scene_control.ml

type id = int

type scene = {
  id: id;
  content: unit -> unit
}

effect Resume : scene -> unit
effect Switch : unit

let control th =
  let h th =
    match th () with
    | _ -> fun () -> ()
    | effect Switch k ->
        fun () -> ignore @@ continue k ()
  in
  let scenes : (id, unit -> unit) Hashtbl.t = Hashtbl.create 10 in
  match th () with
  | x -> x
  | effect (Resume {id; content}) k ->
    let () =
    match Hashtbl.(find_opt scenes id) with
    | None ->
      let rest = h content in
      Hashtbl.(add scenes id rest)
    | Some cache ->
      let rest = h cache in
      Hashtbl.(replace scenes id rest)
    in
    continue k ()

let scene1 () =
  print_endline "Hello";
  perform Switch;
  print_endline "bye"

let scene2 () =
  print_endline "spawn"

let shash content =
  let id = Random.int 1000000 in
  {id; content}

let resume c = perform (Resume (shash c))

let main () =
  control @@ fun () ->
  resume scene1;
  resume scene2;
  resume scene1

scene_log_handle.lua

local eff = require('eff')
local inst, perform, handlers = eff.inst, eff.perform, eff.handlers

local Switch = inst()
local Resume = inst()

local Log = inst()

local scene1 = function()
  perform(Log("hello"))
  perform(Switch())
  print("bye")
end

local scene2 = function()
  print("spawn")
end

local h = handlers{
  function(x) return x end,
  [Switch] = function(k)
    return k
  end
}

local control = function(f)
  local scenes = {}

  return handlers{
    function(x) return x end,
    [Resume] = function(k, scene)
      local cache = scenes[scene]

      if not cache then
        local rest = h(scene)
        scenes[scene] = rest
      else
        local rest = cache()
        scenes[scene] = rest
      end

      return k()
    end
  }(f)
end

local log_collector = handlers {
  function(x) return x end,
  [Log] = function(k, msg)
    io.stderr:write("\27[31;1;4m" .. msg .. "\27[0m\n")
    return k()
  end
}

log_collector(function()
  control(function()
    local resume = function(x) return perform(Resume(x)) end

    resume(scene1)
    resume(scene2)
    resume(scene1)
  end)
end)

local scene1 = coroutine.create(function()
  print("hello")
  coroutine.yield()
  print("world")
end)

local scene2 = coroutine.create(function()
  print("spawn")
end)

do
  coroutine.resume(scene1)
  coroutine.resume(scene2)
  coroutine.resume(scene1)
end