Attribut'ting heads - Hacking Thy Fearful Symmetry

Hacking Thy Fearful Symmetry

Hacker, hacker coding bright

Attribut'ting heads

created: April 5, 2018

Update: a voice arose in the comment section to point out a way to do my final one-two sub manipulation without having to declare two separate functions. Scroll at the end of the blog entry to behold that trick yourself!

A quick one, but it might come in handy as perl 5.28 is right around the corner.

So, in perl 5.27.8 backward-compatibility disaster stroke! The parsing of the experimental signatures and of sub attributes was modified, and as a direct consequence the order in which they must be declared was reversed. Fortunately, only someone stark raving mad would dish out both features on their subs.

Unfortunately, stark raving mad represents.

Basically, the function that worked before 5.27,


sub lazy_list ($generator,$state=undef) :prototype(&@) {
    return List::Lazy->new(
        generator => $generator,
        state     => $state,
    );
}

has to be


sub lazy_list :prototype(&@) ($generator,$state=undef) {
    return List::Lazy->new(
        generator => $generator,
        state     => $state,
    );
}

to work in 5.27 and later. No compromise possible. One wants black, the other requires white. At first glance, it seems I've doomed myself into choosing between supporting perl up to 5.26, or only 5.28 and beyond.

But only at first glance. Now if you would excuse me and hold my beer...

Like a savage

The first and primal solution that the lizard brain comes with first is to craft code on-demand and eval it right on the spot.


# warp in begin so that the prototype is already visible in the module
BEGIN {

    my @proto = ( '($generator,$state=undef)', ':prototype(&@)' );

    @proto = reverse @proto if $] >= 5.027008;

    eval <<"END";
        sub lazy_list @proto {
            return List::Lazy->new(
                generator => \$generator,
                state     => \$state,
            );
        }
END
}

It works. But yeeeeeeah, I feel dirty too. evaling code should only be the last resort. Surely there is a better way.

Make them play in their own corner

Something slightly less icky is to use the if module (ah! I bet you also forgot it existed) and redirect the different perls to the implementation that will please them.



# in Lazy/List.pm

if $] <  5.027008, 'Lazy::List::Implv26';
if $] >= 5.027008, 'Lazy::List::Implv28';

# in Lazy/List/Implv26.pm

package  # we don't want CPAN to see our shame
    Lazy::List::Implv26;

use parent 'Exporter';

our @EXPORT_OK = qw/ lazy_list /;

sub lazy_list ($generator,$state=undef) :prototype(&@) {
    return List::Lazy->new(
        generator => $generator,
        state     => $state,
    );
}

1;

A little bit of code repetition, but at least we're not evaling like brutes.

Just side-step the issue altogether

Then, finally, I realized that I was like the monkey with its fist struck in the jar. Or Neo staring blindly at the spoon. The issue is with the order of the declarations. But if a function only has one such declaration... Then that declaration doesn't have an order. It simply is.

In other words, let's switch the conflict-ridden


sub lazy_list ($generator,$state=undef) :prototype(&@) {
    return List::Lazy->new(
        generator => $generator,
        state     => $state,
    );
}

for


sub _lazy_list ($generator,$state=undef) {
    return List::Lazy->new(
        generator => $generator,
        state     => $state,
    );
}

sub lazy_list :prototype(&@) { goto &_lazy_list }

There. Each sub has either a prototype or a signature, so the perls have no reason to argue about it, and the goto even mask our shenanigans from the user. No eval, no new perl-specific module. Just one short line for each subroutine. I can live with that.

The one-two punch without the two subs

Graham pointed out in the comment section that the prototype can be slatered atop the signature without the definition of an inner function via the use of Sub::Util:


use Sub::Util 1.40;
sub lazy_list ($generator,$state=undef) {
    List::Lazy->new( generator => $generator, state => $state );
}
BEGIN { Sub::Util::set_prototype('&@', \&lazy_list) }

Enjoy!

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About the author

Yanick Champoux
Perl necrohacker , ACP writer, orchid lover. Slightly bonker all around.