getMethod {methods}

R Documentation

Get or Test for the Definition of a Method

Description

Functions to look for a method corresponding to a given generic function and signature. The functions getMethod and selectMethod return the method; the functions existsMethod and hasMethod test for its existence. In both cases the first function only gets direct definitions and the second uses inheritance. In all cases, the search is in the generic function itself or in the package/environment specified by argument where.

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,
          mlist, fdef)

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

findMethod(f, signature, where)

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

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

Arguments

f	A generic function or the character-string name of one.
signature	the signature of classes to match to the arguments of f. See the details below.
where	The position or environment in which to look for the method(s): by default, the table of methods defined in the generic function itself is used.
optional	If the selection in selectMethod does find a valid method an error is generated, unless this argument is TRUE. In that case, the value returned is NULL if no method matches.
mlist, fdef, useInherited, verbose	Optional arguments to getMethod and selectMethod. Avoid these: some will work and others will not, and none of them is required for normal use of the functions.

Details

The signature argument specifies classes, corresponding to formal arguments of the generic function; to be precise, to the signature slot of the generic function object. 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.

Value

The call to selectMethod or getMethod returns the selected method, if one is found. (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 and NULL is returned if optional is TRUE.

The returned method object is a MethodDefinition object, except that the default method for a primitive function is required to be the primitive itself. Note therefore that the only reliable test that the search failed is is.null().

References

Chambers, John M. (2008) Software for Data Analysis: Programming with R Springer. (For the R version.)

Chambers, John M. (1998) Programming with Data Springer (For the original S4 version.)

See Also

Methods for the details of method selection; GenericFunctions for other functions manipulating methods and generic function objects; MethodDefinition for the class that represents method definitions.

Examples

setGeneric("testFun", function(x)standardGeneric("testFun"))
setMethod("testFun", "numeric", function(x)x+1)
hasMethod("testFun", "numeric")
## Not run: [1] TRUE
hasMethod("testFun", "integer") #inherited
## Not run: [1] TRUE
existsMethod("testFun", "integer")
## Not run: [1] FALSE
hasMethod("testFun") # default method
## Not run: [1] FALSE
hasMethod("testFun", "ANY")
## Not run: [1] FALSE

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