open Effect
open Effect.Deep

type _ Effect.t += Yield : unit t

type status =
    | Complete
    | Suspended of (unit, status) continuation

let start (f : unit -> unit) : status =
    match f () with
        | () ->
            Complete
        | effect Yield, cont ->
            Suspended cont

let resume co =
    match co with
        | Complete ->
            Invalid_argument "Can't resume completed quotation" |> raise
        | Suspended k ->
            continue k ()

let yield () = perform Yield

let task_a () =
    "X" |> print_endline |> yield ;
    "Y" |> print_endline |> yield ;
    "Z" |> print_endline

let task_b () =
    "A" |> print_endline |> yield ;
    let co = start task_a in
    "B" |> print_endline |> yield ;
    let co = resume co in
    "C" |> print_endline |> yield ;
    resume co |> ignore

let () =
    1 |> print_int |> print_newline ;
    let co = start task_b in
    2 |> print_int |> print_newline ;
    let co = resume co in
    3 |> print_int |> print_newline ;
    let co = resume co in
    4 |> print_int |> print_newline ;
    resume co |> ignore