% File src/library/base/man/library.Rd
% Part of the R package, https://www.R-project.org
% Copyright 1995-2014 R Core Team
% Distributed under GPL 2 or later

\name{library}
\title{Loading/Attaching and Listing of Packages}
\alias{library}
\alias{require}
\alias{conflictRules}

\alias{.noGenerics}
\alias{format.libraryIQR}
\alias{print.libraryIQR}
\alias{format.packageInfo}
\alias{print.packageInfo}
\description{
  \code{library} and \code{require} load and attach add-on packages.
}
\usage{
library(package, help, pos = 2, lib.loc = NULL,
        character.only = FALSE, logical.return = FALSE,
        warn.conflicts, quietly = FALSE,
        verbose = getOption("verbose"),
        mask.ok, exclude, include.only,
        attach.required = missing(include.only))

require(package, lib.loc = NULL, quietly = FALSE,
        warn.conflicts,
        character.only = FALSE,
        mask.ok, exclude, include.only,
        attach.required = missing(include.only))

conflictRules(pkg, mask.ok = NULL, exclude = NULL)

}
\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 namespace.  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 to
    \code{\link{.libPaths}()}.
    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.  A conflict is a function masking a function,
    or a non-function masking a non-function. The default is \code{TRUE}
    unless specified as \code{FALSE} in the \code{conflicts.policy} option.
  }
  \item{verbose}{a logical.  If \code{TRUE}, additional diagnostics are
    printed.}
  \item{quietly}{a logical.  If \code{TRUE}, no message confirming
    package attaching is printed, and most often, no errors/warnings are
    printed if package attaching fails.}
  \item{pkg}{character string naming a package.}
  \item{mask.ok}{character vector of names of objects that can mask
    objects on the search path without signaling an error when strict
    conflict checking is enabled}
  \item{exclude,include.only}{character vector of names of objects to
    exclude or include in the attached frame. Only one of these arguments
    may be used in a call to \code{library} or \code{require}.}
  \item{attach.required}{logical specifying whether required packages
    listed in the \code{Depends} clause of the \code{DESCRIPTION} file
    should be attached automatically.}
}
\details{
  \code{library(package)} and \code{require(package)} both load the
  namespace of the package with name \code{package} and attach it on the
  search list.  \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 attached
  packages and do not reload a namespace which is already loaded.  (If
  you want to reload such a package, call \code{\link{detach}(unload =
  TRUE)} or \code{\link{unloadNamespace}} first.)  If you want to load a
  package without attaching it on the search list, see
  \code{\link{requireNamespace}}.

  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.)  Use \code{.packages(all = TRUE)} to
  obtain just the names of all available packages, and
  \code{\link{installed.packages}()} for even more information.

  \code{library(help = somename)} computes basic information about the
  package \pkg{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 attached packages are searched before the libraries.
}
\value{
  Normally \code{library} returns (invisibly) the list of attached
  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{Conflicts}{
  Handling of conflicts depends on the setting of the
  \code{conflicts.policy} option. If this option is not set, then
  conflicts result in warning messages if the argument
  \code{warn.conflicts} is \code{TRUE}. If the option is set to the
  character string \code{"strict"}, then all unresolved conflicts signal
  errors.  Conflicts can be resolved using the \code{mask.ok},
  \code{exclude}, and \code{include.only} arguments to \code{library} and
  \code{require}. Defaults for \code{mask.ok} and \code{exclude} can be
  specified using \code{conflictRules}.

  If the \code{conflicts.policy} option is set to the string
  \code{"depends.ok"} then conflicts resulting from attaching declared
  dependencies will not produce errors, but other conflicts will.
  This is likely to be the best setting for most users wanting some
  additional protection against unexpected conflicts.

  The policy can be tuned further by specifying the
  \code{conflicts.policy} option as a named list with the following
  fields:
  \describe{
    \item{\code{error}:}{logical; if \code{TRUE} treat unresolved
      conflicts as errors.}
    \item{\code{warn}:}{logical; unless \code{FALSE} issue a warning
      message when conflicts are found.}
    \item{\code{generics.ok}:}{logical; if \code{TRUE} ignore conflicts
      created by defining S4 generics for functions on the search path.}
    \item{\code{depends.ok}:}{logical; if \code{TRUE} do not treat
      conflicts with required packages as errors.}
    \item{\code{can.mask}:}{character vector of names of packages that
      are allowed to be masked. These would typically be base packages
      attached by default.}
  }
}

\section{Licenses}{
  Some packages have restrictive licenses, and there is a mechanism to
  allow users to be aware of such licenses.  If
  \code{\link{getOption}("checkPackageLicense") == TRUE}, then at first
  use of a package with a not-known-to-be-FOSS (see below) license the
  user is asked to view and accept the license: a list of accepted
  licenses is stored in file \file{~/.R/licensed}.  In a non-interactive
  session it is an error to use such a package whose license has not
  already been recorded as accepted.

  As from \R 3.4.0 the license check is done when the namespace is loaded.

  Free or Open Source Software (FOSS,
  e.g.\sspace{}\url{https://en.wikipedia.org/wiki/FOSS}) packages are
  determined by the same filters used by
  \code{\link{available.packages}} but applied to just the current
  package, not its dependencies.

  There can also be a site-wide file \file{R_HOME/etc/licensed.site} of
  packages (one per line).
}
\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 namespace 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 namespace.  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/attach 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.\sspace{}\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 are common on Windows and
  macOS.
}
\seealso{
  \code{\link{.libPaths}}, \code{\link{.packages}}.

  \code{\link{attach}}, \code{\link{detach}}, \code{\link{search}},
  \code{\link{objects}}, \code{\link{autoload}},
  \code{\link{requireNamespace}},
  \code{\link{library.dynam}}, \code{\link{data}},
  \code{\link{install.packages}} and
  \code{\link{installed.packages}};
  \code{\link{INSTALL}}, \code{\link{REMOVE}}.

  The initial set of packages attached is set by
  \code{\link{options}(defaultPackages=)}: see also \code{\link{Startup}}.
}
\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
\donttest{library(help = splines)     # documentation on package 'splines'}
library(splines)            # attach 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{
## if you want to mask as little as possible, use
library(mypkg, pos = "package:base")
}}
\keyword{data}