Re^4: RFC: Perl meta programming

[chromatic]

Beware.

The problem I have with isa() is that I either need to use the verbose UNIVERSAL::isa( $foo, 'Object::Foo' ) syntax...

Unfortunately, it's completely wrong. See Acme::UNIVERSAL::new if you think it's a good idea.

Wrapping every isa() check in an eval{} seems cumbersome, too.

Tough. If you care about getting the right answer, that's what you need to do. Yes, the syntax is a little clunky, but it will give you the right answer. Nothing else will.

I suppose I could just import isa(), as well.

Not if you care about getting the right answer.

[TGI]

There's some real food for thought in your replies, thanks.

So if get the point, there is no easy way to test whether something can be treated as a hash ref (for instance) short of trying to do so. So the best way to test is to try and catch any exceptions that indicate failure.

If I want to verify membership in a class, I'd need to do something like:

# Option 1.
if ( eval { $foo->isa('Some::Class') } ) {
  
  # Do stuff with Some::Class
}

# or Option 2.

use Scalar::Util;

if ( blessed $foo and $foo->isa('Some::Class') ) {
  # Do stuff with Some::Class
}

Are there any big ugly problems with either option? I've tested both and they seem OK, but there's no point trying to learn a new habbit, if it's a bad one!

[diotalevi]

So if get the point, there is no easy way to test whether something can be treated as a hash ref (for instance) short of trying to do so. So the best way to test is to try and catch any exceptions that indicate failure.

The alternative is that you use B to ask whether you *really* has a hash reference and overload to ask whether your object of some other type has %{} dereferencing magic. This exposes you to more magic than you want to have to know about.

use B 'svref_2object';
use overload ();

sub bad_idea {
    my $thing = shift @_;

    local $@; # Protect against $@ stomping

    return

      # Is a real hash reference
      eval { svref_2object( $thing )->isa( 'B::HV' ) }

      # Pretends to be a hash reference
      or ( overload::Overloaded( $thing )
            and overload::Method( $thing, '%{}' ) );
}

[druud]

perl -wle 'sub x{ return eval{die} or 42 } print x'

[diotalevi]

If I want to verify membership in a class, I'd need to do something like: eval { $foo->isa( 'Some::Class' ) }

Right. That ->isa call might throw an exception because $foo isn't an object, it might resolve to UNIVERSAL::isa, or $foo might have a different ->isa method. Using UNIVERSAL::isa on all objects and classes breaks objects and classes which implement their own ->isa methods. That's much of the interesting code being written these days so you'd best not lock yourself off from the future by hardcoding UNIVERSAL::isa in.

Here's one additional note. Try to remember to localize $@ when you're using eval. It protects your caller's view of $@ from your eval blocks.

[chromatic]

Those are the best two options I know; they're also in the documentation (somewhere) as of a few months ago.