Reader monad

reader.js

function Reader(x) {
    this.x = x;
}

Reader.prototype.run = function(env) {
    return this.x(env);
}

Reader.of = function(x) {
    return new Reader(() => x);
}

Reader.prototype.map = function(fn) {
    return new Reader(env => fn(this.run(env)));   
}

Reader.prototype.join = function() {
    return new Reader(env => this.run(env).run(env));
}

Reader.prototype.chain = function(fn) {
    return this.map(fn).join();
}

Reader.ask = new Reader(env => env);

Reader.of(2)
    .map(x => x * 2)
    .chain(x => Reader.ask.map(env => env + x))
    .map(x => x * 3)
    .chain(x => Reader.ask.map(env => x - env))
    .run(3); // 18