getMethod {methods} | R Documentation

## Get the Definition of a Method

### Description

A call to `getMethod` returns the method corresponding a particular generic function and signature; a call to `findMethod` returns the vector of package names in which a matching method is found.

The functions `MethodsListSelect` and `selectMethod` select methods using an environment in which arguments to the function have been assigned (they differ in the treatment of inheritance). Neither is intended to be called directly, except perhaps in debugging. The function `getMethods` returns all the methods for the generic function.

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)

    findMethod(f, signature, where)

    getMethods(f, where=-1)

    MethodsListSelect(f, env, mlist, fEnv, finalDefault, evalArgs, useInherited)
    selectMethod(f, signature, optional=FALSE, useInherited, mlist=getMethods(f))

### Arguments

`f` | The character-string name of the generic function. In `getMethods` only, this argument may be a function definition, in which case the special methods list object, if any, stored in the environment of the function is returned. (This usage is largely for internal purposes; you aren't likely to have such a function definition for direct use.)
`signature` | The signature of classes to match to the arguments of `f`. The vector of strings for the classes should be named, and the names must match formal argument names of `f`. If not named, the signature is assumed to apply to the arguments of `f` in order, but note below for `selectMethod`. For `selectMethod`, the signature can optionally be an environment with classes assigned to the names of the corresponding arguments. If the signature is not an environment, it must currently be named.
`where` | The position or environment in which to look for the method: 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` | In `selectMethod`, the `MethodsList` 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` | Internal-use arguments for the function's environment, the method to use as the overall default, whether to evaluate arguments, and which arguments should use inheritance.

### Details

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 `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.

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.)

The `selectMethod` function is similar to `MethodsListSelect` (and calls it) but differs in that it is intended to be used with control over which arguments are allowed to use inheritance. The restriction is needed to make sensible method selection for coerce methods (the `as` function).

### Value

The call to `selectMethod` or `getMethod` returns a function object, the selected method, if a unique selection exists. 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.

### Author(s)

John Chambers

### References

The web page http://www.omegahat.org/RSMethods/index.html is the primary documentation.

The functions in this package implement a facility for classes and methods as described in Programming with Data (John M. Chambers, Springer, 1998). See this book for further details and examples.

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