MethodsList Objects

Description

These functions create and manipulate MethodsList objects, the objects formerly used in R to store methods for dispatch. Use of these objects is no longer required, and while not yet formally deprecated, will rarely be a good idea. Where methods dispatch is to be studied, see selectMethod. For computations that iterate over methods or over method signatures, see findMethods, which returns a linearized methods list to hold method definitions, usually more convenient for iteration than the recursive MethodsList objects.

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() )

## S3 method for class 'MethodsList'
print(x, ...)

listFromMlist(mlist, prefix = list(), sigs. = TRUE, methods. = TRUE)

linearizeMlist(mlist, inherited = TRUE)

finalDefaultMethod(mlist, fname = "NULL")

mergeMethods(m1, m2, genericLabel)

loadMethod(method, fname, envir)

MethodsListSelect(f, env, mlist, fEnv, finalDefault, evalArgs,
                  useInherited, fdef, resetAllowed)

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 corresponding definitions will be printed; otherwise, only the signatures.

listFromMlist:

Undo the recursive nature of the methods list, making a list of list(sigs,methods) of function definitions, i.e. of matching signatures and methods. prefix is the partial signature (a named list of classes) to be prepended to the signatures in this object. If sigs. or methods. are FALSE, the resulting part of the return value will be empty.

A utility function used to iterate over all the individual methods in the object, it calls itself recursively.

linearizeMlist:

Undo the recursive nature of the methods list, making a list of function definitions, with the names of the list being the corresponding signatures.

Designed for printing; for looping over the methods, use the above listFromMlist instead.

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 or no matching method is found, 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. For a primitive function, genericLabel is supplied as the name of the generic.

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. Class MethodWithNext has a method taking account of the mechanism for storing the method to be used in a call to callNextMethod.

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

MethodsListSelect

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. This search was used by the evaluator when methods lists were the metadata for methods dispatch. This function is now deprecated.

Note

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.

References

Chambers, John M. (2008) Software for Data Analysis: Programming with R Springer. (For the R version.)

Chambers, John M. (1998) Programming with Data Springer (For the original S4 version.)