redux'ish thing with generators and adhoc handlers

flux.js

const createStore = (state) => {
  const noop_handler = state => state
  state = state || {}
  var handlers = {}
  var subs = []
  
  const store = (function * () {
    while (true) {
      const { type, payload } = yield null
      state = (handlers[type] || noop_handler)(state, ...payload)
      subs.forEach(fn => fn(state))
    }
  })()
  
  const dispatch = (type, ...payload) =>
    (store.next({ type, payload }), [type, ...payload])

  const getState = () => state
  const handler = (type, fn) => handlers[type] = fn
  const subscribe = fn => {
    subs.push(fn)
    fn(state) // emit after we register so subscriptions are primed
    return () => subs.splice(subs.indexOf(fn), 1)
  }

  dispatch('@@init|0') // force past yield null so we can pass to next
  dispatch('@@init|1') // initialize state
  
  return {
    dispatch,
    getState,
    handler,
    subscribe,
  }
}

const { dispatch, getState, handler, subscribe } = createStore({ foo:"bar" })
subscribe(d => console.log(d)) // will emit right away
handler('a', $$$ => ({ ...$$$, foo:"baz" }))
handler('b', $$$ => ({ ...$$$, foo:"bar" }))
dispatch("a")
dispatch("b")
dispatch("c") // does nothing
handler('c', $$$ => ({ ...$$$, foo:"omg" }))
dispatch("c") // does something