multimethod

Multimethod refers to a programming construct in which a function or operation is selected at runtime based on the types of more than one of its arguments. Also called multiple dispatch, it generalizes single-dispatch methods by allowing the dispatch decision to consider the dynamic types of several parameters rather than only the receiver. In practice, a multimethod is realized as a set of specialized methods, each defined for a particular combination of argument types, and a generic function or dispatcher that selects the most specific applicable method at call time.

Origins and languages: The concept originated in the object-oriented Lisp family, notably the Common Lisp Object System (CLOS), where generic functions support multimethod dispatch. It has since become a core feature in languages such as Julia, which employs widespread multiple dispatch, and in Dylan, Cecil, and some implementations of Python and Ruby via libraries. The term consists of a generic function plus multiple methods.

Dispatch mechanism: When a multimethod is invoked, the runtime examines the dynamic types of its argument(s) and locates the candidate methods whose parameter type declarations are compatible. A resolution strategy chooses the most specific method; if no method or ambiguous choice exists, the language can raise an error or fall back to a default. Some systems use type hierarchies to determine specificity, while others consider equality or union types.

Advantages and considerations: Multimethods can improve modularity, enabling new types and operations without modifying existing code, and can express operations that depend on the combination of operand types. Drawbacks include potential performance overhead, increased complexity in method resolution, and the risk of ambiguous or conflicting definitions. Good practice includes careful design of type hierarchies and clear resolution rules.