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% File src/library/utils/man/packageDescription.Rd
% Part of the R package, https://www.R-project.org
% Copyright 1995-2017 R Core Team
% Copyright 2005-2017 The R Foundation
% Distributed under GPL 2 or later

\name{packageDescription}
\title{Package Description}
\alias{packageDescription}
\alias{packageVersion}
\alias{packageDate}
\alias{asDateBuilt}
\alias{print.packageDescription}
\description{
  Parses and returns the \file{DESCRIPTION} file of a package as a
  \code{"packageDescription"}.

  Utility functions return (transformed) parts of that.
}
\usage{
packageDescription(pkg, lib.loc = NULL, fields = NULL,
                   drop = TRUE, encoding = "")
packageVersion(pkg, lib.loc = NULL)
packageDate(pkg, lib.loc = NULL,
            date.fields = c("Date", "Packaged", "Date/Publication", "Built"),
            tryFormats = c("\%Y-\%m-\%d", "\%Y/\%m/\%d", "\%D", "\%m/\%d/\%y"),
            desc = packageDescription(pkg, lib.loc=lib.loc, fields=date.fields))
asDateBuilt(built)
}
\arguments{
  \item{pkg}{a character string with the package name.}
  \item{lib.loc}{a character vector of directory names of \R libraries,
    or \code{NULL}.  The default value of \code{NULL} corresponds to all
    libraries currently known.  If the default is used, the loaded
    packages and namespaces are searched before the libraries.}
  \item{fields}{a character vector giving the tags of fields to return
    (if other fields occur in the file they are ignored).}
  \item{drop}{If \code{TRUE} and the length of \code{fields} is 1, then
    a single character string with the value of the respective field is
    returned instead of an object of class \code{"packageDescription"}.}
  \item{encoding}{If there is an \code{Encoding} field, to what encoding
    should re-encoding be attempted?  If \code{NA}, no re-encoding.  The
    other values are as used by \code{\link{iconv}}, so the default
    \code{""} indicates the encoding of the current locale.}
  \item{date.fields}{character vector of field tags to be tried.  The
    first for which \code{\link{as.Date}(.)} is not \code{\link{NA}}
    will be returned. (Partly experimental, see \emph{Note}.)}
  \item{tryFormats}{date formats to try, see \code{\link{as.Date.character}()}.}
  \item{desc}{optionally, a named \code{\link{list}} with components
    named from \code{date.fields};  where the default is fine, a
    complete \code{packageDescription()} maybe specified as well.}
  \item{built}{for \code{asDateBuilt()}, a \code{\link{character}}
    string as from \code{packageDescription(*, fields="Built")}.}
}
\details{
  A package will not be \sQuote{found} unless it has a \file{DESCRIPTION} file
  which contains a valid \code{Version} field.  Different warnings are
  given when no package directory is found and when there is a suitable
  directory but no valid \file{DESCRIPTION} file.

  An \link{attach}ed environment named to look like a package
  (e.g., \code{package:utils2}) will be ignored.

  \code{packageVersion()} is a convenience shortcut, allowing things
  like \code{if (packageVersion("MASS") < "7.3") { do.things } }.

  For \code{packageDate()}, if \code{desc} is valid, both \code{pkg} and
  \code{lib.loc} are not made use of.
}
\note{The default behavior of \code{packageDate()}, notably for
  \code{date.fields}, is somewhat experimental and may change.
}
\value{
  If a \file{DESCRIPTION} file for the given package is found and can
  successfully be read, \code{packageDescription} returns an object of
  class \code{"packageDescription"}, which is a named list with the
  values of the (given) fields as elements and the tags as names, unless
  \code{drop = TRUE}.

  If parsing the \file{DESCRIPTION} file was not successful, it returns
  a named list of \code{NA}s with the field tags as names if \code{fields}
  is not null, and \code{NA} otherwise.

  \code{packageVersion()} returns a (length-one) object of class
  \code{"\link{package_version}"}.

  \code{packageDate()} will return a \code{"Date"} object from
  \code{\link{as.Date}()} or \code{NA}.

  \code{asDateBuilt(built)} returns a \code{"Date"} object or signals an
  error if \code{built} is invalid.
}
\seealso{
  \code{\link{read.dcf}}
}
\examples{\donttest{
packageDescription("stats")
packageDescription("stats", fields = c("Package", "Version"))

packageDescription("stats", fields = "Version")
packageDescription("stats", fields = "Version", drop = FALSE)
}%dont
if(requireNamespace("MASS") && packageVersion("MASS") < "7.3.29")
  message("you need to update 'MASS'")

pu <- packageDate("utils")
\donttest{str(pu)}
stopifnot(identical(pu, packageDate(desc = packageDescription("utils"))),
          identical(pu, packageDate("stats"))) # as "utils" and "stats" are
                                   # both 'base R' and "Built" at same time
}
\keyword{utilities}