Friday, June 20th, 2008
Algebraic Data Types in JavaScript
Category: JavaScript
ADT.js is written in JavaScript 1.8 "which means that as of this writing it only runs in Firefox 3.0. I have chosen to keep it 1.8, because the code is a lot cleaner, thanks to the new expression closures. But there is nothing that cannot be made to work in ECMAScript edition 3. And I have no doubt that the same thing could be done in Python or Ruby."
Now you have the library, you can create types like this (with Haskell to compare):
JAVASCRIPT:
-
-
// Haskell:
-
// data Color = Red | Green | Blue | Yellow
-
// data Point = Pt Float Float Color
-
Color = Data(function() ({ Red: {}, Green: {}, Blue: {}, Yellow: {} }))
-
Point = Data(function() ({ Pt: { x: Number, y: Number, color: Color } }))
-
-
// Haskell:
-
// data Attrs n v = Attr n v
-
// data Node n v = Elem n (List (Attrs n v)) (List (Node n v)) | Text v
-
Attrs = Data(function(_, n, v) ({ Attr: {name: n, value: v} }))
-
Node = Data(function(node, n, v) ({
-
Elem: { name: n, attributes: List(Attrs(n, v)), childNodes: List(node) },
-
Text: v
-
}))
-
Now you have the types, you can do things to them:
JAVASCRIPT:
-
-
// With unfold you can easily generate algebraic data structures from other data. The following example shows how to generate a decreasing list of numbers.
-
var counter = List.unfold(function(n, c) n ? c.Cons(n, n - 1) : c.Nil)
-
-
// Fold is the reverse of unfold. It destructs data into a return value. Here is an example that multiplies a list of numbers.
-
var prod = List.fold({ Nil: 1, Cons: function(h, t) h * t })
-
-
// With map you provide a function for each type parameter. Here are 2 examples using the XML data types
-
var addPrefix = Node.map(function (name) "x:" + name, id)
-
var normalizeSpace = Node.map(id, function (value) value.replace(/^s+|s+$/g, ""))
-
And, a lot more.
Related Content:
3.9 rating from 15 votes
Leave a comment