1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
|
(***********************************************************************)
(* *)
(* OCaml *)
(* *)
(* Damien Doligez, projet Para, INRIA Rocquencourt *)
(* *)
(* Copyright 1997 Institut National de Recherche en Informatique et *)
(* en Automatique. All rights reserved. This file is distributed *)
(* under the terms of the GNU Library General Public License, with *)
(* the special exception on linking described in file ../LICENSE. *)
(* *)
(***********************************************************************)
(* $Id$ *)
(** Deferred computations. *)
type 'a t = 'a lazy_t;;
(** A value of type ['a Lazy.t] is a deferred computation, called
a suspension, that has a result of type ['a]. The special
expression syntax [lazy (expr)] makes a suspension of the
computation of [expr], without computing [expr] itself yet.
"Forcing" the suspension will then compute [expr] and return its
result.
Note: [lazy_t] is the built-in type constructor used by the compiler
for the [lazy] keyword. You should not use it directly. Always use
[Lazy.t] instead.
Note: [Lazy.force] is not thread-safe. If you use this module in
a multi-threaded program, you will need to add some locks.
Note: if the program is compiled with the [-rectypes] option,
ill-founded recursive definitions of the form [let rec x = lazy x]
or [let rec x = lazy(lazy(...(lazy x)))] are accepted by the type-checker
and lead, when forced, to ill-formed values that trigger infinite
loops in the garbage collector and other parts of the run-time system.
Without the [-rectypes] option, such ill-founded recursive definitions
are rejected by the type-checker.
*)
exception Undefined;;
external force : 'a t -> 'a = "%lazy_force";;
(* val force : 'a t -> 'a ;; *)
(** [force x] forces the suspension [x] and returns its result.
If [x] has already been forced, [Lazy.force x] returns the
same value again without recomputing it. If it raised an exception,
the same exception is raised again.
Raise [Undefined] if the forcing of [x] tries to force [x] itself
recursively.
*)
val force_val : 'a t -> 'a;;
(** [force_val x] forces the suspension [x] and returns its
result. If [x] has already been forced, [force_val x]
returns the same value again without recomputing it.
Raise [Undefined] if the forcing of [x] tries to force [x] itself
recursively.
If the computation of [x] raises an exception, it is unspecified
whether [force_val x] raises the same exception or [Undefined].
*)
val lazy_from_fun : (unit -> 'a) -> 'a t;;
(** [lazy_from_fun f] is the same as [lazy (f ())] but slightly more
efficient. *)
val lazy_from_val : 'a -> 'a t;;
(** [lazy_from_val v] returns an already-forced suspension of [v]
This is for special purposes only and should not be confused with
[lazy (v)]. *)
val lazy_is_val : 'a t -> bool;;
(** [lazy_is_val x] returns [true] if [x] has already been forced and
did not raise an exception. *)
|
