string_capitalize (str) Return a freshly allocated string with the characters in STR, where the first character of every word is capitalized. -- Scheme Procedure: string-capitalize! str -- C Function: scm_string_capitalize_x (str) Upcase the first character of every word in STR destructively and return STR. y => "hello world" (string-capitalize! y) => "Hello World" y => "Hello World"  File: guile.info, Node: Appending Strings, Prev: Alphabetic Case Mapping, Up: Strings 21.4.10 Appending Strings ------------------------- The procedure `string-append' appends several strings together to form a longer result string. -- Scheme Procedure: string-append . args -- C Function: scm_string_append (args) Return a newly allocated string whose characters form the concatenation of the given strings, ARGS. (let ((h "hello ")) (string-append h "world")) => "hello world"  File: guile.info, Node: Regular Expressions, Next: Symbols, Prev: Strings, Up: Simple Data Types 21.5 Regular Expressions ======================== A "regular expression" (or "regexp") is a pattern that describes a whole class of strings. A full description of regular expressions and their syntax is beyond the scope of this manual; an introduction can be found in the Emacs manual (*note Syntax of Regular Expressions: (emacs)Regexps.), or in many general Unix reference books. If your system does not include a POSIX regular expression library, and you have not linked Guile with a third-party regexp library such as Rx, these functions will not be available. You can tell whether your Guile installation includes regular expression support by checking whether `(provided? 'regex)' returns true. The following regexp and string matching features are provided by the `(ice-9 regex)' module. Before using the described functions, you should load this module by executing `(use-modules (ice-9 regex))'. * Menu: * Regexp Functions:: Functions that create and match regexps. * Match Structures:: Finding what was matched by a regexp. * Backslash Escapes:: Removing the special meaning of regexp meta-characters.  File: guile.info, Node: Regexp Functions, Next: Match Structures, Up: Regular Expressions 21.5.1 Regexp Functions ----------------------- By default, Guile supports POSIX extended regular expressions. That means that the characters `(', `)', `+' and `?' are special, and must be escaped if you wish to match the literal characters. This regular expression interface was modeled after that implemented by SCSH, the Scheme Shell. It is intended to be upwardly compatible with SCSH regular expressions. -- Scheme Procedure: string-match pattern str [start] Compile the string PATTERN into a regular expression and compare it with STR. The optional numeric argument START specifies the position of STR at which to begin matching. `string-match' returns a "match structure" which describes what, if anything, was matched by the regular expression. *Note Match Structures::. If STR does not match PATTERN at all, `string-match' returns `#f'. Two examples of a match follow. In the first example, the pattern matches the four digits in the match string. In the second, the pattern matches nothing. (string-match "[0-9][0-9][0-9][0-9]" "blah2002") => #("blah2002" (4 . 8)) (string-match "[A-Za-z]" "123456") => #f Each time `string-match' is called, it must compile its PATTERN argument into a regular expression structure. This operation is expensive, which makes `string-match' inefficient if the same regular expression is used several times (for example, in a loop). For better performance, you can compile a regular expression in advance and then match strings against the compiled regexp. -- Scheme Procedure: make-regexp pat . flags -- C Function: scm_make_regexp (pat, flags) Compile the regular expression described by PAT, and return the compiled regexp structure. If PAT does not describe a legal regular expression, `make-regexp' throws a `regular-expression-syntax' error. The FLAGS arguments change the behavior of the compiled regular expression. The following flags may be supplied: `regexp/icase' Consider uppercase and lowercase letters to be the same when matching. `regexp/newline' If a newline appears in the target string, then permit the `^' and `$' operators to match immediately after or immediately before the newline, respectively. Also, the `.' and `[^...]' operators will never match a newline character. The intent of this flag is to treat the target string as a buffer containing many lines of text, and the regular expression as a pattern that may match a single one of those lines. `regexp/basic' Compile a basic ("obsolete") regexp instead of the extended ("modern") regexps that are the default. Basic regexps do not consider `|', `+' or `?' to be special characters, and require the `{...}' and `(...)' metacharacters to be backslash-escaped (*note Backslash Escapes::). There are several other differences between basic and extended regular expressions, but these are the most significant. `regexp/extended' Compile an extended regular expression rather than a basic regexp. This is the default behavior; this flag will not usually be needed. If a call to `make-regexp' includes both `regexp/basic' and `regexp/extended' flags, the one which comes last will override the earlier one. -- Scheme Procedure: regexp-exec rx str [start [flags]] -- C Function: scm_regexp_exec (rx, str, start, flags) Match the compiled regular expression RX against `str'. If the optional integer START argument is provided, begin matching from that position in the string. Return a match structure describing the results of the match, or `#f' if no match could be found. The FLAGS arguments change the matching behavior. The following flags may be supplied: `regexp/notbol' Operator `^' always fails (unless `regexp/newline' is used). Use this when the beginning of the string should not be considered the beginning of a line. `regexp/noteol' Operator `$' always fails (unless `regexp/newline' is used). Use this when the end of the string should not be considered the end of a line. ;; Regexp to match uppercase letters (define r (make-regexp "[A-Z]*")) ;; Regexp to match letters, ignoring case (define ri (make-regexp "[A-Z]*" regexp/icase)) ;; Search for bob using regexp r (match:substring (regexp-exec r "bob")) => "" ; no match ;; Search for bob using regexp ri (match:substring (regexp-exec ri "Bob")) => "Bob" ; matched case insensitive -- Scheme Procedure: regexp? obj -- C Function: scm_regexp_p (obj) Return `#t' if OBJ is a compiled regular expression, or `#f' otherwise. Regular expressions are commonly used to find patterns in one string and replace them with the contents of another string. -- Scheme Procedure: regexp-substitute port match [item...] Write to the output port PORT selected contents of the match structure MATCH. Each ITEM specifies what should be written, and may be one of the following arguments: * A string. String arguments are written out verbatim. * An integer. The submatch with that number is written. * The symbol `pre'. The portion of the matched string preceding the regexp match is written. * The symbol `post'. The portion of the matched string following the regexp match is written. The PORT argument may be `#f', in which case nothing is written; instead, `regexp-substitute' constructs a string from the specified ITEMs and returns that. The following example takes a regular expression that matches a standard YYYYMMDD-format date such as `"20020828"'. The `regexp-substitute' call returns a string computed from the information in the match structure, consisting of the fields and text from the original string reordered and reformatted. (define date-regex "([0-9][0-9][0-9][0-9])([0-9][0-9])([0-9][0-9])") (define s "Date 20020429 12am.") (define sm (string-match date-regex s)) (regexp-substitute #f sm 'pre 2 "-" 3 "-" 1 'post " (" 0 ")") => "Date 04-29-2002 12am. (20020429)" -- Scheme Procedure: regexp-substitute/global port regexp target [item...] Similar to `regexp-substitute', but can be used to perform global substitutions on STR. Instead of taking a match structure as an argument, `regexp-substitute/global' takes two string arguments: a REGEXP string describing a regular expression, and a TARGET string which should be matched against this regular expression. Each ITEM behaves as in REGEXP-SUBSTITUTE, with the following exceptions: * A function may be supplied. When this function is called, it will be passed one argument: a match structure for a given regular expression match. It should return a string to be written out to PORT. * The `post' symbol causes `regexp-substitute/global' to recurse on the unmatched portion of STR. This _must_ be supplied in order to perform global search-and-replace on STR; if it is not present among the ITEMs, then `regexp-substitute/global' will return after processing a single match. The example above for `regexp-substitute' could be rewritten as follows to remove the `string-match' stage: (define date-regex "([0-9][0-9][0-9][0-9])([0-9][0-9])([0-9][0-9])") (define s "Date 20020429 12am.") (regexp-substitute/global #f date-regex s 'pre 2 "-" 3 "-" 1 'post " (" 0 ")") => "Date 04-29-2002 12am. (20020429)"  File: guile.info, Node: Match Structures, Next: Backslash Escapes, Prev: Regexp Functions, Up: Regular Expressions 21.5.2 Match Structures ----------------------- A "match structure" is the object returned by `string-match' and `regexp-exec'. It describes which portion of a string, if any, matched the given regular expression. Match structures include: a reference to the string that was checked for matches; the starting and ending positions of the regexp match; and, if the regexp included any parenthesized subexpressions, the starting and ending positions of each submatch. In each of the regexp match functions described below, the `match' argument must be a match structure returned by a previous call to `string-match' or `regexp-exec'. Most of these functions return some information about the original target string that was matched against a regular expression; we will call that string TARGET for easy reference. -- Scheme Procedure: regexp-match? obj Return `#t' if OBJ is a match structure returned by a previous call to `regexp-exec', or `#f' otherwise. -- Scheme Procedure: match:substring match [n] Return the portion of TARGET matched by subexpression number N. Submatch 0 (the default) represents the entire regexp match. If the regular expression as a whole matched, but the subexpression number N did not match, return `#f'. (define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo")) (match:substring s) => "2002" ;; match starting at offset 6 in the string (match:substring (string-match "[0-9][0-9][0-9][0-9]" "blah987654" 6)) => "7654" -- Scheme Procedure: match:start match [n] Return the starting position of submatch number N. In the following example, the result is 4, since the match starts at character index 4: (define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo")) (match:start s) => 4 -- Scheme Procedure: match:end match [n] Return the ending position of submatch number N. In the following example, the result is 8, since the match runs between characters 4 and 8 (i.e. the "2002"). (define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo")) (match:end s) => 8 -- Scheme Procedure: match:prefix match Return the unmatched portion of TARGET preceding the regexp match. (define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo")) (match:prefix s) => "blah" -- Scheme Procedure: match:suffix match Return the unmatched portion of TARGET following the regexp match. (define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo")) (match:suffix s) => "foo" -- Scheme Procedure: match:count match Return the number of parenthesized subexpressions from MATCH. Note that the entire regular expression match itself counts as a subexpression, and failed submatches are included in the count. -- Scheme Procedure: match:string match Return the original TARGET string. (define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo")) (match:string s) => "blah2002foo"  File: guile.info, Node: Backslash Escapes, Prev: Match Structures, Up: Regular Expressions 21.5.3 Backslash Escapes ------------------------ Sometimes you will want a regexp to match characters like `*' or `$' exactly. For example, to check whether a particular string represents a menu entry from an Info node, it would be useful to match it against a regexp like `^* [^:]*::'. However, this won't work; because the asterisk is a metacharacter, it won't match the `*' at the beginning of the string. In this case, we want to make the first asterisk un-magic. You can do this by preceding the metacharacter with a backslash character `\'. (This is also called "quoting" the metacharacter, and is known as a "backslash escape".) When Guile sees a backslash in a regular expression, it considers the following glyph to be an ordinary character, no matter what special meaning it would ordinarily have. Therefore, we can make the above example work by changing the regexp to `^\* [^:]*::'. The `\*' sequence tells the regular expression engine to match only a single asterisk in the target string. Since the backslash is itself a metacharacter, you may force a regexp to match a backslash in the target string by preceding the backslash with itself. For example, to find variable references in a TeX program, you might want to find occurrences of the string `\let\' followed by any number of alphabetic characters. The regular expression `\\let\\[A-Za-z]*' would do this: the double backslashes in the regexp each match a single backslash in the target string. -- Scheme Procedure: regexp-quote str Quote each special character found in STR with a backslash, and return the resulting string. *Very important:* Using backslash escapes in Guile source code (as in Emacs Lisp or C) can be tricky, because the backslash character has special meaning for the Guile reader. For example, if Guile encounters the character sequence `\n' in the middle of a string while processing Scheme code, it replaces those characters with a newline character. Similarly, the character sequence `\t' is replaced by a horizontal tab. Several of these "escape sequences" are processed by the Guile reader before your code is executed. Unrecognized escape sequences are ignored: if the characters `\*' appear in a string, they will be translated to the single character `*'. This translation is obviously undesirable for regular expressions, since we want to be able to include backslashes in a string in order to escape regexp metacharacters. Therefore, to make sure that a backslash is preserved in a string in your Guile program, you must use _two_ consecutive backslashes: (define Info-menu-entry-pattern (make-regexp "^\\* [^:]*")) The string in this example is preprocessed by the Guile reader before any code is executed. The resulting argument to `make-regexp' is the string `^\* [^:]*', which is what we really want. This also means that in order to write a regular expression that matches a single backslash character, the regular expression string in the source code must include _four_ backslashes. Each consecutive pair of backslashes gets translated by the Guile reader to a single backslash, and the resulting double-backslash is interpreted by the regexp engine as matching a single backslash character. Hence: (define tex-variable-pattern (make-regexp "\\\\let\\\\=[A-Za-z]*")) The reason for the unwieldiness of this syntax is historical. Both regular expression pattern matchers and Unix string processing systems have traditionally used backslashes with the special meanings described above. The POSIX regular expression specification and ANSI C standard both require these semantics. Attempting to abandon either convention would cause other kinds of compatibility problems, possibly more severe ones. Therefore, without extending the Scheme reader to support strings with different quoting conventions (an ungainly and confusing extension when implemented in other languages), we must adhere to this cumbersome escape syntax.  File: guile.info, Node: Symbols, Next: Keywords, Prev: Regular Expressions, Up: Simple Data Types 21.6 Symbols ============ Symbols in Scheme are widely used in three ways: as items of discrete data, as lookup keys for alists and hash tables, and to denote variable references. A "symbol" is similar to a string in that it is defined by a sequence of characters. The sequence of characters is known as the symbol's "name". In the usual case -- that is, where the symbol's name doesn't include any characters that could be confused with other elements of Scheme syntax -- a symbol is written in a Scheme program by writing the sequence of characters that make up the name, _without_ any quotation marks or other special syntax. For example, the symbol whose name is "multiply-by-2" is written, simply: multiply-by-2 Notice how this differs from a _string_ with contents "multiply-by-2", which is written with double quotation marks, like this: "multiply-by-2" Looking beyond how they are written, symbols are different from strings in two important respects. The first important difference is uniqueness. If the same-looking string is read twice from two different places in a program, the result is two _different_ string objects whose contents just happen to be the same. If, on the other hand, the same-looking symbol is read twice from two different places in a program, the result is the _same_ symbol object both times. Given two read symbols, you can use `eq?' to test whether they are the same (that is, have the same name). `eq?' is the most efficient comparison operator in Scheme, and comparing two symbols like this is as fast as comparing, for example, two numbers. Given two strings, on the other hand, you must use `equal?' or `string=?', which are much slower comparison operators, to determine whether the strings have the same contents. (define sym1 (quote hello)) (define sym2 (quote hello)) (eq? sym1 sym2) => #t (define str1 "hello") (define str2 "hello") (eq? str1 str2) => #f (equal? str1 str2) => #t The second important difference is that symbols, unlike strings, are not self-evaluating. This is why we need the `(quote ...)'s in the example above: `(quote hello)' evaluates to the symbol named "hello" itself, whereas an unquoted `hello' is _read_ as the symbol named "hello" and evaluated as a variable reference ... about which more below (*note Symbol Variables::). * Menu: * Symbol Data:: Symbols as discrete data. * Symbol Keys:: Symbols as lookup keys. * Symbol Variables:: Symbols as denoting variables. * Symbol Primitives:: Operations related to symbols. * Symbol Props:: Function slots and property lists. * Symbol Read Syntax:: Extended read syntax for symbols.  File: guile.info, Node: Symbol Data, Next: Symbol Keys, Up: Symbols 21.6.1 Symbols as Discrete Data ------------------------------- Numbers and symbols are similar to the extent that they both lend themselves to `eq?' comparison. But symbols are more descriptive than numbers, because a symbol's name can be used directly to describe the concept for which that symbol stands. For example, imagine that you need to represent some colours in a computer program. Using numbers, you would have to choose arbitrarily some mapping between numbers and colours, and then take care to use that mapping consistently: ;; 1=red, 2=green, 3=purple (if (eq? (colour-of car) 1) ...) You can make the mapping more explicit and the code more readable by defining constants: (define red 1) (define green 2) (define purple 3) (if (eq? (colour-of car) red) ...) But the simplest and clearest approach is not to use numbers at all, but symbols whose names specify the colours that they refer to: (if (eq? (colour-of car) 'red) ...) The descriptive advantages of symbols over numbers increase as the set of concepts that you want to describe grows. Suppose that a car object can have other properties as well, such as whether it has or uses: * automatic or manual transmission * leaded or unleaded fuel * power steering (or not). Then a car's combined property set could be naturally represented and manipulated as a list of symbols: (properties-of car1) => (red manual unleaded power-steering) (if (memq 'power-steering (properties-of car1)) (display "Unfit people can drive this car.\n") (display "You'll need strong arms to drive this car!\n")) -| Unfit people can drive this car. Remember, the fundamental property of symbols that we are relying on here is that an occurrence of `'red' in one part of a program is an _indistinguishable_ symbol from an occurrence of `'red' in another part of a program; this means that symbols can usefully be compared using `eq?'. At the same time, symbols have naturally descriptive names. This combination of efficiency and descriptive power makes them ideal for use as discrete data.  File: guile.info, Node: Symbol Keys, Next: Symbol Variables, Prev: Symbol Data, Up: Symbols 21.6.2 Symbols as Lookup Keys ----------------------------- Given their efficiency and descriptive power, it is natural to use symbols as the keys in an association list or hash table. To illustrate this, consider a more structured representation of the car properties example from the preceding subsection. Rather than mixing all the properties up together in a flat list, we could use an association list like this: (define car1-properties '((colour . red) (transmission . manual) (fuel . unleaded) (steering . power-assisted))) Notice how this structure is more explicit and extensible than the flat list. For example it makes clear that `manual' refers to the transmission rather than, say, the windows or the locking of the car. It also allows further properties to use the same symbols among their possible values without becoming ambiguous: (define car1-properties '((colour . red) (transmission . manual) (fuel . unleaded) (steering . power-assisted) (seat-colour . red) (locking . manual))) With a representation like this, it is easy to use the efficient `assq-XXX' family of procedures (*note Association Lists::) to extract or change individual pieces of information: (assq-ref car1-properties 'fuel) => unleaded (assq-ref car1-properties 'transmission) => manual (assq-set! car1-properties 'seat-colour 'black) => ((colour . red) (transmission . manual) (fuel . unleaded) (steering . power-assisted) (seat-colour . black) (locking . manual))) Hash tables also have keys, and exactly the same arguments apply to the use of symbols in hash tables as in association lists. The hash value that Guile uses to decide where to add a symbol-keyed entry to a hash table can be obtained by calling the `symbol-hash' procedure: -- Scheme Procedure: symbol-hash symbol -- C Function: scm_symbol_hash (symbol) Return a hash value for SYMBOL. See *Note Hash Tables:: for information about hash tables in general, and for why you might choose to use a hash table rather than an association list.  File: guile.info, Node: Symbol Variables, Next: Symbol Primitives, Prev: Symbol Keys, Up: Symbols 21.6.3 Symbols as Denoting Variables ------------------------------------ When an unquoted symbol in a Scheme program is evaluated, it is interpreted as a variable reference, and the result of the evaluation is the appropriate variable's value. For example, when the expression `(string-length "abcd")' is read and evaluated, the sequence of characters `string-length' is read as the symbol whose name is "string-length". This symbol is associated with a variable whose value is the procedure that implements string length calculation. Therefore evaluation of the `string-length' symbol results in that procedure. The details of the connection between an unquoted symbol and the variable to which it refers are explained elsewhere. See *Note Binding Constructs::, for how associations between symbols and variables are created, and *Note Modules::, for how those associations are affected by Guile's module system.  File: guile.info, Node: Symbol Primitives, Next: Symbol Props, Prev: Symbol Variables, Up: Symbols 21.6.4 Operations Related to Symbols ------------------------------------ Given any Scheme value, you can determine whether it is a symbol using the `symbol?' primitive: -- Scheme Procedure: symbol? obj -- C Function: scm_symbol_p (obj) Return `#t' if OBJ is a symbol, otherwise return `#f'. Once you know that you have a symbol, you can obtain its name as a string by calling `symbol->string'. Note that Guile differs by default from R5RS on the details of `symbol->string' as regards case-sensitivity: -- Scheme Procedure: symbol->string s -- C Function: scm_symbol_to_string (s) Return the name of symbol S as a string. By default, Guile reads symbols case-sensitively, so the string returned will have the same case variation as the sequence of characters that caused S to be created. If Guile is set to read symbols case-insensitively (as specified by R5RS), and S comes into being as part of a literal expression (*note Literal expressions: (r5rs)Literal expressions.) or by a call to the `read' or `string-ci->symbol' procedures, Guile converts any alphabetic characters in the symbol's name to lower case before creating the symbol object, so the string returned here will be in lower case. If S was created by `string->symbol', the case of characters in the string returned will be the same as that in the string that was passed to `string->symbol', regardless of Guile's case-sensitivity setting at the time S was created. It is an error to apply mutation procedures like `string-set!' to strings returned by this procedure. Most symbols are created by writing them literally in code. However it is also possible to create symbols programmatically using the following `string->symbol' and `string-ci->symbol' procedures: -- Scheme Procedure: string->symbol string -- C Function: scm_string_to_symbol (string) Return the symbol whose name is STRING. This procedure can create symbols with names containing special characters or letters in the non-standard case, but it is usually a bad idea to create such symbols because in some implementations of Scheme they cannot be read as themselves. -- Scheme Procedure: string-ci->symbol str -- C Function: scm_string_ci_to_symbol (str) Return the symbol whose name is STR. If Guile is currently reading symbols case-insensitively, STR is converted to lowercase before the returned symbol is looked up or created. The following examples illustrate Guile's detailed behaviour as regards the case-sensitivity of symbols: (read-enable 'case-insensitive) ; R5RS compliant behaviour (symbol->string 'flying-fish) => "flying-fish" (symbol->string 'Martin) => "martin" (symbol->string (string->symbol "Malvina")) => "Malvina" (eq? 'mISSISSIppi 'mississippi) => #t (string->symbol "mISSISSIppi") => mISSISSIppi (eq? 'bitBlt (string->symbol "bitBlt")) => #f (eq? 'LolliPop (string->symbol (symbol->string 'LolliPop))) => #t (string=? "K. Harper, M.D." (symbol->string (string->symbol "K. Harper, M.D."))) => #t (read-disable 'case-insensitive) ; Guile default behaviour (symbol->string 'flying-fish) => "flying-fish" (symbol->string 'Martin) => "Martin" (symbol->string (string->symbol "Malvina")) => "Malvina" (eq? 'mISSISSIppi 'mississippi) => #f (string->symbol "mISSISSIppi") => mISSISSIppi (eq? 'bitBlt (string->symbol "bitBlt")) => #t (eq? 'LolliPop (string->symbol (symbol->string 'LolliPop))) => #t (string=? "K. Harper, M.D." (symbol->string (string->symbol "K. Harper, M.D."))) => #t Finally, some applications, especially those that generate new Scheme code dynamically, need to generate symbols for use in the generated code. The `gensym' primitive meets this need: -- Scheme Procedure: gensym [prefix] -- C Function: scm_gensym (prefix) Create a new symbol with a name constructed from a prefix and a counter value. The string PREFIX can be specified as an optional argument. Default prefix is ` g'. The counter is increased by 1 at each call. There is no provision for resetting the counter. The symbols generated by `gensym' are _likely_ to be unique, since their names begin with a space and it is only otherwise possible to generate such symbols if a programmer goes out of their way to do so. The 1.8 release of Guile will include a way of creating symbols that are _guaranteed_ to be unique.  File: guile.info, Node: Symbol Props, Next: Symbol Read Syntax, Prev: Symbol Primitives, Up: Symbols 21.6.5 Function Slots and Property Lists ---------------------------------------- In traditional Lisp dialects, symbols are often understood as having three kinds of value at once: * a "variable" value, which is used when the symbol appears in code in a variable reference context * a "function" value, which is used when the symbol appears in code in a function name position (i.e. as the first element in an unquoted list) * a "property list" value, which is used when the symbol is given as the first argument to Lisp's `put' or `get' functions. Although Scheme (as one of its simplifications with respect to Lisp) does away with the distinction between variable and function namespaces, Guile currently retains some elements of the traditional structure in case they turn out to be useful when implementing translators for other languages, in particular Emacs Lisp. Specifically, Guile symbols have two extra slots. for a symbol's property list, and for its "function value." The following procedures are provided to access these slots. -- Scheme Procedure: symbol-fref symbol -- C Function: scm_symbol_fref (symbol) Return the contents of SYMBOL's "function slot". -- Scheme Procedure: symbol-fset! symbol value -- C Function: scm_symbol_fset_x (symbol, value) Set the contents of SYMBOL's function slot to VALUE. -- Scheme Procedure: symbol-pref symbol -- C Function: scm_symbol_pref (symbol) Return the "property list" currently associated with SYMBOL. -- Scheme Procedure: symbol-pset! symbol value -- C Function: scm_symbol_pset_x (symbol, value) Set SYMBOL's property list to VALUE. -- Scheme Procedure: symbol-property sym prop From SYM's property list, return the value for property PROP. The assumption is that SYM's property list is an association list whose keys are distinguished from each other using `equal?'; PROP should be one of the keys in that list. If the property list has no entry for PROP, `symbol-property' returns `#f'. -- Scheme Procedure: set-symbol-property! sym prop val In SYM's property list, set the value for property PROP to VAL, or add a new entry for PROP, with value VAL, if none already exists. For the structure of the property list, see `symbol-property'. -- Scheme Procedure: symbol-property-remove! sym prop From SYM's property list, remove the entry for property PROP, if there is one. For the structure of the property list, see `symbol-property'. Support for these extra slots may be removed in a future release, and it is probably better to avoid using them. (In release 1.6, Guile itself uses the property list slot sparingly, and the function slot not at all.) For a more modern and Schemely approach to properties, see *Note Object Properties::.  File: guile.info, Node: Symbol Read Syntax, Prev: Symbol Props, Up: Symbols 21.6.6 Extended Read Syntax for Symbols --------------------------------------- The read syntax for a symbol is a sequence of letters, digits, and "extended alphabetic characters", beginning with a character that cannot begin a number. In addition, the special cases of `+', `-', and `...' are read as symbols even though numbers can begin with `+', `-' or `.'. Extended alphabetic characters may be used within identifiers as if they were letters. The set of extended alphabetic characters is: ! $ % & * + - . / : < = > ? @ ^ _ ~ In addition to the standard read syntax defined above (which is taken from R5RS (*note Formal syntax: (r5rs)Formal syntax.)), Guile provides an extended symbol read syntax that allows the inclusion of unusual characters such as space characters, newlines and parentheses. If (for whatever reason) you need to write a symbol containing characters not mentioned above, you can do so as follows. * Begin the symbol with the characters `#{', * write the characters of the symbol and * finish the symbol with the characters `}#'. Here are a few examples of this form of read syntax. The first symbol needs to use extended syntax because it contains a space character, the second because it contains a line break, and the last because it looks like a number. #{foo bar}# #{what ever}# #{4242}# Although Guile provides this extended read syntax for symbols, widespread usage of it is discouraged because it is not portable and not very readable.  File: guile.info, Node: Keywords, Next: Other Types, Prev: Symbols, Up: Simple Data Types 21.7 Keywords ============= Keywords are self-evaluating objects with a convenient read syntax that makes them easy to type. Guile's keyword support conforms to R5RS, and adds a (switchable) read syntax extension to permit keywords to begin with `:' as well as `#:'. * Menu: * Why Use Keywords?:: Motivation for keyword usage. * Coding With Keywords:: How to use keywords. * Keyword Read Syntax:: Read syntax for keywords. * Keyword Procedures:: Procedures for dealing with keywords. * Keyword Primitives:: The underlying primitive procedures.  File: guile.info, Node: Why Use Keywords?, Next: Coding With Keywords, Up: Keywords 21.7.1 Why Use Keywords? ------------------------ Keywords are useful in contexts where a program or procedure wants to be able to accept a large number of optional arguments without making its interface unmanageable. To illustrate this, consider a hypothetical `make-window' procedure, which creates a new window on the screen for drawing into using some graphical toolkit. There are many parameters that the caller might like to specify, but which could also be sensibly defaulted, for example: * color depth - Default: the color depth for the screen * background color - Default: white * width - Default: 600 * height - Default: 400 If `make-window' did not use keywords, the caller would have to pass in a value for each possible argument, remembering the correct argument order and using a special value to indicate the default value for that argument: (make-window 'default ;; Color depth 'default ;; Background color 800 ;; Width 100 ;; Height ...) ;; More make-window arguments With keywords, on the other hand, defaulted arguments are omitted, and non-default arguments are clearly tagged by the appropriate keyword. As a result, the invocation becomes much clearer: (make-window #:width 800 #:height 100) On the other hand, for a simpler procedure with few arguments, the use of keywords would be a hindrance rather than a help. The primitive procedure `cons', for example, would not be improved if it had to be invoked as (cons #:car x #:cdr y) So the decision whether to use keywords or not is purely pragmatic: use them if they will clarify the procedure invocation at point of call.  File: guile.info, Node: Coding With Keywords, Next: Keyword Read Syntax, Prev: Why Use Keywords?, Up: Keywords 21.7.2 Coding With Keywords --------------------------- If a procedure wants to support keywords, it should take a rest argument and then use whatever means is convenient to extract keywords and their corresponding arguments from the contents of that rest argument. The following example illustrates the principle: the code for `make-window' uses a helper procedure called `get-keyword-value' to extract individual keyword arguments from the rest argument. (define (get-keyword-value args keyword default) (let ((kv (memq keyword args))) (if (and kv (>= (length kv) 2)) (cadr kv) default))) (define (make-window . args) (let ((depth (get-keyword-value args #:depth screen-depth)) (bg (get-keyword-value args #:bg "white")) (width (get-keyword-value args #:width 800)) (height (get-keyword-value args #:height 100)) ...) ...)) But you don't need to write `get-keyword-value'. The `(ice-9 optargs)' module provides a set of powerful macros that you can use to implement keyword-supporting procedures like this: (use-modules (ice-9 optargs)) (define (make-window . args) (let-keywords args #f ((depth screen-depth) (bg "white") (width 800) (height 100)) ...)) Or, even more economically, like this: (use-modules (ice-9 optargs)) (define* (make-window #:key (depth screen-depth) (bg "white") (width 800) (height 100)) ...) For further details on `let-keywords', `define*' and other facilities provided by the `(ice-9 optargs)' module, see *Note Optional Arguments::.  File: guile.info, Node: Keyword Read Syntax, Next: Keyword Procedures, Prev: Coding With Keywords, Up: Keywords 21.7.3 Keyword Read Syntax -------------------------- Guile, by default, only recognizes the keyword syntax specified by R5RS. A token of the form `#:NAME', where `NAME' has the same syntax as a Scheme symbol (*note Symbol Read Syntax::), is the external representation of the keyword named `NAME'. Keyword objects print using this syntax as well, so values containing keyword objects can be read back into Guile. When used in an expression, keywords are self-quoting objects. If the `keyword' read option is set to `'prefix', Guile also recognizes the alternative read syntax `:NAME'. Otherwise, tokens of the form `:NAME' are read as symbols, as required by R5RS. To enable and disable the alternative non-R5RS keyword syntax, you use the `read-set!' procedure documented in *Note User level options interfaces:: and *Note Reader options::. (read-set! keywords 'prefix) #:type => #:type :type => #:type (read-set! keywords #f) #:type => #:type :type -| ERROR: In expression :type: ERROR: Unbound variable: :type ABORT: (unbound-variable)  File: guile.info, Node: Keyword Procedures, Next: Keyword Primitives, Prev: Keyword Read Syntax, Up: Keywords 21.7.4 Keyword Procedures ------------------------- The following procedures can be used for converting symbols to keywords and back. -- Scheme Procedure: symbol->keyword sym Return a keyword with the same characters as in SYM. -- Scheme Procedure: keyword->symbol kw Return a symbol with the same characters as in KW.  File: guile.info, Node: Keyword Primitives, Prev: Keyword Procedures, Up: Keywords 21.7.5 Keyword Primitives ------------------------- Internally, a keyword is implemented as something like a tagged symbol, where the tag identifies the keyword as being self-evaluating, and the symbol, known as the keyword's "dash symbol" has the same name as the keyword name but prefixed by a single dash. For example, the keyword `#:name' has the corresponding dash symbol `-name'. Most keyword objects are constructed automatically by the reader when it reads a token beginning with `#:'. However, if you need to construct a keyword object programmatically, you can do so by calling `make-keyword-from-dash-symbol' with the corresponding dash symbol (as the reader does). The dash symbol for a keyword object can be retrieved using the `keyword-dash-symbol' procedure. -- Scheme Procedure: make-keyword-from-dash-symbol symbol -- C Function: scm_make_keyword_from_dash_symbol (symbol) Make a keyword object from a SYMBOL that starts with a dash. -- Scheme Procedure: keyword? obj -- C Function: scm_keyword_p (obj) Return `#t' if the argument OBJ is a keyword, else `#f'. -- Scheme Procedure: keyword-dash-symbol keyword -- C Function: scm_keyword_dash_symbol (keyword) Return the dash symbol for KEYWORD. This is the inverse of `make-keyword-from-dash-symbol'.  File: guile.info, Node: Other Types, Prev: Keywords, Up: Simple Data Types 21.8 "Functionality-Centric" Data Types ======================================= Procedures and macros are documented in their own chapter: see *Note Procedures and Macros::. Variable objects are documented as part of the description of Guile's module system: see *Note Variables::. Asyncs, dynamic roots and fluids are described in the chapter on scheduling: see *Note Scheduling::. Hooks are documented in the chapter on general utility functions: see *Note Hooks::. Ports are described in the chapter on I/O: see *Note Input and Output::.  File: guile.info, Node: Compound Data Types, Next: Procedures and Macros, Prev: Simple Data Types, Up: Top 22 Compound Data Types ********************** This chapter describes Guile's compound data types. By "compound" we mean that the primary purpose of these data types is to act as containers for other kinds of data (including other compound objects). For instance, a (non-uniform) vector with length 5 is a container that can hold five arbitrary Scheme objects. The various kinds of container object differ from each other in how their memory is allocated, how they are indexed, and how particular values can be looked up within them. * Menu: * Pairs:: Scheme's basic building block. * Lists:: Special list functions supported by Guile. * Vectors:: One-dimensional arrays of Scheme objects. * Records:: * Structures:: * Arrays:: Arrays of values. * Association Lists and Hash Tables:: Dictionary data types.  File: guile.info, Node: Pairs, Next: Lists, Up: Compound Data Types 22.1 Pairs ========== Pairs are used to combine two Scheme objects into one compound object. Hence the name: A pair stores a pair of objects. The data type "pair" is extremely important in Scheme, just like in any other Lisp dialect. The reason is that pairs are not only used to make two values available as one object, but that pairs are used for constructing lists of values. Because lists are so important in Scheme, they are described in a section of their own (*note Lists::). Pairs can literally get entered in source code or at the REPL, in the so-called "dotted list" syntax. This syntax consists of an opening parentheses, the first element of the pair, a dot, the second element and a closing parentheses. The following example shows how a pair consisting of the two numbers 1 and 2, and a pair containing the symbols `foo' and `bar' can be entered. It is very important to write the whitespace before and after the dot, because otherwise the Scheme parser would not be able to figure out where to split the tokens. (1 . 2) (foo . bar) But beware, if you want to try out these examples, you have to "quote" the expressions. More information about quotation is available in the section (REFFIXME). The correct way to try these examples is as follows. '(1 . 2) => (1 . 2) '(foo . bar) => (foo . bar) A new pair is made by calling the procedure `cons' with two arguments. Then the argument values are stored into a newly allocated pair, and the pair is returned. The name `cons' stands for "construct". Use the procedure `pair?' to test whether a given Scheme object is a pair or not. -- Scheme Procedure: cons x y -- C Function: scm_cons (x, y) Return a newly allocated pair whose car is X and whose cdr is Y. The pair is guaranteed to be different (in the sense of `eq?') from every previously existing object. -- Scheme Procedure: pair? x -- C Function: scm_pair_p (x) Return `#t' if X is a pair; otherwise return `#f'. The two parts of a pair are traditionally called "car" and "cdr". They can be retrieved with procedures of the same name (`car' and `cdr'), and can be modified with the procedures `set-car!' and `set-cdr!'. Since a very common operation in Scheme programs is to access the car of a pair, or the car of the cdr of a pair, etc., the procedures called `caar', `cadr' and so on are also predefined. -- Scheme Procedure: car pair -- Scheme Procedure: cdr pair Return the car or the cdr of PAIR, respectively. -- Scheme Procedure: caar pair -- Scheme Procedure: cadr pair ... -- Scheme Procedure: cdddar pair -- Scheme Procedure: cddddr pair These procedures are compositions of `car' and `cdr', where for example `caddr' could be defined by (define caddr (lambda (x) (car (cdr (cdr x))))) -- Scheme Procedure: set-car! pair value -- C Function: scm_set_car_x (pair, value) Stores VALUE in the car field of PAIR. The value returned by `set-car!' is unspecified. -- Scheme Procedure: set-cdr! pair value -- C Function: scm_set_cdr_x (pair, value) Stores VALUE in the cdr field of PAIR. The value returned by `set-cdr!' is unspecified.  File: guile.info, Node: Lists, Next: Vectors, Prev: Pairs, Up: Compound Data Types 22.2 Lists ========== A very important data type in Scheme--as well as in all other Lisp dialects--is the data type "list".(1) This is the short definition of what a list is: * Either the empty list `()', * or a pair which has a list in its cdr. * Menu: * List Syntax:: Writing literal lists. * List Predicates:: Testing lists. * List Constructors:: Creating new lists. * List Selection:: Selecting from lists, getting their length. * Append/Reverse:: Appending and reversing lists. * List Modification:: Modifying existing lists. * List Searching:: Searching for list elements * List Mapping:: Applying procedures to lists. ---------- Footnotes ---------- (1) Strictly speaking, Scheme does not have a real datatype "list". Lists are made up of "chained pairs", and only exist by definition--a list is a chain of pairs which looks like a list.  File: guile.info, Node: List Syntax, Next: List Predicates, Up: Lists 22.2.1 List Read Syntax ----------------------- The syntax for lists is an opening parentheses, then all the elements of the list (separated by whitespace) and finally a closing parentheses.(1). (1 2 3) ; a list of the numbers 1, 2 and 3 ("foo" bar 3.1415) ; a string, a symbol and a real number () ; the empty list The last example needs a bit more explanation. A list with no elements, called the "empty list", is special in some ways. It is used for terminating lists by storing it into the cdr of the last pair that makes up a list. An example will clear that up: (car '(1)) => 1 (cdr '(1)) => () This example also shows that lists have to be quoted (REFFIXME) when written, because they would otherwise be mistakingly taken as procedure applications (*note Simple Invocation::). ---------- Footnotes ---------- (1) Note that there is no separation character between the list elements, like a comma or a semicolon.  File: guile.info, Node: List Predicates, Next: List Constructors, Prev: List Syntax, Up: Lists 22.2.2 List Predicates ---------------------- Often it is useful to test whether a given Scheme object is a list or not. List-processing procedures could use this information to test whether their input is valid, or they could do different things depending on the datatype of their arguments. -- Scheme Procedure: list? x -- C Function: scm_list_p (x) Return `#t' iff X is a proper list, else `#f'. The predicate `null?' is often used in list-processing code to tell whether a given list has run out of elements. That is, a loop somehow deals with the elements of a list until the list satisfies `null?'. Then, the algorithm terminates. -- Scheme Procedure: null? x -- C Function: scm_null_p (x) Return `#t' iff X is the empty list, else `#f'.  File: guile.info, Node: List Constructors, Next: List Selection, Prev: List Predicates, Up: Lists 22.2.3 List Constructors ------------------------ This section describes the procedures for constructing new lists. `list' simply returns a list where the elements are the arguments, `cons*' is similar, but the last argument is stored in the cdr of the last pair of the list. -- Scheme Procedure: list . objs -- C Function: scm_list (objs) Return a list containing OBJS, the arguments to `list'. -- Scheme Procedure: cons* arg1 arg2 ... -- C Function: scm_cons_star (arg1, rest) Like `list', but the last arg provides the tail of the constructed list, returning `(cons ARG1 (cons ARG2 (cons ... ARGN)))'. Requires at least one argument. If given one argument, that argument is returned as result. This function is called `list*' in some other Schemes and in Common LISP. -- Scheme Procedure: list-copy lst -- C Function: scm_list_copy (lst) Return a (newly-created) copy of LST. -- Scheme Procedure: make-list n [init] Create a list containing of N elements, where each element is initialized to INIT. INIT defaults to the empty list `()' if not given. Note that `list-copy' only makes a copy of the pairs which make up the spine of the lists. The list elements are not copied, which means that modifying the elements of the new list also modifies the elements of the old list. On the other hand, applying procedures like `set-cdr!' or `delv!' to the new list will not alter the old list. If you also need to copy the list elements (making a deep copy), use the procedure `copy-tree' (*note Copying::).  File: guile.info, Node: List Selection, Next: Append/Reverse, Prev: List Constructors, Up: Lists 22.2.4 List Selection --------------------- These procedures are used to get some information about a list, or to retrieve one or more elements of a list. -- Scheme Procedure: length lst -- C Function: scm_length (lst) Return the number of elements in list LST. -- Scheme Procedure: last-pair lst -- C Function: scm_last_pair (lst) Return a pointer to the last pair in LST, signalling an error if LST is circular. -- Scheme Procedure: list-ref list k -- C Function: scm_list_ref (list, k) Return the Kth element from LIST. -- Scheme Procedure: list-tail lst k -- Scheme Procedure: list-cdr-ref lst k -- C Function: scm_list_tail (lst, k) Return the "tail" of LST beginning with its Kth element. The first element of the list is considered to be element 0. `list-tail' and `list-cdr-ref' are identical. It may help to think of `list-cdr-ref' as accessing the Kth cdr of the list, or returning the results of cdring K times down LST. -- Scheme Procedure: list-head lst k -- C Function: scm_list_head (lst, k) Copy the first K elements from LST into a new list, and return it.  File: guile.info, Node: Append/Reverse, Next: List Modification, Prev: List Selection, Up: Lists 22.2.5 Append and Reverse ------------------------- `append' and `append!' are used to concatenate two or more lists in order to form a new list. `reverse' and `reverse!' return lists with the same elements as their arguments, but in reverse order. The procedure variants with an `!' directly modify the pairs which form the list, whereas the other procedures create new pairs. This is why you should be careful when using the side-effecting variants. -- Scheme Procedure: append . args -- C Function: scm_append (args) Return a list consisting of the elements the lists passed as arguments. (append '(x) '(y)) => (x y) (append '(a) '(b c d)) => (a b c d) (append '(a (b)) '((c))) => (a (b) (c)) The resulting list is always newly allocated, except that it shares structure with the last list argument. The last argument may actually be any object; an improper list results if the last argument is not a proper list. (append '(a b) '(c . d)) => (a b c . d) (append '() 'a) => a -- Scheme Procedure: append! . lists -- C Function: scm_append_x (lists) A destructive version of `append' (*note Pairs and lists: (r5rs)Pairs and lists.). The cdr field of each list's final pair is changed to point to the head of the next list, so no consing is performed. Return a pointer to the mutated list. -- Scheme Procedure: reverse lst -- C Function: scm_reverse (lst) Return a new list that contains the elements of LST but in reverse order. -- Scheme Procedure: reverse! lst [new_tail] -- C Function: scm_reverse_x (lst, new_tail) A destructive version of `reverse' (*note Pairs and lists: (r5rs)Pairs and lists.). The cdr of each cell in LST is modified to point to the previous list element. Return a pointer to the head of the reversed list. Caveat: because the list is modified in place, the tail of the original list now becomes its head, and the head of the original list now becomes the tail. Therefore, the LST symbol to which the head of the original list was bound now points to the tail. To ensure that the head of the modified list is not lost, it is wise to save the return value of `reverse!'  File: guile.info, Node: List Modification, Next: List Searching, Prev: Append/Reverse, Up: Lists 22.2.6 List Modification ------------------------ The following procedures modify an existing list, either by changing elements of the list, or by changing the list structure itself. -- Scheme Procedure: list-set! list k val -- C Function: scm_list_set_x (list, k, val) Set the Kth element of LIST to VAL. -- Scheme Procedure: list-cdr-set! list k val -- C Function: scm_list_cdr_set_x (list, k, val) Set the Kth cdr of LIST to VAL. -- Scheme Procedure: delq item lst -- C Function: scm_delq (item, lst) Return a newly-created copy of LST with elements `eq?' to ITEM removed. This procedure mirrors `memq': `delq' compares elements of LST against ITEM with `eq?'. -- Scheme Procedure: delv item lst -- C Function: scm_delv (item, lst) Return a newly-created copy of LST with elements `eqv?' to ITEM removed. This procedure mirrors `memv': `delv' compares elements of LST against ITEM with `eqv?'. -- Scheme Procedure: delete item lst -- C Function: scm_delete (item, lst) Return a newly-created copy of LST with elements `equal?' to ITEM removed. This procedure mirrors `member': `delete' compares elements of LST against ITEM with `equal?'. -- Scheme Procedure: delq! item lst -- Scheme Procedure: delv! item lst -- Scheme Procedure: delete! item lst -- C Function: scm_delq_x (item, lst) -- C Function: scm_delv_x (item, lst) -- C Function: scm_delete_x (item, lst) These procedures are destructive versions of `delq', `delv' and `delete': they modify the pointers in the existing LST rather than creating a new list. Caveat evaluator: Like other destructive list functions, these functions cannot modify the binding of LST, and so cannot be used to delete the first element of LST destructively. -- Scheme Procedure: delq1! item lst -- C Function: scm_delq1_x (item, lst) Like `delq!', but only deletes the first occurrence of ITEM from LST. Tests for equality using `eq?'. See also `delv1!' and `delete1!'. -- Scheme Procedure: delv1! item lst -- C Function: scm_delv1_x (item, lst) Like `delv!', but only deletes the first occurrence of ITEM from LST. Tests for equality using `eqv?'. See also `delq1!' and `delete1!'. -- Scheme Procedure: delete1! item lst -- C Function: scm_delete1_x (item, lst) Like `delete!', but only deletes the first occurrence of ITEM from LST. Tests for equality using `equal?'. See also `delq1!' and `delv1!'.  File: guile.info, Node: List Searching, Next: List Mapping, Prev: List Modification, Up: Lists 22.2.7 List Searching --------------------- The following procedures search lists for particular elements. They use different comparison predicates for comparing list elements with the object to be searched. When they fail, they return `#f', otherwise they return the sublist whose car is equal to the search object, where equality depends on the equality predicate used. -- Scheme Procedure: memq x lst -- C Function: scm_memq (x, lst) Return the first sublist of LST whose car is `eq?' to X where the sublists of LST are the non-empty lists returned by `(list-tail LST K)' for K less than the length of LST. If X does not occur in LST, then `#f' (not the empty list) is returned. -- Scheme Procedure: memv x lst -- C Function: scm_memv (x, lst) Return the first sublist of LST whose car is `eqv?' to X where the sublists of LST are the non-empty lists returned by `(list-tail LST K)' for K less than the length of LST. If X does not occur in LST, then `#f' (not the empty list) is returned. -- Scheme Procedure: member x lst -- C Function: scm_member (x, lst) Return the first sublist of LST whose car is `equal?' to X where the sublists of LST are the non-empty lists returned by `(list-tail LST K)' for K less than the length of LST. If X does not occur in LST, then `#f' (not the empty list) is returned.  File: guile.info, Node: List Mapping, Prev: List Searching, Up: Lists 22.2.8 List Mapping ------------------- List processing is very convenient in Scheme because the process of iterating over the elements of a list can be highly abstracted. The procedures in this section are the most basic iterating procedures for lists. They take a procedure and one or more lists as arguments, and apply the procedure to each element of the list. They differ in their return value. -- Scheme Procedure: map proc arg1 arg2 ... -- Scheme Procedure: map-in-order proc arg1 arg2 ... -- C Function: scm_map (proc, arg1, args) Apply PROC to each element of the list ARG1 (if only two arguments are given), or to the corresponding elements of the argument lists (if more than two arguments are given). The result(s) of the procedure applications are saved and returned in a list. For `map', the order of procedure applications is not specified, `map-in-order' applies the procedure from left to right to the list elements. -- Scheme Procedure: for-each proc arg1 arg2 ... Like `map', but the procedure is always applied from left to right, and the result(s) of the procedure applications are thrown away. The return value is not specified.  File: guile.info, Node: Vectors, Next: Records, Prev: Lists, Up: Compound Data Types 22.3 Vectors ============ Vectors are sequences of Scheme objects. Unlike lists, the length of a vector, once the vector is created, cannot be changed. The advantage of vectors over lists is that the time required to access one element of a vector given its "position" (synonymous with "index"), a zero-origin number, is constant, whereas lists have an access time linear to the position of the accessed element in the list. Vectors can contain any kind of Scheme object; it is even possible to have different types of objects in the same vector. For vectors containing vectors, you may wish to use arrays, instead. Note, too, that some array procedures operate happily on vectors (*note Arrays::). * Menu: * Vector Syntax:: Read syntax for vectors. * Vector Creation:: Dynamic vector creation and validation. * Vector Accessors:: Accessing and modifying vector contents.  File: guile.info, Node: Vector Syntax, Next: Vector Creation, Up: Vectors 22.3.1 Read Syntax for Vectors ------------------------------ Vectors can literally be entered in source code, just like strings, characters or some of the other data types. The read syntax for vectors is as follows: A sharp sign (`#'), followed by an opening parentheses, all elements of the vector in their respective read syntax, and finally a closing parentheses. The following are examples of the read syntax for vectors; where the first vector only contains numbers and the second three different object types: a string, a symbol and a number in hexadecimal notation. #(1 2 3) #("Hello" foo #xdeadbeef)  File: guile.info, Node: Vector Creation, Next: Vector Accessors, Prev: Vector Syntax, Up: Vectors 22.3.2 Dynamic Vector Creation and Validation --------------------------------------------- Instead of creating a vector implicitly by using the read syntax just described, you can create a vector dynamically by calling one of the `vector' and `list->vector' primitives with the list of Scheme values that you want to place into a vector. The size of the vector thus created is determined implicitly by the number of arguments given. -- Scheme Procedure: vector . l -- Scheme Procedure: list->vector l -- C Function: scm_vector (l) Return a newly allocated vector composed of the given arguments. Analogous to `list'. (vector 'a 'b 'c) => #(a b c) (As an aside, an interesting implementation detail is that the Guile reader reads the `#(...)' syntax by reading everything but the initial `#' as a _list_, and then passing the list that results to `list->vector'. Notice how neatly this fits with the similarity between the read (and print) syntaxes for lists and vectors.) The inverse operation is `vector->list': -- Scheme Procedure: vector->list v -- C Function: scm_vector_to_list (v) Return a newly allocated list composed of the elements of V. (vector->list '#(dah dah didah)) => (dah dah didah) (list->vector '(dididit dah)) => #(dididit dah) To allocate a vector with an explicitly specified size, use `make-vector'. With this primitive you can also specify an initial value for the vector elements (the same value for all elements, that is): -- Scheme Procedure: make-vector k [fill] -- C Function: scm_make_vector (k, fill) Return a newly allocated vector of K elements. If a second argument is given, then each position is initialized to FILL. Otherwise the initial contents of each position is unspecified. To check whether an arbitrary Scheme value _is_ a vector, use the `vector?' primitive: -- Scheme Procedure: vector? obj -- C Function: scm_vector_p (obj) Return `#t' if OBJ is a vector, otherwise return `#f'.  File: guile.info, Node: Vector Accessors, Prev: Vector Creation, Up: Vectors 22.3.3 Accessing and Modifying Vector Contents ---------------------------------------------- `vector-length' and `vector-ref' return information about a given vector, respectively its size and the elements that are contained in the vector. -- Scheme Procedure: vector-length vector -- C Function: scm_vector_length vector Return the number of elements in VECTOR as an exact integer. -- Scheme Procedure: vector-ref vector k -- C Function: scm_vector_ref vector k Return the contents of position K of VECTOR. K must be a valid index of VECTOR. (vector-ref '#(1 1 2 3 5 8 13 21) 5) => 8 (vector-ref '#(1 1 2 3 5 8 13 21) (let ((i (round (* 2 (acos -1))))) (if (inexact? i) (inexact->exact i) i))) => 13 A vector created by one of the dynamic vector constructor procedures (*note Vector Creation::) can be modified using the following procedures. _NOTE:_ According to R5RS, it is an error to use any of these procedures on a literally read vector, because such vectors should be considered as constants. Currently, however, Guile does not detect this error. -- Scheme Procedure: vector-set! vector k obj -- C Function: scm_vector_set_x vector k obj Store OBJ in position K of VECTOR. K must be a valid index of VECTOR. The value returned by `vector-set!' is unspecified. (let ((vec (vector 0 '(2 2 2 2) "Anna"))) (vector-set! vec 1 '("Sue" "Sue")) vec) => #(0 ("Sue" "Sue") "Anna") -- Scheme Procedure: vector-fill! v fill -- C Function: scm_vector_fill_x (v, fill) Store FILL in every position of VECTOR. The value returned by `vector-fill!' is unspecified. -- Scheme Procedure: vector-move-left! vec1 start1 end1 vec2 start2 -- C Function: scm_vector_move_left_x (vec1, start1, end1, vec2, start2) Copy elements from VEC1, positions START1 to END1, to VEC2 starting at position START2. START1 and START2 are inclusive indices; END1 is exclusive. `vector-move-left!' copies elements in leftmost order. Therefore, in the case where VEC1 and VEC2 refer to the same vector, `vector-move-left!' is usually appropriate when START1 is greater than START2. -- Scheme Procedure: vector-move-right! vec1 start1 end1 vec2 start2 -- C Function: scm_vector_move_right_x (vec1, start1, end1, vec2, start2) Copy elements from VEC1, positions START1 to END1, to VEC2 starting at position START2. START1 and START2 are inclusive indices; END1 is exclusive. `vector-move-right!' copies elements in rightmost order. Therefore, in the case where VEC1 and VEC2 refer to the same vector, `vector-move-right!' is usually appropriate when START1 is less than START2.  File: guile.info, Node: Records, Next: Structures, Prev: Vectors, Up: Compound Data Types 22.4 Records ============ A "record type" is a first class object representing a user-defined data type. A "record" is an instance of a record type. -- Scheme Procedure: record? obj Return `#t' if OBJ is a record of any type and `#f' otherwise. Note that `record?' may be true of any Scheme value; there is no promise that records are disjoint with other Scheme types. -- Scheme Procedure: make-record-type type-name field-names Return a "record-type descriptor", a value representing a new data type disjoint from all others. The TYPE-NAME argument must be a string, but is only used for debugging purposes (such as the printed representation of a record of the new type). The FIELD-NAMES argument is a list of symbols naming the "fields" of a record of the new type. It is an error if the list contains any duplicates. It is unspecified how record-type descriptors are represented. -- Scheme Procedure: record-constructor rtd [field-names] Return a procedure for constructing new members of the type represented by RTD. The returned procedure accepts exactly as many arguments as there are symbols in the given list, FIELD-NAMES; these are used, in order, as the initial values of those fields in a new record, which is returned by the constructor procedure. The values of any fields not named in that list are unspecified. The FIELD-NAMES argument defaults to the list of field names in the call to `make-record-type' that created the type represented by RTD; if the FIELD-NAMES argument is provided, it is an error if it contains any duplicates or any symbols not in the default list. -- Scheme Procedure: record-predicate rtd Return a procedure for testing membership in the type represented by RTD. The returned procedure accepts exactly one argument and returns a true value if the argument is a member of the indicated record type; it returns a false value otherwise. -- Scheme Procedure: record-accessor rtd field-name Return a procedure for reading the value of a particular field of a member of the type represented by RTD. The returned procedure accepts exactly one argument which must be a record of the appropriate type; it returns the current value of the field named by the symbol FIELD-NAME in that record. The symbol FIELD-NAME must be a member of the list of field-names in the call to `make-record-type' that created the type represented by RTD. -- Scheme Procedure: record-modifier rtd field-name Return a procedure for writing the value of a particular field of a member of the type represented by RTD. The returned procedure accepts exactly two arguments: first, a record of the appropriate type, and second, an arbitrary Scheme value; it modifies the field named by the symbol FIELD-NAME in that record to contain the given value. The returned value of the modifier procedure is unspecified. The symbol FIELD-NAME must be a member of the list of field-names in the call to `make-record-type' that created the type represented by RTD. -- Scheme Procedure: record-type-descriptor record Return a record-type descriptor representing the type of the given record. That is, for example, if the returned descriptor were passed to `record-predicate', the resulting predicate would return a true value when passed the given record. Note that it is not necessarily the case that the returned descriptor is the one that was passed to `record-constructor' in the call that created the constructor procedure that created the given record. -- Scheme Procedure: record-type-name rtd Return the type-name associated with the type represented by rtd. The returned value is `eqv?' to the TYPE-NAME argument given in the call to `make-record-type' that created the type represented by RTD. -- Scheme Procedure: record-type-fields rtd Return a list of the symbols naming the fields in members of the type represented by RTD. The returned value is `equal?' to the field-names argument given in the call to `make-record-type' that created the type represented by RTD.  File: guile.info, Node: Structures, Next: Arrays, Prev: Records, Up: Compound Data Types 22.5 Structures =============== [FIXME: this is pasted in from Tom Lord's original guile.texi and should be reviewed] A "structure type" is a first class user-defined data type. A "structure" is an instance of a structure type. A structure type is itself a structure. Structures are less abstract and more general than traditional records. In fact, in Guile Scheme, records are implemented using structures. * Menu: * Structure Concepts:: The structure of Structures * Structure Layout:: Defining the layout of structure types * Structure Basics:: make-, -ref and -set! procedures for structs * Vtables:: Accessing type-specific data  File: guile.info, Node: Structure Concepts, Next: Structure Layout, Up: Structures 22.5.1 Structure Concepts ------------------------- A structure object consists of a handle, structure data, and a vtable. The handle is a Scheme value which points to both the vtable and the structure's data. Structure data is a dynamically allocated region of memory, private to the structure, divided up into typed fields. A vtable is another structure used to hold type-specific data. Multiple structures can share a common vtable. Three concepts are key to understanding structures. * "layout specifications" Layout specifications determine how memory allocated to structures is divided up into fields. Programmers must write a layout specification whenever a new type of structure is defined. * "structural accessors" Structure access is by field number. There is only one set of accessors common to all structure objects. * "vtables" Vtables, themselves structures, are first class representations of disjoint sub-types of structures in general. In most cases, when a new structure is created, programmers must specify a vtable for the new structure. Each vtable has a field describing the layout of its instances. Vtables can have additional, user-defined fields as well.  File: guile.info, Node: Structure Layout, Next: Structure Basics, Prev: Structure Concepts, Up: Structures 22.5.2 Structure Layout ----------------------- When a structure is created, a region of memory is allocated to hold its state. The "layout" of the structure's type determines how that memory is divided into fields. Each field has a specified type. There are only three types allowed, each corresponding to a one letter code. The allowed types are: * 'u' - unprotected The field holds binary data that is not GC protected. * 'p' - protected The field holds a Scheme value and is GC protected. * 's' - self The field holds a Scheme value and is GC protected. When a structure is created with this type of field, the field is initialized to refer to the structure's own handle. This kind of field is mainly useful when mixing Scheme and C code in which the C code may need to compute a structure's handle given only the address of its malloc'd data. Each field also has an associated access protection. There are only three kinds of protection, each corresponding to a one letter code. The allowed protections are: * 'w' - writable The field can be read and written. * 'r' - readable The field can be read, but not written. * 'o' - opaque The field can be neither read nor written. This kind of protection is for fields useful only to built-in routines. A layout specification is described by stringing together pairs of letters: one to specify a field type and one to specify a field protection. For example, a traditional cons pair type object could be described as: ; cons pairs have two writable fields of Scheme data "pwpw" A pair object in which the first field is held constant could be: "prpw" Binary fields, (fields of type "u"), hold one "word" each. The size of a word is a machine dependent value defined to be equal to the value of the C expression: `sizeof (long)'. The last field of a structure layout may specify a tail array. A tail array is indicated by capitalizing the field's protection code ('W', 'R' or 'O'). A tail-array field is replaced by a read-only binary data field containing an array size. The array size is determined at the time the structure is created. It is followed by a corresponding number of fields of the type specified for the tail array. For example, a conventional Scheme vector can be described as: ; A vector is an arbitrary number of writable fields holding Scheme ; values: "pW" In the above example, field 0 contains the size of the vector and fields beginning at 1 contain the vector elements. A kind of tagged vector (a constant tag followed by conventional vector elements) might be: "prpW" Structure layouts are represented by specially interned symbols whose name is a string of type and protection codes. To create a new structure layout, use this procedure: -- Scheme Procedure: make-struct-layout fields -- C Function: scm_make_struct_layout (fields) Return a new structure layout object. FIELDS must be a string made up of pairs of characters strung together. The first character of each pair describes a field type, the second a field protection. Allowed types are 'p' for GC-protected Scheme data, 'u' for unprotected binary data, and 's' for a field that points to the structure itself. Allowed protections are 'w' for mutable fields, 'r' for read-only fields, and 'o' for opaque fields. The last field protection specification may be capitalized to indicate that the field is a tail-array.  File: guile.info, Node: Structure Basics, Next: Vtables, Prev: Structure Layout, Up: Structures 22.5.3 Structure Basics ----------------------- This section describes the basic procedures for creating and accessing structures. -- Scheme Procedure: make-struct vtable tail_array_size . init -- C Function: scm_make_struct (vtable, tail_array_size, init) Create a new structure. TYPE must be a vtable structure (*note Vtables::). TAIL-ELTS must be a non-negative integer. If the layout specification indicated by TYPE includes a tail-array, this is the number of elements allocated to that array. The INIT1, ... are optional arguments describing how successive fields of the structure should be initialized. Only fields with protection 'r' or 'w' can be initialized, except for fields of type 's', which are automatically initialized to point to the new structure itself; fields with protection 'o' can not be initialized by Scheme programs. If fewer optional arguments than initializable fields are supplied, fields of type 'p' get default value #f while fields of type 'u' are initialized to 0. Structs are currently the basic representation for record-like data structures in Guile. The plan is to eventually replace them with a new representation which will at the same time be easier to use and more powerful. For more information, see the documentation for `make-vtable-vtable'. -- Scheme Procedure: struct? x -- C Function: scm_struct_p (x) Return `#t' iff X is a structure object, else `#f'. -- Scheme Procedure: struct-ref handle pos -- Scheme Procedure: struct-set! struct n value -- C Function: scm_struct_ref (handle, pos) -- C Function: scm_struct_set_x (struct, n, value) Access (or modify) the Nth field of STRUCT. If the field is of type 'p', then it can be set to an arbitrary value. If the field is of type 'u', then it can only be set to a non-negative integer value small enough to fit in one machine word.  File: guile.info, Node: Vtables, Prev: Structure Basics, Up: Structures 22.5.4 Vtables -------------- Vtables are structures that are used to represent structure types. Each vtable contains a layout specification in field `vtable-index-layout' - instances of the type are laid out according to that specification. Vtables contain additional fields which are used only internally to libguile. The variable `vtable-offset-user' is bound to a field number. Vtable fields at that position or greater are user definable. -- Scheme Procedure: struct-vtable handle -- C Function: scm_struct_vtable (handle) Return the vtable structure that describes the type of STRUCT. -- Scheme Procedure: struct-vtable? x -- C Function: scm_struct_vtable_p (x) Return `#t' iff X is a vtable structure. If you have a vtable structure, `V', you can create an instance of the type it describes by using `(make-struct V ...)'. But where does `V' itself come from? One possibility is that `V' is an instance of a user-defined vtable type, `V'', so that `V' is created by using `(make-struct V' ...)'. Another possibility is that `V' is an instance of the type it itself describes. Vtable structures of the second sort are created by this procedure: -- Scheme Procedure: make-vtable-vtable user_fields tail_array_size . init -- C Function: scm_make_vtable_vtable (user_fields, tail_array_size, init) Return a new, self-describing vtable structure. USER-FIELDS is a string describing user defined fields of the vtable beginning at index `vtable-offset-user' (see `make-struct-layout'). TAIL-SIZE specifies the size of the tail-array (if any) of this vtable. INIT1, ... are the optional initializers for the fields of the vtable. Vtables have one initializable system field--the struct printer. This field comes before the user fields in the initializers passed to `make-vtable-vtable' and `make-struct', and thus works as a third optional argument to `make-vtable-vtable' and a fourth to `make-struct' when creating vtables: If the value is a procedure, it will be called instead of the standard printer whenever a struct described by this vtable is printed. The procedure will be called with arguments STRUCT and PORT. The structure of a struct is described by a vtable, so the vtable is in essence the type of the struct. The vtable is itself a struct with a vtable. This could go on forever if it weren't for the vtable-vtables which are self-describing vtables, and thus terminate the chain. There are several potential ways of using structs, but the standard one is to use three kinds of structs, together building up a type sub-system: one vtable-vtable working as the root and one or several "types", each with a set of "instances". (The vtable-vtable should be compared to the class which is the class of itself.) (define ball-root (make-vtable-vtable "pr" 0)) (define (make-ball-type ball-color) (make-struct ball-root 0 (make-struct-layout "pw") (lambda (ball port) (format port "#" (color ball) (owner ball))) ball-color)) (define (color ball) (struct-ref (struct-vtable ball) vtable-offset-user)) (define (owner ball) (struct-ref ball 0)) (define red (make-ball-type 'red)) (define green (make-ball-type 'green)) (define (make-ball type owner) (make-struct type 0 owner)) (define ball (make-ball green 'Nisse)) ball => # -- Scheme Procedure: struct-vtable-name vtable -- C Function: scm_struct_vtable_name (vtable) Return the name of the vtable VTABLE. -- Scheme Procedure: set-struct-vtable-name! vtable name -- C Function: scm_set_struct_vtable_name_x (vtable, name) Set the name of the vtable VTABLE to NAME. -- Scheme Procedure: struct-vtable-tag handle -- C Function: scm_struct_vtable_tag (handle) Return the vtable tag of the structure HANDLE.  File: guile.info, Node: Arrays, Next: Association Lists and Hash Tables, Prev: Structures, Up: Compound Data Types 22.6 Arrays =========== * Menu: * Conventional Arrays:: Arrays with arbitrary data. * Array Mapping:: Applying a procedure to the contents of an array. * Uniform Arrays:: Arrays with data of a single type. * Bit Vectors:: Vectors of bits.  File: guile.info, Node: Conventional Arrays, Next: Array Mapping, Up: Arrays 22.6.1 Conventional Arrays -------------------------- "Conventional arrays" are a collection of cells organized into an arbitrary number of dimensions. Each cell can hold any kind of Scheme value and can be accessed in constant time by supplying an index for each dimension. This contrasts with uniform arrays, which use memory more efficiently but can hold data of only a single type, and lists where inserting and deleting cells is more efficient, but more time is usually required to access a particular cell. A conventional array is displayed as `#' followed by the "rank" (number of dimensions) followed by the cells, organized into dimensions using parentheses. The nesting depth of the parentheses is equal to the rank. When an array is created, the number of dimensions and range of each dimension must be specified, e.g., to create a 2x3 array with a zero-based index: (make-array 'ho 2 3) => #2((ho ho ho) (ho ho ho)) The range of each dimension can also be given explicitly, e.g., another way to create the same array: (make-array 'ho '(0 1) '(0 2)) => #2((ho ho ho) (ho ho ho)) A conventional array with one dimension based at zero is identical to a vector: (make-array 'ho 3) => #(ho ho ho) The following procedures can be used with conventional arrays (or vectors). -- Scheme Procedure: array? v [prot] -- C Function: scm_array_p (v, prot) Return `#t' if the OBJ is an array, and `#f' if not. The PROTOTYPE argument is used with uniform arrays and is described elsewhere. -- Scheme Procedure: make-array initial-value bound1 bound2 ... Create and return an array that has as many dimensions as there are BOUNDs and fill it with INITIAL-VALUE. Each BOUND may be a positive non-zero integer N, in which case the index for that dimension can range from 0 through N-1; or an explicit index range specifier in the form `(LOWER UPPER)', where both LOWER and UPPER are integers, possibly less than zero, and possibly the same number (however, LOWER cannot be greater than UPPER). -- Scheme Procedure: uniform-vector-ref v args -- Scheme Procedure: array-ref v . args -- C Function: scm_uniform_vector_ref (v, args) Return the element at the `(index1, index2)' element in ARRAY. -- Scheme Procedure: array-in-bounds? v . args -- C Function: scm_array_in_bounds_p (v, args) Return `#t' if its arguments would be acceptable to `array-ref'. -- Scheme Procedure: array-set! v obj . args -- Scheme Procedure: uniform-array-set1! v obj args -- C Function: scm_array_set_x (v, obj, args) Set the element at the `(index1, index2)' element in ARRAY to NEW-VALUE. The value returned by array-set! is unspecified. -- Scheme Procedure: make-shared-array oldra mapfunc . dims -- C Function: scm_make_shared_array (oldra, mapfunc, dims) `make-shared-array' can be used to create shared subarrays of other arrays. The MAPPER is a function that translates coordinates in the new array into coordinates in the old array. A MAPPER must be linear, and its range must stay within the bounds of the old array, but it can be otherwise arbitrary. A simple example: (define fred (make-array #f 8 8)) (define freds-diagonal (make-shared-array fred (lambda (i) (list i i)) 8)) (array-set! freds-diagonal 'foo 3) (array-ref fred 3 3) => foo (define freds-center (make-shared-array fred (lambda (i j) (list (+ 3 i) (+ 3 j))) 2 2)) (array-ref freds-center 0 0) => foo -- Scheme Procedure: shared-array-increments ra -- C Function: scm_shared_array_increments (ra) For each dimension, return the distance between elements in the root vector. -- Scheme Procedure: shared-array-offset ra -- C Function: scm_shared_array_offset (ra) Return the root vector index of the first element in the array. -- Scheme Procedure: shared-array-root ra -- C Function: scm_shared_array_root (ra) Return the root vector of a shared array. -- Scheme Procedure: transpose-array ra . args -- C Function: scm_transpose_array (ra, args) Return an array sharing contents with ARRAY, but with dimensions arranged in a different order. There must be one DIM argument for each dimension of ARRAY. DIM0, DIM1, ... should be integers between 0 and the rank of the array to be returned. Each integer in that range must appear at least once in the argument list. The values of DIM0, DIM1, ... correspond to dimensions in the array to be returned, their positions in the argument list to dimensions of ARRAY. Several DIMs may have the same value, in which case the returned array will have smaller rank than ARRAY. (transpose-array '#2((a b) (c d)) 1 0) => #2((a c) (b d)) (transpose-array '#2((a b) (c d)) 0 0) => #1(a d) (transpose-array '#3(((a b c) (d e f)) ((1 2 3) (4 5 6))) 1 1 0) => #2((a 4) (b 5) (c 6)) -- Scheme Procedure: enclose-array ra . axes -- C Function: scm_enclose_array (ra, axes) DIM0, DIM1 ... should be nonnegative integers less than the rank of ARRAY. ENCLOSE-ARRAY returns an array resembling an array of shared arrays. The dimensions of each shared array are the same as the DIMth dimensions of the original array, the dimensions of the outer array are the same as those of the original array that did not match a DIM. An enclosed array is not a general Scheme array. Its elements may not be set using `array-set!'. Two references to the same element of an enclosed array will be `equal?' but will not in general be `eq?'. The value returned by ARRAY-PROTOTYPE when given an enclosed array is unspecified. examples: (enclose-array '#3(((a b c) (d e f)) ((1 2 3) (4 5 6))) 1) => # (enclose-array '#3(((a b c) (d e f)) ((1 2 3) (4 5 6))) 1 0) => # -- Scheme Procedure: array-shape array Return a list of inclusive bounds of integers. (array-shape (make-array 'foo '(-1 3) 5)) => ((-1 3) (0 4)) -- Scheme Procedure: array-dimensions ra -- C Function: scm_array_dimensions (ra) `Array-dimensions' is similar to `array-shape' but replaces elements with a `0' minimum with one greater than the maximum. So: (array-dimensions (make-array 'foo '(-1 3) 5)) => ((-1 3) 5) -- Scheme Procedure: array-rank ra -- C Function: scm_array_rank (ra) Return the number of dimensions of OBJ. If OBJ is not an array, `0' is returned. -- Scheme Procedure: array->list v -- C Function: scm_array_to_list (v) Return a list consisting of all the elements, in order, of ARRAY. -- Scheme Procedure: array-copy! src dst -- Scheme Procedure: array-copy-in-order! src dst -- C Function: scm_array_copy_x (src, dst) Copy every element from vector or array SOURCE to the corresponding element of DESTINATION. DESTINATION must have the same rank as SOURCE, and be at least as large in each dimension. The order is unspecified. -- Scheme Procedure: array-fill! ra fill -- C Function: scm_array_fill_x (ra, fill) Store FILL in every element of ARRAY. The value returned is unspecified. -- Scheme Procedure: array-equal? ra0 ra1 Return `#t' iff all arguments are arrays with the same shape, the same type, and have corresponding elements which are either `equal?' or `array-equal?'. This function differs from `equal?' in that a one dimensional shared array may be ARRAY-EQUAL? but not EQUAL? to a vector or uniform vector. -- Scheme Procedure: array-contents array [strict] -- C Function: scm_array_contents (array, strict) If ARRAY may be "unrolled" into a one dimensional shared array without changing their order (last subscript changing fastest), then `array-contents' returns that shared array, otherwise it returns `#f'. All arrays made by MAKE-ARRAY and MAKE-UNIFORM-ARRAY may be unrolled, some arrays made by MAKE-SHARED-ARRAY may not be. If the optional argument STRICT is provided, a shared array will be returned only if its elements are stored internally contiguous in memory.  File: guile.info, Node: Array Mapping, Next: Uniform Arrays, Prev: Conventional Arrays, Up: Arrays 22.6.2 Array Mapping -------------------- -- Scheme Procedure: array-map! ra0 proc . lra -- Scheme Procedure: array-map-in-order! ra0 proc . lra -- C Function: scm_array_map_x (ra0, proc, lra) ARRAY1, ... must have the same number of dimensions as ARRAY0 and have a range for each index which includes the range for the corresponding index in ARRAY0. PROC is applied to each tuple of elements of ARRAY1 ... and the result is stored as the corresponding element in ARRAY0. The value returned is unspecified. The order of application is unspecified. -- Scheme Procedure: array-for-each proc ra0 . lra -- C Function: scm_array_for_each (proc, ra0, lra) Apply PROC to each tuple of elements of ARRAY0 ... in row-major order. The value returned is unspecified. -- Scheme Procedure: array-index-map! ra proc -- C Function: scm_array_index_map_x (ra, proc) Apply PROC to the indices of each element of ARRAY in turn, storing the result in the corresponding element. The value returned and the order of application are unspecified. One can implement ARRAY-INDEXES as (define (array-indexes array) (let ((ra (apply make-array #f (array-shape array)))) (array-index-map! ra (lambda x x)) ra)) Another example: (define (apl:index-generator n) (let ((v (make-uniform-vector n 1))) (array-index-map! v (lambda (i) i)) v))  File: guile.info, Node: Uniform Arrays, Next: Bit Vectors, Prev: Array Mapping, Up: Arrays 22.6.3 Uniform Arrays --------------------- "Uniform arrays" have elements all of the same type and occupy less storage than conventional arrays. Uniform arrays with a single zero-based dimension are also known as "uniform vectors". The procedures in this section can also be used on conventional arrays, vectors, bit-vectors and strings. When creating a uniform array, the type of data to be stored is indicated with a PROTOTYPE argument. The following table lists the types available and example prototypes: prototype type printing character #t boolean (bit-vector) b #\a char (string) a #\nul byte (integer) y 's short (integer) h 1 unsigned long (integer) u -1 signed long (integer) e 'l signed long long (integer) l 1.0 float (single precision) s 1/3 double (double precision float) i 0+i complex (double precision) c () conventional vector Unshared uniform arrays of characters with a single zero-based dimension are identical to strings: (make-uniform-array #\a 3) => "aaa" Unshared uniform arrays of booleans with a single zero-based dimension are identical to *Note bit-vectors: Bit Vectors. (make-uniform-array #t 3) => #*111 Other uniform vectors are written in a form similar to that of vectors, except that a single character from the above table is put between `#' and `('. For example, a uniform vector of signed long integers is displayed in the form `'#e(3 5 9)'. -- Scheme Procedure: array? v [prot] Return `#t' if the OBJ is an array, and `#f' if not. The PROTOTYPE argument is used with uniform arrays and is described elsewhere. -- Scheme Procedure: make-uniform-array prototype bound1 bound2 ... Create and return a uniform array of type corresponding to PROTOTYPE that has as many dimensions as there are BOUNDs and fill it with PROTOTYPE. -- Scheme Procedure: array-prototype ra -- C Function: scm_array_prototype (ra) Return an object that would produce an array of the same type as ARRAY, if used as the PROTOTYPE for `make-uniform-array'. -- Scheme Procedure: list->uniform-array ndim prot lst -- Scheme Procedure: list->uniform-vector prot lst -- C Function: scm_list_to_uniform_array (ndim, prot, lst) Return a uniform array of the type indicated by prototype PROT with elements the same as those of LST. Elements must be of the appropriate type, no coercions are done. -- Scheme Procedure: uniform-vector-fill! uve fill Store FILL in every element of UVE. The value returned is unspecified. -- Scheme Procedure: uniform-vector-length v -- C Function: scm_uniform_vector_length (v) Return the number of elements in UVE. -- Scheme Procedure: dimensions->uniform-array dims prot [fill] -- Scheme Procedure: make-uniform-vector length prototype [fill] -- C Function: scm_dimensions_to_uniform_array (dims, prot, fill) Create and return a uniform array or vector of type corresponding to PROTOTYPE with dimensions DIMS or length LENGTH. If FILL is supplied, it's used to fill the array, otherwise PROTOTYPE is used. -- Scheme Procedure: uniform-array-read! ra [port_or_fd [start [end]]] -- Scheme Procedure: uniform-vector-read! uve [port-or-fdes] [start] [end] -- C Function: scm_uniform_array_read_x (ra, port_or_fd, start, end) Attempt to read all elements of URA, in lexicographic order, as binary objects from PORT-OR-FDES. If an end of file is encountered, the objects up to that point are put into URA (starting at the beginning) and the remainder of the array is unchanged. The optional arguments START and END allow a specified region of a vector (or linearized array) to be read, leaving the remainder of the vector unchanged. `uniform-array-read!' returns the number of objects read. PORT-OR-FDES may be omitted, in which case it defaults to the value returned by `(current-input-port)'. -- Scheme Procedure: uniform-array-write v [port_or_fd [start [end]]] -- Scheme Procedure: uniform-vector-write uve [port-or-fdes] [start] [end] -- C Function: scm_uniform_array_write (v, port_or_fd, start, end) Writes all elements of URA as binary objects to PORT-OR-FDES. The optional arguments START and END allow a specified region of a vector (or linearized array) to be written. The number of objects actually written is returned. PORT-OR-FDES may be omitted, in which case it defaults to the value returned by `(current-output-port)'.  File: guile.info, Node: Bit Vectors, Prev: Uniform Arrays, Up: Arrays 22.6.4 Bit Vectors ------------------ Bit vectors are a specific type of uniform array: an array of booleans with a single zero-based index. They are displayed as a sequence of `0's and `1's prefixed by `#*', e.g., (make-uniform-vector 8 #t #f) => #*00000000 #b(#t #f #t) => #*101 -- Scheme Procedure: bit-count b bitvector -- C Function: scm_bit_count (b, bitvector) Return the number of occurrences of the boolean B in BITVECTOR. -- Scheme Procedure: bit-position item v k -- C Function: scm_bit_position (item, v, k) Return the minimum index of an occurrence of BOOL in BV which is at least K. If no BOOL occurs within the specified range `#f' is returned. -- Scheme Procedure: bit-invert! v -- C Function: scm_bit_invert_x (v) Modify BV by replacing each element with its negation. -- Scheme Procedure: bit-set*! v kv obj -- C Function: scm_bit_set_star_x (v, kv, obj) If uve is a bit-vector BV and uve must be of the same length. If BOOL is `#t', uve is OR'ed into BV; If BOOL is `#f', the inversion of uve is AND'ed into BV. If uve is a unsigned long integer vector all the elements of uve must be between 0 and the `length' of BV. The bits of BV corresponding to the indexes in uve are set to BOOL. The return value is unspecified. -- Scheme Procedure: bit-count* v kv obj -- C Function: scm_bit_count_star (v, kv, obj) Return (bit-count (bit-set*! (if bool bv (bit-invert! bv)) uve #t) #t). BV is not modified.  File: guile.info, Node: Association Lists and Hash Tables, Prev: Arrays, Up: Compound Data Types 22.7 Association Lists and Hash Tables ====================================== This chapter discusses dictionary objects: data structures that are useful for organizing and indexing large bodies of information. * Menu: * Dictionary Types:: About dictionary types; what they're good for. * Association Lists:: List-based dictionaries. * Hash Tables:: Table-based dictionaries.  File: guile.info, Node: Dictionary Types, Next: Association Lists, Up: Association Lists and Hash Tables 22.7.1 Dictionary Types ----------------------- A "dictionary" object is a data structure used to index information in a user-defined way. In standard Scheme, the main aggregate data types are lists and vectors. Lists are not really indexed at all, and vectors are indexed only by number (e.g. `(vector-ref foo 5)'). Often you will find it useful to index your data on some other type; for example, in a library catalog you might want to look up a book by the name of its author. Dictionaries are used to help you organize information in such a way. An "association list" (or "alist" for short) is a list of key-value pairs. Each pair represents a single quantity or object; the `car' of the pair is a key which is used to identify the object, and the `cdr' is the object's value. A "hash table" also permits you to index objects with arbitrary keys, but in a way that makes looking up any one object extremely fast. A well-designed hash system makes hash table lookups almost as fast as conventional array or vector references. Alists are popular among Lisp programmers because they use only the language's primitive operations (lists, "car", "cdr" and the equality primitives). No changes to the language core are necessary. Therefore, with Scheme's built-in list manipulation facilities, it is very convenient to handle data stored in an association list. Also, alists are highly portable and can be easily implemented on even the most minimal Lisp systems. However, alists are inefficient, especially for storing large quantities of data. Because we want Guile to be useful for large software systems as well as small ones, Guile provides a rich set of tools for using either association lists or hash tables.  File: guile.info, Node: Association Lists, Next: Hash Tables, Prev: Dictionary Types, Up: Association Lists and Hash Tables 22.7.2 Association Lists ------------------------ An association list is a conventional data structure that is often used to implement simple key-value databases. It consists of a list of entries in which each entry is a pair. The "key" of each entry is the `car' of the pair and the "value" of each entry is the `cdr'. ASSOCIATION LIST ::= '( (KEY1 . VALUE1) (KEY2 . VALUE2) (KEY3 . VALUE3) ... ) Association lists are also known, for short, as "alists". The structure of an association list is just one example of the infinite number of possible structures that can be built using pairs and lists. As such, the keys and values in an association list can be manipulated using the general list structure procedures `cons', `car', `cdr', `set-car!', `set-cdr!' and so on. However, because association lists are so useful, Guile also provides specific procedures for manipulating them. * Menu: * Alist Key Equality:: * Adding or Setting Alist Entries:: * Retrieving Alist Entries:: * Removing Alist Entries:: * Sloppy Alist Functions:: * Alist Example::  File: guile.info, Node: Alist Key Equality, Next: Adding or Setting Alist Entries, Up: Association Lists 22.7.2.1 Alist Key Equality ........................... All of Guile's dedicated association list procedures, apart from `acons', come in three flavours, depending on the level of equality that is required to decide whether an existing key in the association list is the same as the key that the procedure call uses to identify the required entry. * Procedures with "assq" in their name use `eq?' to determine key equality. * Procedures with "assv" in their name use `eqv?' to determine key equality. * Procedures with "assoc" in their name use `equal?' to determine key equality. `acons' is an exception because it is used to build association lists which do not require their entries' keys to be unique.  File: guile.info, Node: Adding or Setting Alist Entries, Next: Retrieving Alist Entries, Prev: Alist Key Equality, Up: Association Lists 22.7.2.2 Adding or Setting Alist Entries ........................................ `acons' adds a new entry to an association list and returns the combined association list. The combined alist is formed by consing the new entry onto the head of the alist specified in the `acons' procedure call. So the specified alist is not modified, but its contents become shared with the tail of the combined alist that `acons' returns. In the most common usage of `acons', a variable holding the original association list is updated with the combined alist: (set! address-list (acons name address address-list)) In such cases, it doesn't matter that the old and new values of `address-list' share some of their contents, since the old value is usually no longer independently accessible. Note that `acons' adds the specified new entry regardless of whether the alist may already contain entries with keys that are, in some sense, the same as that of the new entry. Thus `acons' is ideal for building alists where there is no concept of key uniqueness. (set! task-list (acons 3 "pay gas bill" '())) task-list => ((3 . "pay gas bill")) (set! task-list (acons 3 "tidy bedroom" task-list)) task-list => ((3 . "tidy bedroom") (3 . "pay gas bill")) `assq-set!', `assv-set!' and `assoc-set!' are used to add or replace an entry in an association list where there _is_ a concept of key uniqueness. If the specified association list already contains an entry whose key is the same as that specified in the procedure call, the existing entry is replaced by the new one. Otherwise, the new entry is consed onto the head of the old association list to create the combined alist. In all cases, these procedures return the combined alist. `assq-set!' and friends _may_ destructively modify the structure of the old association list in such a way that an existing variable is correctly updated without having to `set!' it to the value returned: address-list => (("mary" . "34 Elm Road") ("james" . "16 Bow Street")) (assoc-set! address-list "james" "1a London Road") => (("mary" . "34 Elm Road") ("james" . "1a London Road")) address-list => (("mary" . "34 Elm Road") ("james" . "1a London Road")) Or they may not: (assoc-set! address-list "bob" "11 Newington Avenue") => (("bob" . "11 Newington Avenue") ("mary" . "34 Elm Road") ("james" . "1a London Road")) address-list => (("mary" . "34 Elm Road") ("james" . "1a London Road")) The only safe way to update an association list variable when adding or replacing an entry like this is to `set!' the variable to the returned value: (set! address-list (assoc-set! address-list "bob" "11 Newington Avenue")) address-list => (("bob" . "11 Newington Avenue") ("mary" . "34 Elm Road") ("james" . "1a London Road")) Because of this slight inconvenience, you may find it more convenient to use hash tables to store dictionary data. If your application will not be modifying the contents of an alist very often, this may not make much difference to you. If you need to keep the old value of an association list in a form independent from the list that results from modification by `acons', `assq-set!', `assv-set!' or `assoc-set!', use `list-copy' to copy the old association list before modifying it. -- Scheme Procedure: acons key value alist -- C Function: scm_acons (key, value, alist) Add a new key-value pair to ALIST. A new pair is created whose car is KEY and whose cdr is VALUE, and the pair is consed onto ALIST, and the new list is returned. This function is _not_ destructive; ALIST is not modified. -- Scheme Procedure: assq-set! alist key val -- Scheme Procedure: assv-set! alist key value -- Scheme Procedure: assoc-set! alist key value -- C Function: scm_assq_set_x (alist, key, val) -- C Function: scm_assv_set_x (alist, key, val) -- C Function: scm_assoc_set_x (alist, key, val) Reassociate KEY in ALIST with VALUE: find any existing ALIST entry for KEY and associate it with the new VALUE. If ALIST does not contain an entry for KEY, add a new one. Return the (possibly new) alist. These functions do not attempt to verify the structure of ALIST, and so may cause unusual results if passed an object that is not an association list.  File: guile.info, Node: Retrieving Alist Entries, Next: Removing Alist Entries, Prev: Adding or Setting Alist Entries, Up: Association Lists 22.7.2.3 Retrieving Alist Entries ................................. `assq', `assv' and `assoc' take an alist and a key as arguments and return the entry for that key if an entry exists, or `#f' if there is no entry for that key. Note that, in the cases where an entry exists, these procedures return the complete entry, that is `(KEY . VALUE)', not just the value. -- Scheme Procedure: assq key alist -- Scheme Procedure: assv key alist -- Scheme Procedure: assoc key alist -- C Function: scm_assq (key, alist) -- C Function: scm_assv (key, alist) -- C Function: scm_assoc (key, alist) Fetch the entry in ALIST that is associated with KEY. To decide whether the argument KEY matches a particular entry in ALIST, `assq' compares keys with `eq?', `assv' uses `eqv?' and `assoc' uses `equal?'. If KEY cannot be found in ALIST (according to whichever equality predicate is in use), then return `#f'. These functions return the entire alist entry found (i.e. both the key and the value). `assq-ref', `assv-ref' and `assoc-ref', on the other hand, take an alist and a key and return _just the value_ for that key, if an entry exists. If there is no entry for the specified key, these procedures return `#f'. This creates an ambiguity: if the return value is `#f', it means either that there is no entry with the specified key, or that there _is_ an entry for the specified key, with value `#f'. Consequently, `assq-ref' and friends should only be used where it is known that an entry exists, or where the ambiguity doesn't matter for some other reason. -- Scheme Procedure: assq-ref alist key -- Scheme Procedure: assv-ref alist key -- Scheme Procedure: assoc-ref alist key -- C Function: scm_assq_ref (alist, key) -- C Function: scm_assv_ref (alist, key) -- C Function: scm_assoc_ref (alist, key) Like `assq', `assv' and `assoc', except that only the value associated with KEY in ALIST is returned. These functions are equivalent to (let ((ent (ASSOCIATOR KEY ALIST))) (and ent (cdr ent))) where ASSOCIATOR is one of `assq', `assv' or `assoc'.  File: guile.info, Node: Removing Alist Entries, Next: Sloppy Alist Functions, Prev: Retrieving Alist Entries, Up: Association Lists 22.7.2.4 Removing Alist Entries ............................... To remove the element from an association list whose key matches a specified key, use `assq-remove!', `assv-remove!' or `assoc-remove!' (depending, as usual, on the level of equality required between the key that you specify and the keys in the association list). As with `assq-set!' and friends, the specified alist may or may not be modified destructively, and the only safe way to update a variable containing the alist is to `set!' it to the value that `assq-remove!' and friends return. address-list => (("bob" . "11 Newington Avenue") ("mary" . "34 Elm Road") ("james" . "1a London Road")) (set! address-list (assoc-remove! address-list "mary")) address-list => (("bob" . "11 Newington Avenue") ("james" . "1a London Road")) Note that, when `assq/v/oc-remove!' is used to modify an association list that has been constructed only using the corresponding `assq/v/oc-set!', there can be at most one matching entry in the alist, so the question of multiple entries being removed in one go does not arise. If `assq/v/oc-remove!' is applied to an association list that has been constructed using `acons', or an `assq/v/oc-set!' with a different level of equality, or any mixture of these, it removes only the first matching entry from the alist, even if the alist might contain further matching entries. For example: (define address-list '()) (set! address-list (assq-set! address-list "mary" "11 Elm Street")) (set! address-list (assq-set! address-list "mary" "57 Pine Drive")) address-list => (("mary" . "57 Pine Drive") ("mary" . "11 Elm Street")) (set! address-list (assoc-remove! address-list "mary")) address-list => (("mary" . "11 Elm Street")) In this example, the two instances of the string "mary" are not the same when compared using `eq?', so the two `assq-set!' calls add two distinct entries to `address-list'. When compared using `equal?', both "mary"s in `address-list' are the same as the "mary" in the `assoc-remove!' call, but `assoc-remove!' stops after removing the first matching entry that it finds, and so one of the "mary" entries is left in place. -- Scheme Procedure: assq-remove! alist key -- Scheme Procedure: assv-remove! alist key -- Scheme Procedure: assoc-remove! alist key -- C Function: scm_assq_remove_x (alist, key) -- C Function: scm_assv_remove_x (alist, key) -- C Function: scm_assoc_remove_x (alist, key) Delete the first entry in ALIST associated with KEY, and return the resulting alist.  File: guile.info, Node: Sloppy Alist Functions, Next: Alist Example, Prev: Removing Alist Entries, Up: Association Lists 22.7.2.5 Sloppy Alist Functions ............................... `sloppy-assq', `sloppy-assv' and `sloppy-assoc' behave like the corresponding non-`sloppy-' procedures, except that they return `#f' when the specified association list is not well-formed, where the non-`sloppy-' versions would signal an error. Specifically, there are two conditions for which the non-`sloppy-' procedures signal an error, which the `sloppy-' procedures handle instead by returning `#f'. Firstly, if the specified alist as a whole is not a proper list: (assoc "mary" '((1 . 2) ("key" . "door") . "open sesame")) => ERROR: In procedure assoc in expression (assoc "mary" (quote #)): ERROR: Wrong type argument in position 2 (expecting NULLP): "open sesame" ABORT: (wrong-type-arg) (sloppy-assoc "mary" '((1 . 2) ("key" . "door") . "open sesame")) => #f Secondly, if one of the entries in the specified alist is not a pair: (assoc 2 '((1 . 1) 2 (3 . 9))) => ERROR: In procedure assoc in expression (assoc 2 (quote #)): ERROR: Wrong type argument in position 2 (expecting CONSP): 2 ABORT: (wrong-type-arg) (sloppy-assoc 2 '((1 . 1) 2 (3 . 9))) => #f Unless you are explicitly working with badly formed association lists, it is much safer to use the non-`sloppy-' procedures, because they help to highlight coding and data errors that the `sloppy-' versions would silently cover up. -- Scheme Procedure: sloppy-assq key alist -- C Function: scm_sloppy_assq (key, alist) Behaves like `assq' but does not do any error checking. Recommended only for use in Guile internals. -- Scheme Procedure: sloppy-assv key alist -- C Function: scm_sloppy_assv (key, alist) Behaves like `assv' but does not do any error checking. Recommended only for use in Guile internals. -- Scheme Procedure: sloppy-assoc key alist -- C Function: scm_sloppy_assoc (key, alist) Behaves like `assoc' but does not do any error checking. Recommended only for use in Guile internals.  File: guile.info, Node: Alist Example, Prev: Sloppy Alist Functions, Up: Association Lists 22.7.2.6 Alist Example ...................... Here is a longer example of how alists may be used in practice. (define capitals '(("New York" . "Albany") ("Oregon" . "Salem") ("Florida" . "Miami"))) ;; What's the capital of Oregon? (assoc "Oregon" capitals) => ("Oregon" . "Salem") (assoc-ref capitals "Oregon") => "Salem" ;; We left out South Dakota. (set! capitals (assoc-set! capitals "South Dakota" "Pierre")) capitals => (("South Dakota" . "Pierre") ("New York" . "Albany") ("Oregon" . "Salem") ("Florida" . "Miami")) ;; And we got Florida wrong. (set! capitals (assoc-set! capitals "Florida" "Tallahassee")) capitals => (("South Dakota" . "Pierre") ("New York" . "Albany") ("Oregon" . "Salem") ("Florida" . "Tallahassee")) ;; After Oregon secedes, we can remove it. (set! capitals (assoc-remove! capitals "Oregon")) capitals => (("South Dakota" . "Pierre") ("New York" . "Albany") ("Florida" . "Tallahassee"))  File: guile.info, Node: Hash Tables, Prev: Association Lists, Up: Association Lists and Hash Tables 22.7.3 Hash Tables ------------------ Hash tables are dictionaries which offer similar functionality as association lists: They provide a mapping from keys to values. The difference is that association lists need time linear in the size of elements when searching for entries, whereas hash tables can normally search in constant time. The drawback is that hash tables require a little bit more memory, and that you can not use the normal list procedures (*note Lists::) for working with them. * Menu: * Hash Table Examples:: Demonstration of hash table usage. * Hash Table Reference:: Hash table procedure descriptions.  File: guile.info, Node: Hash Table Examples, Next: Hash Table Reference, Up: Hash Tables 22.7.3.1 Hash Table Examples ............................ For demonstration purposes, this section gives a few usage examples of some hash table procedures, together with some explanation what they do. First we start by creating a new hash table with 31 slots, and populate it with two key/value pairs. (define h (make-hash-table 31)) (hashq-create-handle! h 'foo "bar") => (foo . "bar") (hashq-create-handle! h 'braz "zonk") => (braz . "zonk") (hashq-create-handle! h 'frob #f) => (frob . #f) You can get the value for a given key with the procedure `hashq-ref', but the problem with this procedure is that you cannot reliably determine whether a key does exists in the table. The reason is that the procedure returns `#f' if the key is not in the table, but it will return the same value if the key is in the table and just happens to have the value `#f', as you can see in the following examples. (hashq-ref h 'foo) => "bar" (hashq-ref h 'frob) => #f (hashq-ref h 'not-there) => #f Better is to use the procedure `hashq-get-handle', which makes a distinction between the two cases. Just like `assq', this procedure returns a key/value-pair on success, and `#f' if the key is not found. (hashq-get-handle h 'foo) => (foo . "bar") (hashq-get-handle h 'not-there) => #f There is no procedure for calculating the number of key/value-pairs in a hash table, but `hash-fold' can be used for doing exactly that. (hash-fold (lambda (key value seed) (+ 1 seed)) 0 h) => 3  File: guile.info, Node: Hash Table Reference, Prev: Hash Table Examples, Up: Hash Tables 22.7.3.2 Hash Table Reference ............................. Like the association list functions, the hash table functions come in several varieties: `hashq', `hashv', and `hash'. The `hashq' functions use `eq?' to determine whether two keys match. The `hashv' functions use `eqv?', and the `hash' functions use `equal?'. In each of the functions that follow, the TABLE argument must be a vector. The KEY and VALUE arguments may be any Scheme object. -- Scheme Procedure: make-hash-table size Create a new hash table of SIZE slots. Note that the number of slots does not limit the size of the table, it just tells how large the underlying vector will be. The SIZE should be similar to the expected number of elements which will be added to the table, but they need not match. For good performance, it might be a good idea to use a prime number as the SIZE. -- Scheme Procedure: hashq-ref table key [dflt] -- C Function: scm_hashq_ref (table, key, dflt) Look up KEY in the hash table TABLE, and return the value (if any) associated with it. If KEY is not found, return DEFAULT (or `#f' if no DEFAULT argument is supplied). Uses `eq?' for equality testing. -- Scheme Procedure: hashv-ref table key [dflt] -- C Function: scm_hashv_ref (table, key, dflt) Look up KEY in the hash table TABLE, and return the value (if any) associated with it. If KEY is not found, return DEFAULT (or `#f' if no DEFAULT argument is supplied). Uses `eqv?' for equality testing. -- Scheme Procedure: hash-ref table key [dflt] -- C Function: scm_hash_ref (table, key, dflt) Look up KEY in the hash table TABLE, and return the value (if any) associated with it. If KEY is not found, return DEFAULT (or `#f' if no DEFAULT argument is supplied). Uses `equal?' for equality testing. -- Scheme Procedure: hashq-set! table key val -- C Function: scm_hashq_set_x (table, key, val) Find the entry in TABLE associated with KEY, and store VALUE there. Uses `eq?' for equality testing. -- Scheme Procedure: hashv-set! table key val -- C Function: scm_hashv_set_x (table, key, val) Find the entry in TABLE associated with KEY, and store VALUE there. Uses `eqv?' for equality testing. -- Scheme Procedure: hash-set! table key val -- C Function: scm_hash_set_x (table, key, val) Find the entry in TABLE associated with KEY, and store VALUE there. Uses `equal?' for equality testing. -- Scheme Procedure: hashq-remove! table key -- C Function: scm_hashq_remove_x (table, key) Remove KEY (and any value associated with it) from TABLE. Uses `eq?' for equality tests. -- Scheme Procedure: hashv-remove! table key -- C Function: scm_hashv_remove_x (table, key) Remove KEY (and any value associated with it) from TABLE. Uses `eqv?' for equality tests. -- Scheme Procedure: hash-remove! table key -- C Function: scm_hash_remove_x (table, key) Remove KEY (and any value associated with it) from TABLE. Uses `equal?' for equality tests. The standard hash table functions may be too limited for some applications. For example, you may want a hash table to store strings in a case-insensitive manner, so that references to keys named "foobar", "FOOBAR" and "FooBaR" will all yield the same item. Guile provides you with "extended" hash tables that permit you to specify a hash function and associator function of your choosing. The functions described in the rest of this section can be used to implement such custom hash table structures. If you are unfamiliar with the inner workings of hash tables, then this facility will probably be a little too abstract for you to use comfortably. If you are interested in learning more, see an introductory textbook on data structures or algorithms for an explanation of how hash tables are implemented. -- Scheme Procedure: hashq key size -- C Function: scm_hashq (key, size) Determine a hash value for KEY that is suitable for lookups in a hash table of size SIZE, where `eq?' is used as the equality predicate. The function returns an integer in the range 0 to SIZE - 1. Note that `hashq' may use internal addresses. Thus two calls to hashq where the keys are `eq?' are not guaranteed to deliver the same value if the key object gets garbage collected in between. This can happen, for example with symbols: `(hashq 'foo n) (gc) (hashq 'foo n)' may produce two different values, since `foo' will be garbage collected. -- Scheme Procedure: hashv key size -- C Function: scm_hashv (key, size) Determine a hash value for KEY that is suitable for lookups in a hash table of size SIZE, where `eqv?' is used as the equality predicate. The function returns an integer in the range 0 to SIZE - 1. Note that `(hashv key)' may use internal addresses. Thus two calls to hashv where the keys are `eqv?' are not guaranteed to deliver the same value if the key object gets garbage collected in between. This can happen, for example with symbols: `(hashv 'foo n) (gc) (hashv 'foo n)' may produce two different values, since `foo' will be garbage collected. -- Scheme Procedure: hash key size -- C Function: scm_hash (key, size) Determine a hash value for KEY that is suitable for lookups in a hash table of size SIZE, where `equal?' is used as the equality predicate. The function returns an integer in the range 0 to SIZE - 1. -- Scheme Procedure: hashx-ref hash assoc table key [dflt] -- C Function: scm_hashx_ref (hash, assoc, table, key, dflt) This behaves the same way as the corresponding `ref' function, but uses HASH as a hash function and ASSOC to compare keys. `hash' must be a function that takes two arguments, a key to be hashed and a table size. `assoc' must be an associator function, like `assoc', `assq' or `assv'. By way of illustration, `hashq-ref table key' is equivalent to `hashx-ref hashq assq table key'. -- Scheme Procedure: hashx-set! hash assoc table key val -- C Function: scm_hashx_set_x (hash, assoc, table, key, val) This behaves the same way as the corresponding `set!' function, but uses HASH as a hash function and ASSOC to compare keys. `hash' must be a function that takes two arguments, a key to be hashed and a table size. `assoc' must be an associator function, like `assoc', `assq' or `assv'. By way of illustration, `hashq-set! table key' is equivalent to `hashx-set! hashq assq table key'. -- Scheme Procedure: hashq-get-handle table key -- C Function: scm_hashq_get_handle (table, key) This procedure returns the `(key . value)' pair from the hash table TABLE. If TABLE does not hold an associated value for KEY, `#f' is returned. Uses `eq?' for equality testing. -- Scheme Procedure: hashv-get-handle table key -- C Function: scm_hashv_get_handle (table, key) This procedure returns the `(key . value)' pair from the hash table TABLE. If TABLE does not hold an associated value for KEY, `#f' is returned. Uses `eqv?' for equality testing. -- Scheme Procedure: hash-get-handle table key -- C Function: scm_hash_get_handle (table, key) This procedure returns the `(key . value)' pair from the hash table TABLE. If TABLE does not hold an associated value for KEY, `#f' is returned. Uses `equal?' for equality testing. -- Scheme Procedure: hashx-get-handle hash assoc table key -- C Function: scm_hashx_get_handle (hash, assoc, table, key) This behaves the same way as the corresponding `-get-handle' function, but uses HASH as a hash function and ASSOC to compare keys. `hash' must be a function that takes two arguments, a key to be hashed and a table size. `assoc' must be an associator function, like `assoc', `assq' or `assv'. -- Scheme Procedure: hashq-create-handle! table key init -- C Function: scm_hashq_create_handle_x (table, key, init) This function looks up KEY in TABLE and returns its handle. If KEY is not already present, a new handle is created which associates KEY with INIT. -- Scheme Procedure: hashv-create-handle! table key init -- C Function: scm_hashv_create_handle_x (table, key, init) This function looks up KEY in TABLE and returns its handle. If KEY is not already present, a new handle is created which associates KEY with INIT. -- Scheme Procedure: hash-create-handle! table key init -- C Function: scm_hash_create_handle_x (table, key, init) This function looks up KEY in TABLE and returns its handle. If KEY is not already present, a new handle is created which associates KEY with INIT. -- Scheme Procedure: hashx-create-handle! hash assoc table key init -- C Function: scm_hashx_create_handle_x (hash, assoc, table, key, init) This behaves the same way as the corresponding `-create-handle' function, but uses HASH as a hash function and ASSOC to compare keys. `hash' must be a function that takes two arguments, a key to be hashed and a table size. `assoc' must be an associator function, like `assoc', `assq' or `assv'. -- Scheme Procedure: hash-fold proc init table -- C Function: scm_hash_fold (proc, init, table) An iterator over hash-table elements. Accumulates and returns a result by applying PROC successively. The arguments to PROC are "(key value prior-result)" where key and value are successive pairs from the hash table TABLE, and prior-result is either INIT (for the first application of PROC) or the return value of the previous application of PROC. For example, `(hash-fold acons '() tab)' will convert a hash table into an a-list of key-value pairs.  File: guile.info, Node: Procedures and Macros, Next: Utility Functions, Prev: Compound Data Types, Up: Top 23 Procedures and Macros ************************ * Menu: * Lambda:: Basic procedure creation using lambda. * Optional Arguments:: Handling keyword, optional and rest arguments. * Procedure Properties:: Procedure properties and meta-information. * Procedures with Setters:: Procedures with setters. * Macros:: Lisp style macro definitions. * Syntax Rules:: Support for R5RS `syntax-rules'. * Syntax Case:: Support for the `syntax-case' system. * Internal Macros:: Guile's internal representation.  File: guile.info, Node: Lambda, Next: Optional Arguments, Up: Procedures and Macros 23.1 Lambda: Basic Procedure Creation ===================================== A `lambda' expression evaluates to a procedure. The environment which is in effect when a `lambda' expression is evaluated is enclosed in the newly created procedure, this is referred to as a "closure" (*note About Closure::). When a procedure created by `lambda' is called with some actual arguments, the environment enclosed in the procedure is extended by binding the variables named in the formal argument list to new locations and storing the actual arguments into these locations. Then the body of the `lambda' expression is evaluation sequentially. The result of the last expression in the procedure body is then the result of the procedure invocation. The following examples will show how procedures can be created using `lambda', and what you can do with these procedures. (lambda (x) (+ x x)) => a procedure ((lambda (x) (+ x x)) 4) => 8 The fact that the environment in effect when creating a procedure is enclosed in the procedure is shown with this example: (define add4 (let ((x 4)) (lambda (y) (+ x y)))) (add4 6) => 10 -- syntax: lambda formals body FORMALS should be a formal argument list as described in the following table. `(VARIABLE1 ...)' The procedure takes a fixed number of arguments; when the procedure is called, the arguments will be stored into the newly created location for the formal variables. `VARIABLE' The procedure takes any number of arguments; when the procedure is called, the sequence of actual arguments will converted into a list and stored into the newly created location for the formal variable. `(VARIABLE1 ... VARIABLEN . VARIABLEN+1)' If a space-delimited period precedes the last variable, then the procedure takes N or more variables where N is the number of formal arguments before the period. There must be at least one argument before the period. The first N actual arguments will be stored into the newly allocated locations for the first N formal arguments and the sequence of the remaining actual arguments is converted into a list and the stored into the location for the last formal argument. If there are exactly N actual arguments, the empty list is stored into the location of the last formal argument. BODY is a sequence of Scheme expressions which are evaluated in order when the procedure is invoked.  File: guile.info, Node: Optional Arguments, Next: Procedure Properties, Prev: Lambda, Up: Procedures and Macros 23.2 Optional Arguments ======================= Scheme procedures, as defined in R5RS, can either handle a fixed number of actual arguments, or a fixed number of actual arguments followed by arbitrarily many additional arguments. Writing procedures of variable arity can be useful, but unfortunately, the syntactic means for handling argument lists of varying length is a bit inconvenient. It is possible to give names to the fixed number of argument, but the remaining (optional) arguments can be only referenced as a list of values (*note Lambda::). Guile comes with the module `(ice-9 optargs)', which makes using optional arguments much more convenient. In addition, this module provides syntax for handling keywords in argument lists (*note Keywords::). Before using any of the procedures or macros defined in this section, you have to load the module `(ice-9 optargs)' with the statement: (use-modules (ice-9 optargs)) * Menu: * let-optional Reference:: Locally binding optional arguments. * let-keywords Reference:: Locally binding keywords arguments. * lambda* Reference:: Creating advanced argument handling procedures. * define* Reference:: Defining procedures and macros.  File: guile.info, Node: let-optional Reference, Next: let-keywords Reference, Up: Optional Arguments 23.2.1 let-optional Reference ----------------------------- The syntax `let-optional' and `let-optional*' are for destructuring rest argument lists and giving names to the various list elements. `let-optional' binds all variables simultaneously, while `let-optional*' binds them sequentially, consistent with `let' and `let*' (*note Local Bindings::). -- library syntax: let-optional rest-arg (binding ...) expr ... -- library syntax: let-optional* rest-arg (binding ...) expr ... These two macros give you an optional argument interface that is very "Schemey" and introduces no fancy syntax. They are compatible with the scsh macros of the same name, but are slightly extended. Each of BINDING may be of one of the forms VAR or `(VAR DEFAULT-VALUE)'. REST-ARG should be the rest-argument of the procedures these are used from. The items in REST-ARG are sequentially bound to the variable names are given. When REST-ARG runs out, the remaining vars are bound either to the default values or `#f' if no default value was specified. REST-ARG remains bound to whatever may have been left of REST-ARG. After binding the variables, the expressions EXPR ... are evaluated in order.  File: guile.info, Node: let-keywords Reference, Next: lambda* Reference, Prev: let-optional Reference, Up: Optional Arguments 23.2.2 let-keywords Reference ----------------------------- `let-keywords' and `let-keywords*' are used for extracting values from argument lists which use keywords instead of argument position for binding local variables to argument values. `let-keywords' binds all variables simultaneously, while `let-keywords*' binds them sequentially, consistent with `let' and `let*' (*note Local Bindings::). -- library syntax: let-keywords rest-arg allow-other-keys? (binding ...) expr ... -- library syntax: let-keywords rest-arg allow-other-keys? (binding ...) expr ... These macros pick out keyword arguments from REST-ARG, but do not modify it. This is consistent at least with Common Lisp, which duplicates keyword arguments in the rest argument. More explanation of what keyword arguments in a lambda list look like can be found below in the documentation for `lambda*' (*note lambda* Reference::). BINDINGs can have the same form as for `let-optional'. If ALLOW-OTHER-KEYS? is false, an error will be thrown if anything that looks like a keyword argument but does not match a known keyword parameter will result in an error. After binding the variables, the expressions EXPR ... are evaluated in order.  File: guile.info, Node: lambda* Reference, Next: define* Reference, Prev: let-keywords Reference, Up: Optional Arguments 23.2.3 lambda* Reference ------------------------ When using optional and keyword argument lists, using `lambda' for creating procedures and using `let-optional' or `let-keywords' is a bit lengthy. Therefore, `lambda*' is provided, which combines the features of those macros into a single convenient syntax. For quick reference, here is the syntax of the formal argument list for `lambda*' (brackets are used to indicate grouping only): ext-param-list ::= [identifier]* [#:optional [ext-var-decl]+]? [#:key [ext-var-decl]+ [#:allow-other-keys]?]? [[#:rest identifier]|[. identifier]]? ext-var-decl ::= identifier | ( identifier expression ) The characters `*', `+' and `?' are not to be taken literally; they mean respectively, zero or more occurrences, one or more occurrences, and one or zero occurrences. -- library syntax: lambda* formals body `lambda*' creates a procedure that takes optional arguments. These are specified by putting them inside brackets at the end of the parameter list, but before any dotted rest argument. For example, (lambda* (a b #:optional c d . e) '()) creates a procedure with fixed arguments A and B, optional arguments C and D, and rest argument E. If the optional arguments are omitted in a call, the variables for them are bound to `#f'. `lambda*' can also take keyword arguments. For example, a procedure defined like this: (lambda* (#:key xyzzy larch) '()) can be called with any of the argument lists `(#:xyzzy 11)' `(#:larch 13)' `(#:larch 42 #:xyzzy 19)' `()'. Whichever arguments are given as keywords are bound to values. Optional and keyword arguments can also be given default values which they take on when they are not present in a call, by giving a two-item list in place of an optional argument, for example in: (lambda* (foo #:optional (bar 42) #:key (baz 73)) (list foo bar baz)) FOO is a fixed argument, BAR is an optional argument with default value 42, and baz is a keyword argument with default value 73. Default value expressions are not evaluated unless they are needed and until the procedure is called. `lambda*' also supports two more special parameter list keywords. `lambda*'-defined procedures now throw an error by default if a keyword other than one of those specified is found in the actual passed arguments. However, specifying `#:allow-other-keys' immediately after the keyword argument declarations restores the previous behavior of ignoring unknown keywords. `lambda*' also now guarantees that if the same keyword is passed more than once, the last one passed is the one that takes effect. For example, ((lambda* (#:key (heads 0) (tails 0)) (display (list heads tails))) #:heads 37 #:tails 42 #:heads 99) would result in (99 47) being displayed. `#:rest' is also now provided as a synonym for the dotted syntax rest argument. The argument lists `(a . b)' and `(a #:rest b)' are equivalent in all respects to `lambda*'. This is provided for more similarity to DSSSL, MIT-Scheme and Kawa among others, as well as for refugees from other Lisp dialects.  File: guile.info, Node: define* Reference, Prev: lambda* Reference, Up: Optional Arguments 23.2.4 define* Reference ------------------------ Just like `define' has a shorthand notation for defining procedures (*note Lambda Alternatives::), `define*' is provided as an abbreviation of the combination of `define' and `lambda*'. `define*-public' is the `lambda*' version of `define-public'; `defmacro*' and `defmacro*-public' exist for defining macros with the improved argument list handling possibilities. The `-public' versions not only define the procedures/macros, but also export them from the current module. -- library syntax: define* formals body -- library syntax: define*-public formals body `define*' and `define*-public' support optional arguments with a similar syntax to `lambda*'. They also support arbitrary-depth currying, just like Guile's define. Some examples: (define* (x y #:optional a (z 3) #:key w . u) (display (list y z u))) defines a procedure `x' with a fixed argument Y, an optional argument A, another optional argument Z with default value 3, a keyword argument W, and a rest argument U. (define-public* ((foo #:optional bar) #:optional baz) '()) This illustrates currying. A procedure `foo' is defined, which, when called with an optional argument BAR, returns a procedure that takes an optional argument BAZ. Of course, `define*[-public]' also supports `#:rest' and `#:allow-other-keys' in the same way as `lambda*'. -- library syntax: defmacro* name formals body -- library syntax: defmacro*-public name formals body These are just like `defmacro' and `defmacro-public' except that they take `lambda*'-style extended parameter lists, where `#:optional', `#:key', `#:allow-other-keys' and `#:rest' are allowed with the usual semantics. Here is an example of a macro with an optional argument: (defmacro* transmorgify (a #:optional b) (a 1))  File: guile.info, Node: Procedure Properties, Next: Procedures with Setters, Prev: Optional Arguments, Up: Procedures and Macros 23.3 Procedure Properties and Meta-information ============================================== Procedures always have attached the environment in which they were created and information about how to apply them to actual arguments. In addition to that, properties and meta-information can be stored with procedures. The procedures in this section can be used to test whether a given procedure satisfies a condition; and to access and set a procedure's property. The first group of procedures are predicates to test whether a Scheme object is a procedure, or a special procedure, respectively. `procedure?' is the most general predicates, it returns `#t' for any kind of procedure. `closure?' does not return `#t' for primitive procedures, and `thunk?' only returns `#t' for procedures which do not accept any arguments. -- Scheme Procedure: procedure? obj -- C Function: scm_procedure_p (obj) Return `#t' if OBJ is a procedure. -- Scheme Procedure: closure? obj -- C Function: scm_closure_p (obj) Return `#t' if OBJ is a closure. -- Scheme Procedure: thunk? obj -- C Function: scm_thunk_p (obj) Return `#t' if OBJ is a thunk. Procedure properties are general properties to be attached to procedures. These can be the name of a procedure or other relevant information, such as debug hints. -- Scheme Procedure: procedure-name proc -- C Function: scm_procedure_name (proc) Return the name of the procedure PROC -- Scheme Procedure: procedure-source proc -- C Function: scm_procedure_source (proc) Return the source of the procedure PROC. -- Scheme Procedure: procedure-environment proc -- C Function: scm_procedure_environment (proc) Return the environment of the procedure PROC. -- Scheme Procedure: procedure-properties proc -- C Function: scm_procedure_properties (proc) Return OBJ's property list. -- Scheme Procedure: procedure-property obj key -- C Function: scm_procedure_property (obj, key) Return the property of OBJ with name KEY. -- Scheme Procedure: set-procedure-properties! proc alist -- C Function: scm_set_procedure_properties_x (proc, alist) Set OBJ's property list to ALIST. -- Scheme Procedure: set-procedure-property! obj key value -- C Function: scm_set_procedure_property_x (obj, key, value) In OBJ's property list, set the property named KEY to VALUE. Documentation for a procedure can be accessed with the procedure `procedure-documentation'. -- Scheme Procedure: procedure-documentation proc -- C Function: scm_procedure_documentation (proc) Return the documentation string associated with `proc'. By convention, if a procedure contains more than one expression and the first expression is a string constant, that string is assumed to contain documentation for that procedure. Source properties are properties which are related to the source code of a procedure, such as the line and column numbers, the file name etc. -- Scheme Procedure: set-source-properties! obj plist -- C Function: scm_set_source_properties_x (obj, plist) Install the association list PLIST as the source property list for OBJ. -- Scheme Procedure: set-source-property! obj key datum -- C Function: scm_set_source_property_x (obj, key, datum) Set the source property of object OBJ, which is specified by KEY to DATUM. Normally, the key will be a symbol. -- Scheme Procedure: source-properties obj -- C Function: scm_source_properties (obj) Return the source property association list of OBJ. -- Scheme Procedure: source-property obj key -- C Function: scm_source_property (obj, key) Return the source property specified by KEY from OBJ's source property list.  File: guile.info, Node: Procedures with Setters, Next: Macros, Prev: Procedure Properties, Up: Procedures and Macros 23.4 Procedures with Setters ============================ A "procedure with setter" is a special kind of procedure which normally behaves like any accessor procedure, that is a procedure which accesses a data structure. The difference is that this kind of procedure has a so-called "setter" attached, which is a procedure for storing something into a data structure. Procedures with setters are treated specially when the procedure appears in the special form `set!' (REFFIXME). How it works is best shown by example. Suppose we have a procedure called `foo-ref', which accepts two arguments, a value of type `foo' and an integer. The procedure returns the value stored at the given index in the `foo' object. Let `f' be a variable containing such a `foo' data structure.(1) (foo-ref f 0) => bar (foo-ref f 1) => braz Also suppose that a corresponding setter procedure called `foo-set!' does exist. (foo-set! f 0 'bla) (foo-ref f 0) => bla Now we could create a new procedure called `foo', which is a procedure with setter, by calling `make-procedure-with-setter' with the accessor and setter procedures `foo-ref' and `foo-set!'. Let us call this new procedure `foo'. (define foo (make-procedure-with-setter foo-ref foo-set!)) `foo' can from now an be used to either read from the data structure stored in `f', or to write into the structure. (set! (foo f 0) 'dum) (foo f 0) => dum -- Scheme Procedure: make-procedure-with-setter procedure setter -- C Function: scm_make_procedure_with_setter (procedure, setter) Create a new procedure which behaves like PROCEDURE, but with the associated setter SETTER. -- Scheme Procedure: procedure-with-setter? obj -- C Function: scm_procedure_with_setter_p (obj) Return `#t' if OBJ is a procedure with an associated setter procedure. -- Scheme Procedure: procedure proc -- C Function: scm_procedure (proc) Return the procedure of PROC, which must be either a procedure with setter, or an operator struct. -- Scheme Procedure: setter proc Return the setter of PROC, which must be either a procedure with setter or an operator struct. ---------- Footnotes ---------- (1) Working definitions would be: (define foo-ref vector-ref) (define foo-set! vector-set!) (define f (make-vector 2 #f))  File: guile.info, Node: Macros, Next: Syntax Rules, Prev: Procedures with Setters, Up: Procedures and Macros 23.5 Lisp Style Macro Definitions ================================= Macros are objects which cause the expression that they appear in to be transformed in some way _before_ being evaluated. In expressions that are intended for macro transformation, the identifier that names the relevant macro must appear as the first element, like this: (MACRO-NAME MACRO-ARGS ...) In Lisp-like languages, the traditional way to define macros is very similar to procedure definitions. The key differences are that the macro definition body should return a list that describes the transformed expression, and that the definition is marked as a macro definition (rather than a procedure definition) by the use of a different definition keyword: in Lisp, `defmacro' rather than `defun', and in Scheme, `define-macro' rather than `define'. Guile supports this style of macro definition using both `defmacro' and `define-macro'. The only difference between them is how the macro name and arguments are grouped together in the definition: (defmacro NAME (ARGS ...) BODY ...) is the same as (define-macro (NAME ARGS ...) BODY ...) The difference is analogous to the corresponding difference between Lisp's `defun' and Scheme's `define'. `false-if-exception', from the `boot-9.scm' file in the Guile distribution, is a good example of macro definition using `defmacro': (defmacro false-if-exception (expr) `(catch #t (lambda () ,expr) (lambda args #f))) The effect of this definition is that expressions beginning with the identifier `false-if-exception' are automatically transformed into a `catch' expression following the macro definition specification. For example: (false-if-exception (open-input-file "may-not-exist")) == (catch #t (lambda () (open-input-file "may-not-exist")) (lambda args #f))  File: guile.info, Node: Syntax Rules, Next: Syntax Case, Prev: Macros, Up: Procedures and Macros 23.6 The R5RS `syntax-rules' System =================================== R5RS defines an alternative system for macro and syntax transformations using the keywords `define-syntax', `let-syntax', `letrec-syntax' and `syntax-rules'. The main difference between the R5RS system and the traditional macros of the previous section is how the transformation is specified. In R5RS, rather than permitting a macro definition to return an arbitrary expression, the transformation is specified in a pattern language that * does not require complicated quoting and extraction of components of the source expression using `caddr' etc. * is designed such that the bindings associated with identifiers in the transformed expression are well defined, and such that it is impossible for the transformed expression to construct new identifiers. The last point is commonly referred to as being "hygienic": the R5RS `syntax-case' system provides "hygienic macros". For example, the R5RS pattern language for the `false-if-exception' example of the previous section looks like this: (syntax-rules () ((_ expr) (catch #t (lambda () expr) (lambda args #f)))) In Guile, the `syntax-rules' system is provided by the `(ice-9 syncase)' module. To make these facilities available in your code, include the expression `(use-syntax (ice-9 syncase))' (*note Using Guile Modules::) before the first usage of `define-syntax' etc. If you are writing a Scheme module, you can alternatively include the form `#:use-syntax (ice-9 syncase)' in your `define-module' declaration (*note Creating Guile Modules::). * Menu: * Pattern Language:: The `syntax-rules' pattern language. * Define-Syntax:: Top level syntax definitions. * Let-Syntax:: Local syntax definitions.  File: guile.info, Node: Pattern Language, Next: Define-Syntax, Up: Syntax Rules 23.6.1 The `syntax-rules' Pattern Language ------------------------------------------  File: guile.info, Node: Define-Syntax, Next: Let-Syntax, Prev: Pattern Language, Up: Syntax Rules 23.6.2 Top Level Syntax Definitions ----------------------------------- define-syntax: The gist is (define-syntax ) makes the into a macro so that ( ...) expands at _compile_ or _read_ time (i.e. before any evaluation begins) into some expression that is given by the .  File: guile.info, Node: Let-Syntax, Prev: Define-Syntax, Up: Syntax Rules 23.6.3 Local Syntax Definitions -------------------------------  File: guile.info, Node: Syntax Case, Next: Internal Macros, Prev: Syntax Rules, Up: Procedures and Macros 23.7 Support for the `syntax-case' System =========================================  File: guile.info, Node: Internal Macros, Prev: Syntax Case, Up: Procedures and Macros 23.8 Internal Representation of Macros and Syntax ================================================= Internally, Guile uses three different flavors of macros. The three flavors are called "acro" (or "syntax"), "macro" and "mmacro". Given the expression (foo ...) with `foo' being some flavor of macro, one of the following things will happen when the expression is evaluated. * When `foo' has been defined to be an "acro", the procedure used in the acro definition of `foo' is passed the whole expression and the current lexical environment, and whatever that procedure returns is the value of evaluating the expression. You can think of this a procedure that receives its argument as an unevaluated expression. * When `foo' has been defined to be a "macro", the procedure used in the macro definition of `foo' is passed the whole expression and the current lexical environment, and whatever that procedure returns is evaluated again. That is, the procedure should return a valid Scheme expression. * When `foo' has been defined to be a "mmacro", the procedure used in the mmacro definition of `foo' is passed the whole expression and the current lexical environment, and whatever that procedure returns replaces the original expression. Evaluation then starts over from the new expression that has just been returned. The key difference between a "macro" and a "mmacro" is that the expression returned by a "mmacro" procedure is remembered (or "memoized") so that the expansion does not need to be done again next time the containing code is evaluated. The primitives `procedure->syntax', `procedure->macro' and `procedure->memoizing-macro' are used to construct acros, macros and mmacros respectively. However, if you do not have a very special reason to use one of these primitives, you should avoid them: they are very specific to Guile's current implementation and therefore likely to change. Use `defmacro', `define-macro' (*note Macros::) or `define-syntax' (*note Syntax Rules::) instead. (In low level terms, `defmacro', `define-macro' and `define-syntax' are all implemented as mmacros.) -- Scheme Procedure: procedure->syntax code -- C Function: scm_makacro (code) Return a macro which, when a symbol defined to this value appears as the first symbol in an expression, returns the result of applying CODE to the expression and the environment. -- Scheme Procedure: procedure->macro code -- C Function: scm_makmacro (code) Return a macro which, when a symbol defined to this value appears as the first symbol in an expression, evaluates the result of applying CODE to the expression and the environment. For example: (define trace (procedure->macro (lambda (x env) `(set! ,(cadr x) (tracef ,(cadr x) ',(cadr x)))))) (trace foo) == (set! foo (tracef foo 'foo)). -- Scheme Procedure: procedure->memoizing-macro code -- C Function: scm_makmmacro (code) Return a macro which, when a symbol defined to this value appears as the first symbol in an expression, evaluates the result of applying CODE to the expression and the environment. `procedure->memoizing-macro' is the same as `procedure->macro', except that the expression returned by CODE replaces the original macro expression in the memoized form of the containing code. In the following primitives, "acro" flavor macros are referred to as "syntax transformers". -- Scheme Procedure: macro? obj -- C Function: scm_macro_p (obj) Return `#t' if OBJ is a regular macro, a memoizing macro or a syntax transformer. -- Scheme Procedure: macro-type m -- C Function: scm_macro_type (m) Return one of the symbols `syntax', `macro' or `macro!', depending on whether M is a syntax transformer, a regular macro, or a memoizing macro, respectively. If M is not a macro, `#f' is returned. -- Scheme Procedure: macro-name m -- C Function: scm_macro_name (m) Return the name of the macro M. -- Scheme Procedure: macro-transformer m -- C Function: scm_macro_transformer (m) Return the transformer of the macro M. -- Scheme Procedure: cons-source xorig x y -- C Function: scm_cons_source (xorig, x, y) Create and return a new pair whose car and cdr are X and Y. Any source properties associated with XORIG are also associated with the new pair.  File: guile.info, Node: Utility Functions, Next: Binding Constructs, Prev: Procedures and Macros, Up: Top 24 General Utility Functions **************************** This chapter contains information about procedures which are not cleanly tied to a specific data type. Because of their wide range of applications, they are collected in a "utility" chapter. * Menu: * Equality:: When are two values `the same'? * Object Properties:: A modern interface to object properties. * Sorting:: Sort utility procedures. * Copying:: Copying deep structures. * General Conversion:: Converting objects to strings. * Hooks:: User-customizable event lists.  File: guile.info, Node: Equality, Next: Object Properties, Up: Utility Functions 24.1 Equality ============= Three different kinds of "sameness" are defined in Scheme. * Two values can refer to exactly the same object. * Two objects can have the same "value". * Two objects can be structurally equivalent. The differentiation between these three kinds is important, because determining whether two values are the same objects is very efficient, while determining structural equivalence can be quite expensive (consider comparing two very long lists). Therefore, three different procedures for testing for equality are provided, which correspond to the three kinds of "sameness" defined above. -- Scheme Procedure: eq? x y Return `#t' iff X references the same object as Y. `eq?' is similar to `eqv?' except that in some cases it is capable of discerning distinctions finer than those detectable by `eqv?'. -- Scheme Procedure: eqv? x y The `eqv?' procedure defines a useful equivalence relation on objects. Briefly, it returns `#t' if X and Y should normally be regarded as the same object. This relation is left slightly open to interpretation, but works for comparing immediate integers, characters, and inexact numbers. -- Scheme Procedure: equal? x y Return `#t' iff X and Y are recursively `eqv?' equivalent. `equal?' recursively compares the contents of pairs, vectors, and strings, applying `eqv?' on other objects such as numbers and symbols. A rule of thumb is that objects are generally `equal?' if they print the same. `equal?' may fail to terminate if its arguments are circular data structures.  File: guile.info, Node: Object Properties, Next: Sorting, Prev: Equality, Up: Utility Functions 24.2 Object Properties ====================== It's often useful to associate a piece of additional information with a Scheme object even though that object does not have a dedicated slot available in which the additional information could be stored. Object properties allow you to do just that. An object property is most commonly used to associate one kind of additional information with each instance of a class of similar Scheme objects. For example, all procedures have a `name' property, which stores the name of the variable in which the procedure was stored by a `define' expression, or `#f' if the procedure wasn't created by that kind of expression. Guile's representation of an object property is a procedure-with-setter (*note Procedures with Setters::) that can be used with the generalized form of `set!' (REFFIXME) to set and retrieve that property for any Scheme object. So, setting a property looks like this: (set! (my-property obj1) value-for-obj1) (set! (my-property obj2) value-for-obj2) And retrieving values of the same property looks like this: (my-property obj1) => value-for-obj1 (my-property obj2) => value-for-obj2 To create an object property in the first place, use the `make-object-property' procedure: (define my-property (make-object-property)) -- Scheme Procedure: make-object-property Create and return an object property. An object property is a procedure-with-setter that can be called in two ways. `(set! (PROPERTY OBJ) VAL)' sets OBJ's PROPERTY to VAL. `(PROPERTY OBJ)' returns the current setting of OBJ's PROPERTY. A single object property created by `make-object-property' can associate distinct property values with all Scheme values that are distinguishable by `eq?' (including, for example, integers). Internally, object properties are implemented using a weak key hash table. This means that, as long as a Scheme value with property values is protected from garbage collection, its property values are also protected. When the Scheme value is collected, its entry in the property table is removed and so the (ex-) property values are no longer protected by the table. * Menu: * Property Primitives:: Low level property implementation. * Old-fashioned Properties:: An older approach to properties.  File: guile.info, Node: Property Primitives, Next: Old-fashioned Properties, Up: Object Properties 24.2.1 Low Level Property Implementation. ----------------------------------------- -- Scheme Procedure: primitive-make-property not_found_proc -- C Function: scm_primitive_make_property (not_found_proc) Create a "property token" that can be used with `primitive-property-ref' and `primitive-property-set!'. See `primitive-property-ref' for the significance of NOT_FOUND_PROC. -- Scheme Procedure: primitive-property-ref prop obj -- C Function: scm_primitive_property_ref (prop, obj) Return the property PROP of OBJ. When no value has yet been associated with PROP and OBJ, call NOT-FOUND-PROC instead (see `primitive-make-property') and use its return value. That value is also associated with OBJ via `primitive-property-set!'. When NOT-FOUND-PROC is `#f', use `#f' as the default value of PROP. -- Scheme Procedure: primitive-property-set! prop obj val -- C Function: scm_primitive_property_set_x (prop, obj, val) Associate CODE with PROP and OBJ. -- Scheme Procedure: primitive-property-del! prop obj -- C Function: scm_primitive_property_del_x (prop, obj) Remove any value associated with PROP and OBJ.  File: guile.info, Node: Old-fashioned Properties, Prev: Property Primitives, Up: Object Properties 24.2.2 An Older Approach to Properties -------------------------------------- Traditionally, Lisp systems provide a different object property interface to that provided by `make-object-property', in which the object property that is being set or retrieved is indicated by a symbol. Guile includes this older kind of interface as well, but it may well be removed in a future release, as it is less powerful than `make-object-property' and so increases the size of the Guile library for no benefit. (And it is trivial to write a compatibility layer in Scheme.) -- Scheme Procedure: object-properties obj -- C Function: scm_object_properties (obj) Return OBJ's property list. -- Scheme Procedure: set-object-properties! obj alist -- C Function: scm_set_object_properties_x (obj, alist) Set OBJ's property list to ALIST. -- Scheme Procedure: object-property obj key -- C Function: scm_object_property (obj, key) Return the property of OBJ with name KEY. -- Scheme Procedure: set-object-property! obj key value -- C Function: scm_set_object_property_x (obj, key, value) In OBJ's property list, set the property named KEY to VALUE.  File: guile.info, Node: Sorting, Next: Copying, Prev: Object Properties, Up: Utility Functions 24.3 Sorting ============ Sorting is very important in computer programs. Therefore, Guile comes with several sorting procedures built-in. As always, procedures with names ending in `!' are side-effecting, that means that they may modify their parameters in order to produce their results. The first group of procedures can be used to merge two lists (which must be already sorted on their own) and produce sorted lists containing all elements of the input lists. -- Scheme Procedure: merge alist blist less -- C Function: scm_merge (alist, blist, less) Merge two already sorted lists into one. Given two lists ALIST and BLIST, such that `(sorted? alist less?)' and `(sorted? blist less?)', return a new list in which the elements of ALIST and BLIST have been stably interleaved so that `(sorted? (merge alist blist less?) less?)'. Note: this does _not_ accept vectors. -- Scheme Procedure: merge! alist blist less -- C Function: scm_merge_x (alist, blist, less) Takes two lists ALIST and BLIST such that `(sorted? alist less?)' and `(sorted? blist less?)' and returns a new list in which the elements of ALIST and BLIST have been stably interleaved so that `(sorted? (merge alist blist less?) less?)'. This is the destructive variant of `merge' Note: this does _not_ accept vectors. The following procedures can operate on sequences which are either vectors or list. According to the given arguments, they return sorted vectors or lists, respectively. The first of the following procedures determines whether a sequence is already sorted, the other sort a given sequence. The variants with names starting with `stable-' are special in that they maintain a special property of the input sequences: If two or more elements are the same according to the comparison predicate, they are left in the same order as they appeared in the input. -- Scheme Procedure: sorted? items less -- C Function: scm_sorted_p (items, less) Return `#t' iff ITEMS is a list or a vector such that for all 1 <= i <= m, the predicate LESS returns true when applied to all elements i - 1 and i -- Scheme Procedure: sort items less -- C Function: scm_sort (items, less) Sort the sequence ITEMS, which may be a list or a vector. LESS is used for comparing the sequence elements. This is not a stable sort. -- Scheme Procedure: sort! items less -- C Function: scm_sort_x (items, less) Sort the sequence ITEMS, which may be a list or a vector. LESS is used for comparing the sequence elements. The sorting is destructive, that means that the input sequence is modified to produce the sorted result. This is not a stable sort. -- Scheme Procedure: stable-sort items less -- C Function: scm_stable_sort (items, less) Sort the sequence ITEMS, which may be a list or a vector. LESS is used for comparing the sequence elements. This is a stable sort. -- Scheme Procedure: stable-sort! items less -- C Function: scm_stable_sort_x (items, less) Sort the sequence ITEMS, which may be a list or a vector. LESS is used for comparing the sequence elements. The sorting is destructive, that means that the input sequence is modified to produce the sorted result. This is a stable sort. The procedures in the last group only accept lists or vectors as input, as their names indicate. -- Scheme Procedure: sort-list items less -- C Function: scm_sort_list (items, less) Sort the list ITEMS, using LESS for comparing the list elements. This is a stable sort. -- Scheme Procedure: sort-list! items less -- C Function: scm_sort_list_x (items, less) Sort the list ITEMS, using LESS for comparing the list elements. The sorting is destructive, that means that the input list is modified to produce the sorted result. This is a stable sort. -- Scheme Procedure: restricted-vector-sort! vec less startpos endpos -- C Function: scm_restricted_vector_sort_x (vec, less, startpos, endpos) Sort the vector VEC, using LESS for comparing the vector elements. STARTPOS and ENDPOS delimit the range of the vector which gets sorted. The return value is not specified.  File: guile.info, Node: Copying, Next: General Conversion, Prev: Sorting, Up: Utility Functions 24.4 Copying Deep Structures ============================ The procedures for copying lists (*note Lists::) only produce a flat copy of the input list, and currently Guile does not even contain procedures for copying vectors. `copy-tree' can be used for these application, as it does not only copy the spine of a list, but also copies any pairs in the cars of the input lists. -- Scheme Procedure: copy-tree obj -- C Function: scm_copy_tree (obj) Recursively copy the data tree that is bound to OBJ, and return a pointer to the new data structure. `copy-tree' recurses down the contents of both pairs and vectors (since both cons cells and vector cells may point to arbitrary objects), and stops recursing when it hits any other object.  File: guile.info, Node: General Conversion, Next: Hooks, Prev: Copying, Up: Utility Functions 24.5 General String Conversion ============================== When debugging Scheme programs, but also for providing a human-friendly interface, a procedure for converting any Scheme object into string format is very useful. Conversion from/to strings can of course be done with specialized procedures when the data type of the object to convert is known, but with this procedure, it is often more comfortable. `object->string' converts an object by using a print procedure for writing to a string port, and then returning the resulting string. Converting an object back from the string is only possible if the object type has a read syntax and the read syntax is preserved by the printing procedure. -- Scheme Procedure: object->string obj [printer] -- C Function: scm_object_to_string (obj, printer) Return a Scheme string obtained by printing OBJ. Printing function can be specified by the optional second argument PRINTER (default: `write').  File: guile.info, Node: Hooks, Prev: General Conversion, Up: Utility Functions 24.6 Hooks ========== A hook is a list of procedures to be called at well defined points in time. Typically, an application provides a hook H and promises its users that it will call all of the procedures in H at a defined point in the application's processing. By adding its own procedure to H, an application user can tap into or even influence the progress of the application. Guile itself provides several such hooks for debugging and customization purposes: these are listed in a subsection below. When an application first creates a hook, it needs to know how many arguments will be passed to the hook's procedures when the hook is run. The chosen number of arguments (which may be none) is declared when the hook is created, and all the procedures that are added to that hook must be capable of accepting that number of arguments. A hook is created using `make-hook'. A procedure can be added to or removed from a hook using `add-hook!' or `remove-hook!', and all of a hook's procedures can be removed together using `reset-hook!'. When an application wants to run a hook, it does so using `run-hook'. * Menu: * Hook Example:: Hook usage by example. * Hook Reference:: Reference of all hook procedures. * C Hooks:: Hooks for use from C code. * Guile Hooks:: Hooks provided by Guile.  File: guile.info, Node: Hook Example, Next: Hook Reference, Up: Hooks 24.6.1 Hook Usage by Example ---------------------------- Hook usage is shown by some examples in this section. First, we will define a hook of arity 2 -- that is, the procedures stored in the hook will have to accept two arguments. (define hook (make-hook 2)) hook => # Now we are ready to add some procedures to the newly created hook with `add-hook!'. In the following example, two procedures are added, which print different messages and do different things with their arguments. (add-hook! hook (lambda (x y) (display "Foo: ") (display (+ x y)) (newline))) (add-hook! hook (lambda (x y) (display "Bar: ") (display (* x y)) (newline))) Once the procedures have been added, we can invoke the hook using `run-hook'. (run-hook hook 3 4) -| Bar: 12 -| Foo: 7 Note that the procedures are called in the reverse of the order with which they were added. This is because the default behaviour of `add-hook!' is to add its procedure to the _front_ of the hook's procedure list. You can force `add-hook!' to add its procedure to the _end_ of the list instead by providing a third `#t' argument on the second call to `add-hook!'. (add-hook! hook (lambda (x y) (display "Foo: ") (display (+ x y)) (newline))) (add-hook! hook (lambda (x y) (display "Bar: ") (display (* x y)) (newline)) #t) ; <- Change here! (run-hook hook 3 4) -| Foo: 7 -| Bar: 12  File: guile.info, Node: Hook Reference, Next: C Hooks, Prev: Hook Example, Up: Hooks 24.6.2 Hook Reference --------------------- When you create a hook with `make-hook', you must specify the arity of the procedures which can be added to the hook. If the arity is not given explicitly as an argument to `make-hook', it defaults to zero. All procedures of a given hook must have the same arity, and when the procedures are invoked using `run-hook', the number of arguments passed must match the arity specified at hook creation time. The order in which procedures are added to a hook matters. If the third parameter to `add-hook!' is omitted or is equal to `#f', the procedure is added in front of the procedures which might already be on that hook, otherwise the procedure is added at the end. The procedures are always called from the front to the end of the list when they are invoked via `run-hook'. The ordering of the list of procedures returned by `hook->list' matches the order in which those procedures would be called if the hook was run using `run-hook'. Note that the C functions in the following entries are for handling "Scheme-level" hooks in C. There are also "C-level" hooks which have their own interface (*note C Hooks::). -- Scheme Procedure: make-hook [n_args] -- C Function: scm_make_hook (n_args) Create a hook for storing procedure of arity N_ARGS. N_ARGS defaults to zero. The returned value is a hook object to be used with the other hook procedures. -- Scheme Procedure: hook? x -- C Function: scm_hook_p (x) Return `#t' if X is a hook, `#f' otherwise. -- Scheme Procedure: hook-empty? hook -- C Function: scm_hook_empty_p (hook) Return `#t' if HOOK is an empty hook, `#f' otherwise. -- Scheme Procedure: add-hook! hook proc [append_p] -- C Function: scm_add_hook_x (hook, proc, append_p) Add the procedure PROC to the hook HOOK. The procedure is added to the end if APPEND_P is true, otherwise it is added to the front. The return value of this procedure is not specified. -- Scheme Procedure: remove-hook! hook proc -- C Function: scm_remove_hook_x (hook, proc) Remove the procedure PROC from the hook HOOK. The return value of this procedure is not specified. -- Scheme Procedure: reset-hook! hook -- C Function: scm_reset_hook_x (hook) Remove all procedures from the hook HOOK. The return value of this procedure is not specified. -- Scheme Procedure: hook->list hook -- C Function: scm_hook_to_list (hook) Convert the procedure list of HOOK to a list. -- Scheme Procedure: run-hook hook . args -- C Function: scm_run_hook (hook, args) Apply all procedures from the hook HOOK to the arguments ARGS. The order of the procedure application is first to last. The return value of this procedure is not specified. If, in C code, you are certain that you have a hook object and well formed argument list for that hook, you can also use `scm_c_run_hook', which is identical to `scm_run_hook' but does no type checking. -- C Function: void scm_c_run_hook (SCM hook, SCM args) The same as `scm_run_hook' but without any type checking to confirm that HOOK is actually a hook object and that ARGS is a well-formed list matching the arity of the hook. For C code, `SCM_HOOKP' is a faster alternative to `scm_hook_p': -- C Macro: int SCM_HOOKP (x) Return 1 if X is a Scheme-level hook, 0 otherwise. 24.6.3 Handling Scheme-level hooks from C code ---------------------------------------------- Here is an example of how to handle Scheme-level hooks from C code using the above functions. if (SCM_NFALSEP (scm_hook_p (obj))) /* handle Scheme-level hook using C functions */ scm_reset_hook_x (obj); else /* do something else (obj is not a hook) */  File: guile.info, Node: C Hooks, Next: Guile Hooks, Prev: Hook Reference, Up: Hooks 24.6.4 Hooks For C Code. ------------------------ The hooks already described are intended to be populated by Scheme-level procedures. In addition to this, the Guile library provides an independent set of interfaces for the creation and manipulation of hooks that are designed to be populated by functions implemented in C. The original motivation here was to provide a kind of hook that could safely be invoked at various points during garbage collection. Scheme-level hooks are unsuitable for this purpose as running them could itself require memory allocation, which would then invoke garbage collection recursively ... However, it is also the case that these hooks are easier to work with than the Scheme-level ones if you only want to register C functions with them. So if that is mainly what your code needs to do, you may prefer to use this interface. To create a C hook, you should allocate storage for a structure of type `scm_t_c_hook' and then initialize it using `scm_c_hook_init'. -- C Type: scm_t_c_hook Data type for a C hook. The internals of this type should be treated as opaque. -- C Enum: scm_t_c_hook_type Enumeration of possible hook types, which are: `SCM_C_HOOK_NORMAL' Type of hook for which all the registered functions will always be called. `SCM_C_HOOK_OR' Type of hook for which the sequence of registered functions will be called only until one of them returns C true (a non-NULL pointer). `SCM_C_HOOK_AND' Type of hook for which the sequence of registered functions will be called only until one of them returns C false (a NULL pointer). -- C Function: void scm_c_hook_init (scm_t_c_hook *hook, void *hook_data, scm_t_c_hook_type type) Initialize the C hook at memory pointed to by HOOK. TYPE should be one of the values of the `scm_t_c_hook_type' enumeration, and controls how the hook functions will be called. HOOK_DATA is a closure parameter that will be passed to all registered hook functions when they are called. To add or remove a C function from a C hook, use `scm_c_hook_add' or `scm_c_hook_remove'. A hook function must expect three `void *' parameters which are, respectively: HOOK_DATA The hook closure data that was specified at the time the hook was initialized by `scm_c_hook_init'. FUNC_DATA The function closure data that was specified at the time that that function was registered with the hook by `scm_c_hook_add'. DATA The call closure data specified by the `scm_c_hook_run' call that runs the hook. -- C Type: scm_t_c_hook_function Function type for a C hook function: takes three `void *' parameters and returns a `void *' result. -- C Function: void scm_c_hook_add (scm_t_c_hook *hook, scm_t_c_hook_function func, void *func_data, int appendp) Add function FUNC, with function closure data FUNC_DATA, to the C hook HOOK. The new function is appended to the hook's list of functions if APPENDP is non-zero, otherwise prepended. -- C Function: void scm_c_hook_remove (scm_t_c_hook *hook, scm_t_c_hook_function func, void *func_data) Remove function FUNC, with function closure data FUNC_DATA, from the C hook HOOK. `scm_c_hook_remove' checks both FUNC and FUNC_DATA so as to allow for the same FUNC being registered multiple times with different closure data. Finally, to invoke a C hook, call the `scm_c_hook_run' function specifying the hook and the call closure data for this run: -- C Function: void * scm_c_hook_run (scm_t_c_hook *hook, void *data) Run the C hook HOOK will call closure data DATA. Subject to the variations for hook types `SCM_C_HOOK_OR' and `SCM_C_HOOK_AND', `scm_c_hook_run' calls HOOK's registered functions in turn, passing them the hook's closure data, each function's closure data, and the call closure data. `scm_c_hook_run''s return value is the return value of the last function to be called.  File: guile.info, Node: Guile Hooks, Prev: C Hooks, Up: Hooks 24.6.5 Hooks Provided by Guile ------------------------------ * Menu: * GC Hooks:: Garbage collection hooks. * REPL Hooks:: Hooks into the Guile REPL.  File: guile.info, Node: GC Hooks, Next: REPL Hooks, Up: Guile Hooks 24.6.5.1 Hooks for Garbage Collection ..................................... Whenever Guile performs a garbage collection, it calls the following hooks in the order shown. -- C Hook: scm_before_gc_c_hook C hook called at the very start of a garbage collection, after setting `scm_gc_running_p' to 1, but before entering the GC critical section. If garbage collection is blocked because `scm_block_gc' is non-zero, GC exits early soon after calling this hook, and no further hooks will be called. -- C Hook: scm_before_mark_c_hook C hook called before beginning the mark phase of garbage collection, after the GC thread has entered a critical section. -- C Hook: scm_before_sweep_c_hook C hook called before beginning the sweep phase of garbage collection. This is the same as at the end of the mark phase, since nothing else happens between marking and sweeping. -- C Hook: scm_after_sweep_c_hook C hook called after the end of the sweep phase of garbage collection, but while the GC thread is still inside its critical section. -- C Hook: scm_after_gc_c_hook C hook called at the very end of a garbage collection, after the GC thread has left its critical section. -- Scheme Hook: after-gc-hook Scheme hook with arity 0. This hook is run asynchronously (*note Asyncs::) soon after the GC has completed and any other events that were deferred during garbage collection have been processed. (Also accessible from C with the name `scm_after_gc_hook'.) All the C hooks listed here have type `SCM_C_HOOK_NORMAL', are initialized with hook closure data NULL, are are invoked by `scm_c_hook_run' with call closure data NULL. The Scheme hook `after-gc-hook' is particularly useful in conjunction with guardians (*note Guardians::). Typically, if you are using a guardian, you want to call the guardian after garbage collection to see if any of the objects added to the guardian have been collected. By adding a thunk that performs this call to `after-gc-hook', you can ensure that your guardian is tested after every garbage collection cycle.  File: guile.info, Node: REPL Hooks, Prev: GC Hooks, Up: Guile Hooks 24.6.5.2 Hooks into the Guile REPL ..................................  File: guile.info, Node: Binding Constructs, Next: Control Mechanisms, Prev: Utility Functions, Up: Top 25 Definitions and Variable Bindings ************************************ Scheme supports the definition of variables in different contexts. Variables can be defined at the top level, so that they are visible in the entire program, and variables can be defined locally to procedures and expressions. This is important for modularity and data abstraction. * Menu: * Top Level:: Top level variable definitions. * Local Bindings:: Local variable bindings. * Internal Definitions:: Internal definitions. * Binding Reflection:: Querying variable bindings.  File: guile.info, Node: Top Level, Next: Local Bindings, Up: Binding Constructs 25.1 Top Level Variable Definitions =================================== On the top level of a program (i.e. when not inside the body of a procedure definition or a `let', `let*' or `letrec' expression), a definition of the form (define a VALUE) defines a variable called `a' and sets it to the value VALUE. If the variable already exists, because it has already been created by a previous `define' expression with the same name, its value is simply changed to the new VALUE. In this case, then, the above form is completely equivalent to (set! a VALUE) This equivalence means that `define' can be used interchangeably with `set!' to change the value of variables at the top level of the REPL or a Scheme source file. It is useful during interactive development when reloading a Scheme file that you have modified, because it allows the `define' expressions in that file to work as expected both the first time that the file is loaded and on subsequent occasions. Note, though, that `define' and `set!' are not always equivalent. For example, a `set!' is not allowed if the named variable does not already exist, and the two expressions can behave differently in the case where there are imported variables visible from another module. -- Scheme Syntax: define name value Create a top level variable named NAME with value VALUE. If the named variable already exists, just change its value. The return value of a `define' expression is unspecified. The C API equivalents of `define' are `scm_define' and `scm_c_define', which differ from each other in whether the variable name is specified as a `SCM' symbol or as a null-terminated C string. -- C Function: scm_define (sym, value) -- C Function: scm_c_define (const char *name, value) C equivalents of `define', with variable name specified either by SYM, a symbol, or by NAME, a null-terminated C string. Both variants return the new or preexisting variable object. `define' (when it occurs at top level), `scm_define' and `scm_c_define' all create or set the value of a variable in the top level environment of the current module. If there was not already a variable with the specified name belonging to the current module, but a similarly named variable from another module was visible through having been imported, the newly created variable in the current module will shadow the imported variable, such that the imported variable is no longer visible. Attention: Scheme definitions inside local binding constructs (*note Local Bindings::) act differently (*note Internal Definitions::).  File: guile.info, Node: Local Bindings, Next: Internal Definitions, Prev: Top Level, Up: Binding Constructs 25.2 Local Variable Bindings ============================ As opposed to definitions at the top level, which are visible in the whole program (or current module, when Guile modules are used), it is also possible to define variables which are only visible in a well-defined part of the program. Normally, this part of a program will be a procedure or a subexpression of a procedure. With the constructs for local binding (`let', `let*' and `letrec'), the Scheme language has a block structure like most other programming languages since the days of ALGOL 60. Readers familiar to languages like C or Java should already be used to this concept, but the family of `let' expressions has a few properties which are well worth knowing. The first local binding construct is `let'. The other constructs `let*' and `letrec' are specialized versions for usage where using plain `let' is a bit inconvenient. -- syntax: let bindings body BINDINGS has the form ((VARIABLE1 INIT1) ...) that is zero or more two-element lists of a variable and an arbitrary expression each. All VARIABLE names must be distinct. A `let' expression is evaluated as follows. * All INIT expressions are evaluated. * New storage is allocated for the VARIABLES. * The values of the INIT expressions are stored into the variables. * The expressions in BODY are evaluated in order, and the value of the last expression is returned as the value of the `let' expression. * The storage for the VARIABLES is freed. The INIT expressions are not allowed to refer to any of the VARIABLES. -- syntax: let* bindings body Similar to `let', but the variable bindings are performed sequentially, that means that all INIT expression are allowed to use the variables defined on their left in the binding list. A `let*' expression can always be expressed with nested `let' expressions. (let* ((a 1) (b a)) b) == (let ((a 1)) (let ((b a)) b)) -- syntax: letrec bindings body Similar to `let', but it is possible to refer to the VARIABLE from lambda expression created in any of the INITS. That is, procedures created in the INIT expression can recursively refer to the defined variables. (letrec ((even? (lambda (n) (if (zero? n) #t (odd? (- n 1))))) (odd? (lambda (n) (if (zero? n) #f (even? (- n 1)))))) (even? 88)) => #t There is also an alternative form of the `let' form, which is used for expressing iteration. Because of the use as a looping construct, this form (the "named let") is documented in the section about iteration (*note Iteration: while do.)  File: guile.info, Node: Internal Definitions, Next: Binding Reflection, Prev: Local Bindings, Up: Binding Constructs 25.3 Internal definitions ========================= A `define' form which appears inside the body of a `lambda', `let', `let*', `letrec' or equivalent expression is called an "internal definition". An internal definition differs from a top level definition (*note Top Level::), because the definition is only visible inside the complete body of the enclosing form. Let us examine the following example. (let ((frumble "froz")) (define banana (lambda () (apple 'peach))) (define apple (lambda (x) x)) (banana)) => peach Here the enclosing form is a `let', so the `define's in the `let'-body are internal definitions. Because the scope of the internal definitions is the *complete* body of the `let'-expression, the `lambda'-expression which gets bound to the variable `banana' may refer to the variable `apple', even though it's definition appears lexically _after_ the definition of `banana'. This is because a sequence of internal definition acts as if it were a `letrec' expression. (let () (define a 1) (define b 2) (+ a b)) is equivalent to (let () (letrec ((a 1) (b 2)) (+ a b))) Another noteworthy difference to top level definitions is that within one group of internal definitions all variable names must be distinct. That means where on the top level a second define for a given variable acts like a `set!', an exception is thrown for internal definitions with duplicate bindings.  File: guile.info, Node: Binding Reflection, Prev: Internal Definitions, Up: Binding Constructs 25.4 Querying variable bindings =============================== Guile provides a procedure for checking whether a symbol is bound in the top level environment. -- Scheme Procedure: defined? sym [env] -- C Function: scm_definedp (sym, env) Return `#t' if SYM is defined in the lexical environment ENV. When ENV is not specified, look in the top-level environment as defined by the current module.  File: guile.info, Node: Control Mechanisms, Next: Input and Output, Prev: Binding Constructs, Up: Top 26 Controlling the Flow of Program Execution ******************************************** * Menu: * begin:: Evaluating a sequence of expressions. * if cond case:: Simple conditional evaluation. * and or:: Conditional evaluation of a sequence. * while do:: Iteration mechanisms. * Continuations:: Continuations. * Multiple Values:: Returning and accepting multiple values. * Exceptions:: Throwing and catching exceptions. * Error Reporting:: Procedures for signaling errors. * Dynamic Wind:: Guarding against non-local entrance/exit. * Handling Errors:: How to handle errors in C code.  File: guile.info, Node: begin, Next: if cond case, Up: Control Mechanisms 26.1 Evaluating a Sequence of Expressions ========================================= `begin' is used for grouping several expression together so that they syntactically are treated as if they were one expression. This is particularly important when syntactic expressions are used which only allow one expression, but the programmer wants to use more than one expression in that place. As an example, consider the conditional expression below: (if (> x 0) (begin (display "greater") (newline))) If the two calls to `display' and `newline' were not embedded in a `begin'-statement, the call to `newline' would get misinterpreted as the else-branch of the `if'-expression. -- syntax: begin expr1 expr2 ... The expression(s) are evaluated in left-to-right order and the value of the last expression is returned as the value of the `begin'-expression. This expression type is used when the expressions before the last one are evaluated for their side effects.  File: guile.info, Node: if cond case, Next: and or, Prev: begin, Up: Control Mechanisms 26.2 Simple Conditional Evaluation ================================== Guile provides three syntactic constructs for conditional evaluation. `if' is the normal if-then-else expression (with an optional else branch), `cond' is a conditional expression with multiple branches and `case' branches if an expression has one of a set of constant values. -- syntax: if test consequent [alternate] All arguments may be arbitrary expressions. First, TEST is evaluated. If it returns a true value, the expression CONSEQUENT is evaluated and ALTERNATE is ignored. If TEST evaluates to `#f', ALTERNATE is evaluated instead. The value of the evaluated branch (CONSEQUENT or ALTERNATE) is returned as the value of the `if' expression. When ALTERNATE is omitted and the TEST evaluates to `#f', the value of the expression is not specified. -- syntax: cond clause1 clause2 ... Each `cond'-clause must look like this: (TEST EXPRESSION ...) where TEST and EXPRESSION are arbitrary expression, or like this (TEST => EXPRESSION where EXPRESSION must evaluate to a procedure. The TESTs of the clauses are evaluated in order and as soon as one of them evaluates to a true values, the corresponding EXPRESSIONs are evaluated in order and the last value is returned as the value of the `cond'-expression. For the `=>' clause type, EXPRESSION is evaluated and the resulting procedure is applied to the value of TEST. The result of this procedure application is then the result of the `cond'-expression. The TEST of the last CLAUSE may be the keyword `else'. Then, if none of the preceding TESTs is true, the EXPRESSIONs following the `else' are evaluated to produce the result of the `cond'-expression. -- syntax: case key clause1 clause2 ... KEY may be any expression, the CLAUSEs must have the form ((DATUM1 ...) EXPR1 EXPR2 ...) and the last CLAUSE may have the form (else EXPR1 EXPR2 ...) All DATUMs must be distinct. First, KEY is evaluated. The the result of this evaluation is compared against all DATUMs using `eqv?'. When this comparison succeeds, the expression(s) following the DATUM are evaluated from left to right, returning the value of the last expression as the result of the `case' expression. If the KEY matches no DATUM and there is an `else'-clause, the expressions following the `else' are evaluated. If there is no such clause, the result of the expression is unspecified.  File: guile.info, Node: and or, Next: while do, Prev: if cond case, Up: Control Mechanisms 26.3 Conditional Evaluation of a Sequence of Expressions ======================================================== `and' and `or' evaluate all their arguments, similar to `begin', but evaluation stops as soon as one of the expressions evaluates to false or true, respectively. -- syntax: and expr ... Evaluate the EXPRs from left to right and stop evaluation as soon as one expression evaluates to `#f'; the remaining expressions are not evaluated. The value of the last evaluated expression is returned. If no expression evaluates to `#f', the value of the last expression is returned. If used without expressions, `#t' is returned. -- syntax: or expr ... Evaluate the EXPRs from left to right and stop evaluation as soon as one expression evaluates to a true value (that is, a value different from `#f'); the remaining expressions are not evaluated. The value of the last evaluated expression is returned. If all expressions evaluate to `#f', `#f' is returned. If used without expressions, `#f' is returned.  File: guile.info, Node: while do, Next: Continuations, Prev: and or, Up: Control Mechanisms 26.4 Iteration mechanisms ========================= Scheme has only few iteration mechanisms, mainly because iteration in Scheme programs is normally expressed using recursion. Nevertheless, R5RS defines a construct for programming loops, calling `do'. In addition, Guile has an explicit looping syntax called `while'. -- syntax: do ((variable1 init1 step1) ...) (test expr ...) command ... The INIT expressions are evaluated and the VARIABLES are bound to their values. Then looping starts with testing the TEST expression. If TEST evaluates to a true value, the EXPR following the TEST are evaluated and the value of the last EXPR is returned as the value of the `do' expression. If TEST evaluates to false, the COMMANDs are evaluated in order, the STEPs are evaluated and stored into the VARIABLES and the next iteration starts. Any of the STEP expressions may be omitted, so that the corresponding variable is not changed during looping. -- syntax: while cond body ... Evaluate all expressions in BODY in order, as long as COND evaluates to a true value. The COND expression is tested before every iteration, so that the body is not evaluated at all if COND is `#f' right from the start. Another very common way of expressing iteration in Scheme programs is the use of the so-called "named let". Named let is a variant of `let' which creates a procedure and calls it in one step. Because of the newly created procedure, named let is more powerful than `do'-it can be used for iteration, but also for arbitrary recursion. -- syntax: let variable bindings body For the definition of BINDINGS see the documentation about `let' (*note Local Bindings::). Named `let' works as follows: * A new procedure which accepts as many arguments as are in BINDINGS is created and bound locally (using `let') to VARIABLE. The new procedure's formal argument names are the name of the VARIABLES. * The BODY expressions are inserted into the newly created procedure. * The procedure is called with the INIT expressions as the formal arguments. The next example implements a loop which iterates (by recursion) 1000 times. (let lp ((x 1000)) (if (positive? x) (lp (- x 1)) x)) => 0  File: guile.info, Node: Continuations, Next: Multiple Values, Prev: while do, Up: Control Mechanisms 26.5 Continuations ================== The ability to explicitly capture continuations using `call-with-current-continuation' (also often called `call/cc' for short), and to invoke such continuations later any number of times, and from any other point in a program, provides maybe the most powerful control structure known. All other control structures, such as loops and coroutines, can be emulated using continuations. The implementation of continuations in Guile is not as efficient as one might hope, because it is constrained by the fact that Guile is designed to cooperate with programs written in other languages, such as C, which do not know about continuations. So continuations should be used when there is no other simple way of achieving the desired behaviour, or where the advantages of the elegant continuation mechanism outweigh the need for optimum performance. If you find yourself using `call/cc' for escape procedures and your program is running too slow, you might want to use exceptions (*note Exceptions::) instead. -- Scheme Procedure: call-with-current-continuation proc Capture the current continuation and call PROC with the captured continuation as the single argument. This continuation can then be called with arbitrarily many arguments. Such a call will work like a goto to the invocation location of `call-with-current-continuation', passing the arguments in a way that they are returned by the call to `call-with-current-continuation'. Since it is legal to store the captured continuation in a variable or to pass it to other procedures, it is possible that a procedure returns more than once, even if it is called only one time. This can be confusing at times. (define kont #f) (call-with-current-continuation (lambda (k) (set! kont k) 1)) => 1 (kont 2) => 2  File: guile.info, Node: Multiple Values, Next: Exceptions, Prev: Continuations, Up: Control Mechanisms 26.6 Returning and Accepting Multiple Values ============================================ Scheme allows a procedure to return more than one value to its caller. This is quite different to other languages which only allow single-value returns. Returning multiple values is different from returning a list (or pair or vector) of values to the caller, because conceptually not _one_ compound object is returned, but several distinct values. The primitive procedures for handling multiple values are `values' and `call-with-values'. `values' is used for returning multiple values from a procedure. This is done by placing a call to `values' with zero or more arguments in tail position in a procedure body. `call-with-values' combines a procedure returning multiple values with a procedure which accepts these values as parameters. -- Scheme Procedure: values . args -- C Function: scm_values (args) Delivers all of its arguments to its continuation. Except for continuations created by the `call-with-values' procedure, all continuations take exactly one value. The effect of passing no value or more than one value to continuations that were not created by `call-with-values' is unspecified. -- Scheme Procedure: call-with-values producer consumer Calls its PRODUCER argument with no values and a continuation that, when passed some values, calls the CONSUMER procedure with those values as arguments. The continuation for the call to CONSUMER is the continuation of the call to `call-with-values'. (call-with-values (lambda () (values 4 5)) (lambda (a b) b)) ==> 5 (call-with-values * -) ==> -1 In addition to the fundamental procedures described above, Guile has a module which exports a syntax called `receive', which is much more convenient. If you want to use it in your programs, you have to load the module `(ice-9 receive)' with the statement (use-modules (ice-9 receive)) -- library syntax: receive formals expr body ... Evaluate the expression EXPR, and bind the result values (zero or more) to the formal arguments in the formal argument list FORMALS. FORMALS must have the same syntax like the formal argument list used in `lambda' (*note Lambda::). After binding the variables, the expressions in BODY ... are evaluated in order.  File: guile.info, Node: Exceptions, Next: Error Reporting, Prev: Multiple Values, Up: Control Mechanisms 26.7 Exceptions =============== A common requirement in applications is to want to jump "non-locally" from the depths of a computation back to, say, the application's main processing loop. Usually, the place that is the target of the jump is somewhere in the calling stack of procedures that called the procedure that wants to jump back. For example, typical logic for a key press driven application might look something like this: main-loop: read the next key press and call dispatch-key dispatch-key: lookup the key in a keymap and call an appropriate procedure, say find-file find-file: interactively read the required file name, then call find-specified-file find-specified-file: check whether file exists; if not, jump back to main-loop ... The jump back to `main-loop' could be achieved by returning through the stack one procedure at a time, using the return value of each procedure to indicate the error condition, but Guile (like most modern programming languages) provides an additional mechanism called "exception handling" that can be used to implement such jumps much more conveniently. * Menu: * Exception Terminology:: Different ways to say the same thing. * Catch:: Setting up to catch exceptions. * Throw:: Throwing an exception. * Lazy Catch:: Catch without unwinding the stack. * Exception Implementation:: How Guile implements exceptions.  File: guile.info, Node: Exception Terminology, Next: Catch, Up: Exceptions 26.7.1 Exception Terminology ---------------------------- There are several variations on the terminology for dealing with non-local jumps. It is useful to be aware of them, and to realize that they all refer to the same basic mechanism. * Actually making a non-local jump may be called "raising an exception", "raising a signal", "throwing an exception" or "doing a long jump". When the jump indicates an error condition, people may talk about "signalling", "raising" or "throwing" "an error". * Handling the jump at its target may be referred to as "catching" or "handling" the "exception", "signal" or, where an error condition is involved, "error". Where "signal" and "signalling" are used, special care is needed to avoid the risk of confusion with POSIX signals. (Especially considering that Guile handles POSIX signals by throwing a corresponding kind of exception: REFFIXME.) This manual prefers to speak of throwing and catching exceptions, since this terminology matches the corresponding Guile primitives.  File: guile.info, Node: Catch, Next: Throw, Prev: Exception Terminology, Up: Exceptions 26.7.2 Catching Exceptions -------------------------- `catch' is used to set up a target for a possible non-local jump. The arguments of a `catch' expression are a "key", which restricts the set of exceptions to which this `catch' applies, a thunk that specifies the "normal case" code -- i.e. what should happen if no exceptions are thrown -- and a "handler" procedure that says what to do if an exception is thrown. Note that if the "normal case" thunk executes "normally", which means without throwing any exceptions, the handler procedure is not executed at all. When an exception is thrown using the `throw' primitive, the first argument of the `throw' is a symbol that indicates the type of the exception. For example, Guile throws an exception using the symbol `numerical-overflow' to indicate numerical overflow errors such as division by zero: (/ 1 0) => ABORT: (numerical-overflow) The KEY argument in a `catch' expression corresponds to this symbol. KEY may be a specific symbol, such as `numerical-overflow', in which case the `catch' applies specifically to exceptions of that type; or it may be `#t', which means that the `catch' applies to all exceptions, irrespective of their type. The second argument of a `catch' expression should be a thunk (i.e. a procedure that accepts no arguments) that specifies the normal case code. The `catch' is active for the execution of this thunk, including any code called directly or indirectly by the thunk's body. Evaluation of the `catch' expression activates the catch and then calls this thunk. The third argument of a `catch' expression is a handler procedure. If an exception is thrown, this procedure is called with exactly the arguments specified by the `throw'. Therefore, the handler procedure must be designed to accept a number of arguments that corresponds to the number of arguments in all `throw' expressions that can be caught by this `catch'. -- Scheme Procedure: catch key thunk handler -- C Function: scm_catch (key, thunk, handler) Invoke THUNK in the dynamic context of HANDLER for exceptions matching KEY. If thunk throws to the symbol KEY, then HANDLER is invoked this way: (handler key args ...) KEY is a symbol or `#t'. THUNK takes no arguments. If THUNK returns normally, that is the return value of `catch'. Handler is invoked outside the scope of its own `catch'. If HANDLER again throws to the same key, a new handler from further up the call chain is invoked. If the key is `#t', then a throw to _any_ symbol will match this call to `catch'. If the handler procedure needs to match a variety of `throw' expressions with varying numbers of arguments, you should write it like this: (lambda (key . args) ...) The KEY argument is guaranteed always to be present, because a `throw' without a KEY is not valid. The number and interpretation of the ARGS varies from one type of exception to another, but should be specified by the documentation for each exception type. Note that, once the handler procedure is invoked, the catch that led to the handler procedure being called is no longer active. Therefore, if the handler procedure itself throws an exception, that exception can only be caught by another active catch higher up the call stack, if there is one.  File: guile.info, Node: Throw, Next: Lazy Catch, Prev: Catch, Up: Exceptions 26.7.3 Throwing Exceptions -------------------------- The `throw' primitive is used to throw an exception. One argument, the KEY, is mandatory, and must be a symbol; it indicates the type of exception that is being thrown. Following the KEY, `throw' accepts any number of additional arguments, whose meaning depends on the exception type. The documentation for each possible type of exception should specify the additional arguments that are expected for that kind of exception. -- Scheme Procedure: throw key . args -- C Function: scm_throw (key, args) Invoke the catch form matching KEY, passing ARGS to the HANDLER. KEY is a symbol. It will match catches of the same symbol or of `#t'. If there is no handler at all, Guile prints an error and then exits. When an exception is thrown, it will be caught by the innermost `catch' expression that applies to the type of the thrown exception; in other words, the innermost `catch' whose KEY is `#t' or is the same symbol as that used in the `throw' expression. Once Guile has identified the appropriate `catch', it handles the exception by applying that `catch' expression's handler procedure to the arguments of the `throw'. If there is no appropriate `catch' for a thrown exception, Guile prints an error to the current error port indicating an uncaught exception, and then exits. In practice, it is quite difficult to observe this behaviour, because Guile when used interactively installs a top level `catch' handler that will catch all exceptions and print an appropriate error message _without_ exiting. For example, this is what happens if you try to throw an unhandled exception in the standard Guile REPL; note that Guile's command loop continues after the error message: guile> (throw 'badex) :3:1: In procedure gsubr-apply ... :3:1: unhandled-exception: badex ABORT: (misc-error) guile> The default uncaught exception behaviour can be observed by evaluating a `throw' expression from the shell command line: $ guile -c "(begin (throw 'badex) (display \"here\\n\"))" guile: uncaught throw to badex: () $ That Guile exits immediately following the uncaught exception is shown by the absence of any output from the `display' expression, because Guile never gets to the point of evaluating that expression.  File: guile.info, Node: Lazy Catch, Next: Exception Implementation, Prev: Throw, Up: Exceptions 26.7.4 Catch Without Unwinding ------------------------------ A "lazy catch" is used in the same way as a normal `catch', with KEY, THUNK and HANDLER arguments specifying the exception type, normal case code and handler procedure, but differs in one important respect: the handler procedure is executed without unwinding the call stack from the context of the `throw' expression that caused the handler to be invoked. -- Scheme Procedure: lazy-catch key thunk handler -- C Function: scm_lazy_catch (key, thunk, handler) This behaves exactly like `catch', except that it does not unwind the stack before invoking HANDLER. The HANDLER procedure is not allowed to return: it must throw to another catch, or otherwise exit non-locally. Typically, HANDLER should save any desired state associated with the stack at the point where the corresponding `throw' occurred, and then throw an exception itself -- usually the same exception as the one it caught. If HANDLER is invoked and does _not_ throw an exception, Guile itself throws an exception with key `misc-error'. Not unwinding the stack means that throwing an exception that is caught by a `lazy-catch' is _almost_ equivalent to calling the `lazy-catch''s handler inline instead of each `throw', and then omitting the surrounding `lazy-catch'. In other words, (lazy-catch 'key (lambda () ... (throw 'key args ...) ...) handler) is _almost_ equivalent to ((lambda () ... (handler 'key args ...) ...)) But why only _almost_? The difference is that with `lazy-catch' (as with normal `catch'), the dynamic context is unwound back to just outside the `lazy-catch' expression before invoking the handler. (For an introduction to what is meant by dynamic context, *Note Dynamic Wind::.) Then, when the handler _itself_ throws an exception, that exception must be caught by some kind of `catch' (including perhaps another `lazy-catch') higher up the call stack. The dynamic context also includes `with-fluids' blocks (REFFIXME), so the effect of unwinding the dynamic context can also be seen in fluid variable values. This is illustrated by the following code, in which the normal case thunk uses `with-fluids' to temporarily change the value of a fluid: (define f (make-fluid)) (fluid-set! f "top level value") (define (handler . args) (cons (fluid-ref f) args)) (lazy-catch 'foo (lambda () (with-fluids ((f "local value")) (throw 'foo))) handler) => ("top level value" foo) ((lambda () (with-fluids ((f "local value")) (handler 'foo)))) => ("local value" foo) In the `lazy-catch' version, the unwinding of dynamic context restores `f' to its value outside the `with-fluids' block before the handler is invoked, so the handler's `(fluid-ref f)' returns the external value. `lazy-catch' is useful because it permits the implementation of debuggers and other reflective programming tools that need to access the state of the call stack at the exact point where an exception or an error is thrown. For an example of this, see REFFIXME:stack-catch.  File: guile.info, Node: Exception Implementation, Prev: Lazy Catch, Up: Exceptions 26.7.5 How Guile Implements Exceptions -------------------------------------- It is traditional in Scheme to implement exception systems using `call-with-current-continuation'. Continuations (*note Continuations::) are such a powerful concept that any other control mechanism -- including `catch' and `throw' -- can be implemented in terms of them. Guile does not implement `catch' and `throw' like this, though. Why not? Because Guile is specifically designed to be easy to integrate with applications written in C. In a mixed Scheme/C environment, the concept of "continuation" must logically include "what happens next" in the C parts of the application as well as the Scheme parts, and it turns out that the only reasonable way of implementing continuations like this is to save and restore the complete C stack. So Guile's implementation of `call-with-current-continuation' is a stack copying one. This allows it to interact well with ordinary C code, but means that creating and calling a continuation is slowed down by the time that it takes to copy the C stack. The more targeted mechanism provided by `catch' and `throw' does not need to save and restore the C stack because the `throw' always jumps to a location higher up the stack of the code that executes the `throw'. Therefore Guile implements the `catch' and `throw' primitives independently of `call-with-current-continuation', in a way that takes advantage of this _upwards only_ nature of exceptions.  File: guile.info, Node: Error Reporting, Next: Dynamic Wind, Prev: Exceptions, Up: Control Mechanisms 26.8 Procedures for Signaling Errors ==================================== Guile provides a set of convenience procedures for signaling error conditions that are implemented on top of the exception primitives just described. -- Scheme Procedure: error msg args ... Raise an error with key `misc-error' and a message constructed by displaying MSG and writing ARGS. -- Scheme Procedure: scm-error key subr message args data -- C Function: scm_error_scm (key, subr, message, args, data) Raise an error with key KEY. SUBR can be a string naming the procedure associated with the error, or `#f'. MESSAGE is the error message string, possibly containing `~S' and `~A' escapes. When an error is reported, these are replaced by formatting the corresponding members of ARGS: `~A' (was `%s' in older versions of Guile) formats using `display' and `~S' (was `%S') formats using `write'. DATA is a list or `#f' depending on KEY: if KEY is `system-error' then it should be a list containing the Unix `errno' value; If KEY is `signal' then it should be a list containing the Unix signal number; otherwise it will usually be `#f'. -- Scheme Procedure: strerror err -- C Function: scm_strerror (err) Return the Unix error message corresponding to ERR, which must be an integer value. -- syntax: false-if-exception expr Returns the result of evaluating its argument; however if an exception occurs then `#f' is returned instead.  File: guile.info, Node: Dynamic Wind, Next: Handling Errors, Prev: Error Reporting, Up: Control Mechanisms 26.9 Dynamic Wind ================= [FIXME: this is pasted in from Tom Lord's original guile.texi and should be reviewed] -- Scheme Procedure: dynamic-wind in_guard thunk out_guard -- C Function: scm_dynamic_wind (in_guard, thunk, out_guard) All three arguments must be 0-argument procedures. IN_GUARD is called, then THUNK, then OUT_GUARD. If, any time during the execution of THUNK, the continuation of the `dynamic_wind' expression is escaped non-locally, OUT_GUARD is called. If the continuation of the dynamic-wind is re-entered, IN_GUARD is called. Thus IN_GUARD and OUT_GUARD may be called any number of times. (define x 'normal-binding) => x (define a-cont (call-with-current-continuation (lambda (escape) (let ((old-x x)) (dynamic-wind ;; in-guard: ;; (lambda () (set! x 'special-binding)) ;; thunk ;; (lambda () (display x) (newline) (call-with-current-continuation escape) (display x) (newline) x) ;; out-guard: ;; (lambda () (set! x old-x))))))) ;; Prints: special-binding ;; Evaluates to: => a-cont x => normal-binding (a-cont #f) ;; Prints: special-binding ;; Evaluates to: => a-cont ;; the value of the (define a-cont...) x => normal-binding a-cont => special-binding