getMethod {methods}

R Documentation

Get or Test for the Definition of a Method

Description

The functions getMethod and selectMethod get the definition of a particular method; the functions existsMethod and hasMethod test for the existence of a method. In both cases the first function only gets direct definitions and the second uses inheritance. 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.

The other functions are support functions: see the details below.

Usage

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

findMethod(f, signature, where)

getMethods(f, where=-1)

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

hasMethod(f, signature=character())

selectMethod(f, signature, optional=FALSE, useInherited,
             mlist=getMethods(f), fdef = getGeneric(f))

MethodsListSelect(f, env, mlist, fEnv, finalDefault, evalArgs,
                  useInherited, fdef, resetAllowed)

Arguments

f	The character-string name of the generic function.
signature	The signature of classes to match to the arguments of f. See the details below. For selectMethod, the signature can optionally be an environment with classes assigned to the names of the corresponding arguments. Note: the names correspond to the names of the classes, not to the objects supplied in a call to the generic function. (You are not likely to find this approach convenient, but it is used internally and is marginally more efficient.)
where	The position or environment in which to look for the method: by default, anywhere in the current 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, fdef	In selectMethod, the MethodsList object and/or the generic function 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, resetAllowed	Internal-use arguments for the function's environment, the method to use as the overall default, whether to evaluate arguments, which arguments should use inheritance, and whether the cached methods are allowed to be reset.

Details

The signature argument specifies classes, in an extended sense, corresponding to formal arguments of the generic function. As supplied, 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 .... 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 match.call).

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

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 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 getMethod, selectMethod returns NULL or generates an error if the method is not found, depending on the argument optional.

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

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.)

Value

The call to selectMethod or getMethod returns a MethodDefinition-class object, the selected method, if a unique selection exists. (This class extends function, so you can use the result directly as a function if that is what you want.) 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.

References

The R package methods implements, with a few exceptions, the programming interface for classes and methods in the book 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 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 http://developer.r-project.org/methodsPackage.html and the pointers from that page.

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