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Multiple Dispatch

Table of Contents

1. Introduction

Multimethods, or multiple dispatch, is a feature of Common Lisp. The exact details depend on the Common Lisp Object System.

2. Common Lisp

We can have multiple inheritance in Common Lisp, which complicates things. If we avoid multiple inheritance, we can use defgeneric to create a generic function, and provide a defmethod implementation for each data type or class.

The dispatch mechanism proceeds as follows:

  1. Compute the list of applicable methods
  2. If no method is applicable, signal an error
  3. Sort the applicable methods in order of specificity
  4. Invoke the most specific method.

3. Predicate Dispatching

Apparently more sophisticated than Common Lisp's builtin defgeneric and defmethod. Predicates are used to determine if a generic method applies to its arguments, and method m1 overrides m2 if the predicate for m1 implies m2. This relationship is computed at compile-time.

One advantage predicate dispatching has over multiple dispatch, predicate dispatching allows for some degree of pattern matching. This is impossible with multiple dispatching.

See also:

  • filtered-functions (github) for an implementation of predicate dispatching for Common Lisp (see also associated paper)

4. References

  • Ernst, Kaplan, Chambers, "Predicate Dispatching: A Unified Theory of Dispatch". In ECOOP'98, LNCS vol 1445, Springer, pp 186-211 (1998). Eprint.
  • Aaron Mark Ucko, "Predicate Dispatching in the Common Lisp Object System". MIT AI Tech Report 2001-006, dated June 2001. Eprint.
  • C2 WikiPage on Predicate Dispatching

Last Updated 2021-11-21 Sun 10:29.