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% File src/library/methods/man/GenericFunctions.Rd

% Part of the R package, https://www.R-project.org

% Copyright 1995-2016 R Core Team

% Distributed under GPL 2 or later

\name{GenericFunctions}

\alias{GenericFunctions}

\alias{isGeneric}

\alias{isGroup}

\alias{removeGeneric}

\alias{getGenerics}

\alias{dumpMethod}

\alias{findFunction}

\alias{dumpMethods}

\alias{removeMethods}

\alias{signature}

\alias{setReplaceMethod}

%% FIXME:  These are basically not documented at all:

%% -----  isGroup, setReplaceMethod

\title{Tools for Managing Generic Functions}

\description{

  The functions documented here manage collections of methods associated

  with a generic function, as well as providing information about the

  generic functions themselves.

}

\usage{

isGeneric(f, where, fdef, getName = FALSE)

isGroup(f, where, fdef)

removeGeneric(f, where)

dumpMethod(f, signature, file, where, def)

findFunction(f, generic = TRUE, where = topenv(parent.frame()))

dumpMethods(f, file, signature, methods, where)

signature(\dots)

removeMethods(f, where = topenv(parent.frame()), all = missing(where))

setReplaceMethod(f, \dots, where = topenv(parent.frame()))

getGenerics(where, searchForm = FALSE)

}

\section{Summary of Functions}{

  \describe{

    \item{\code{isGeneric}:}{

      Is there a function named \code{f}, and if so, is it a generic?

      The \code{getName} argument allows a function to find the name

      from a function definition.  If it is \code{TRUE} then the name of

      the generic is returned, or \code{FALSE} if this is not a generic

      function definition.

      The behavior of \code{isGeneric} and \code{\link{getGeneric}} for

      primitive functions is slightly different.  These functions don't

      exist as formal function objects (for efficiency and historical

      reasons), regardless of whether methods have been defined for

      them.  A call to \code{isGeneric} tells you whether methods have

      been defined for this primitive function, anywhere in the current

      search list, or in the specified position \code{where}.  In

      contrast, a call to \code{\link{getGeneric}} will return what the

      generic for that function would be, even if no methods have been

      currently defined for it.

    }

    \item{\code{removeGeneric}, \code{removeMethods}:}{

      Remove all the methods for the generic function of this

      name.  In addition, \code{removeGeneric} removes the function

      itself; \code{removeMethods} restores the non-generic function

      which was the default method.   If there was no default method,

      \code{removeMethods} leaves a generic function with no methods.

    }

    \item{\code{standardGeneric}:}{

      Dispatches a method from the current function call for the generic

      function \code{f}.  It is an error to call

      \code{standardGeneric} anywhere except in the body of the

      corresponding generic function.

      Note that \code{\link{standardGeneric}} is a primitive function in

      the \pkg{base} package

      for efficiency % << MM: this is just my guess, what *are* the reasons?

      reasons, but rather documented here where it belongs naturally.

    }

    \item{\code{dumpMethod}:}{

      Dump the method for this generic function and signature.

    }

    \item{\code{findFunction}:}{

      return a list of either the positions on the search list, or the

      current top-level environment, on which a function object

      for \code{name} exists.  The returned value is \emph{always} a

      list, use the first element to access the first visible version

      of the function.  See the example.

      \emph{NOTE:} Use this rather than \code{\link{find}} with

      \code{mode="function"}, which is not as meaningful, and has a few

      subtle bugs from its use of regular expressions.  Also,

      \code{findFunction} works correctly in the code for a package

      when attaching the package via a call to \code{\link{library}}.

    }

    \item{\code{dumpMethods}:}{

      Dump all the methods for this generic.

    }

    \item{\code{signature}:}{

      Returns a named list of classes to be matched to arguments of a

      generic function.

    }

    \item{\code{getGenerics}:}{returns the names of the generic

      functions that have methods defined on \code{where}; this

      argument can be an environment or an index into the search

      list.  By default, the whole search list is used.

      The methods definitions are stored with

      package qualifiers; for example, methods for function

      \code{"initialize"} might refer to two different functions

      of that name, on different packages.  The package names

      corresponding to the method list object are contained in the

      slot \code{package} of the returned object.  The form of

      the returned name can be plain (e.g., \code{"base"}), or in

      the form used in the search list (\code{"package:base"})

      according to the value of \code{searchForm}}

  }

}

\arguments{

  \item{f}{ The character string naming the function. }

  \item{where}{ The environment, namespace, or search-list position

  from which to search for objects.  By default, start at the

  top-level environment of the calling function, typically the global

  environment (i.e., use the search list), or the namespace of a

  package from which the call came.  It is important to supply this

  argument when calling any of these functions indirectly.  With

  package namespaces, the default is likely to be wrong in such calls.}

  \item{signature}{ The class signature of the relevant method.  A

    signature is a named or unnamed vector of character strings.  If

    named, the names must be formal argument names for the generic

    function.  Signatures are matched to the arguments specified in

    the signature slot of the generic function (see the Details

    section of the \code{\link{setMethod}} documentation).

  The \code{signature} argument to \code{dumpMethods} is ignored (it

  was used internally in previous implementations).}

  \item{file}{ The file or connection on which to dump method definitions. }

  \item{def}{ The function object defining the method; if omitted, the

    current method definition corresponding to the signature. }

  \item{\dots}{Named or unnamed arguments to form a signature.}

  \item{generic}{In testing or finding functions, should generic

    functions be included.  Supply as \code{FALSE} to get only

    non-generic functions.}

  \item{fdef}{Optional, the generic function definition.

   Usually omitted in calls to \code{isGeneric}}

  \item{getName}{If \code{TRUE}, \code{isGeneric} returns the name of

    the generic.  By default, it returns \code{TRUE}. }

  \item{methods}{

    The methods object containing the methods to be dumped.  By default,

    the methods defined for this generic (optionally on the specified

    \code{where} location).

  }

  \item{all}{in \code{removeMethods}, logical indicating if all

    (default) or only the first method found should be removed.}

  \item{searchForm}{In \code{getGenerics}, if \code{TRUE}, the

    \code{package} slot of the returned result is in the form used

    by \code{search()}, otherwise as the simple package name (e.g,

    \code{"package:base"} vs \code{"base"}).

  }

}

\section{Details}{

  \describe{

    \item{\code{isGeneric}:}{

      If the \code{fdef} argument is supplied, take this as the

      definition of the generic, and test whether it is really a

      generic, with \code{f} as the name of the generic.  (This argument

      is not available in S-Plus.)

    }

    \item{\code{removeGeneric}:}{

      If \code{where} supplied, just remove the version on this element

      of the search list; otherwise, removes the first version

      encountered.

    }

    \item{\code{standardGeneric}:}{

      Generic functions should usually have a call to

      \code{standardGeneric} as their entire body.  They can, however,

      do any other computations as well.

      The usual \code{setGeneric} (directly or through calling

      \code{setMethod}) creates a function with a call to

      \code{standardGeneric}.

    }

    \item{\code{dumpMethod}:}{

      The resulting source file will recreate the method.

    }

    \item{\code{findFunction}:}{

      If \code{generic} is \code{FALSE}, ignore generic functions.

    }

    \item{\code{dumpMethods}:}{

      If \code{signature} is supplied only the methods matching this

      initial signature are dumped.  (This feature is not found in

      S-Plus:  don't use it if you want compatibility.)

    }

    \item{\code{signature}:}{

      The advantage of using \code{signature} is to provide a check on

      which arguments you meant, as well as clearer documentation in

      your method specification.  In addition, \code{signature} checks

      that each of the elements is a single character string.

    }

    \item{\code{removeMethods}:}{

      Returns \code{TRUE} if \code{f} was a generic function,

      \code{FALSE} (silently) otherwise.

      If there is a default method, the function will be re-assigned as

      a simple function with this definition.

      Otherwise, the generic function remains but with no methods (so

      any call to it will generate an error).  In either case, a

      following call to \code{setMethod} will consistently

      re-establish the same generic function as before.

    }

  }

}

\references{

 Chambers, John M. (2016)

 \emph{Extending R},

  Chapman & Hall.

(Chapters 9 and 10.)

}

\seealso{

  \code{\link{getMethod}} (also for \code{selectMethod}),

  \code{\link{setGeneric}},

  \code{\link{setClass}},

  \code{\link{showMethods}}

}

\examples{

require(stats) # for lm

## get the function "myFun" -- throw an error if 0 or > 1 versions visible:

findFuncStrict <- function(fName) {

  allF <- findFunction(fName)

  if(length(allF) == 0)

    stop("No versions of ",fName," visible")

  else if(length(allF) > 1)

    stop(fName," is ambiguous: ", length(allF), " versions")

  else

    get(fName, allF[[1]])

}

try(findFuncStrict("myFun"))# Error: no version

lm <- function(x) x+1

try(findFuncStrict("lm"))#    Error: 2 versions

findFuncStrict("findFuncStrict")# just 1 version

rm(lm)

\dontshow{

## because nosegfault runs standardGeneric w/o the methods package, nothing

## really gets tested.  The following check that it catches some errors

mustDie <- function(expr)

   stopifnot(is(tryCatch(expr, error=function(e)e), "error"))

mustDie(standardGeneric()) # 3 tests of requiring a single string

mustDie(standardGeneric(NULL))

mustDie(standardGeneric(""))

mustDie(standardGeneric("notAGenericFunction"))

mustDie(standardGeneric("show"))  # a generic, but not called from its body

}

## method dumping ------------------------------------

setClass("A", slots = c(a="numeric"))

setMethod("plot", "A", function(x,y,...){ cat("A meth\n") })

dumpMethod("plot","A", file="")

\dontrun{

setMethod("plot", "A",

function (x, y, ...)

{

    cat("AAAAA\n")

}

)

}%dont

tmp <- tempfile()

dumpMethod("plot","A", file=tmp)

## now remove, and see if we can parse the dump

stopifnot(removeMethod("plot", "A"))

source(tmp)

stopifnot(is(getMethod("plot", "A"), "MethodDefinition"))

## same with dumpMethods() :

setClass("B", contains="A")

setMethod("plot", "B", function(x,y,...){ cat("B ...\n") })

dumpMethods("plot", file=tmp)

stopifnot(removeMethod("plot", "A"),

          removeMethod("plot", "B"))

source(tmp)

stopifnot(is(getMethod("plot", "A"), "MethodDefinition"),

          is(getMethod("plot", "B"), "MethodDefinition"))

}

\keyword{programming}

\keyword{classes}

\keyword{methods}