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Postings from March 2007
Michael G Schwern wrote:
> Yuval Kogman wrote:
>>> Likewise with ref in boolean context, I almost never want the object
>>> to be able to lie to me.
>> But if it has to work hard to lie, then does it matter?
>
> Yeah, I'm with Yuval here. There seem to be a cold war going on here wrt
> identifying an object.
>
> In the beginning we used UNIVERSAL::isa() and UNIVERSAL::can(). And it
> could identify the nature of any scalar, blessed or not. Reference or not.
> And it was good.
>
> And then someone got clever and overrode isa() and can(). And there was
> much wailing and nashing of teeth and slaughtering of lambs. Now we're
> supposed to use C<< eval { $obj->isa($class) } >> and for a while balance
> was restored.
Well, except you aren't supposed to do that, because if ->isa can be
overloaded, it can also die or throw an exception. So eval {
$obj->isa($class) } falsely ignores errors.
> And then someone defined a $SIG{__DIE__} so now its C<<{ local
> $SIG{__DIE__}; eval { $obj->isa($class) } }>> or you must depend on having
> a 3rd party module written in XS, Scalar::Util, available and compiled and
> do C<<use Scalar::Util qw(blessed); blessed $obj && $obj->isa($class) >>.
This just compounds the above-mentioned mistake.
> And then someone got really clever and blessed their object into class "0".
> Now we must do C<<use Scalar::Util qw(blessed); defined blessed $obj &&
> $obj->isa($class) >>. Or if you can't rely on Scalar::Util: C<< local
> $SIG{__DIE__}; eval { $obj->isa($class) } >>.
And this compounds it further.
> But what if isa() is broken or has side effects, hmm? And what if its lying
> to me and I *really* want to know what methods are *really* available and
> not autoloaded? And what if someone got even more clever and blessed into
> class ""...
>
> ARGHAGHAGHGGGGHHH!
>
> IT SHOULD NOT BE THIS COMPLICATED TO IDENTIFY THE HERITAGE OF AN OBJECT!
use Params::Util '_INSTANCE';
if ( _INSTANCE($obj, 'Whatever') ) {
}
> The best practice should be straightforward: Ask the object. If it wants
> to lie to you and override isa() or use class "0" (and yes, there really is
> no other reason to use such a class name but to lie) then damned well let
> it. We call this "encapsulation" and it is good.
Yep, I was pissed off too, so the above tries to compensate for the
problems and re-encapsulate what you originally meant by UNIVERSAL::isa().
> 99.999% of the time you do not want to really know how something is
> implemented, you want to remain ignorant.
I concur, which is what really pisses me off them
IO::String->new(\$string)->isa('IO::Handle') returns false, because the
author believes in duck typing over actually having isa describe the
interfaces.
It is detrimental for your
> shopping cart code to pierce the facade surrounding the objects it is using.
> The remaining 0.001% of the time you're writing something like Data::Dumper
> or a serialization module and fine, employ all the magic you like, but don't
> try to apply the needs of a module like Data::Dumper to Joe Business
> Programmer! If I caught someone blessing into class "0" at my work he'd
> have to give such a damned good reason that I wouldn't use his eye sockets
> for USB ports.
>
> All the trouble here seems to stem from the fact that its so damned hard to
> reliably determine what is and is not an object. And the only reason we
> care about that is so we can call isa() on it. The proper way to identify
> the nature of an object is C<< $obj->isa($class) >>. The proper way to
> identify whether a class has a method is C<< $obj->can($method) >>. Done.
> All the other junk we do these days is determining if its blessed so there's
> not an error when we try to call a method on it.
>
> Solution... make everything an object. Or at least everything callable with
> methods. Or for bloody screaming hell at least the UNIVERSAL ones! This is
> called "autoboxing" and there's already a module out there to do it.
> http://search.cpan.org/dist/autobox. In one fell swoop all this nonsense
> goes away. It doesn't even require a core patch anymore.
>
> $ perl -Mautobox -wle 'print []->isa("ARRAY");'
> 1
>
> autobox.pm was proposed as a core module many moons ago but it was rejected
> as not being necessary. As its a central tenet of any decent OO language
> that everything should be (or at least act like) an object I was very
> dismayed at that. Well, here's your use case.
>
> Please reconsider autobox.
heh :)
Adam K
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