R: MethodsList Objects

MethodsList {methods} | R Documentation

## 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.)

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