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Hello, so my talk is going to be on multiple dispatch.

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First for starters what is method dispatch?

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If you have a polymorphic method

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that mean if you have multiple implementations

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and we'll soon see an example to make it clear what I mean

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if you have those multiple implementations, the dispatch determines

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which of those implementations will be called when you

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invoke the method.

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JavaScript has simple dispatch, so a single value is used for dispatch

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we will then look at something new

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and that is called multiple dispatch.

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You can either dispatch via an object or

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via a function. And if you dispatch via an

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object this is what it looks like.

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You have a so-called polymorphic method

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and the type of jane determines which of the two functions

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will be called if you make that method call up here.

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So it depends on whether jane is a person or an employee

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If on the other hand you have a generic function

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then things work differently

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jane is now just a normal argument and

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the function internally checks the type of the argument

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and then depending on the type of the argument

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chooses between two different implementations.

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The same principle can be used if you have more than one parameter

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and naturally you then have to check the type of each

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of those two parameters.

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So if you implemented multiple dispatch as a library in JavaScript

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this is what it would look like.

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You would first create a generic function that's called plus

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then you would add two methods

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and finally you would invoke it as you would with any other function

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and then the types of the parameters

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determine which of the two implementations are invoked when you make the call

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as you can see it's like a table that maps function types

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or parameters types to methods.

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What are the benefits? Why do you do that?

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What's good about it?

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So what's actually happens is

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you get more symmetry

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Normally, the implicit parameter "this" is more powerful

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than the other explicit parameters when you have

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single dispatch like in JavaScript

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So that's one of the properties of multiple dispatch

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And then functions also become object-oriented

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they become aware of the type hierachy of objects

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The benefits are that whenever you have a polymorphic algorithm

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that applies to several objects, then you can put that into a generic function

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and here you have the generic function foo

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it works with three objects and then it's obvious that foo

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doesn't belong to a single object

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it belongs to all of the objects at the same time.

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So generic functions allow you to do that

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and you have the visitor pattern for

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single dispatch languages and that is actually just

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a generic function implemented as an object

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an you use single dispatch in the host to trigger the right visitor method

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and languages with multiple dispatch include

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Common Lisp, Haskell, R, Groovy, Clojure and C#

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That's it, thank you!