// The single impure place in the course (with browser.js and main.js): everything else only builds values.
// Here we interpret IO descriptions against a Platform and hold the one mutable variable, the current stream.
// Platform = { fillRect, fillText, randomSeed, startClock, onKeyDown, onKeyUp }

// Perform one op on the platform and return its result.
const perform = (op, platform) => {
  if (op.tag === 'RandomSeed') return platform.randomSeed()
  const s = op.shape
  return s.tag === 'Rect'
    ? platform.fillRect(s.x, s.y, s.w, s.h, s.color)
    : platform.fillText(s.x, s.y, s.text, s.color, s.size, s.align)
}

// A loop, not recursion: a scene is hundreds of ops and the stack must not grow with them.
export const runIO = (io, platform) => {
  let current = io
  while (current.tag === 'Effect') current = current.k(perform(current.op, platform))
  return current.value
}

export const runApp = (main, platform) => {
  let stream = runIO(main, platform)
  const feed = (input) => {
    const [ios, next] = stream.step(input)
    stream = next
    ios.forEach((io) => runIO(io, platform))
  }
  platform.onKeyDown((key) => feed({ tag: 'KeyDown', key }))
  platform.onKeyUp((key) => feed({ tag: 'KeyUp', key }))
  platform.startClock((t) => feed({ tag: 'Time', t }))
}