Multiple dispatch

Transcript

1. Multiple dispatch Axel Rauschmayer 2ality.com 2012-11-30
2. What is method dispatch? Polymorphic method: multiple implementations ⇒ dispatch determines which implementation to call Single dispatch (JavaScript): a single value is used for dispatch. We’ll look at something new – multiple dispatch. Axel Rauschmayer (2ality.com) Multiple dispatch 2012-11-30 2 / 8
3. Dispatch: via an object or via a function Polymorphic method: dispatch via an object. jane.describe() Person.prototype.describe = function (...) {...}; Employee.prototype.describe = function (...) {...}; Axel Rauschmayer (2ality.com) Multiple dispatch 2012-11-30 3 / 8
4. Dispatch: via an object or via a function 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 2012-11-30 3 / 8
5. 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 2012-11-30 4 / 8
6. Adding methods 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) Axel Rauschmayer (2ality.com) Multiple dispatch 2012-11-30 5 / 8
7. 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 types: foo(obj1, obj2, obj3) // collaborators, not data In which object should we put the algorithm? Visitor pattern – encapsulate an algorithm: The visitor is a generic function Single dispatch in host is used to trigger visitor methods Axel Rauschmayer (2ality.com) Multiple dispatch 2012-11-30 6 / 8
8. Languages with multiple dispatch Common Lisp Haskell R Groovy Clojure C# 4.0 Axel Rauschmayer (2ality.com) Multiple dispatch 2012-11-30 7 / 8
9. Thank you! Questions? Ask me off-stage! Axel Rauschmayer (2ality.com) Multiple dispatch 2012-11-30 8 / 8