Functions to look for a method corresponding to a given generic function and signature. The functions
selectMethod return the method; the functions
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
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.
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 = , doCache = ) hasMethod(f, signature=character(), where)
- A generic function or the character-string name of one.
- the signature of classes to match to the arguments of
f. See the details below.
- 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.
- If the selection in
selectMethoddoes not find a valid method an error is generated, unless this argument is
TRUE. In that case, the value returned is
NULLif no method matches.
- mlist, fdef, useInherited, verbose, doCache
- Optional arguments to
selectMethodfor internal use. Avoid these: some will work as expected and others will not, and none of them is required for normal use of the functions.
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
The strings in the signature may be class names,
"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.
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
NULL or generates an error if the method is not found, depending on the argument
The call to
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
NULL is returned if
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
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.)
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.
setGeneric("testFun", function(x)standardGeneric("testFun")) setMethod("testFun", "numeric", function(x)x+1) hasMethod("testFun", "numeric") ## Not run: TRUE## End(Not run) hasMethod("testFun", "integer") #inherited ## Not run: TRUE## End(Not run) existsMethod("testFun", "integer") ## Not run: FALSE## End(Not run) hasMethod("testFun") # default method ## Not run: FALSE## End(Not run) hasMethod("testFun", "ANY") ## Not run: FALSE## End(Not run)
Documentation reproduced from R 3.0.2. License: GPL-2.