open Printf

exception Goto of string

let label (name: string) = ()
let goto (name: string) = raise (Goto name)

let goto_block (blocks: (string * (unit -> unit)) list): unit =
  let rec goto_block_impl (name: string option): unit =
    try
      let exec (blocks: (string * (unit -> unit)) list): unit =
        blocks |> List.iter (fun (_, block) -> block ())
      in
      let rec skip (blocks: (string * (unit -> unit)) list) (entry: string) =
        match blocks with
        | [] -> ()
        | (name, _) :: rest ->
           if String.equal name entry
           then exec blocks
           else skip rest entry
      in
      match name with
      | None -> exec blocks
      | (Some entry) -> skip blocks entry
    with
      Goto name -> goto_block_impl (Some name)
  in goto_block_impl None

let () =
  let i = ref 0 in
  goto_block
    [("loop", (fun () ->
        if !i >= 10 then goto "out" else ();
        printf "%d: Hello, World\n" !i;
        i := !i + 1;
        goto "loop"));
     ("out", (fun () ->
        printf "Done!\n"))]