63 lines
1.3 KiB
Standard ML
63 lines
1.3 KiB
Standard ML
(*
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* Copyright 2020, Data61, CSIRO (ABN 41 687 119 230)
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*
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* SPDX-License-Identifier: BSD-2-Clause
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*)
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(*
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* Title: Pure/Concurrent/lazy_sequential.ML
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* Author: Florian Haftmann and Makarius, TU Muenchen
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* David Greenaway, NICTA
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*
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* Thread-safe lazy evaluation with memoing, without using futures.
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*)
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structure SimpleLazy: LAZY =
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struct
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(* datatype *)
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datatype 'a expr =
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Expr of unit -> 'a |
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Result of 'a Exn.result;
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abstype 'a lazy = Lazy of 'a expr Synchronized.var
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with
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fun peek (Lazy r) =
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(case (Synchronized.value r) of
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Expr _ => NONE
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| Result res => SOME res);
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fun lazy e = Lazy (Synchronized.var "simple_lazy" (Expr e));
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fun value a = Lazy (Synchronized.var "simple_lazy" (Result (Exn.Res a)));
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(* force result *)
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fun force_result (Lazy r) =
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Synchronized.change_result r (fn x =>
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(case x of
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Expr e =>
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let
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val x = Exn.capture e ()
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in
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(x, Result x)
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end
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| Result res => (res, Result res)))
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fun force r = Exn.release (force_result r);
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fun is_finished x = is_some (peek x)
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fun map f x = lazy (fn () => f (force x));
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fun future params x =
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if is_finished x then Future.value_result (force_result x)
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else (singleton o Future.forks) params (fn () => force x);
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end;
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end;
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type 'a simple_lazy = 'a SimpleLazy.lazy;
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