// m07 io: effects as values. An IO is a plain-data description of a program; nothing runs until interpreted.
//   {tag:'Pure', value}      - done, with a value
//   {tag:'Effect', op, k}    - perform `op`, then continue with `k(result)`

export const pure = (value) => ({ tag: 'Pure', value })

export const effect = (op) => ({ tag: 'Effect', op, k: pure })

// Pure: apply f now. Effect: keep the op, and extend the continuation so f runs after k.
// Nothing is performed here, we only build a bigger description.
export const flatMap = (io, f) =>
  io.tag === 'Pure'
    ? f(io.value)
    : { tag: 'Effect', op: io.op, k: (result) => flatMap(io.k(result), f) }

export const map = (io, f) => flatMap(io, (a) => pure(f(a)))

export const sequence = (ios) =>
  ios.reduceRight((rest, io) => flatMap(io, () => rest), pure(undefined))

export const draw = (shape) => effect({ tag: 'Draw', shape })

export const randomSeed = effect({ tag: 'RandomSeed' })

export const drawScene = (scene) => sequence(scene.map(draw))

// Answer each op with a canned result, collecting the ops as we go (recursion, no mutation).
const answer = (op, seed) => (op.tag === 'RandomSeed' ? seed : undefined)

export const trace = (io, seed) => {
  if (io.tag === 'Pure') return { result: io.value, ops: [] }
  const rest = trace(io.k(answer(io.op, seed)), seed)
  return { result: rest.result, ops: [io.op, ...rest.ops] }
}