a0.nim

import deques
type
  AsyncQueue[Input, Output] = ref object
    list: Deque[Input]
    futures: Deque[Future[Output]]
    running: bool
    handler: proc(input: Input): Future[Output]

proc initAsyncQueue[Input, Output](handler: proc(input: Input): Future[Output]): AsyncQueue[Input, Output] =
  AsyncQueue[Input, Output](list: initDeque[Input](), futures: initDeque[Future[Output]](), running: false, handler: handler)

proc run[Input, Output](queue: AsyncQueue[Input, Output]): Future[void] {.async.} =
  queue.running = true
  while queue.list.len > 0:
    var next = queue.list.popLast
    var future = queue.futures.popLast
    # echo "next ", $next
    future.complete(await queue.handler(next))
  queue.running = false

proc pushOrSend[Input, Output](queue: AsyncQueue[Input, Output], input: Input): Future[Output] {.async.} =
  var future = newFuture[Output]("pushOrSend " & $input)
  # echo "pushOrSend " & $input
  queue.list.addFirst(input)
  queue.futures.addFirst(future)
  if not queue.running:
    await queue.run()
  return await future