Encoding algebraic data type in JavaScript

JavaScript lacks algebraic data type, but we can emulate it because JavaScript has first-class functions.

Here’s a simple Haskell program declaring Tree data type with Bud, Leaf and Tree data constructors. treeSum calculates the sum of leaf nodes by iterating all nodes in the given tree.

data Tree = Bud | Leaf Int | Tree Tree Tree deriving Show

treeSum :: Tree -> Int
treeSum Bud = 0
treeSum (Leaf n) = n
treeSum (Tree t1 t2) = treeSum t1 + treeSum t2

main = do
	let t0 = treeSum Bud
	putStrLn $ show t0
	let t1 = treeSum (Leaf 2)
	putStrLn $ show t1
	let t2 = treeSum (Tree (Leaf 3) (Tree (Leaf 5) Bud))
	putStrLn $ show t2

In JavaScript, we can emulate the data constructors of Tree with higher order functions. The trick used here is to pass b, l and p as arguments to data constructors. Only one of these 3 arguments is used by each data constructor:

  • b is a value for Bud.
  • l is a function taking an integer n as an argument and returns a value for Leaf.
  • p is a function taking left and right trees as arguments and returns a value for Tree.

(ES6 let and arrow are used for conciseness.)

let Bud = b => l => p => b;
let Leaf = n => b => l => p => l(n);
let Tree = t1 => t2 => b => l => p =>
           p(t1(b)(l)(p))(t2(b)(l)(p))

You can think of Bud as a value by itself. For Leaf and Tree, you can create each node by partially applying arguments.

var t0 = Bud;
var t1 = Leaf(2);
var t2 = Tree(Leaf(2), Bud);

With this encoding scheme, defining treeSum is trivial. You just need to pass 3 functions to handle Bud, Leaf and Tree respectively. If w is Bud, it returns 0. If w is a Leaf node, it returns n. If w is a Tree node, it returns the sum of left and right trees.

let treeSum = w => w(0)(n => n)(x => y => x + y);

The client code looks almost the same to Haskell.

var v0 = treeSum(Bud);
console.log(v0);

var v1 = treeSum(Leaf(2));
console.log(v1);

var v2 = treeSum(Tree(Leaf(3))(Tree(Leaf(5))(Bud)));
console.log(v2);
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