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% File src/library/methods/man/getMethod.Rd
% Part of the R package, https://www.R-project.org
% Copyright 1995-2012 R Core Team
% Distributed under GPL 2 or later

\name{getMethod}
\alias{getMethod}
\alias{findMethod}
\alias{existsMethod}
\alias{selectMethod}
\alias{hasMethod}
\title{ Get or Test for the Definition of a Method }
\description{
  Functions to look for a method corresponding to a given generic function and signature.
  The functions \code{getMethod} and \code{selectMethod} return the method; the functions
  \code{existsMethod} and \code{hasMethod} test for its existence.  In both
  cases the first function only gets direct definitions and the second
  uses inheritance.  In all cases, the search is in the generic function itself or in
  the package/environment specified by argument \code{where}.

  The function \code{findMethod} returns the package(s) in the search
  list (or in the packages specified by the \code{where} argument) that
  contain a method for this function and signature.
}
\usage{
getMethod(f, signature=character(), where, optional = FALSE,
          mlist, fdef)

existsMethod(f, signature = character(), where)

findMethod(f, signature, where)

selectMethod(f, signature, optional = FALSE, useInherited =,
             mlist = , fdef = , verbose = , doCache = )

hasMethod(f, signature=character(), where)

}
\arguments{
  \item{f}{A  generic function or the character-string name of one.}
  \item{signature}{the signature of classes to match to the arguments
    of \code{f}.  See the details below.

  }
  \item{where}{The position or environment in which to look for the
    method(s):  by default, the table of methods defined in the generic function itself is used.}

  \item{optional}{If the selection in \code{selectMethod} does not find a valid method
    an error is generated, unless this argument is \code{TRUE}.  In that
    case, the value returned is \code{NULL} if no method matches.}
  \item{mlist, fdef, useInherited, verbose, doCache}{Optional
      arguments  to  \code{getMethod} and \code{selectMethod} for
      internal use.  Avoid
      these: some will work as expected and others will not, and none of them is required for normal use of the functions.
    }

}
\details{
  The \code{signature} argument specifies classes, corresponding to formal arguments of the generic function; to be precise, to the \code{signature} slot of the generic function object.
    The argument may be a vector of strings identifying
    classes, and may be named or not.  Names, if supplied, match the
    names of those formal arguments included in the signature of the
    generic.  That signature is normally all the arguments except
    \dots.  However, generic functions can be specified with only a
    subset of the arguments permitted, or with the signature taking
    the arguments in a different order.

    It's a good idea to name the arguments in the signature to avoid
    confusion, if you're dealing with a generic that does something
    special with its signature.   In any case, the elements of the
    signature are matched to the formal signature by the same rules
    used in matching arguments in function calls (see
    \code{\link{match.call}}).

    The strings in the signature may be class names, \code{"missing"}
    or \code{"ANY"}.  See \link{Methods} for the meaning of these in
    method selection.  Arguments not supplied in the signature
    implicitly correspond to class \code{"ANY"}; in particular, giving
    an empty signature means to look for the default method.

  A call to \code{getMethod} returns the method for a particular
  function and signature.  As with other \code{get} functions,
  argument \code{where} controls where the function looks (by default
  anywhere in the search list) and argument \code{optional} controls
  whether the function returns \code{NULL} or generates an error if
  the method is not found.  The search for the method makes no use of
  inheritance.

  The function \code{selectMethod} also looks for a method given the
  function and signature, but makes full use of the method dispatch
  mechanism; i.e., inherited methods and group generics are taken into
  account just as they would be in dispatching a method for the
  corresponding signature, with the one exception that conditional
  inheritance is not used.  Like \code{getMethod}, \code{selectMethod}
  returns \code{NULL} or generates an error if
  the method is not found, depending on the argument \code{optional}.

  The functions \code{existsMethod} and \code{hasMethod} return
  \code{TRUE} or \code{FALSE} according to whether a method is found,
  the first corresponding to \code{getMethod} (no inheritance) and the
  second to \code{selectMethod}.

}
\value{
  The call to \code{selectMethod} or \code{getMethod} returns  the selected method, if
 one is found.
  (This class extends \code{function}, so you can use the result
  directly as a function if that is what you want.)
  Otherwise an error is thrown if \code{optional} is \code{FALSE}  and  \code{NULL} is returned if
  \code{optional} is \code{TRUE}.

The returned method object is a
  \code{\linkS4class{MethodDefinition}} object, \emph{except} that the default method for a primitive function is required to be the primitive itself.
Note therefore that the only reliable test that the search failed is \code{is.null()}.

}
\references{
 Chambers, John M. (2008)
 \emph{Software for Data Analysis: Programming with R}
  Springer.  (For the R version.)

 Chambers, John M. (1998)
 \emph{Programming with Data}
 Springer (For the original S4 version.)
}
\seealso{\code{\link{Methods}} for the details of method
  selection; \code{\link{GenericFunctions}} for other functions
  manipulating methods and generic function objects;
  \code{\linkS4class{MethodDefinition}} for the class that represents
  method definitions.}
\examples{
setGeneric("testFun", function(x)standardGeneric("testFun"))
setMethod("testFun", "numeric", function(x)x+1)
hasMethod("testFun", "numeric")
\dontrun{[1] TRUE}
hasMethod("testFun", "integer") #inherited
\dontrun{[1] TRUE}
existsMethod("testFun", "integer")
\dontrun{[1] FALSE}
hasMethod("testFun") # default method
\dontrun{[1] FALSE}
hasMethod("testFun", "ANY")
\dontrun{[1] FALSE}
\dontshow{
stopifnot(isGeneric("testFun"),
          hasMethod("testFun", "numeric"),
          hasMethod("testFun", "integer"),
          !existsMethod("testFun", "integer"),
          !hasMethod("testFun"),
          !hasMethod("testFun", "ANY") )
removeGeneric("testFun")
}
}
\keyword{programming}
\keyword{classes}
\keyword{methods}