Loading and Listing of Packages

Description

library and require load add-on packages.

.First.lib is called when a package is loaded; .Last.lib is called when a package is detached.

Usage

library(package, help, pos = 2, lib.loc = NULL,
        character.only = FALSE, logical.return = FALSE,
        warn.conflicts = TRUE,
        keep.source = getOption("keep.source.pkgs"),
        verbose = getOption("verbose"),
        version)

require(package, quietly = FALSE, warn.conflicts = TRUE,
        keep.source = getOption("keep.source.pkgs"),
        character.only = FALSE, version, save = TRUE)

.First.lib(libname, pkgname)
.Last.lib(libpath)

Arguments

Details

library(package) and require(package) both load the package with name package. require is designed for use inside other functions; it returns FALSE and gives a warning (rather than an error as library() does) if the package does not exist. Both functions check and update the list of currently loaded packages and do not reload code that is already loaded.

For large packages, setting keep.source = FALSE may save quite a bit of memory.

If library is called with no package or help argument, it lists all available packages in the libraries specified by lib.loc, and returns the corresponding information in an object of class "libraryIQR". The structure of this class may change in future versions. In earlier versions of R, only the names of all available packages were returned; use .packages(all = TRUE) for obtaining these. Note that installed.packages() returns even more information.

library(help = somename) computes basic information about the package somename, and returns this in an object of class "packageInfo". The structure of this class may change in future versions. When used with the default value (NULL) for lib.loc, the loaded packages are searched before the libraries.

.First.lib is called when a package without a namespace is loaded by library. (For packages with namespaces see .onLoad.) It is called with two arguments, the name of the library directory where the package was found (i.e., the corresponding element of lib.loc), and the name of the package (in that order). It is a good place to put calls to library.dynam which are needed when loading a package into this function (don't call library.dynam directly, as this will not work if the package is not installed in a “standard” location). .First.lib is invoked after the search path interrogated by search() has been updated, so as.environment(match("package:name", search())) will return the environment in which the package is stored. If calling .First.lib gives an error the loading of the package is abandoned, and the package will be unavailable. Similarly, if the option ".First.lib" has a list element with the package's name, this element is called in the same manner as .First.lib when the package is loaded. This mechanism allows the user to set package “load hooks” in addition to startup code as provided by the package maintainers, but setHook is preferred.

.Last.lib is called when a package is detached. Beware that it might be called if .First.lib has failed, so it should be written defensively. (It is called within try, so errors will not stop the package being detached.)

Value

library returns the list of loaded (or available) packages (or TRUE if logical.return is TRUE). require returns a logical indicating whether the required package is available.

Packages that require other packages

The source code for a package that requires one or more other packages should have a call to require, preferably near the beginning of the source, and of course before any code that uses functions, classes or methods from the other package. The default for argument save will save the names of all required packages in the environment of the new package. The saved package names are used by detach when a package is detached to warn if other packages still require the detached package. Also, if a package is installed with saved image (see INSTALL), the saved package names are used to require these packages when the new package is attached.

Formal methods

library takes some further actions when package methods is attached (as it is by default). Packages may define formal generic functions as well as re-defining functions in other packages (notably base) to be generic, and this information is cached whenever such a package is loaded after methods and re-defined functions are excluded from the list of conflicts. The check requires looking for a pattern of objects; the pattern search may be avoided by defining an object .noGenerics (with any value) in the package. Naturally, if the package does have any such methods, this will prevent them from being used.

Note

library and require can only load an installed package, and this is detected by having a ‘DESCRIPTION’ file containing a Built: field.

Under Unix-alikes, the code checks that the package was installed under a similar operating system as given by R.version$platform (the canonical name of the platform under which R was compiled), provided it contains compiled code. Packages which do not contain compiled code can be shared between Unix-alikes, but not to other OSes because of potential problems with line endings and OS-specific help files.

As of version 2.0.0, the package name given to library and require must match the name given in the package's DESCRIPTION file exactly, even on case-insensitive file systems such as MS Windows.

References

Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.

See Also

.libPaths, .packages.

attach, detach, search, objects, autoload, library.dynam, data, install.packages and installed.packages; INSTALL, REMOVE.

Examples

library()                   # list all available packages
library(lib = .Library)     # list all packages in the default library
library(help = splines)     # documentation on package 'splines'
library(splines)            # load package 'splines'
require(splines)            # the same
search()                    # "splines", too
detach("package:splines")

# if the package name is in a character vector, use
pkg <- "splines"
library(pkg, character.only = TRUE)
detach(pos = match(paste("package", pkg, sep=":"), search()))

require(pkg, character.only = TRUE)
detach(pos = match(paste("package", pkg, sep=":"), search()))

require(nonexistent)        # FALSE
## Not run: 
## Suppose a package needs to call a shared library named 'fooEXT',
## where 'EXT' is the system-specific extension.  Then you should use
.First.lib <- function(lib, pkg) {
  library.dynam("foo", pkg, lib)
}

## if you want to mask as little as possible, use
library(mypkg, pos = "package:base")
## End(Not run)