Algebraic Effects for the Rest of Us

Re:Algebraic Effects for the Rest of Us by sweetsocks21 on Saturday May 30, 2026 @07:43PM (#48341683)

I think the parent may be getting at the continuation aspect of effects? Effect systems make the stack a first class object you can reuse, I think a standard example is implementing a scheduler. I'm not familiar with your Bluefin library so maybe it already handles this:

  effect Sched =
    yield : unit -> unit
    fork  : (unit -> unit) -> unit
  end
  
  let mut run_queue = []
  let enqueue t = run_queue := List.concat run_queue [t]
  
  let dequeue () =
    match run_queue with
    | [] -> ()
    | t :: rest ->
      run_queue := rest;
      t ()

  let rec spawn task =
    handle
      task ()
    with
    | return _ -> dequeue ()
    | yield () k ->
      enqueue (fn () -> resume k ());
      dequeue ()
    | fork f k ->
      enqueue (fn () -> resume k ());
      spawn f

  let run main = spawn main

  let worker name steps =
    let rec loop i =
      if i > steps do ()
      else do
        print $"{name}: step {i}";
        perform yield ();
        loop (i + 1)
      end
    in
    loop 1
  
  let () =
    run (fn () ->
      print "main: starting";
      perform fork (fn () -> worker "A" 3);
      perform fork (fn () -> worker "B" 3);
      print "main: forked workers, now yielding";
      perform yield ();
      print "main: done")

output:

  main: starting
  A: step 1
  B: step 1
  A: step 2
  main: forked workers, now yielding
  B: step 2
  A: step 3
  main: done
  B: step 3

[ Parent | on HN ]

Re:Algebraic Effects for the Rest of Us by tome on Sunday May 31, 2026 @07:05AM (#48344760)

Ah yes, OK, I missed the point that the timeout is applied to the entire continuation, not just the part of the computation until the next await. Bluefin can't currently do that. I think I could make it do that, using the same implementation strategy as awaitYield (fork a thread, communicate through an MVar) but I wonder what the point is, given that Bluefin allows you to run the continuation at most once. Is the use case of "run the continuation in a modified environment (e.g. with a timeout)" really that compelling? Maybe it is! But I don't see it yet.

On the other hand, I don't see any difficulty with implementing a scheduler using Await/Yield. I don't think it needs access to the full continuation.

[ Parent | on HN ]