An example of deferred execution with thunks and hashtables (OCaml)

thunksandhashes.ml

module Logs = struct

let hash : (string, string) Hashtbl.t = Hashtbl.create 256

  let show_all = fun () ->
    Printf.printf "- - - HASH CONTENTS:\n%!";
    Hashtbl.iter (fun key value -> Printf.printf "- - - '%s': '%s'\n%!" key value) hash

  let register_msg key value =
    Printf.printf "- Registering '%s': '%s'\n%!" key value;
    Hashtbl.add hash key value

  let close key =
    Printf.printf "- Closing: '%s'\n%!" key;
    let value = Hashtbl.find hash key in
    Printf.printf "- - Closed: '%s'\n%!" value

  let close_all = fun () ->
    Printf.printf "- Closing all.\n%!";
    Hashtbl.iter (fun key _ -> close key) hash

end

let show s = Printf.printf "Showing: '%s'\n%!" s

let main () =
  at_exit Logs.close_all;
  Logs.show_all ();

  Printf.printf "Starting\n%!";
  Logs.register_msg "foo" "bar";
  Logs.register_msg "biz" "baz";

  Unix.sleep 1;

  show "A message.";
  show "Another message.";

  Logs.show_all ()

let () = main ()