## Loading and Listing of Packages

## Usage

    library(name, help = NULL, lib.loc = .lib.loc,
    	character.only = FALSE, logical.return = FALSE)
    require(name, quietly = FALSE)
    provide(name)

    .First.lib(libname, pkgname)

    .packages()
    .lib.loc
    .Library
    .Provided
    .Autoloaded

## Arguments

`name, help` | `name` or character string giving the name of a package.
`lib.loc` | a character vector describing the location of R library trees to search through.
`character.only` | a logical indicating whether `name` or `help` can be assumed to be character strings.
`quietly` | a logical. If `TRUE`, a warning will not be printed if the package cannot be found.
`libname` | a character string giving the library directory where the package was found.
`pkgname` | a character string giving the name of the package.

## Description

`library(name)` and `require(name)` both load the package with name `name`. `require` is designed for use inside other functions; it returns `FALSE` and optionally gives a warning, rather than giving an error, 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. `require` also checks the list `.Provided`.

`provide` allows code to register services that it provides. The argument is stored in the list `.Provided`. `provide` returns `FALSE` if the name was already present in `.Provided` or among the packages in `search()`. The main use for `provide` is when multiple packages share code. This is most likely when the code implements features present in S(-PLUS) but not in R. For example, the spline functions `ns`, `bs` and so on are not included in the R distribution. A package containing these functions can use `provide(splines)` to register this fact. Another package that needs the functions can execute `require(splines)` rather than `library(splines)` to load the spline package only if their functionality is not already available.

If `library` is called with no argument, it gives a list of all available packages. `library(help = name)` prints information on the package `name`, typically by listing the most important user level objects it contains.

`.First.lib()` is called when a package is loaded by `library()`. It is called with two arguments, the name of the library tree 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 `search()` has been updated, so `pos.to.env(match("package:name"), search())` will return the environment in which the package is stored.

`.packages()` returns the ``base names'' of the currently attached packages invisibly.

`.Autoloaded` contains the ``base names'' of the packages for which autoloading has been promised.

`.Library` is a character string giving the location of the default library, the ``library'' subdirectory of `RHOME`. `.lib.loc` is a character vector with the locations of all library trees that R should use. It is initialized at startup from the environment variable `RLIBS`, which should be a colon-separated list of directories at which R library trees are rooted, and `.Library`.

## Value

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

## Creating Packages

Packages provide a mechanism for loading optional code and attached documentation as needed. The R distribution provides the example packages `eda`, `mva`, and `stepfun`.

A package consists of a subdirectory containing the files `DESCRIPTION', `INDEX', and `TITLE', and the subdirectories `R', `data', `exec', `man', and `src' (some of which can be missing).

The `DESCRIPTION' file contains information about authors, version, copyright, etc., and looks like

Package:
Version:
Author:
Description:
Depends:
License:

Continuation lines (e.g., for descriptions longer than one line) start with a whitespace character. The license field should contain an explicit statement or a well-known abbreviation (such as `GPL', `LGPL', `BSD' and `Artistic'), maybe followed by a reference to the actual license file. It is very important that this information is included---otherwise, it may not even be legally correct for others to distribute copies of the package.

The `TITLE' file contains a line giving the name of the package and a brief description. `INDEX' contains a line for each sufficiently interesting object in the package, giving its name and a description (functions such as print methods not usually called explicitly might not be included). Note that you can automatically create this file using the `Rdindex` program in `RHOME/etc', provided that Perl is available on your system.

The `R' subdirectory contains R code files. The code files to be installed must start with a (lower- or uppercase) letter and have one of the extensions `.R', `.S', `.q', `.r', or `.s'. We recommend using `.R', as this extension seems to be not used by any other software. If necessary, one of these files (historically `zzz.R') should use `library.dynam()` inside `.First.lib()` to load compiled code.

The `man' subdirectory should contain R documentation files for the objects in the package. The documentation files to be installed must also start with a (lower- or uppercase) letter and have the extension `.Rd' (the default) or `.rd'.

Source and a Makefile for the compiled code is in `src', containing C, FORTRAN, or Ratfor source. The Makefile will be passed various machine-dependent compile and link flags, examples of which can be seen in the `eda` package.

The `data' subdirectory is for additional data files the package makes available for loading using `data()`. Note that (at least currently) all such files are in fact R code files, and must have the extension `.R'.

Finally, `exec' could contain additional executables the package needs, typically Shell or Perl scripts. This mechanism is currently not used by any package, and still experimental.

## See Also

`attach`, `detach`, `search`, `objects`, `autoload`, `library.dynam`; `INSTALL`, `REMOVE`

## Examples

    ( .packages() )	    	    # maybe just "base"
    library()		    # list all available packages
    library(lib = .Library)	    # list all packages in the default library
    library(help = eda)	    # documentation on package "eda"
    library(eda)		    # load package "eda"
    require(eda)		    # the same
    ( .packages() )             # "eda", too
    require(nonexistent)	    # FALSE
    ## Suppose the a package needs to call a shared library named "foo.EXT",
    ## where "EXT" is the system-specific extension.  Then you should use
    .First.lib <- function(lib, pkg) {
      library.dynam("foo", pkg, lib)
    }

[Package Contents]