NAME
provide - easily switch between different implementations depending on
which version of Perl is detected
SYNOPSIS
package My::Module;
use provide (
if => ge => '5.013000' => 'My::Module::v5_013000',
else => 'My::Module::v5_080000',
);
DESCRIPTION
Good code is free of side effects, avoids tight coupling, and solves a
useful problem in an understandable way. This module, on the other hand,
is ball of frightened octopuses clinging together.
The simple act of adding
use provide (...)
to an otherwise well-behaved class performs the following changes to it:
* The calling module automatically inherits from Exporter.
* provide.pm finds suitable modules to load and loads them.
* Any functions exported by the loaded modules get re-exported by the
calling module.
This module is marginally useful if you are implementing your own module
and you end up stumbling over some bug in your code caused by a change
to the Perl core. Here's a worked example of when you might use this
module:
hash_pop v1.0 - pass by value
Let's pretend you want to implement your own version of pop, but for
hashes: it'll return the last key+value pair of the hash (whatever
"last" means in the context of an inherently unordered list!). You might
start out like this:
use strict;
use warnings;
package My::Module;
use base qw(Exporter);
our @EXPORT = qw(hash_pop);
sub hash_pop {
my (%hash) = @_;
my ($last_key) = reverse keys %hash;
return ($last_key, delete $hash{$last_key});
}
1;
__END__
Well, this is about as good an implementation as you can expect. It is
easy enough to call:
my %hash = (1..10); # Belden's default "just give me some kind of hash" hash
my ($last_key, $last_value) = hash_pop(%hash);
But unlike the `pop' that we're mimicking, our `hash_pop' doesn't mutate
the %hash that we pass in, so it's not very `pop'-like yet.
hash_pop v1.1 - explicit pass by reference
To mutate our subject %hash, we'll need pass by reference:
sub hash_pop {
my $hash = shift;
my ($last_key) = reverse keys %$hash;
return ($last_key, delete $hash->{$last_key});
}
And since we're passing by reference, we'd darn well better change our
call pattern:
my %hash = (1..10);
my ($last_key, $last_value) = hash_pop(\%hash);
hash_pop v1.2 - implicit pass by reference
If only there were a way to implicitly pass %hash by reference to
`hash_pop' - then we'd have the best of both worlds, wouldn't we? (Would
we? I really don't know.)
Ruby and Python aren't the only languages that have built-in
documentation; look at this marvelous interaction with the Perl
debugger:
$ perl -de 1
DB<1> p prototype 'CORE::keys'
\%
That's pretty good stuff! Take that, highly self-documenting languages!
Now we know how to change `hash_pop':
sub hash_pop (\%) {
my $hash = shift;
my ($last_key) = reverse keys %$hash;
return ($last_key, delete $hash->{$last_key});
}
And now here's someone using this ridiculous function:
my %hash = (1..10);
my ($last_key, $last_value) = hash_pop(%hash);
Sweet! All done, let's stick it on CPAN!
Uh-oh, implementing CORE::-like functions means we have to respect the CORE
Except: you're not done until you run it against every version of Perl
you can shake a perlbrew at. And when you go through and do that, you'll
discover a break between Perl v5.12 and v5.13:
$ perlbrew list | \
cut -b 3- | (while read ver; do \
perlbrew use $ver; \
perl -le 'print $] . "\t" . prototype q,CORE::keys,'; \
done)
__END__
5.006002 \%
5.008009 \%
5.010001 \%
5.012005 \[@%]
5.014003 +
5.016002 +
5.017008 +
Aww, nerds! there's two difference prototypes in play here: \% and +.
One valid option is to just give up on supporting older versions of
Perl. Another is to implement your own version-specific loading code.
And yet another option is to use this module to gloss away implementing
your own version-specific code:
hash_pop v1.3 - version-specific prototypes for implicit reference passing
use strict;
use warnings;
package My::Module;
use provide (
if => ge => '5.013000' => 'My::Module::hash_pop::v5_013000',
else => 'My::Module::hash_pop::v5_006000',
);
sub _hash_pop {
my $hash = shift;
my ($last_key) = reverse keys %$hash;
return ($last_key, delete $hash->{$last_key});
}
1;
We're collecting common behavior between the two version-specific
modules in My::Module::_hash_pop.
Now all that's left is to write your version-specific modules. Here's
the one for v5.013000 and above:
use strict;
use warnings;
package My::Module::hash_pop::v5_013000;
our @EXPORT = qw(hash_pop);
require My::Module;
sub hash_pop (+) { goto &My::Module::_hash_pop }
1;
The module for v5.006000 would be nearly identical:
use strict;
use warnings;
package My::Module::hash_pop::v5_006000;
our @EXPORT = qw(hash_pop);
require My::Module;
sub hash_pop (\%) { goto &My::Module::_hash_pop }
1;
And now someone can go and use our module:
#!/usr/bin/env perl
use strict;
use warnings;
use My::Module qw(hash_pop);
my %hash = (1..10);
my ($nine, $ten) = hash_pop %hash;
SYNTAX
Currently two statements are supported: `if' and `else'.
if => TEST => VALUE => RESULT
TEST
TEST may be any of:
gt "greater than"
ge "greater or equal to"
eq "equal to"
ne "not equal to"
le "less than or equal to"
lt "less than"
VALUE
VALUE should be a string which describes something you might get back in
$]. See also: perldoc.
RESULT
RESULT is the resulting module that will be loaded if this condition is
true.
else => RESULT
In the event that the preceding `if' condition is false, the `else'
RESULT will be loaded.
BUGS
This doesn't handle elsif conditions. It probably won't handle if+if+if
conditions. Please report bugs on this project's Github issues page.
APOLOGY
Too often the explanation for crufty code is, "It seemed like a good
idea at the time." To the contrary, this seems like a bad idea. It was a
bit of fun to write, but I probably missed a great learning opportunity
with my poor implementation.
I really don't know if this will be useful to anyone at all. One of the
challenges to us portraying the Perl community as actively growing is
that there are so many well-tested implementations on CPAN to the
various Big Problems we all face: processing a CGI form, connecting to a
DB_File, writing EBCDIC things (whatever those are!); and more
modernishly, Dancing with Mooses and Catalytic Test frameworks.
ACKNOWLEDGEMENTS
Sam Merrit coined the phrase "a ball of frightened octopuses clinging
together".
Logan Bell practically dared me to release this. Well, maybe he would if
I were to ask him.
John Napiorkowski originally put in my head the notion that, "A CPAN
module is a unit of conversation between developers. It says, 'Here is a
problem, and here is my take on how to solve it.'"
My employer, Shutterstock, Inc., is a staunch supporter of open-source
software. It's a shame I've worked so hard to link them to this amusing
but disingenuous implementation.
CONTRIBUTING
Feel free to use and improve this software in whatever way you see fit.
This code is hosted on Github.com at
http://github.com/belden/perl-provide.
COPYRIGHT AND LICENSE
(c) 2013 by Belden Lyman
This library is free software: you may redistribute it and/or modify it
under the same terms as Perl itself; either Perl version 5.8.8 or, at
your option, any later version of Perl 5 you may have available.