State monad in JS

State.js

const composeK = (...fns) =>
  fns.reduce((f, g) => (...args) => g(...args).chain(f));

const liftA2 = (f, m1, m2) => m1.map(f).ap(m2);

const K = a => b => a;

const State = computation => {
  const map = f =>
    State(state => {
      const prev = computation(state);
      return { value: f(prev.value), state: prev.state };
    });

  const ap = other =>
    State(state => {
      const prev = computation(state);
      const fn = prev.value;
      return other.map(fn).runWith(prev.state);
    });

  const chain = fn =>
    State(state => {
      const prev = computation(state);
      const next = fn(prev.value);
      return next.runWith(prev.state);
    });

  const runWith = computation;

  const evalWith = initState => computation(initState).value;

  const execWith = initState => computation(initState).state;

  return Object.freeze({
    map,
    ap,
    chain,
    evalWith,
    runWith,
    execWith
  });
};

const modify = f => State(state => ({ value: undefined, state: f(state) }));

State.get = (f = x => x) => State(state => ({ value: f(state), state }));

State.modify = modify;

State.put = state => modify(K(state));

State.of = value => State(state => ({ value, state }));