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    Acme::ComeFrom - Parallel Goto-in-reverse

VERSION
    This document describes version 0.10 of Acme::ComeFrom, released October
    14, 2007.

SYNOPSIS
        use Acme::ComeFrom;

        sub func { print "@_" }; func("Start\n");
        print "This won't happen\n";

        comefrom &func; print "Branch 1\n"; exit;
        comefrom &func; print "Branch 2\n";

        label: print "This won't happen either\n";

        comefrom label; print "Branch 2.1\n"; exit;
        comefrom label; print "Branch 2.2\n";

        EXPR0: print "To be\n"; exit;
        comefrom "EXPR".int(rand(2)); print "Not to be\n";

DESCRIPTION
    INTERCAL programmers have for a long time monopolized the enormously
    powerful construct "COME FROM", both as a flow-control replacement to
    "goto", and as a simple way to mark parallel execution branches in the
    multi-thread variant.

    But now, with Acme::ComeFrom, we Perl hackers can finally be on par with
    them in terms of wackiness, if not in obfuscation.

    Just like "goto", "comefrom" comes in three different flavors:

    comefrom LABEL
        The "comefrom-LABEL" form finds the statement labeled with "LABEL"
        and jumps to the "comefrom" each time just *before* that statement's
        execution. The "comefrom" may not be inside any construct that
        requires initialization, such as a subroutine or a "foreach" loop,
        unless the targeting "LABEL" is also in the same construct.

    comefrom EXPR
        The "comefrom-EXPR" form expects a label name, whose scope will be
        resolved dynamically. This allows for computed "comefrom"s by
        checking the "EXPR" before every label (a.k.a. watchpoints), so you
        can write:

            # $i below evaluates in the LABEL's scope
            comefrom ("FOO", "BAR", "GLARCH")[$i];

        Starting from version 0.05, the value of EXPR is evaluated each
        time, instead of the old *frozen at the first check* behaviour. If
        this breaks your code -- as if there's any code based on comefrom --
        You may retain the original behaviour by assigning a true value to
        $Acme::ComeFrom::CacheEXPR.

    comefrom &NAME
        The "comefrom-&NAME" form is quite different from the other forms of
        "comefrom". In fact, it isn't a comefrom in the normal sense at all,
        and doesn't have the stigma associated with other "comefrom"s.
        Instead, it installs a post-processing handler for the subroutine,
        and a jump would be made just *after* the subroutine's execution.

    If two or more "comefrom" were applied to the same LABEL, EXPR or NAME,
    they will be executed simultaneously via "fork()". The forking are
    ordered by their occurrances, with the parent process receiving the last
    one.

BUGS
    This module does not really parse perl; it guesses label names quite
    accurately, but the regex matching the "comefrom" itself could catch
    many false-positives. Perhaps some day a brave soul somewhere will
    volunteer to patch this module to use PPI instead...

ACKNOWLEDGEMENTS
    To the INTERCAL language, for its endless inspiration.

    As its manual states: "The earliest known description of the COME FROM
    statement in the computing literature is in [R. L. Clark, "A linguistic
    contribution to GOTO-less programming," Commun. ACM 27 (1984), pp.
    349-350], part of the famous April Fools issue of CACM. The subsequent
    rush by language designers to include the statement in their languages
    was underwhelming, one might even say nonexistent. It was therefore
    decided that COME FROM would be an appropriate addition to C-INTERCAL."

    To Maestro Damian Conway, the source of all magic bits in Hook::LexWrap
    and Filter::Simple, on which this module is based.

    To Ton Hospel, for his tolerance on my semantic hackeries, and for
    suggesting the correct behaviour of "comefrom-LABEL" and
    "comefrom-EXPR".

SEE ALSO
    Hook::LexWrap, Filter::Simple, "goto" in perlfunc

AUTHORS
    Audrey Tang <cpan@audreyt.org>

COPYRIGHT
    Copyright 2001, 2002, 2003, 2004, 2005, 2006, 2007 by Audrey Tang
    <cpan@audreyt.org>.

    This software is released under the MIT license cited below.

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    copy of this software and associated documentation files (the
    "Software"), to deal in the Software without restriction, including
    without limitation the rights to use, copy, modify, merge, publish,
    distribute, sublicense, and/or sell copies of the Software, and to
    permit persons to whom the Software is furnished to do so, subject to
    the following conditions:

    The above copyright notice and this permission notice shall be included
    in all copies or substantial portions of the Software.

    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
    OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
    MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
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