batReturn.mli

(*
 * Return -- fast return in OCaml
 * Copyright (C) 2008 David Teller
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version,
 * with the special exception on linking described in file LICENSE.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 *)

(**
   Local exceptions/labels/goto/return.

   This module defines a mechanism akin to SML's exception generators
   or to a generalization of C's [return], i.e. the ability to define
   local {i labels}, which may be used for immediately terminating an
   expression and returning a value. By opposition to usual OCaml
   exceptions, this mechanism
   - allows polymorphic return values
   - makes accidental exception catching slightly harder (while a local
   exception can escape its scope, it cannot be caught again by accident
   from this module).

   Example:
   {[
     let find_in_array a e =
       label (fun label ->
         for i = 0 to Array.length a - 1 do
           if Array.get a i = e then return label (Some i)
         done;
         None)
   ]}

   @author David Teller

   @documents Return
*)

type 'a t
(** A label which may be used to return values of type ['a]*)

val label : ('a t -> 'a) -> 'a
(** [label f] creates a new label [x] and invokes
    [f x]. If, during the execution of [f], [return x v]
    is invoked, the execution of [f x] stops
    immediately and [label f] returns [v].
    Otherwise, if [f x] terminates normally and
    returns [y], [label f] returns [y].

    Calling [return x v] from outside scope [f]
    is a run-time error and causes termination
    of the program.*)
val with_label  : ('a t -> 'a) -> 'a
(**as [label]*)

val return : 'a t -> 'a -> _
  (** Return to a label. [return l v] returns
      to the point where label [l] was obtained
      and produces value [l].

      Calling [return l v] from outside the scope
      of [l] (i.e. the call to function [label]
      which produced [l]) is a run-time error
      and causes termination of the program.*)