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

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

% Copyright 1995-2007 R Core Team

% Distributed under GPL 2 or later

\name{MethodsList}

\alias{MethodsList}

\alias{makeMethodsList}

\alias{SignatureMethod}

\alias{insertMethod}

\alias{inheritedSubMethodLists}

\alias{showMlist}

\alias{print.MethodsList}

\alias{emptyMethodsList}

\alias{insertMethodInEmptyList}

\alias{listFromMlist}

\alias{linearizeMlist}

\alias{finalDefaultMethod}

\alias{mergeMethods}

\alias{loadMethod}

\alias{loadMethod-methods}

\alias{loadMethod,ANY-method}

\alias{loadMethod,MethodDefinition-method}

\alias{loadMethod,MethodWithNext-method}

\alias{MethodsListSelect}

\title{MethodsList Objects}

\description{

  These functions create and manipulate \code{MethodsList} objects, the

  objects formerly used in R to store methods for dispatch.  Use of

  these objects is no longer required, and while not yet formally

  deprecated, will rarely be a good idea.  Where methods dispatch is

  to be studied, see \code{\link{selectMethod}}.  For computations

  that iterate over methods or over method signatures, see

  \code{\link{findMethods}}, which returns a linearized methods list

  to hold method definitions, usually more convenient for iteration

  than the recursive \code{MethodsList} objects.

}

\usage{

MethodsList(.ArgName, ...)

makeMethodsList(object, level=1)

SignatureMethod(names, signature, definition)

insertMethod(mlist, signature, args, def, cacheOnly)

inheritedSubMethodLists(object, thisClass, mlist, ev)

showMlist(mlist, includeDefs = TRUE, inherited = TRUE,

   classes, useArgNames, printTo = stdout() )

\method{print}{MethodsList}(x, ...)

listFromMlist(mlist, prefix = list(), sigs. = TRUE, methods. = TRUE)

linearizeMlist(mlist, inherited = TRUE)

finalDefaultMethod(method)

mergeMethods(m1, m2, genericLabel)

loadMethod(method, fname, envir)

MethodsListSelect(f, env, mlist, fEnv, finalDefault, evalArgs,

                  useInherited, fdef, resetAllowed)

}

\section{Details}{

  \describe{

    \item{\code{MethodsList}:}{

      Create a MethodsList object out of the arguments.

      Conceptually, this object is a named collection of methods to be

      dispatched when the (first) argument in a function call matches

      the class corresponding to one of the names.  A final, unnamed

      element (i.e., with name \code{""}) corresponds to the default

      method.

      The elements can be either a function, or another MethodsList.  In

      the second case, this list implies dispatching on the second

      argument to the function using that list, given a selection of

      this element on the first argument.  Thus, method dispatching on

      an arbitrary number of arguments is defined.

      MethodsList objects are used primarily to dispatch OOP-style

      methods and, in R, to emulate S4-style methods.

    }

    \item{\code{SignatureMethod}:}{

      construct a MethodsList object containing (only) this method,

      corresponding to the signature; i.e., such that

      \code{signature[[1]]} is the match for the first argument,

      \code{signature[[2]]} for the second argument, and so on.  The

      string \code{"missing"} means a match for a missing argument, and

      \code{"ANY"} means use this as the default setting at this level.

      The first argument is the argument names to be used for dispatch

      corresponding to the signatures.

    }

    \item{\code{insertMethod}:}{

      insert the definition \code{def} into the MethodsList object,

      \code{mlist}, corresponding to the signature.  By default, insert

      it in the slot \code{"methods"}, but \code{cacheOnly=TRUE} inserts

      it into the \code{"allMethods"} slot (used for dispatch but not saved).

    }

    \item{\code{inheritedSubMethodLists}:}{

      Utility function to match the object  or the class (if the object

      is \code{NULL}) to the elements of a methods list.

      Used in finding inherited methods, and not meant to be called

      directly.

    }

    \item{\code{showMlist}:}{

      Prints the contents of the MethodsList.  If \code{includeDefs} the

      signatures and the corresponding definitions will be printed;

      otherwise, only the signatures.

    }

    \item{\code{listFromMlist}:}{

      Undo the recursive nature of the methods list, making a list of

      \code{list(sigs,methods)} of function definitions, i.e. of

      matching signatures and methods.

      \code{prefix} is the partial signature (a named list of classes)

      to be prepended to the signatures in this object.  If \code{sigs.}

      or \code{methods.} are \code{FALSE}, the resulting part of the

      return value will be empty.

      A utility function used to iterate over all the individual methods

      in the object, it calls itself recursively.

    }

    \item{\code{linearizeMlist}:}{

      Undo the recursive nature of the methods list, making a list of

      function definitions, with the names of the list being the

      corresponding signatures.

      Designed for printing; for looping over the methods, use the above

      \code{listFromMlist} instead.

    }

    \item{\code{finalDefaultMethod}:}{

      The default method or NULL.  With the demise of

      \code{"MethodsList"} objects, this function only checks that the

      value given it is a method definition, primitive or NULL.

    }

    \item{\code{mergeMethods}:}{

      Merges the methods in the second MethodsList object into the

      first, and returns the merged result.  Called from

      \code{\link{getAllMethods}}.  For a primitive function,

      \code{genericLabel} is supplied as the name of the generic.

    }

    \item{\code{loadMethod}:}{

      Called, if necessary, just before a call to \code{method} is

      dispatched in the frame \code{envir}.  The function exists so that

      methods can be defined for special classes of objects.  Usually

      the point is to assign or modify information in the frame

      environment to be used evaluation.  For example, the standard

      class \code{MethodDefinition} has a method that stores the target

      and defined signatures in the environment.  Class

      \code{MethodWithNext} has a method taking account of the

      mechanism for storing the method to be used in a call to

      \code{\link{callNextMethod}}.

      Any methods defined for \code{loadMethod} must return the function

      definition to be used for this call; typically, this is just the

      \code{method} argument.

    }

  \item{\code{MethodsListSelect}}{

      The function \code{MethodsListSelect} performs a full search

      (including all inheritance and group generic information: see the

      \link{Methods} documentation page for details on how this works).

      The call returns a possibly revised methods list object,

      incorporating any method found as part of the \code{allMethods}

      slot.  This search was used by the evaluator when methods lists

      were the metadata for methods dispatch.  This function is now deprecated.

    }

  }

}

\note{

  Note that \code{MethodsList} objects represent methods only in the \R

  implementation.  You can use them to find or manipulate information about

  methods, but avoid doing so if you want your code to port to S-Plus.

}

\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.)

}

\keyword{internal}