Some of you may remember that I (like Mr. Cawley) am implementing a programing language in Perl (though mine is just a toy). I finally got down to working on the code to implement variables, and happened upon attributes in the docs. Useing lvalue subs I was able to make a delightfully simple API for handling complex data structures with arrays, scalars, and hashes, alng with namespaces and lexical variables. The only problem is that it doesn't work.

use strict;
use warnings;

{
    my %hash;

        sub assign_to_hash : lvalue {
            my $key = shift;
            return $hash{ $key };
        }

}

print "Couldn't set value!" unless assign_to_hash('foo') = (['array', 
+'of', 'values']);
print "Value defined" if defined assign_to_hash('foo');

[download]

This gives no output.

This reduces the problem to the basics. Essentially, the lvalue sub does the hard part (be assigned to) but otherwise returns undef, as if the hash key is autovivifiying . . .which it shouldn't be, unless I'm badly mistaken (which I could be :-).

I understand that attributes are an experimental don't-use-this-even-if-your-life-depended-on-it feature (which is maturing nicely in Perl 6) but I'm still curious if anyone can explain it.

Cheers, (and thanks)
Erik

In reply to lvalue subs return undef, playing with experimental features, the End of the World, etc by erikharrison

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