Multiple dispatch

Transcript

1. Multiple dispatch Axel Rauschmayer 2ality.com 2013-11-11
2. What is method dispatch? Polymorphic method: multiple implementations ⇒ dispatch determines which implementation to call Single dispatch (JavaScript): single value used for dispatch. Multiple dispatch: uses multiple values. Axel Rauschmayer (2ality.com) Multiple dispatch 2013-11-11 2 / 9
3. Polymorphic method: dispatch via an object. jane.describe() Person.prototype.describe = function (...) {...}; Employee.prototype.describe = function (...) {...}; Generic function: dispatch via a function. describe(jane) function describe(self) { if (self instanceof Person) { ... } else if (self instanceof Employee) { ... } } Axel Rauschmayer (2ality.com) Multiple dispatch 2013-11-11 3 / 9
4. Generic function: more than one parameter function plus(op1, op2) { if (typeof op1 === 'number' && typeof op2 === 'string') { ... } else if (typeof op1 === 'string' && typeof op2 === 'number') { ... } } Axel Rauschmayer (2ality.com) Multiple dispatch 2013-11-11 4 / 9
5. Adding cases to a generic function var plus = new GenericFunction(); plus.addMethod([num, str], function (op1, op2) { ... }); plus.addMethod([str, num], function (op1, op2) { ... }); plus(3, 'abc') plus('abc', 3) (In languages with multiple dispatch, the cases are called methods.) Axel Rauschmayer (2ality.com) Multiple dispatch 2013-11-11 5 / 9
6. Benefits of multiple dispatch What happens? Symmetry: all parameters as important as this (implicit parameter in single dispatch) Functions become object-oriented, aware of type hierarchy. Benefits: Useful whenever a polymorphic algorithm spans several objects: foo(obj1, obj2, obj3) // collaborators, not data In which object should we put the algorithm? Extend existing “classes” in an easy, modular fashion. Axel Rauschmayer (2ality.com) Multiple dispatch 2013-11-11 6 / 9
7. Multiple dispatch is better than visitor pattern Visitor pattern (single dispatch): Algorithm encapsulated as object: visitor Data: client client.accept(visitor) The visitor is (almost) a generic function! Multiple dispatch: visitor(client) Axel Rauschmayer (2ality.com) Multiple dispatch 2013-11-11 7 / 9
8. Languages with multiple dispatch Common Lisp Haskell R Groovy Clojure C# 4.0 Axel Rauschmayer (2ality.com) Multiple dispatch 2013-11-11 8 / 9
9. Thank you! Questions? Ask me off-stage! Axel Rauschmayer (2ality.com) Multiple dispatch 2013-11-11 9 / 9