gistfile1.ts

interface Lens<C, X> {
	get(c: C): X;
	set(c: C, x: X): C;
}

function get<C, X>(l: Lens<C, X>, c: C): X {
	return l.get(c);
}

function set<C, X>(l: Lens<C, X>, c: C, x: X): C {
	return l.set(c, x);
}

function modify<C, X>(l: Lens<C, X>, c: C, f: (X) => X): C {
	return l.set(c, f(l.get(c)));
}

function lens<C, X>(get: (C) => X, set: (C, X) => C): Lens<C, X> {
	return {
		get: get,
		set: set,
	};
}

function compose<C, X, Y>(a: Lens<C, X>, b: Lens<X, Y>): Lens<C, Y> {
	return lens(
		(x) => get(b, get(a, x)),
		(c, x) => modify(a, c, (d) => set(b, d, x))
	);
}

function ix<C>(index: number): Lens<C[], C> {
	return lens(
		(c) => c[index],
		(c, x) => (() => {
			var result = c.slice();
			result[index] = x;
			return result;
		})()
	);
}

function prop<C, T>(name: string): Lens<C, T> {
	return lens(
		(c) => c[name],
		(c, x) => (() => {
			var result = Object.create(Object.getPrototypeOf(c));
			for (var k in c) {
				if (c.hasOwnProperty(k)) {
					result[k] = c[k];
				}
			}
			result[name] = x;
			return result;
		})()
	)
}

var xs = [{ x: 1, y: 2 }, { x: 2, y: 3 }];
type T = { x: number; y: number };
console.log(set(compose(ix<T>(1), prop<T, number>('y')), xs, 8));