Re: How Are Attributes Useful?

[Zaxo]

I recall you have been studying the :lvalue attribute for subs. Reflect that that attribute is more than a tag that you may query at runtime like some kind of metadata. I'm not sure how I'd go about querying that.

When the :lvalue attribute is applied to a sub, it changes how the sub is compiled and what usage is permitted of it.

The description of attributes as tags to be queried sounds a lot like the "properties" Lisp can attach to a symbol. I think that's a mischaracterization.

A read of the Attribute::Handlers pod suggests that magic outside runtime is what attributes are for. Those docs give some non-trivial examples. I suspect that perldoc -m Attribute::Handlers will yield lots of wisdom on the matter, but I know for a fact it won't be easy ;-)

I'm sure that perl attributes would benefit from more of us experimenting with them.

[Limbic~Region]

Zaxo,
Reflect that that attribute is more than a tag that you may query at runtime like some kind of metadata. I'm not sure how I'd go about querying that.

Really. I haven't found a single example of how one might modify the behavior of a subroutine using attributes outside the few built-ins. As far as querying the attributes - it was in the synopsis of perldoc attributes

use attributes ();    # optional, to get subroutine declarations
my @attrlist = attributes::get(\&foo);

I have looked at Attribute::Handlers and still felt unsatisfied. Attribute::Handlers::Prospective looks promising after glancing at the documentation but I find it hard to believe that there are no code examples outside the module. Why is this I wonder. It seems to me that you are right - the majority of us are in the dark and so attributes, if useful at all as others have implied, are way underdeveloped in perl.

[redlemon]

Check Bot-CPAN. It's Glue module uses Attribute::Handlers to define well.. attributes to methods. That lets you define a bot command like:

    sub list
         :help ("lists available modules")
         :private
         :adminCmd
    {
       # implements list()
    }

I thought that was a rather elegant way of doing this.

[tye]

No, they are just text tags. Perl checks for the "lvalue" tag and, if found, does special processing. But the attribute is just a way to tell Perl to do that special processing.

[Zaxo]

That's an implementation detail. The point is that thay represent metadata for the perl compiler, not the user's runtime. It's magic, I say!

[tye]

s/not/or/

Attributes are just syntactic sugar for implementation details.

If you think attributes are the magic, then you'll never understand how to use attributes in general.