state.js

/**
 * Model stateful computations (those that return a result, but also
 * have the side effect of altering some shared context).
 * Each stateful computation, is a function that takes a current
 * state, and returns two values, the result, `r`, and the new shared
 * state `s'`.
 *
 * In other words, a function `s => [r, s']`
 *
 * The State monad allows you to compose these effectful computations
 * into a starting state that allows you to pipeline values.
 */
class State {
  static of(x) {
    return new State(s => [x,s]);
  }

  constructor(compute) {
    this.compute = compute;
  }

  flatMap(fn) {
    let { compute } = this;
    return new State(s => {
      let [ result, newState ] = compute(s);
      let computation = fn(result);
      return computation.compute(newState);
    });
  }

  set(value) {
    return new State(() => [null, value]);
  }
}

export default function Constructor(compute) {
  return new State(compute);
}
Constructor.of = State.of;