src/Pure/Concurrent/par_list.ML

subject to Multithreading.enabled;
raw_map: join sequentially, less overhead;

(*  Title:      Pure/Concurrent/par_list.ML
    ID:         $Id$
    Author:     Makarius

Parallel list combinators.

Notes:

  * These combinators only make sense if the operator (function or
    predicate) applied to the list of operands takes considerable
    time.  The overhead of scheduling is significantly higher than
    just traversing the list of operands sequentially.

  * The order of operator application is non-deterministic.  Watch out
    for operators that have side-effects or raise exceptions!
*)

signature PAR_LIST =
sig
  val map: ('a -> 'b) -> 'a list -> 'b list
  val get_some: ('a -> 'b option) -> 'a list -> 'b option
  val find_some: ('a -> bool) -> 'a list -> 'a option
  val exists: ('a -> bool) -> 'a list -> bool
  val forall: ('a -> bool) -> 'a list -> bool
end;

structure ParList: PAR_LIST =
struct

fun raw_map f xs =
  if ! future_scheduler andalso Multithreading.enabled () then
    let
      val group = TaskQueue.new_group ();
      val futures = map (fn x => Future.future (SOME group) [] true (fn () => f x)) xs;
      val _ = List.app (ignore o Future.join_results o single) futures;
    in Future.join_results futures end
  else map (Exn.capture f) xs;

fun map f xs = Exn.release_first (raw_map f xs);

fun get_some f xs =
  let
    exception FOUND of 'b option;
    fun found (Exn.Exn (FOUND some)) = some
      | found _ = NONE;
    val results = raw_map (fn x => (case f x of NONE => () | some => raise FOUND some)) xs;
  in
    (case get_first found results of
      SOME y => SOME y
    | NONE => (Exn.release_first results; NONE))
  end;

fun find_some P = get_some (fn x => if P x then SOME x else NONE);

fun exists P = is_some o get_some (fn x => if P x then SOME () else NONE);
fun forall P = not o exists (not o P);

end;