multidispatch

Multidispatch, or multiple dispatch, is a method dispatch mechanism in which the function that executes is chosen based on the runtime types of multiple arguments, not just a single receiver. The central idea is that the behavior of an operation can depend on the combination of types involved in a call, enabling more natural and extensible code for operations on heterogeneous data.

In a multispatch system, functions are organized as generic functions or multimethods. For a given call, the runtime determines all argument types and selects the most specific method whose parameter type signature matches those types. If no exact match exists, a less specific method may be chosen or, in some systems, an error is raised. If multiple methods appear equally specific, the call becomes ambiguous and is typically reported as an error. Some languages provide rules or metatheory to resolve such conflicts, while others require explicit disambiguation.

Languages and systems that support or popularize multispatch include the Common Lisp Object System (CLOS), which introduced robust multiple dispatch and method combinations; Julia, which treats multiple dispatch as a central design principle; and various implementations of Python, R, and Lisp with multimethod libraries. Implementations often use dispatch tables, type hierarchies, and caching to improve performance, with trade-offs between flexibility, extensibility, and lookup cost.

Advantages of multidispatch include easier extension (new methods can be added without modifying existing code) and natural modeling of operations on combinations of types. Drawbacks include increased complexity, potential for ambiguous or conflicting methods, and higher runtime dispatch costs compared to single-dispatch systems.