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% File src/library/methods/man/findMethods.Rd
% Part of the R package, http://www.R-project.org
% Copyright 2008 R Core Development Team
% Distributed under GPL 2 or later

\name{findMethods}
\alias{findMethods}
\alias{findMethodSignatures}
\alias{hasMethods}
\alias{getMethods}
\title{Descriptions of the Methods Defined for a Generic Function}
\description{
  These functions convert the methods defined in a table for a generic
  function (as used for selection of methods) into a list, for study or
  display, or provide other information summarizing the methods.

  The function \code{findMethods} returns a list of the method
  definitions currently existing for generic function \code{f}, limited
  to the methods defined in environment \code{where} if that argument is
  supplied and possibly limited to those including one or more of the
  specified \code{classes} in the method signature.

  The function \code{findMethodSignatures} returns a character matrix
  whose rows are the class names from the signature of the corresponding
  methods; it operates either from a list returned by
  \code{findMethods}, or by computing such a list itself, given the same
  arguments as \code{findMethods} .

  The function \code{hasMethods} returns \code{TRUE} or \code{FALSE}
  according to whether there is a non-empty table of methods for
  function \code{f} in the environment or search position \code{where}
  (or anywhere on the search list if \code{where} is missing).

  The function \code{getMethods} is an older alternative to
  \code{findMethods} , returning information in the form of an object of
  class \code{MethodsList}, previously used for method dispatch.  It is
  not recommended, since the object returned is usually less convenient
  than the value of \code{findMethods}.  
}
\usage{
findMethods(f, where, classes = character(), inherited = FALSE)
findMethodSignatures(..., target = TRUE, methods = )

hasMethods(f, where, package)

getMethods(f, where)
}
\arguments{
  \item{f}{A generic function or the character-string name of one.}
  \item{where}{Optionally, an environment or position on the search list
    to look for methods metadata.

    If \code{where} is missing,  \code{findMethods} uses the current
    table of methods in the generic function itself, and
    \code{hasMethods} looks for metadata anywhere in the search list.
  }
  \item{classes}{If supplied, only methods whose signatures contain at
    least one of the supplied classes will be included in the value
    returned.}
  \item{inherited}{Logical flag; if \code{TRUE}, the table of all
    methods, inherited or defined directly, will be used; otherwise,
    only the methods explicitly defined.  Option \code{TRUE} is
    meaningful only if \code{where} is missing.}

  \item{...}{In the call to \code{findMethodSignatures}, any arguments
    that might be given to \code{findMethods}.}
  \item{target}{Optional flag to \code{findMethodSignatures}; if
    \code{TRUE}, the signatures used are the target signatures (the
    classes for which the method will be selected); if \code{FALSE},
    they will be the signatures are defined.  The difference is only
    meaningful if \code{inherited} is \code{TRUE}.}
  \item{methods}{In the call to \code{findMethodSignatures}, an optional
    list of methods, presumably returned by a previous call to
    \code{findMethods}.  If missing, that function will be call with the
    \dots arguments.}

  \item{package}{In a call to \code{hasMethods}, the package name for
    the generic function (e.g., \code{"base"} for primitives).  If
    missing this will be inferred either from the \code{"package"}
    attribute of the function name, if any, or from the package slot of
    the generic function.  See \sQuote{Details}.}
}
\details{
  
  The functions obtain a table of the defined methods, either from the
  generic function or from the stored metadata object in the environment
  specified by \code{where}.  The information in the table is converted
  as described above to produce the returned value.

  Note that the list returned may contain the primitive function itself,
  when the generic corresponds to a primitive.  (Basically, because
  primitive functions are abnormal R objects, which cannot currently be
  extended as method definitions.)  Computations that use the returned
  list to derive other information need to take account of this
  possibility.  See the implementation of \code{findMethodSignatures}
  for an example.

  Note that \code{hasMethods}, but not the other functions, can be used
  even if no generic function of this name is currently found.  In this
  case \code{package} must either be supplied as an argument or included
  as an attribute of \code{f}, since the package name is part of the
  identification of the methods tables.

  These functions should be used in searching for or summarizing
  methods, rather than using information about how the metadata for
  methods is stored.
}
\seealso{ \code{\link{showMethods}}, \code{\link{MethodsList-class}} }
\examples{
mm <-  findMethods("Ops")
findMethodSignatures(methods = mm)
}
\keyword{programming}
\keyword{classes}
\keyword{methods}