1(**************************************************************************)
2(*                                                                        *)
3(*                                 OCaml                                  *)
4(*                                                                        *)
5(*             Damien Doligez, projet Para, INRIA Rocquencourt            *)
6(*                                                                        *)
7(*   Copyright 1997 Institut National de Recherche en Informatique et     *)
8(*     en Automatique.                                                    *)
9(*                                                                        *)
10(*   All rights reserved.  This file is distributed under the terms of    *)
11(*   the GNU Lesser General Public License version 2.1, with the          *)
12(*   special exception on linking described in the file LICENSE.          *)
13(*                                                                        *)
14(**************************************************************************)
15
16(* Module [Lazy]: deferred computations *)
17
18
19(*
20   WARNING: some purple magic is going on here.  Do not take this file
21   as an example of how to program in OCaml.
22*)
23
24
25(* We make use of two special tags provided by the runtime:
26   [lazy_tag] and [forward_tag].
27
28   A value of type ['a Lazy.t] can be one of three things:
29   1. A block of size 1 with tag [lazy_tag].  Its field is a closure of
30      type [unit -> 'a] that computes the value.
31   2. A block of size 1 with tag [forward_tag].  Its field is the value
32      of type ['a] that was computed.
33   3. Anything else except a float.  This has type ['a] and is the value
34      that was computed.
35   Exceptions are stored in format (1).
36   The GC will magically change things from (2) to (3) according to its
37   fancy.
38
39   We cannot use representation (3) for a [float Lazy.t] because
40   [caml_make_array] assumes that only a [float] value can have tag
41   [Double_tag].
42
43   We have to use the built-in type constructor [lazy_t] to
44   let the compiler implement the special typing and compilation
45   rules for the [lazy] keyword.
46*)
47
48type 'a t = 'a lazy_t
49
50exception Undefined = CamlinternalLazy.Undefined
51
52external make_forward : 'a -> 'a lazy_t = "caml_lazy_make_forward"
53
54external force : 'a t -> 'a = "%lazy_force"
55
56(* let force = force *)
57
58let force_val = CamlinternalLazy.force_val
59
60let from_fun (f : unit -> 'arg) =
61  let x = Obj.new_block Obj.lazy_tag 1 in
62  Obj.set_field x 0 (Obj.repr f);
63  (Obj.obj x : 'arg t)
64
65
66let from_val (v : 'arg) =
67  let t = Obj.tag (Obj.repr v) in
68  if t = Obj.forward_tag || t = Obj.lazy_tag || t = Obj.double_tag then begin
69    make_forward v
70  end else begin
71    (Obj.magic v : 'arg t)
72  end
73
74
75let is_val (l : 'arg t) = Obj.tag (Obj.repr l) <> Obj.lazy_tag
76
77let lazy_from_fun = from_fun
78
79let lazy_from_val = from_val
80
81let lazy_is_val = is_val
82