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, mlist)

    findMethod(f, signature, where)

    getMethods(f, where)

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

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

    selectMethod(f, signature, optional = FALSE, useInherited = TRUE,
                 mlist = (if (is.null(fdef)) NULL else
                          getMethodsForDispatch(f, fdef)),
                 fdef = getGeneric(f, !optional))

    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(s): 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` | Optionally, the list of methods in which to search. By default, the function finds the methods for the corresponding generic function. To restrict the search to a particular package or environment, e.g., supply this argument as `getMethodsMetaData(f,where)`. For `selectMethod`, see the discussion of argument `fdef`.
`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. Note that it gets all the visible methods for the specified functions. If you want only the methods defined explicitly in a particular environment, use the function `getMethodsMetaData` instead.

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.

### See Also

`GenericFunctions`

### 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 2.3.1 Index]