StateMonad in TypeScript

StateMonad.ts

class State {
  private constructor(protected state) {}

  Map = f =>
    new State(s => {
      var prev = this.state(s);
      return { value: f(prev.value), state: prev.state };
    });

  Join = () =>
    new State(s => {
      var prev = this.state(s);
      var inner = prev.value.state(prev.state);
      return inner;
    });

  Bind = f => this.Map(f).Join();

  EvalState = i => this.state(i).value;

  ExecState = i => this.state(i).state;

  static Get = () => new State(s => ({ value: s, state: s }));

  static Put = ns => new State(s => ({ value: undefined, state: ns }));

  static Modify = f => State.Get().Bind(s => State.Put(f(s)));

  static Gets = f => State.Get().Bind(s => State.Pure(f(s)));

  static Pure = v => new State(s => ({ value: v, state: s }));
}

function randVal() {
  return Math.floor(Math.random() * 1000 - 500);
}

const a = randVal();
const b = randVal();
const c = randVal();

const f = x => State.Pure(x + b);
const g = x => State.Pure(x + c);

const m = State.Pure(a);

console.log(
  `Left identity: ${State.Pure(a)
    .Bind(f)
    .EvalState(0) === f(a).EvalState(0)}`
);
console.log(
  `Right identity ${m.Bind(State.Pure).EvalState(0) === m.EvalState(0)}`
);
console.log(
  `Associativity ${m
    .Bind(f)
    .Bind(g)
    .EvalState(0) === m.Bind(x => f(x).Bind(g)).EvalState(0)}`
);