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From: Lawrence D'Oliveiro <ldo@geek-central.gen.new_zealand>
Newsgroups: comp.lang.java.programmer
Subject: Re: Making one or more threads wait for another to produce a value or fail
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Date: Wed, 01 Jun 2011 12:33:39 +1200
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In message <alpine.DEB.2.00.1105311443530.28134@urchin.earth.li>, Tom 
Anderson wrote:

> Penelope is a widow, or at least her husband isn't around any more (she's
> not sure which; long story). There are 108 suitors who would like to marry
> her. She hasn't decided which one she'll marry. So, the 108 suitors are
> sitting about waiting for her to decide. It's possible that instead of
> deciding to marry one of them, she'll deliver some other, exceptional,
> verdict (eg "turns out my husband is still alive, and will now murder you
> all").
> 
> Penelope is a thread, as are her suitors. Penelope has plenty to do after
> she delivers her verdict, so the verdict is not a return value - it's a
> value she'll pass to a method.

procedure penelope_and_her_suitors is

    subtype verdict_string is
        string(1 .. 12);

    protected verdict is

        entry deliver(v : in verdict_string);
        entry obtain(v : out verdict_string);

    private
        v : verdict_string;
        got_v : boolean := false;
    end verdict;

    protected body verdict is

        entry deliver(v : in verdict_string) when not got_v is
        begin
            verdict.v := v;
            got_v := true;
        end deliver;

        entry obtain(v : out verdict_string) when got_v is
        begin
            v := verdict.v;
        end obtain;

    end verdict;

    task penelope is
    end penelope;

    task type suitor is
    end suitor;

    task body penelope is

    begin
        --- think about what verdict to deliver
        verdict.deliver("you all die "); -- or whatever
    end penelope;

    task body suitor is

        the_verdict : verdict_string;

    begin
        verdict.obtain(the_verdict);
        -- do whatever with it
    end suitor;

    suitors : array (1 .. 108) of suitor;

begin -- penelope_and_her_suitors
    null; -- wait for all the fun to finish
end penelope_and_her_suitors;