NAME

Acme::ComeFrom - Parallel Goto-in-reverse

VERSION

This document describes version 0.11 of Acme::ComeFrom, released October 15, 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 comefroms 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 comefroms. 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.

The "MIT" License

Permission is hereby granted, free of charge, to any person obtaining a 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. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.