Multiple dispatch

Multiple dispatch or multimethods is a feature of some programming languages in which a function or method can be dynamically dispatched based on the run-time (dynamic) type or, in the more general case some other attribute, of more than one of its arguments. This is a generalization of single-dispatch polymorphism where a function or method call is dynamically dispatched based on the actual derived type of the object on which the method has been called. Multiple dispatch routes the dynamic dispatch to the implementing function or method using the combined characteristics of one or more arguments.

Developers of computer software typically organize source code into named blocks variously called subroutines, procedures, subprograms, functions, or methods. The code in the function is executed by calling it – executing a piece of code that references its name. This transfers control temporarily to the called function; when the function's execution has completed, control is typically transferred back to the instruction in the caller that follows the reference.

Function names are usually selected so as to be descriptive of the function's purpose. It is sometimes desirable to give several functions the same name, often because they perform conceptually similar tasks, but operate on different types of input data. In such cases, the name reference at the function call site is not sufficient for identifying the block of code to be executed. Instead, the number and type of the arguments to the function call are also used to select among several function implementations.

In more conventional, i.e. single-dispatch object-oriented programming languages, when invoking a method (sending a message in Smalltalk, calling a member function in C++), one of its arguments is treated specially and used to determine which of the (potentially many) methods of that name is to be applied. In many languages, the special argument is indicated syntactically; for example, a number of programming languages put the special argument before a dot in making a method call: special.method(other, arguments, here), so that lion.sound() would produce a roar, whereas sparrow.sound() would produce a cheep.

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