R: Get or Test for the Definition of a Method

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 = , doCache = )

    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 not 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, 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.

### 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

[Package methods version 3.1.2 Index]