Multiple Dispatch¶
Multiple dispatch, as found in Common Lisp (CLOS), Julia, and Dylan, selects which implementation of a function to run based on the runtime types of all of its arguments, not just the first one, the way ordinary object-oriented method calls do. A function defined for (Circle, Rectangle) and again for (Circle, Circle) will resolve to a different body depending on the runtime type of every argument passed, and this resolution happens at the call site, at runtime.
This differs from single dispatch, where a method call $shape->collidesWith($other) only considers the runtime type of $shape; the type of $other is fixed by its declared parameter type and requires manual branching, such as instanceof checks or the double-dispatch visitor pattern, to react differently per combination.
PHP performs single dispatch only: overloaded function declarations do not exist, and function collidesWith(Circle $a, Rectangle $b) cannot coexist with function collidesWith(Circle $a, Circle $b) as two resolvable overloads. Choosing behavior based on the combined runtime types of several arguments must be written by hand with match, instanceof, or the visitor pattern.
<?php
// PHP has no true overloading: this second declaration is a fatal error.
function collide(Circle $a, Circle $b) { /* ... */ }
function collide(Circle $a, Rectangle $b) { /* ... */ } // Fatal error: redeclaration
?>
Related : Overloading, Method Overloading, Polymorphism, instanceof
