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Characters are objects that represent printed characters, such as letters and digits.(5)
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Characters are written using the notation #\character or
#\character-name. For example:
#\a ; lowercase letter #\A ; uppercase letter #\( ; left parenthesis #\space ; the space character #\newline ; the newline character |
Case is significant in #\character, but not in
#\character-name. If character in
#\character is a letter, character must be followed
by a delimiter character such as a space or parenthesis. Characters
written in the #\ notation are self-evaluating; you don't need to
quote them.
A character name may include one or more bucky bit prefixes to indicate that the character includes one or more of the keyboard shift keys Control, Meta, Super, Hyper, or Top (note that the Control bucky bit prefix is not the same as the ASCII control key). The bucky bit prefixes and their meanings are as follows (case is not significant):
Key Bucky bit prefix Bucky bit --- ---------------- --------- Meta M- or Meta- 1 Control C- or Control- 2 Super S- or Super- 4 Hyper H- or Hyper- 8 Top T- or Top- 16 |
For example,
#\c-a ; Control-a #\meta-b ; Meta-b #\c-s-m-h-a ; Control-Meta-Super-Hyper-A |
The following character-names are supported, shown here with their ASCII equivalents:
Character Name ASCII Name -------------- ---------- altmode ESC backnext US backspace BS call SUB linefeed LF page FF return CR rubout DEL space tab HT |
In addition, #\newline is the same as #\linefeed (but this
may change in the future, so you should not depend on it). All of the
standard ASCII names for non-printing characters are supported:
NUL SOH STX ETX EOT ENQ ACK BEL BS HT LF VT FF CR SO SI DLE DC1 DC2 DC3 DC4 NAK SYN ETB CAN EM SUB ESC FS GS RS US DEL |
(char->name #\a) => "a" (char->name #\space) => "Space" (char->name #\c-a) => "C-a" (char->name #\control-a) => "C-a" |
Slashify?, if specified and true, says to insert the necessary
backslash characters in the result so that read will parse it
correctly. In other words, the following generates the external
representation of char:
(string-append "#\\" (char->name char #t)) |
If slashify? is not specified, it defaults to #f.
name->char signals
an error.
(name->char "a") => #\a (name->char "space") => #\Space (name->char "c-a") => #\C-a (name->char "control-a") => #\C-a |
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#t if the specified characters are have the appropriate
order relationship to one another; otherwise returns #f. The
-ci procedures don't distinguish uppercase and lowercase letters.
Character ordering follows these rules:
(char<? #\0 #\9) returns
#t.
(char<? #\A
#\B) returns #t.
(char<? #\a
#\b) returns #t.
In addition, MIT Scheme orders those characters that satisfy
char-standard? the same way that ISO-8859-1 does.
Characters are ordered by first comparing their bucky bits part and then their code part. In particular, characters without bucky bits come before characters with bucky bits.
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#t if object is a character; otherwise returns
#f.
(char-ci=? char
char2).
char->digit returns #f.
Note that this procedure is insensitive to the alphabetic case of char.
(char->digit #\8) => 8 (char->digit #\e 16) => 14 (char->digit #\e) => #f |
digit->char
returns #f.
(digit->char 8) => #\8 (digit->char 14 16) => #\E |
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An MIT Scheme character consists of a code part and a bucky bits part. The MIT Scheme set of characters can represent more characters than ASCII can; it includes characters with Super, Hyper, and Top bucky bits, as well as Control and Meta. Every ASCII character corresponds to some MIT Scheme character, but not vice versa.(6)
MIT Scheme uses a 16-bit character code with 5 bucky bits. Normally, Scheme uses the least significant 8 bits of the character code to contain the ISO-8859-1 representation for the character. The representation is expanded in order to allow for the use of UTF-16 in the future.
char-code and char-bits to
extract the code and bucky bits from the character. If 0 is
specified for bucky-bits, make-char produces an ordinary
character; otherwise, the appropriate bits are turned on as follows:
1 Meta 2 Control 4 Super 8 Hyper 16 Top |
For example,
(make-char 97 0) => #\a (make-char 97 1) => #\M-a (make-char 97 2) => #\C-a (make-char 97 3) => #\C-M-a |
(char-bits #\a) => 0 (char-bits #\m-a) => 1 (char-bits #\c-a) => 2 (char-bits #\c-m-a) => 3 |
(char-code #\a) => 97 (char-code #\c-a) => 97 |
char->integer returns the character code representation for
char. integer->char returns the character whose character
code representation is k.
In MIT Scheme, if (char-ascii? char) is true, then
(eqv? (char->ascii char) (char->integer char)) |
However, this behavior is not required by the Scheme standard, and code that depends on it is not portable to other implementations.
These procedures implement order isomorphisms between the set of
characters under the char<=? ordering and some subset of the
integers under the <= ordering. That is, if
(char<=? a b) => #t and (<= x y) => #t |
and x and y are in the range of char->integer,
then
(<= (char->integer a)
(char->integer b)) => #t
(char<=? (integer->char x)
(integer->char y)) => #t
|
Note: If the argument to char->integer or integer->char is
a constant, the compiler will constant-fold the call, replacing it with
the corresponding result. This is a very useful way to denote unusual
character constants or ASCII codes.
char->integer is defined to be the exact
non-negative integers that are less than the value of this variable
(exclusive).
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MIT Scheme internally uses ISO-8859-1 codes for I/O, and stores character objects in a fashion that makes it convenient to convert between ISO-8859-1 codes and characters. Also, character strings are implemented as byte vectors whose elements are ISO-8859-1 codes; these codes are converted to character objects when accessed. For these reasons it is sometimes desirable to be able to convert between ISO-8859-1 codes and characters.
Not all characters can be represented as ISO-8859-1 codes. A character that has an equivalent ISO-8859-1 representation is called an ISO-8859-1 character.
For historical reasons, the procedures that manipulate ISO-8859-1 characters use the word "ASCII" rather than "ISO-8859-1".
#f.
In the current implementation, the characters that satisfy this predicate are those in which the bucky bits are turned off, and for which the character code is less than 256.
condition-type:bad-range-argument is signalled if char
doesn't have an ISO-8859-1 representation.
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MIT Scheme's character-set abstraction is used to represent groups of characters, such as the letters or digits. Character sets may contain only ISO-8859-1 characters; in the future this may be changed to allow the full range of characters.
There is no meaningful external representation for character sets; use
char-set-members to examine their contents. There is (at
present) no specific equivalence predicate for character sets; use
equal? for this purpose.
#t if object is a character set; otherwise returns
#f.
char-set-members.
Alphabetic characters are the 52 upper and lower case letters.
Numeric characters are the 10 decimal digits. Alphanumeric
characters are those in the union of these two sets. Whitespace
characters are #\space, #\tab, #\page,
#\linefeed, and #\return. Graphic characters are
the printing characters and #\space. Standard characters
are the printing characters, #\space, and #\newline.
These are the printing characters:
! " # $ % & ' ( ) * + , - . /
0 1 2 3 4 5 6 7 8 9
: ; < = > ? @
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
[ \ ] ^ _ `
a b c d e f g h i j k l m n o p q r s t u v w x y z
{ | } ~
|
#t if char is in char-set; otherwise returns
#f.
char-set returns an empty
character set.
(apply
char-set chars).
For historical reasons, the name of this procedure refers to "ASCII" rather than "ISO-8859-1".
predicate->char-set creates and returns a character set
consisting of the ISO-8859-1 characters for which
predicate is true.
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MIT Scheme provides rudimentary support for Unicode characters. In an ideal world, Unicode would be the base character set for MIT Scheme, but this implementation predates the invention of Unicode. And converting an application of this size is a considerable undertaking. So for the time being, the base character set is ISO-8859-1 and Unicode support is grafted on.
This Unicode support was implemented as a part of the XML parser (see section 14.12 XML Parser) implementation. XML uses Unicode as its base character set, and any XML implementation must support Unicode.
The Unicode implementation consists of two parts: I/O
procedures that read and write UTF-8 characters, and an
alphabet abstraction, which is an efficient implementation of
sets of Unicode code points (similar to the char-set
abstraction).
The basic unit in a Unicode implementation is the code point.
#t if object is a Unicode code point. Code
points are implemented as exact non-negative integers. Code points
are further limited, by the Unicode standard, to be strictly less than
#x80000000.
The next few procedures do I/O on code points.
(read-utf8-code-point (string->input-port string)) |
(with-string-output-port (lambda (port) (write-utf8-code-point code-point port))) |
Applications often need to manipulate sets of characters, such as the set of alphabetic characters or the set of whitespace characters. The alphabet abstraction provides an efficient implementation of sets of Unicode code points.
#t if object is a Unicode alphabet, otherwise
returns #f.
well-formed-code-points-list?.
#t if object is a well-formed code-points list,
otherwise returns #f. A well-formed code-points list is a
proper list, each element of which is either a code point or a pair of
code points. A pair of code points represents a contiguous range of
code points. The CAR of the pair is the lower limit, and the
CDR is the upper limit. Both limits are inclusive, and the lower
limit must be strictly less than the upper limit.
#t if code-point is a member of alphabet,
otherwise returns #f.
#t if char is a member of alphabet,
otherwise returns #f. Equivalent to
(code-point-in-alphabet? (char-code char) alphabet) |
Character sets and alphabets can be converted to one another, provided that the alphabet contains only 8-bit code points. This is true because 8-bit code points in Unicode map directly to ISO-8859-1 characters, which is what character sets contain.
(char-set->alphabet (string->char-set string)) |
#t if alphabet contains only 8-bit code points,
otherwise returns #f.
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