getMethod              package:methods              R Documentation

Get the Definition of a Method

Description:

     A call to `getMethod' returns the method corresponding a
     particular generic function and signature (set classes for formal
     arguments). The functions `MethodsListSelect' and `selectMethod'
     select methods using an environment in which arguments to the
     function have been assigned (they differ in the treatment of
     inheritance).  Neither is intended to be called directly, except
     perhaps in debugging. The function `getMethods' returns all the
     methods for the generic function.

     The function `findMethod' returns the package(s) in the search
     list (or in the packages specified by the `where' argument) that
     contain a method for this function and signature.

Usage:

     getMethod(f, signature=character(), where, optional=FALSE)

     findMethod(f, signature, where)

     getMethods(f, where=-1)

     MethodsListSelect(f, env, mlist, fEnv, finalDefault, evalArgs, useInherited)
     selectMethod(f, env, optional=False, useInherited, mlist=getMethods(f))

Arguments:

       f: The character-string name of the generic function.

          In `getMethods' only, this argument may be a function
          definition, in which case the special methods list object, if
          any, stored in the environment of the function is returned. 
          (This usage is largely for internal purposes; you aren't
          likely to have such a function definition for direct use.)

signature: The signature of classes to match to the arguments of `f'. 
          The vector of strings for the classes can be named or not. 
          If named, the names must match formal argument names of `f'. 
          If not named, the signature is assumed to apply to the
          arguments of `f' in order. 

   where: Where to look for the method.  By default, looks in all the
          currently attached databases (i.e., the complete search
          list). 

optional: If the selection does not produce a unique result, an error
          is generated, unless this argument is `TRUE'.  In that case,
          the value returned is either a `MethodsList' object, if more
          than one method matches this signature, or `NULL' if no
          method matches.

   mlist: In `selectMethod', the `MethodsList' object can be explicitly
          supplied.  (Unlikely to be used, except in the recursive call
          that finds matches to more than one argument.)

     env: The environment in which argument evaluations are done in
          `MethodsListSelect'.  Currently must be supplied, but should
          usually be `sys.frame(sys.parent())' when calling the
          function explicitly for debugging purposes.

fEnv, finalDefault, evalArgs, useInherited: Internal-use arguments for
          the function's environment, the method to use as the overall
          default, whether to evaluate arguments, and which arguments
          should use inheritance.

Details:

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

     The function `getMethods' returns all the methods for a particular
     generic (in the form of a generic function with the methods
     information in its environment).  The function is called from the
     evaluator to merge method information, and is not intended to be
     called directly.

     The function `MethodsListSelect' performs a full search (including
     all inheritance and group generic information: see the 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 `allMethods' slot.

     Normally you won't call `MethodsListSelect' directly, but it is
     possible to use it for debugging purposes (only for distinctly
     advanced users!).

     Note that the statement that `MethodsListSelect' corresponds to
     the selection done by the evaluator is a fact, not an assertion,
     in the sense that the evaluator code constructs and executes a
     call to `MethodsListSelect' when it does not already have a cached
     method for this generic function and signature.  (The value
     returned is stored by the evaluator so that the search is not
     required next time.)

     The `selectMethod' function is similar to `MethodsListSelect' (and
     calls it) but differs in that it is intended to be used with
     control over which arguments are allowed to use inheritance.  The
     restriction is needed to make sensible method selection for coerce
     methods (the `as' function).

Value:

     The call to `selectMethod' or `getMethod' returns a function
     object, the selected method, if a unique selection exists.
     Otherwise an error is thrown if `optional' is `FALSE'.  If
     `optional' is `TRUE', the value returned is `NULL' if no method
     matched, or a `MethodsList' object if multiple methods matched.

     The call to `getMethods' returns the `MethodsList' object
     containing all the methods requested.  If there are none, `NULL'
     is returned: `getMethods' does not generate an error in this case.

Author(s):

     John Chambers

References:

     The web page <URL: http://www.omegahat.org/RSMethods/index.html>
     is the primary documentation.

     The functions in this package implement a facility for classes and
     methods as described in Programming with Data, (John M. Chambers,
     Springer, 1998).  See this book for further details and examples.

Examples:

