Converting Coroutine to State monads

state.js

function* incr() {
  return (yield set((yield get) + 1))
}

function* incrX2() {
  return (yield* incr()) + (yield* incr())
}

const main = state(incrX2)

// framework
function state(iter) {
  const i = iter()[Symbol.iterator]()
  return walk()

  function walk(arg) {
    const step = i.next(arg)
    return step.done ?
      state => [state, step.value] :
      state => {
        const [next, value] = step.value(state)
        return walk(value)(next)
      }
  }
}

function set(s) { return _ => [s, s] }
function get(s) { return [s, s] }