Type-safe Lens (TypeScript 2.1)

lens.ts

// requires TypeScript 2.1 or higher

export abstract class Lens<T, U> {
    abstract get: (obj: T) => U;
    abstract set: (value: U) => (obj: T) => T;
    then = <V>(lens: Lens<U, V>) => new ComposedLens(this, lens);
    thenKey = <L extends keyof U>(key: L): Lens<T, U[L]> => this.then(new ObjectLens<U, L>(key));
    modify = (f: (value: U) => U) => (obj: T) => this.set(f(this.get(obj)))(obj);
}
export class IdLens<T> extends Lens<T, T> {
    get = (obj: T) => obj;
    set = (value: T) => (obj: T) => value;
}
export class ComposedLens<T, U, V> extends Lens<T, V> {
    constructor(private l1: Lens<T, U>, private l2: Lens<U, V>) { super(); }
    get = (obj: T) => this.l2.get(this.l1.get(obj));
    set = (value: V) => (obj: T) => this.l1.set(this.l2.set(value)(this.l1.get(obj)))(obj);
}
export class ObjectLens<T, K extends keyof T> extends Lens<T, T[K]> {
    constructor(private k: K) { super(); }
    get = (obj: T) => obj[this.k];
    set = (value: T[K]) => (obj: T) => {
        if (Array.isArray(obj)) {
            const clone: T = obj.concat() as any;
            clone[this.k] = value;
            return clone;
        } else {
            const clone: T = Object.assign({}, obj);
            clone[this.k] = value;
            return clone;
        }
    };
}

export const id = <T>() => new IdLens<T>();
export const key = <T>() => <K extends keyof T>(key: K) => new ObjectLens<T, K>(key);

const exec = <T>(value: T) => (...fs: ((lens: Lens<T, T>) => (value: T) => T)[]): T =>
    fs.reduce((value, f) => f(id<T>())(value), value);

interface Address {
    state: string;
    city: string;
}
interface Person {
    name: string;
    address: Address;
};
let x: Person[] = [{
    name: "Taro",
    address: {
        state: "Hoge-ken",
        city: "Fuga-cho"
    }
}];


console.log("x: ", x);

const y = key<Person[]>()(0)
    .then(key<Person>()("address"))
    .then(key<Address>()("state"))
    .set("Piyo-ken")(x);
console.log("x: ", x);
console.log("y: ", y);

const z = id<Person[]>()
    .thenKey(0)
    .thenKey("address")
    .thenKey("city")
    .modify((c) => c.toUpperCase())(x);
console.log("x: ", x);
console.log("z: ", z);

const w = exec(x)(
    ($) => $.thenKey(0).thenKey("name").set("Jiro"),
    ($) => $.thenKey(0).thenKey("address").thenKey("state").set("Piyo-ken"),
    ($) => $.thenKey(0).thenKey("address").thenKey("city").modify((c) => c.toLowerCase()),
);
console.log("x: ", x);
console.log("w: ", w);