R: Description of the Methods Defined for a Generic Function

findMethods {methods} | R Documentation

## Description of the Methods Defined for a Generic Function

### Description

The function `findMethods` converts the methods defined in a table for a generic function (as used for selection of methods) into a list, for study or display. The list is actually from the class `listOfMethods` (see the section describing the class, below).

The list will be limited to the methods defined in environment `where` if that argument is supplied and limited to those including one or more of the specified `classes` in the method signature if that argument is supplied.

To see the actual table (an environment) used for methods dispatch, call `getMethodsForDispatch`. The names of the list returned by `findMethods` are the names of the objects in the table.

The function `findMethodSignatures` returns a character matrix whose rows are the class names from the signature of the corresponding methods; it operates either from a list returned by `findMethods`, or by computing such a list itself, given the same arguments as `findMethods` .

The function `hasMethods` returns `TRUE` or `FALSE` according to whether there is a non-empty table of methods for function `f` in the environment or search position `where` (or for the generic function generally if `where` is missing).

The deprecated function `getMethods` is an older alternative to `findMethods` , returning information in the form of an object of class `MethodsList`, previously used for method dispatch. It is not recommended, since this class of objects is deprecated generally and will disappear in a future version of R.

### Usage

    findMethods(f, where, classes = character(), inherited = FALSE,
          package = "")

    findMethodSignatures(..., target = TRUE, methods = )

    hasMethods(f, where, package)

    ### DEPRECATED
    getMethods(f, where, table = FALSE)

### Arguments

`f` | A generic function or the character-string name of one.
`where` | Optionally, an environment or position on the search list to look for methods metadata. If `where` is missing, `findMethods` uses the current table of methods in the generic function itself, and `hasMethods` looks for metadata anywhere in the search list.
`table` | If `TRUE` in a call to `getMethods` the returned value is the table used for dispatch, including inherited methods discovered to date. Used internally, but since the default result is the now unused `mlist` object, the default will likely be changed at some point.
`classes` | If supplied, only methods whose signatures contain at least one of the supplied classes will be included in the value returned.
`inherited` | Logical flag; if `TRUE`, the table of all methods, inherited or defined directly, will be used; otherwise, only the methods explicitly defined. Option `TRUE` is meaningful only if `where` is missing.
`...` | In the call to `findMethodSignatures`, any arguments that might be given to `findMethods`.
`target` | Optional flag to `findMethodSignatures`; if `TRUE`, the signatures used are the target signatures (the classes for which the method will be selected); if `FALSE`, they will be the signatures are defined. The difference is only meaningful if `inherited` is `TRUE`.
`methods` | In the call to `findMethodSignatures`, an optional list of methods, presumably returned by a previous call to `findMethods`. If missing, that function will be call with the ... arguments.
`package` | In a call to `hasMethods`, the package name for the generic function (e.g., `"base"` for primitives). If missing this will be inferred either from the `"package"` attribute of the function name, if any, or from the package slot of the generic function. See ‘Details’.

### Details

The functions obtain a table of the defined methods, either from the generic function or from the stored metadata object in the environment specified by `where`. In a call to `getMethods`, the information in the table is converted as described above to produce the returned value, except with the `table` argument.

Note that `hasMethods`, but not the other functions, can be used even if no generic function of this name is currently found. In this case `package` must either be supplied as an argument or included as an attribute of `f`, since the package name is part of the identification of the methods tables.

### The Class for lists of methods

The class `"listOfMethods"` returns the methods as a named list of method definitions (or a primitive function, see the slot documentation below). The names are the strings used to store the corresponding objects in the environment from which method dispatch is computed. The current implementation uses the names of the corresponding classes in the method signature, separated by `"#"` if more than one argument is involved in the signature.

### Slots

`.Data`:

  Object of class `"list"` The method definitions.

  Note that these may include the primitive function itself as default method, when the generic corresponds to a primitive. (Basically, because primitive functions are abnormal R objects, which cannot currently be extended as method definitions.) Computations that use the returned list to derive other information need to take account of this possibility. See the implementation of `findMethodSignatures` for an example.

`arguments`:

  Object of class `"character"`. The names of the formal arguments in the signature of the generic function.

`signatures`:

  Object of class `"list"`. A list of the signatures of the individual methods. This is currently the result of splitting the `names` according to the `"#"` separator.

  If the object has been constructed from a table, as when returned by `findMethods`, the signatures will all have the same length. However, a list rather than a character matrix is used for generality. Calling `findMethodSignatures` as in the example below will always convert to the matrix form.

`generic`:

  Object of class `"genericFunction"`. The generic function corresponding to these methods. There are plans to generalize this slot to allow reference to the function.

`names`:

  Object of class `"character"`. The names as noted are the class names separated by `"#"` .

### Extends

Class `"namedList"`, directly.

Class `"list"`, by class `"namedList"`, distance 2.

Class `"vector"`, by class `"namedList"`, distance 3.

### See Also

`showMethods`, `selectMethod`, Methods

### Examples

    mm <-  findMethods("Ops")
    findMethodSignatures(methods = mm)

[Package methods version 3.1.2 Index]