Comment by mpfundstein

Shameless plug but here is my pattern machter [1] :D

Example:

    // simple factorial
    const factorial = n => match(n)
        .when(0, 1)
        .otherwise(n => n * factorial(n - 1));

    // walking a tree
    class Tree {
        constructor(left, right) {
                this.left = left;
                this.right = right;
        }
    }

    class Node {
        constructor(value) {
                this.value = value;
        }
    }

    const T = (l, r) => new Tree(l, r);
    const N = v => new Node(v);

    const walkT = t => match(t)
        .when(Node, v => console.log(v.value))
        .when(Tree, t => { walkT(t.left); walkT(t.right)})
        .otherwise(_ => 'error');

    const mapT = (f, t) => match(t)
        .when(Node, v => N(f(v.value)))
        .when(Tree, t => T(mapT(f, t.left), mapT(f, t.right)))
        .otherwise(_ => { throw new Error('error') });

Works also on deeply nested objects.

[1] https://github.com/MarkusPfundstein/pmatch-js