Tools for Managing Generic Functions

Description

The functions documented here manage collections of methods associated with a generic function, as well as providing information about the generic functions themselves.

Usage

isGeneric(f, where, fdef, getName = FALSE)

isGroup(f, where, fdef)

removeGeneric(f, where)

standardGeneric(f)

dumpMethod(f, signature, file, where, def)

existsFunction(f, generic = TRUE, where)

findFunction(f, generic=TRUE)

dumpMethods(f, file, signature, methods, where)

signature(...)

removeMethods(f, where)

setReplaceMethod(f, ...)

getGenerics(where, searchForm = FALSE)

allGenerics(where, searchForm = FALSE)

callGeneric(...)

Arguments

Summary of Functions

isGeneric:

Is there a function named f, and if so, is it a generic?

The getName argument allows a function to find the name from a function definition. If it is TRUE then the name of the generic is returned, or FALSE if this is not a generic function definition.

The behavior of isGeneric and getGeneric for primitive functions is slightly different. These functions don't exist as formal function objects (for efficiency and historical reasons), regardless of whether methods have been defined for them. A call to isGeneric tells you whether methods have been defined for this primitive function, anywhere in the current search list, or in the specified position where. In contrast, a call to getGeneric will return what the generic for that function would be, even if no methods have been currently defined for it.

removeGeneric, removeMethods:

Remove the all the methods for the generic function of this name. In addition, removeGeneric removes the function itself; removeMethods restores the non-generic function which was the default method. If there was no default method, removeMethods leaves a generic function with no methods.

standardGeneric:

Dispatches a method from the current function call for the generic function f. It is an error to call standardGeneric anywhere except in the body of the corresponding generic function.

getMethods:

The list of methods for the specified generic.

dumpMethod:

Dump the method for this generic function and signature.

existsFunction:

Is there a function of this name. If generic is FALSE, generic functions are not counted.

findFunction:

return all the elements of the search list on which a function definition for name exists.

NOTE: Use this rather than find with mode="function", which is not as meaningful, and has a few subtle bugs from its use of regular expressions.

selectMethod:

Returns the method (a function) that R would use to evaluate a call to this generic, with arguments corresponding to the specified signature.

f = the name of the generic function, signature is the signature of classes to match to the arguments of f.

dumpMethods:

Dump all the methods for this generic.

signature:

Returns a named list of classes to be matched to arguments of a generic function.

getGenerics:

Returns the names of the generic functions that have methods defined on where; this argument can be an environment or an index into the search list. By default, the whole search list is used.

The methods definitions are stored with package qualifiers; for example, methods for function "initialize" might refer to two different functions of that name, on different packages. The package names corresponding to the method list object are contained in the slot package of the returned object. The form of the returned name can be plain (e.g., "base"), or in the form used in the search list ("package:base") according to the value of searchForm

callGeneric:

In the body of a method, this function will make a call to the current generic function. If no arguments are passed to callGeneric, the arguments to the current call are passed down; otherwise, the arguments are interpreted as in a call to the generic function.

Details

setGeneric:

If there is already a non-generic function of this name, it will be used to define the generic unless def is supplied, and the current function will become the default method for the generic.

If def is supplied, this defines the generic function, and no default method will exist (often a good feature, if the function should only be available for a meaningful subset of all objects).

Arguments group and valueClass are retained for consistency with S-Plus, but are currently not used.

isGeneric:

If the fdef argument is supplied, take this as the definition of the generic, and test whether it is really a generic, with f as the name of the generic. (This argument is not available in S-Plus.)

removeGeneric:

If where supplied, just remove the version on this element of the search list; otherwise, removes the first version encountered.

standardGeneric:

Generic functions should usually have a call to standardGeneric as their entire body. They can, however, do any other computations as well.

The usual setGeneric (directly or through calling setMethod) creates a function with a call to standardGeneric.

getMethods:

If the function is not a generic function, returns NULL. The f argument can be either the character string name of the generic or the object itself.

The where argument optionally says where to look for the function, if f is given as the name.

dumpMethod:

The resulting source file will recreate the method.

findFunction:

If generic is FALSE, ignore generic functions.

selectMethod:

The vector of strings for the classes can be named or not. If named, 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.

If mustFind is TRUE, an error results if there is no method (or no unique method) corresponding to this signature. Otherwise may return NULL or a MethodsList object.

dumpMethods:

If signature is supplied only the methods matching this initial signature are dumped. (This feature is not found in S-Plus: don't use it if you want compatibility.)

signature:

The advantage of using signature is to provide a check on which arguments you meant, as well as clearer documentation in your method specification. In addition, signature checks that each of the elements is a single character string.

removeMethods:

Returns TRUE if f was a generic function, FALSE (silently) otherwise.

If there is a default method, the function will be re-assigned as a simple function with this definition. Otherwise, the generic function remains but with no methods (so any call to it will generate an error). In either case, a following call to setMethod will consistently re-establish the same generic function as before.

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

setGeneric, setClass, showMethods

Examples