% File src/library/base/man/library.Rd
% Part of the R package, http://www.R-project.org
% Copyright 1995-2009 R Core Development Team
% Distributed under GPL 2 or later

\name{library}
\title{Loading and Listing of Packages}
\alias{library}
\alias{require}
\alias{.First.lib}
\alias{.Last.lib}
\alias{.noGenerics}
\alias{print.libraryIQR}
\alias{print.packageInfo}
\description{
  \code{library} and \code{require} load add-on packages.

  \code{.First.lib} is called when a package is loaded;
  \code{.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"))

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

.First.lib(libname, pkgname)
.Last.lib(libpath)
}
\arguments{
  \item{package, help}{the name of a package, given as a \link{name} or
    literal character string, or a character string, depending on
    whether \code{character.only} is \code{FALSE} (default) or
    \code{TRUE}).}
  \item{pos}{the position on the search list at which to attach the
    loaded package.  Note that \code{.First.lib} may attach other
    packages, and \code{pos} is computed \emph{after} \code{.First.lib}
    has been run.  Can also be the name of a position on the current
    search list as given by \code{\link{search}()}.}
  \item{lib.loc}{a character vector describing the location of \R
    library trees to search through, or \code{NULL}.  The default value
    of \code{NULL} corresponds to all libraries currently known.
    Non-existent library trees are silently ignored.}
  \item{character.only}{a logical indicating whether \code{package} or
    \code{help} can be assumed to be character strings.}
  \item{logical.return}{logical.  If it is \code{TRUE},  \code{FALSE} or
    \code{TRUE} is returned to indicate success.}
  \item{warn.conflicts}{logical.  If \code{TRUE}, warnings are
    printed about \code{\link{conflicts}} from attaching the new
    package, unless that package contains an object
    \code{.conflicts.OK}.  A conflict is a function masking a function,
    or a non-function masking a non-function.
  }
  \item{keep.source}{logical.  If \code{TRUE}, functions
    \sQuote{keep their source} including comments, see argument
    \code{keep.source} to \code{\link{options}}.  This applies only to
    the named package, and not to any packages or name spaces which might be
    loaded to satisfy dependencies or imports.

    This argument does not apply to packages using lazy-loading.
    Whether they have kept source is determined when they are installed
    (and is most likely false).
  }
  \item{verbose}{a logical.  If \code{TRUE}, additional diagnostics are
    printed.}
  \item{quietly}{a logical.  If \code{TRUE}, no message confirming
    package loading is printed.}
  \item{save}{logical or environment.  If \code{TRUE}, a call to
    \code{require} from the source for a package will save the name of
    the required package in the variable \code{".required"}, allowing
    function \code{\link{detach}} to warn if a required package is
    detached. See section \sQuote{Packages that require other packages} below.}
  \item{libname}{a character string giving the library directory where
    the package was found.}
  \item{pkgname}{a character string giving the name of the package.}
  \item{libpath}{a character string giving the complete path to the
    package.}
}
\details{
  \code{library(package)} and \code{require(package)} both load the
  package with name \code{package}.  \code{require} is designed for use
  inside other functions; it returns \code{FALSE} and gives a warning
  (rather than an error as \code{library()} does by default) if the
  package does not exist.  Both functions check and update the list of
  currently loaded packages and do not reload a package which is already
  loaded.  (Furthermore, if the package has a name space and a name
  space of that name is already loaded, they work from the existing
  name space rather than reloading from the file system.  If you want to
  reload a package, call \code{\link{detach}} or
  \code{\link{unloadNamespace}} first.)

  To suppress messages during the loading of packages use
  \code{\link{suppressPackageStartupMessages}}: this will suppress all
  messages from \R itself but not necessarily all those from package
  authors.
  
  If \code{library} is called with no \code{package} or \code{help}
  argument, it lists all available packages in the libraries specified
  by \code{lib.loc}, and returns the corresponding information in an
  object of class \code{"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 \code{.packages(all =
    TRUE)} for obtaining these.  Note that
  \code{\link{installed.packages}()}
  returns even more information.

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

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

  \code{.Last.lib} is called when a package is detached.  Beware
  that it might be called if \code{.First.lib} has failed, so it
  should be written defensively. (It is called within \code{\link{try}},
  so errors will not stop the package being detached.)
}
\value{
  Normally \code{library} returns (invisibly) the list of loaded
  packages, but \code{TRUE} or \code{FALSE} if \code{logical.return} is
  \code{TRUE}.  When called as \code{library()} it returns an object of
  class \code{"libraryIQR"}, and for \code{library(help=)}, one of
  class \code{"packageInfo"}.

  \code{require} returns (invisibly) a logical indicating whether the required
  package is available.
}
\section{Packages that require other packages}{
  \bold{NB:} This mechanism has been almost entirely superseded by
  using the \samp{Depends:} field in the \file{DESCRIPTION} file of a
  package. 

  % But this only works without lazy-loading or in .First.lib,
  % and if only works in packages without namespaces.
  The source code for a
  package that requires one or more other packages should have a call
  to \code{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 \code{save} will
  save the names of all required packages in the environment of the
  new package.  The saved package names are used by
  \code{\link{detach}} when a package is detached to warn if other
  packages still require the to-be-detached package.
}
\section{Formal methods}{
  \code{library} takes some further actions when package \pkg{methods}
  is attached (as it is by default).  Packages may define formal generic
  functions as well as re-defining functions in other packages (notably
  \pkg{base}) to be generic, and this information is cached whenever
  such a package is loaded after \pkg{methods} and re-defined functions
  (\link{implicit generic}s) are excluded from the list of conflicts.
  The caching  and check for conflicts require looking for a pattern of
  objects; the search may be avoided by defining an object
  \code{.noGenerics} (with any value) in the package.  Naturally, if the
  package \emph{does} have any such methods, this will prevent them from
  being used.
}
\note{
  \code{library} and \code{require} can only load an \emph{installed}
  package, and this is detected by having a \file{DESCRIPTION} file
  containing a \samp{Built:} field.

  Under Unix-alikes, the code checks that the package was installed
  under a similar operating system as given by \code{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.  If sub-architectures are used, the OS similarity is not
  checked since the OS used to build may differ (e.g.
  \code{i386-pc-linux-gnu} code can be built on an
  \code{x86_64-unknown-linux-gnu} OS).
 

  The package name given to \code{library} and \code{require} must match
  the name given in the package's \file{DESCRIPTION} file exactly, even
  on case-insensitive file systems such as MS Windows.
}
\seealso{
  \code{\link{.libPaths}}, \code{\link{.packages}}.

  \code{\link{attach}}, \code{\link{detach}}, \code{\link{search}},
  \code{\link{objects}}, \code{\link{autoload}},
  \code{\link{library.dynam}}, \code{\link{data}},
  \code{\link{install.packages}} and
  \code{\link{installed.packages}};
  \code{\link{INSTALL}}, \code{\link{REMOVE}}.
}
\references{
  Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988)
  \emph{The New S Language}.
  Wadsworth & Brooks/Cole.
}
\examples{
library()                   # list all available packages
library(lib.loc = .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
\dontrun{
## Suppose a package needs to call a DLL 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")
}}
\keyword{data}
