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1 | (* Copyright (C) 2017 Matthew Fluet. |
2 | * Copyright (C) 1999-2005 Henry Cejtin, Matthew Fluet, Suresh | |
3 | * Jagannathan, and Stephen Weeks. | |
4 | * | |
5 | * MLton is released under a BSD-style license. | |
6 | * See the file MLton-LICENSE for details. | |
7 | *) | |
8 | ||
9 | structure Promise: PROMISE = | |
10 | struct | |
11 | ||
12 | datatype 'a t = T of 'a state ref | |
13 | and 'a state = | |
14 | Unevaluated of unit -> 'a | |
15 | | Evaluating | |
16 | | Evaluated of 'a | |
17 | ||
18 | fun layout l (T r) = | |
19 | let | |
20 | open Layout | |
21 | in | |
22 | case !r of | |
23 | Unevaluated _ => str "Unevaluated" | |
24 | | Evaluating => str "Evaluating" | |
25 | | Evaluated x => seq [str "Evaluated ", l x] | |
26 | end | |
27 | ||
28 | fun delay th = T (ref (Unevaluated th)) | |
29 | ||
30 | fun reset (T r, th) = | |
31 | case !r of | |
32 | Evaluating => Error.bug "Promise.reset" | |
33 | | _ => r := Unevaluated th | |
34 | ||
35 | exception Force | |
36 | fun force (T r) = | |
37 | case !r of | |
38 | Evaluated x => x | |
39 | | Unevaluated th => | |
40 | (let | |
41 | val _ = r := Evaluating | |
42 | val x = th () | |
43 | val _ = r := Evaluated x | |
44 | in | |
45 | x | |
46 | end handle exn => (r := Unevaluated th; raise exn)) | |
47 | | Evaluating => raise Force | |
48 | ||
49 | fun lazy th = | |
50 | let val p = delay th | |
51 | in fn () => force p | |
52 | end | |
53 | ||
54 | fun isUnevaluated (T r) = | |
55 | case !r of | |
56 | Unevaluated _ => true | |
57 | | _ => false | |
58 | ||
59 | end |