getMethod: Get or Test for the Definition of a Method

Description Usage Arguments Details Value Using findMethod() Methods for as() References See Also Examples

Description

The function selectMethod() returns the method that would be selected for a call to function f if the arguments had classes as specified by signature. Failing to find a method is an error, unless argument optional = TRUE, in which case NULL is returned.

The function findMethod() returns a list of environments that contain a method for the specified function and signature; by default, these are a subset of the packages in the current search list. See section “Using findMethod()” for details.

The function getMethod() returns the method corresponding to the function and signature supplied similarly to selectMethod, but without using inheritance or group generics.

The functions hasMethod() and existsMethod() test whether selectMethod() or getMethod(), respectively, finds a matching method.

Usage

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

  findMethod(f, signature, where)

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

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

  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 environment in which to look for the method(s). By default, if the call comes from the command line, the table of methods defined in the generic function itself is used, except for findMethod (see the section below).

optional

if the selection in selectMethod does not find a valid method an error is generated, unless optional is TRUE, in which case the value returned is NULL.

mlist, fdef, useInherited, verbose, doCache

optional arguments to getMethod and selectMethod for 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. But see the section “Methods for as()” for nonstandard inheritance.

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_Details 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. 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.

Both selectMethod and getMethod will normally use the current version of the generic function in the R session, which has a table of the methods obtained from all the packages loaded in the session. Optional arguments can cause a search for the generic function from a specified environment, but this is rarely a useful idea. In contrast, findMethod has a different default and the optional where= argument may be needed. See the section “Using findMethod()”.

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().

The returned value of findMethod is a list of environments in which a corresponding method was found; that is, a table of methods including the one specified.

Using findMethod()

As its name suggests, this function is intended to behave like find, which produces a list of the packages on the current search list which have, and have exported, the object named. That's what findMethod does also, by default. The “exported” part in this case means that the package's namespace has an exportMethods directive for this generic function.

An important distinction is that the absence of such a directive does not prevent methods from the package from being called once the package is loaded. Otherwise, the code in the package could not use un-exported methods.

So, if your question is whether loading package thisPkg will define a method for this function and signature, you need to ask that question about the namespace of the package:

findMethod(f, signature, where = asNamespace("thisPkg"))

If the package did not export the method, attaching it and calling findMethod with no where argument will not find the method.

Notice also that the length of the signature must be what the corresponding package used. If thisPkg had only methods for one argument, only length-1 signatures will match (no trailing "ANY"), even if another currently loaded package had signatures with more arguments.

Methods for as()

The function setAs allows packages to define methods for coercing one class of objects to another class. This works internally by defining methods for the generic function coerce(from, to), which can not be called directly.

The R evaluator selects methods for this purpose using a different form of inheritance. While methods can be inherited for the object being coerced, they cannot inherit for the target class, since the result would not be a valid object from that class. If you want to examine the selection procedure, you must supply the optional argument useInherited = c(TRUE, FALSE) to selectMethod.

References

Chambers, John M. (2016) Extending R, Chapman & Hall. (Chapters 9 and 10.)

Chambers, John M. (2008) Software for Data Analysis: Programming with R Springer. (Section 10.6 for some details of method selection.)

See Also

Methods_Details 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

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testFun <-  function(x)x
setGeneric("testFun")
setMethod("testFun", "numeric", function(x)x+1)

hasMethod("testFun", "numeric") # TRUE

hasMethod("testFun", "integer") #TRUE, inherited

existsMethod("testFun", "integer") #FALSE

hasMethod("testFun") # TRUE, default method

hasMethod("testFun", "ANY")

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