The R Project SVN R

\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}

\title{MethodsList Objects}

\description{

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

  objects used in R to store methods for dispatch.  You should not call

  any of these functions from code that you want to port to S-Plus.

  Instead, use the functions described in the references.

}

\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())

linearizeMlist(mlist, inherited = TRUE)

finalDefaultMethod(mlist, fname = "NULL")

mergeMethods(m1, m2, genericLabel)

loadMethod(method, fname, envir)

}

\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 corresonding definitions will be printed;

      otherwise, only the signatures.

      The function calls itself recursively: \code{prev} is the

      previously selected classes.

    }

    \item{\code{listFromMlistForPrint}:}{

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

      function defintions, with the names of the list being the

      corresponding signatures (designed for printing; for looping over

      the methods, use \code{\link{listFromMlist}} instead).

      The function calls itself recursively: \code{prev} is the

      previously selected classes.

    }

    \item{\code{finalDefaultMethod}:}{

      The true default method for the methods list object \code{mlist}

      (the method that matches class \code{"ANY"} for as many arguments

      as are used in methods matching for this generic function).  If

      \code{mlist} is null, returns the function called \code{fname}, or

      \code{NULL} if there is no such function.

    }

    \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.

    }

  }

}

\details{

  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{

  The R package \pkg{methods} implements, with a few exceptions, the

  programming interface for classes and methods in the book

  \emph{Programming with Data} (John M. Chambers, Springer, 1998), in

  particular sections 1.6, 2.7, 2.8, and chapters 7 and 8.

  While the programming interface for the \pkg{methods} package follows

  the reference, the R software is an original implementation, so

  details in the reference that reflect the S4 implementation may appear

  differently in R.  Also, there are extensions to the programming

  interface developed more recently than the reference.  For a

  discussion of details and ongoing development, see the web page

  \url{http://developer.r-project.org/methodsPackage.html} and the

  pointers from that page.

}

\keyword{internal}