NAME

Acme::ComeFrom - Parallel goto-in-reverse

VERSION

This document describes version 0.07 of Acme::ComeFrom, released November 8, 2002.

SYNOPSIS

    use Acme::ComeFrom;

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

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

    label: print "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 could 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 could write ($i 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. I'm looking forward for ways to change that.

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 suggesting the correct behaviour of comefrom-LABEL and comefrom-EXPR.

SEE ALSO

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

AUTHORS

Autrijus Tang <autrijus@autrijus.org>

COPYRIGHT

Copyright 2001, 2002, 2003 by Autrijus Tang <autrijus@autrijus.org>.

This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

See http://www.perl.com/perl/misc/Artistic.html