R: Tools for Managing Generic Functions

GenericFunctions {methods}

R Documentation

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)

dumpMethod(f, signature, file, where, def)
findFunction(f, generic = TRUE, where = topenv(parent.frame()))
dumpMethods(f, file, signature, methods, where)
signature(...)

removeMethods(f, where = topenv(parent.frame()), all = TRUE)
setReplaceMethod(f, ..., where = topenv(parent.frame()))

getGenerics(where, searchForm = FALSE)
allGenerics(where, searchForm = FALSE)
callGeneric(...)

Arguments

`f`	The character string naming the function.
`where`	The environment, namespace, or search-list position from which to search for objects. By default, start at the top-level environment of the calling function, typically the global environment (i.e., use the search list), or the namespace of a package from which the call came. It is important to supply this argument when calling any of these functions indirectly. With package namespaces, the default is likely to be wrong in such calls.
`signature`	The class signature of the relevant method. A signature is a named or unnamed vector of character strings. If named, the names must be formal argument names for the generic function. If `signature` is unnamed, the default is to use the first `length(signature)` formal arguments of the function.
`file`	The file on which to dump method definitions.
`def`	The function object defining the method; if omitted, the current method definition corresponding to the signature.
`...`	Named or unnamed arguments to form a signature.
`generic`	In testing or finding functions, should generic functions be included. Supply as `FALSE` to get only non-generic functions.
`fdef`	Optional, the generic function definition. Usually omitted in calls to `isGeneric`
`getName`	If `TRUE`, `isGeneric` returns the name of the generic. By default, it returns `TRUE`.
`methods`	The methods object containing the methods to be dumped. By default, the methods defined for this generic (optionally on the specified `where` location).
`all`	in `removeMethods`, logical indicating if all (default) or only the first method found should be removed.
`searchForm`	In `getGenerics`, if `TRUE`, the `package` slot of the returned result is in the form used by `search()`, otherwise as the simple package name (e.g, `"package:base"` vs `"base"`).

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 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.

Note that standardGeneric is a primitive function in the base package for efficiency reasons, but rather documented here where it belongs naturally.

dumpMethod:

Dump the method for this generic function and signature.

findFunction:

return a list of either the positions on the search list, or the current top-level environment, on which a function object for name exists. The returned value is always a list, use the first element to access the first visible version of the function. See the example.

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. Also, findFunction works correctly in the code for a package when attaching the package via a call to library.

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.

dumpMethod:

The resulting source file will recreate the method.

findFunction:

If generic is FALSE, ignore generic functions.

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 see ?Methods and the links from that documentation.

Examples


## get the function "myFun" -- throw an error if 0 or > 1 versions visible:
findFuncStrict <- function(fName) {
  allF <- findFunction(fName)
  if(length(allF) == 0)
    stop("No versions of ",fName," visible")
  else if(length(allF) > 1)
    stop(fName," is ambiguous: ", length(allF), " versions")
  else
    get(fName, allF[[1]])
}

try(findFuncStrict("myFun"))# Error: no version
lm <- function(x) x+1
try(findFuncStrict("lm"))#    Error: 2 versions
findFuncStrict("findFuncStrict")# just 1 version
rm(lm)

[Package methods version 2.4.1 Index]