\HeaderA{library}{Loading and Listing of Packages}{library}
\aliasA{.First.lib}{library}{.First.lib}
\aliasA{.Last.lib}{library}{.Last.lib}
\aliasA{.noGenerics}{library}{.noGenerics}
\aliasA{print.libraryIQR}{library}{print.libraryIQR}
\aliasA{print.packageInfo}{library}{print.packageInfo}
\aliasA{require}{library}{require}
\aliasA{R\_LIBS}{library}{R.Rul.LIBS}
\keyword{data}{library}
\begin{Description}\relax
\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.
\end{Description}
\begin{Usage}
\begin{verbatim}
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)
\end{verbatim}
\end{Usage}
\begin{Arguments}
\begin{ldescription}
\item[\code{package, help}] the name of a package, given as a \LinkA{name}{name} or
literal character string, or a character string, depending on
whether \code{character.only} is \code{FALSE} (default) or
\code{TRUE}).
\item[\code{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{\LinkA{search}{search}()}.
\item[\code{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[\code{character.only}] a logical indicating whether \code{package} or
\code{help} can be assumed to be character strings.
\item[\code{version}] A character string denoting a version number of the
package to be loaded, for use with \emph{versioned installs}: see
the section later in this document.
\item[\code{logical.return}] logical.  If it is \code{TRUE},  \code{FALSE} or
\code{TRUE} is returned to indicate success.
\item[\code{warn.conflicts}] logical.  If \code{TRUE}, warnings are
printed about \code{\LinkA{conflicts}{conflicts}} from attaching the new
package, unless that package contains an object \code{.conflicts.OK}.
\item[\code{keep.source}] logical.  If \code{TRUE}, functions
\dQuote{keep their source} including comments, see argument
\code{keep.source} to \code{\LinkA{options}{options}}.  This applies only to
the named package, and not to any packages or namespaces which might be
loaded to satisfy dependencies or imports.

This argument does not apply to packages using lazy-loading or saved
images.  Whether they have kept source is determined when they are
installed (and is most likely false).

\item[\code{verbose}] a logical.  If \code{TRUE}, additional diagnostics are
printed.
\item[\code{quietly}] a logical.  If \code{TRUE}, no message confirming
package loading is printed.
\item[\code{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{\LinkA{detach}{detach}} to warn if a required package is
detached. See section \sQuote{Packages that require other packages} below.
\item[\code{libname}] a character string giving the library directory where
the package was found.
\item[\code{pkgname}] a character string giving the name of the package,
including the version number if the package was installed with
\code{--with-package-versions}.
\item[\code{libpath}] a character string giving the complete path to the
package.
\end{ldescription}
\end{Arguments}
\begin{Details}\relax
\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.

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{\LinkA{installed.packages}{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 namespace is
loaded by \code{library}.  (For packages with namespaces see
\code{\LinkA{.onLoad}{.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
(in that order, and with the package name including the version for a
versioned install, e.g. \code{tree\_1.0-16}).  It is a good place to
put calls to \code{\LinkA{library.dynam}{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
\dQuote{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
\dQuote{load hooks} in addition to startup code as provided by the
package maintainers, but \code{\LinkA{setHook}{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{\LinkA{try}{try}},
so errors will not stop the package being detached.)
\end{Details}
\begin{Value}
\code{library} returns the list of loaded (or available) packages
(or \code{TRUE} if \code{logical.return} is \code{TRUE}).
\code{require} returns a logical indicating whether the required
package is available.
\end{Value}
\begin{Section}{Packages that require other packages}
\bold{NB:} This mechanism has been almost entirely superseded by
using the \code{Depends:} field in the \file{DESCRIPTION} file of a package.

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{\LinkA{detach}{detach}} when a package is detached to warn if other
packages still require the to-be-detached package.  Also, if a package is
installed with saved image (see \LinkA{INSTALL}{INSTALL}), the saved package
names are used to require these packages when the new package is
attached.
\end{Section}
\begin{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
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 \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.
\end{Section}
\begin{Section}{Versioned installs}
Packages can be installed with version information by \code{R CMD
    \LinkA{INSTALL}{INSTALL} --with-package-versions} or
\code{\LinkA{install.packages}{install.packages}(installWithVers = TRUE)}.  This allows
more than one version of a package to be installed in a library
directory, using package directory names like \code{foo\_1.5-1}.  When
such packages are loaded, it is this \emph{versioned}  name
that \code{\LinkA{search}{search}()} returns.  Some utility functions require
the versioned name and some the unversioned name (here \code{foo}).

If \code{version} \emph{is not} specified, \code{library} looks first
for a packages of that name, and then for versioned installs of the
package, selecting the one with the latest version number.  If
\code{version} \emph{is} specified, a versioned install with an
exactly matching version is looked for.

If \code{version} is not specified, \code{require} will accept any
version that is already loaded, whereas \code{library} will look for
an unversioned install even if a versioned install is already loaded.

Loading more than one version of a package into an \R{} session is not
currently supported.  Support for versioned installs is patchy.
\end{Section}
\begin{Note}\relax
\code{library} and \code{require} can only load an \emph{installed}
package, and this is detected by having a \file{DESCRIPTION} file
containing a \code{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.

As of version 2.0.0, the package name given to \code{library} and
\code{require} must match the name given in the package's
\code{DESCRIPTION} file exactly, even on case-insensitive file systems
such as MS Windows.
\end{Note}
\begin{References}\relax
Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988)
\emph{The New S Language}.
Wadsworth \& Brooks/Cole.
\end{References}
\begin{SeeAlso}\relax
\code{\LinkA{.libPaths}{.libPaths}}, \code{\LinkA{.packages}{.packages}}.

\code{\LinkA{attach}{attach}}, \code{\LinkA{detach}{detach}}, \code{\LinkA{search}{search}},
\code{\LinkA{objects}{objects}}, \code{\LinkA{autoload}{autoload}},
\code{\LinkA{library.dynam}{library.dynam}}, \code{\LinkA{data}{data}},
\code{\LinkA{install.packages}{install.packages}} and
\code{\LinkA{installed.packages}{installed.packages}};
\code{\LinkA{INSTALL}{INSTALL}}, \code{\LinkA{REMOVE}{REMOVE}}.
\end{SeeAlso}
\begin{Examples}
\begin{ExampleCode}
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)\end{ExampleCode}
\end{Examples}

