Anyone using Attribute::Default from CPAN?

[rovf]

Hello Monks,

I would like to use Attribute::Default from CPAN in order to define default arguments for subs in a easily readable way. Here is a simple example which works:

use strict;
use warnings;
use base 'Attribute::Default';
sub f : Default(undef,"xyz") {
  print("@_\nEND\n");
}
f('abc'); # 2nd argument defaults to xyz

This prints abc xyz, followed by a line saying END. Things stop to work, however, when I use Attribute::Default in a package which itself uses the Exporter; or maybe I just may some silly mistake. Here is the first thing I tried:

# This is file Co.pm
package Co;
use strict;
use warnings;
use base qw(Exporter Attribute::Default);
our @EXPORT_OK=qw(f);
sub f : Default(undef,"xyz") {
  print("@_\nEND\n");
}
1;

This will be called in the following way:

# This is file cotest.pl
use warnings;
use strict;
use Co qw(f);
f('abc');

Executing this will output just abc followed by END; i.e. the default argument is not used.

It gets even more bizarre when I handle @ISA by myself instead of letting use base doint it:

# This is file Co1.pm
package Co1;
use strict;
use warnings;
use Exporter;
use Attribute::Default;
our @ISA=qw(Exporter Attribute::Default);
our @EXPORT_OK=qw(f);
sub f : Default(undef,"xyz") {
  print("@_\nEND\n");
}
1;

I call this as usual,

# This is file cotest1.pl
use warnings;
use strict;
use Co qw(f);
f('abc');

but now I get the error message:

Invalid CODE attribute: Default(undef,"xyz") at Co1.pm

Well, I guess there is a reason why the perldoc to Attribute::Default says that you need do use base '...' to access it. Anyway, as a last resort I tried to combine the two approaches:

# This is Co2.pm
package Co2;
use strict;
use warnings;
use Exporter;
our @ISA=qw(Exporter);
use base qw(Attribute::Default);
our @EXPORT_OK=qw(f);
sub f : Default(undef,"xyz") {
  print("@_\nEND\n");
}
1;

The program for calling it is, not surprisingly:

use warnings;
use strict;
use Co2 qw(f);
f('abc');

Now I get the error message:

Can't locate object method "_ATTR_CODE_Default" via package "Co2"

What is going wrong here?

[moritz]

I don't know what went wrong, but you have another alternative. You can use Exporter without inheriting from it:

package Co;
use Exporter qw(import);
our @EXPORT_OK = qw(f);
...

[rovf]

Thank you for this suggestion, but this doesn't work either: Though I don't get any error message, the default argument is not supplied to the function. :-(

[lodin]

I'd recommend against use of attributes in any serious program (with some exceptions, such as Catalyst). You often end up with trouble, as you've already experienced, and there's almost always better alternatives. For some of the issues with attributes, see Re: what would you like to see in perl5.12? (attributes) and Re: What's broken in Perl 5? (attributes) where I lay it out a bit.

lodin

[rovf]

Thanks a lot. The articles you wrote gave me some idea where the problem could be. I now tend to agree with you that I would have been better off without attributes, but at least I could get it working!. I have placed a HowTo on my public scratchpad.

Thanks to all for helping!

[lodin]

Perhaps the comments on your scratchpad would be a good addition to the documentation of the module. You could annotate the POD via AnnoCPAN and e-mail the author of Attribute::Default telling him about your problem and your solution.

lodin