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92 version_store

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% vim: ft=mercury ts=4 sw=4 et wm=0 tw=0
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% Copyright (C) 2004-2006, 2011 The University of Melbourne.
% This file may only be copied under the terms of the GNU Library General
% Public License - see the file COPYING.LIB in the Mercury distribution.
%--------------------------------------------------%
% 
% File: version_store.m.
% Author: Ralph Becket <rafe@cs.mu.oz.au>
% Stability: low.
% 
% (See the header comments in version_array.m for an explanation of version
% types.)
%
% A version_store is similar to, albeit slightly slower than, an ordinary
% store, but does not depend upon uniqueness.
%
% Note that, unlike ordinary stores, liveness of data is via the version store
% rather than the mutvars.  This means that dead data (i.e.  whose mutvar is
% out of scope) in a version_store may not be garbage collected.
% 
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:- module version_store.
:- interface.

%--------------------------------------------------%

:- type version_store(S).

:- type mutvar(T, S).

    % Construct a new version store.  This is distinguished from other
    % version stores by its existentially quantified type.  This means
    % the compiler can automatically detect any attempt to use a
    % mutvar with the wrong version store.
    %
:- some [S] func init = version_store(S).

    % new_mutvar(X, Mutvar, VS0, VS) adds a new mutvar with value reference X
    % to the version store.
    %
:- pred new_mutvar(T::in, mutvar(T, S)::out,
    version_store(S)::in, version_store(S)::out) is det.

    % new_cyclic_mutvar(F, Mutvar, VS0, VS) adds a new mutvar with value
    % reference F(Mutvar) to the version store.  This can be used to
    % construct cyclic terms.
    %
:- pred new_cyclic_mutvar((func(mutvar(T, S)) = T)::in, mutvar(T, S)::out,
    version_store(S)::in, version_store(S)::out) is det.

    % copy_mutvar(Mutvar, NewMutvar, VS0, VS) constructs NewMutvar
    % with the same value reference as Mutvar.
    %
:- pred copy_mutvar(mutvar(T, S)::in, mutvar(T, S)::out,
    version_store(S)::in, version_store(S)::out) is det.

    % VS ^ elem(Mutvar) returns the element referenced by Mutvar in
    % the version store.
    %
:- func version_store(S) ^ elem(mutvar(T, S)) = T.

    % lookup(VS, Mutvar) = VS ^ elem(Mutvar).
    %
    % A predicate version is also provided.
    %
:- func lookup(version_store(S), mutvar(T, S)) = T.
:- pred get_mutvar(mutvar(T, S)::in, T::out,
    version_store(S)::in, version_store(S)::out) is det.

    % ( VS ^ elem(Mutvar) := X ) updates the version store so that
    % Mutvar now refers to value X.
    %
:- func ( version_store(S) ^ elem(mutvar(T, S)) := T ) = version_store(S).

    % set(VS, Mutvar, X) = ( VS ^ elem(Mutvar) := X ).
    %
    % A predicate version is also provided.
    %
:- func set(version_store(S), mutvar(T, S), T) = version_store(S).
:- pred set_mutvar(mutvar(T, S)::in, T::in,
    version_store(S)::in, version_store(S)::out) is det.

    % unsafe_rewind(VS) produces a version of VS for which all accesses are
    % O(1).  Invoking this predicate renders undefined VS and all later
    % versions undefined that were derived by performing individual updates.
    % Only use this when you are absolutely certain there are no live
    % references to VS or later versions of VS.
    %
    % A predicate version is also provided.
    %
:- func unsafe_rewind(version_store(T)) = version_store(T).
:- pred unsafe_rewind(version_store(T)::in, version_store(T)::out) is det.

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