94 lines
3.3 KiB
OCaml
94 lines
3.3 KiB
OCaml
(***********************************************************************)
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(* *)
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(* Objective Caml *)
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(* *)
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(* Damien Doligez, projet Para, INRIA Rocquencourt *)
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(* *)
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(* Copyright 1997 Institut National de Recherche en Informatique et *)
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(* en Automatique. All rights reserved. This file is distributed *)
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(* under the terms of the GNU Library General Public License, with *)
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(* the special exception on linking described in file ../LICENSE. *)
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(* *)
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(***********************************************************************)
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(* $Id$ *)
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(* Module [Lazy]: deferred computations *)
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(*
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WARNING: some purple magic is going on here. Do not take this file
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as an example of how to program in Objective Caml.
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*)
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(* We make use of two special tags provided by the runtime:
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[lazy_tag] and [forward_tag].
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A value of type ['a Lazy.t] can be one of three things:
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1. A block of size 1 with tag [lazy_tag]. Its field is a closure of
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type [unit -> 'a] that computes the value.
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2. A block of size 1 with tag [forward_tag]. Its field is the value
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of type ['a] that was computed.
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3. Anything else. This has type ['a] and is the value that was computed.
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Exceptions are stored in format (1).
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The GC will magically change things from (2) to (3) according to its
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fancy.
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We have to use the built-in type constructor [lazy_t] to
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let the compiler implement the special typing and compilation
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rules for the [lazy] keyword.
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*)
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type 'a t = 'a lazy_t;;
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exception Undefined;;
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let raise_undefined = Obj.repr (fun () -> raise Undefined);;
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let force (l : 'arg t) =
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let x = Obj.repr l in
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if Obj.is_int x then (Obj.obj x : 'arg)
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else if Obj.tag x = Obj.forward_tag then (Obj.obj (Obj.field x 0) : 'arg)
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else if Obj.tag x <> Obj.lazy_tag then (Obj.obj x : 'arg)
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else begin
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let closure = (Obj.obj (Obj.field x 0) : unit -> 'arg) in
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Obj.set_field x 0 raise_undefined;
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try
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let result = closure () in
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Obj.set_field x 0 (Obj.repr result); (* do set_field BEFORE set_tag *)
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Obj.set_tag x (Obj.forward_tag);
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result
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with e ->
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Obj.set_field x 0 (Obj.repr (fun () -> raise e));
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raise e;
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end
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;;
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let force_val (l : 'arg t) =
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let x = Obj.repr l in
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if Obj.is_int x then (Obj.obj x : 'arg)
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else if Obj.tag x = Obj.forward_tag then (Obj.obj (Obj.field x 0) : 'arg)
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else if Obj.tag x <> Obj.lazy_tag then (Obj.obj x : 'arg)
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else begin
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let closure = (Obj.obj (Obj.field x 0) : unit -> 'arg) in
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Obj.set_field x 0 raise_undefined;
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let result = closure () in
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Obj.set_field x 0 (Obj.repr result); (* do set_field BEFORE set_tag *)
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Obj.set_tag x (Obj.forward_tag);
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result
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end
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;;
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let lazy_from_fun (f : unit -> 'arg) =
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let x = Obj.new_block Obj.lazy_tag 1 in
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Obj.set_field x 0 (Obj.repr f);
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(Obj.obj x : 'arg t)
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;;
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let lazy_from_val (v : 'arg) = (Obj.magic v : 'arg t);;
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let lazy_is_val (l : 'arg t) =
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let x = Obj.repr l in
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Obj.is_int x || Obj.tag x <> Obj.lazy_tag
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;;
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