multi shot continuations

multishot-callcc.js

function isGenerator(x) {
  return x != null && typeof x.next === "function"
}

function isFrame(x) {
  return x != null && x._type_ === 'call_frame'
}

function call(genFunc, ...args) {
  return { _type_: 'call_frame', genFunc, args, history: [] }
}

function runFrame(frame, arg, next) {
  const {genFunc, args, history} = frame  
  // rebuilds the state of the Generator
  const gen = genFunc(...args)
  history.forEach(result => gen.next(result))

  const {done, value} = gen.next(arg)
  

  if(done) {
    return next(value)
  }

  const newFrame = {...frame, history: [...history, arg]}

  if(isFrame(value)) {
    runFrame(value, null, function continuation(result) {
      runFrame(newFrame, result, next)
    }) 
  }
  else if(typeof value === "function") {
    value(function continuation(result) {
      runFrame(newFrame, result, next)
    })
  }
  else {
    runFrame(newFrame, value, next)
  }
}

function start(genFunc, next) {
  runFrame(call(genFunc), null, next)
}

function callcc(genFunc) {
  return function (capturedCont) {
    function jump(value) {
      return next => capturedCont(value)
    }
    runFrame(
      call(genFunc, jump), 
      null,
      capturedCont
    )
  }
}

let resume

function* main() {
  const a = 10
  const b = yield callcc(function*(k) {
    resume = k
    yield k(10)
  })
  return a + b
}

runFrame(call(main), null, console.log)
// => 20

resume(30)()
// => 40

resume(50)()
// => 60