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% vim: ft=mercury ts=4 sw=4 et
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% Copyright (C) 2000-2003, 2006-2007, 2011 The University of Melbourne.
% Copyright (C) 2014, 2016, 2018 The Mercury Team.
% This file is distributed under the terms specified in COPYING.LIB.
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%
% File: thread.mvar.m.
% Main author: petdr, fjh.
% Stability: low.
%
% This module provides a Mercury version of Haskell mutable variables.  A
% mutable variable (mvar) is a reference to a mutable location which can
% either contain a value of type T or be empty.
%
% Access to a mvar is thread-safe and can be used to synchronize between
% different threads.
%
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:- module thread.mvar.
:- interface.

:- import_module bool.
:- import_module io.
:- import_module maybe.

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:- type mvar(T).

    % Create an empty mvar.
    %
:- pred init(mvar(T)::out, io::di, io::uo) is det.

    % Create an mvar with the given initial value.
    %
:- pred init(T::in, mvar(T)::out, io::di, io::uo) is det.

    % Create an empty mvar.
    %
:- impure func impure_init = (mvar(T)::uo) is det.

    % Create an mvar with the given initial value.
    %
:- impure func impure_init(T) = mvar(T).

    % Take the contents of the mvar out, leaving the mvar empty.
    % If the mvar is empty, block until some thread fills the mvar.
    %
:- pred take(mvar(T)::in, T::out, io::di, io::uo) is det.

    % Take the contents of the mvar out, leaving the mvar empty.
    % Returns `yes(X)' if the mvar contained X, or `no' if the operation
    % would block.
    %
    % WARNING: a return value of `no' does not necessarily mean the mvar
    % is or was empty. For example, another thread attempting to read or take
    % an item out of the mvar may also cause `try_take' to return `no'.
    %
:- pred try_take(mvar(T)::in, maybe(T)::out, io::di, io::uo) is det.

    % Place the value of type T into an empty mvar.
    % If the mvar is full then block until it becomes empty.
    %
:- pred put(mvar(T)::in, T::in, io::di, io::uo) is det.

    % Place the value of type T into an empty mvar, returning yes on success.
    % If the mvar is full then return no immediately without blocking.
    %
:- pred try_put(mvar(T)::in, T::in, bool::out, io::di, io::uo) is det.

    % Read the contents of mvar without taking it out.
    % If the mvar is empty then block until it is full.
    % This is similar to mvar.take followed by mvar.put, but another value
    % cannot be placed into the mvar between the two operations.
    %
:- pred read(mvar(T)::in, T::out, io::di, io::uo) is det.

    % Try to read the contents of mvar without taking it out.
    % Returns `yes(X)' if the mvar contained X, or `no' if the operation
    % would block.
    %
    % WARNING: a return value of `no' does not necessarily mean the mvar
    % is or was empty. For example, another thread attempting to read or take
    % an item out of the mvar may also cause `try_read' to return `no'.
    %
:- pred try_read(mvar(T)::in, maybe(T)::out, io::di, io::uo) is det.

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