MethodsList Objects

Description

These functions create and manipulate MethodsList objects, the objects used in R to store methods for dispatch. You should not call any of these functions from code that you want to port to S-Plus. Instead, use the functions described in the references.

Usage

MethodsList(.ArgName, ...)

makeMethodsList(object, level=1)

SignatureMethod(names, signature, definition)

insertMethod(mlist, signature, args, def, cacheOnly)

inheritedSubMethodLists(object, thisClass, mlist, ev)

showMlist(mlist, includeDefs = TRUE, inherited = TRUE,
   classes, useArgNames, printTo = stdout() )

print.MethodsList(x, ...)

listFromMlist(mlist, prefix = list())

linearizeMlist(mlist, inherited = TRUE)

finalDefaultMethod(mlist, fname = "NULL")

mergeMethods(m1, m2)

loadMethod(method, fname, envir)

Details

Note that MethodsList objects represent methods only in the R implementation. You can use them to find or manipulate information about methods, but avoid doing so if you want your code to port to S-Plus.

Details

MethodsList:

Create a MethodsList object out of the arguments.

Conceptually, this object is a named collection of methods to be dispatched when the (first) argument in a function call matches the class corresponding to one of the names. A final, unnamed element (i.e., with name "") corresponds to the default method.

The elements can be either a function, or another MethodsList. In the second case, this list implies dispatching on the second argument to the function using that list, given a selection of this element on the first argument. Thus, method dispatching on an arbitrary number of arguments is defined.

MethodsList objects are used primarily to dispatch OOP-style methods and, in R, to emulate S4-style methods.

SignatureMethod:

construct a MethodsList object containing (only) this method, corresponding to the signature; i.e., such that signature[[1]] is the match for the first argument, signature[[2]] for the second argument, and so on. The string "missing" means a match for a missing argument, and "ANY" means use this as the default setting at this level.

The first argument is the argument names to be used for dispatch corresponding to the signatures.

insertMethod:

insert the definition def into the MethodsList object, mlist, corresponding to the signature. By default, insert it in the slot "methods", but cacheOnly=TRUE inserts it into the "allMethods" slot (used for dispatch but not saved).

inheritedSubMethodLists:

Utility function to match the object or the class (if the object is NULL) to the elements of a methods list. Used in finding inherited methods, and not meant to be called directly.

showMlist:

Prints the contents of the MethodsList. If includeDefs the signatures and the corresonding definitions will be printed; otherwise, only the signatures.

The function calls itself recursively: prev is the previously selected classes.

listFromMlistForPrint:

Undo the recursive nature of the methods list, making a list of function defintions, with the names of the list being the corresponding signatures (designed for printing; for looping over the methods, use listFromMlist instead).

The function calls itself recursively: prev is the previously selected classes.

finalDefaultMethod:

The true default method for the methods list object mlist (the method that matches class "ANY" for as many arguments as are used in methods matching for this generic function). If mlist is null, returns the function called fname, or NULL if there is no such function.

mergeMethods:

Merges the methods in the second MethodsList object into the first, and returns the merged result. Called from getAllMethods.

loadMethod:

Called, if necessary, just before a call to method is dispatched in the frame envir. The function exists so that methods can be defined for special classes of objects. Usually the point is to assign or modify information in the frame environment to be used evaluation. For example, the standard class MethodDefinition has a method that stores the target and defined signatures in the environment.

Any methods defined for loadMethod must return the function definition to be used for this call; typically, this is just the method argument.

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.