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Re: Re: Foo is not a Bar, why?

by Mr_Person (Hermit)
on Dec 18, 2003 at 15:39 UTC ( #315540=note: print w/ replies, xml ) Need Help??


in reply to Re: Foo is not a Bar, why?
in thread Foo is not a Bar, why?

Or using the nicer-looking use base. It will complain that the Bar package is empty, so I put a sub in it, but then it works fine.

my $foo = Foo->new; print "foo is".($foo->isa( 'Bar' ) ? '' : ' _not_')." a Bar\n"; package Bar; sub null {} package Foo; use base Bar; sub new { bless {},shift }


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Re: Re: Re: Foo is not a Bar, why?
by liz (Monsignor) on Dec 18, 2003 at 15:49 UTC
    Hmmm... which version of Perl are you using? I don't need a sub null {} inside Bar to get it to run as expected.

    Slaven Rezic pointed out (on p5p) the use of base.pm as an alternative. I assumed it would always require an external file: apparently it doesn't do that (anymore, anyway, I seem to remember that at one point it did. But I am probably wrong about that).

    What I don't like about base.pm is the extra stuff about fields. Have you ever looked under the hood of base.pm? Yuck!

    Liz

    Update:
    Argh. The reason I didn't get any errors about the empty Bar package, was that I actually had a Bar.pm as an external file hanging around from some previous testing. So yes, you need something in there, not necessarily a sub null {} though, a $VERSION = '0.01' also does the trick. Thanks to Mr. Person for his perseverance proving me wrong in this matter.

      It's one of the reasons I wrote vars::i
      my $foo = Foo->new; print "foo is".($foo->isa( 'Bar' ) ? '' : ' _not_')." a Bar\n"; package Bar; sub null {} package Foo; use vars::i qw[ @ISA Bar ]; sub new { bless {},shift } __END__ foo is a Bar

      MJD says "you can't just make shit up and expect the computer to know what you mean, retardo!"
      I run a Win32 PPM repository for perl 5.6.x and 5.8.x -- I take requests (README).
      ** The third rule of perl club is a statement of fact: pod is sexy.

        That's all good and dandy, but even tough your module looks useful (and generic), I can't say I like the resulting interface — i.e. the use statement. I'd rather see something like this:
        use strict; package Foo; use isa qw(Bar); sub new { bless {},shift }

        The module isa.pm can actually quite short:

        # file isa.pm package isa; sub import { my $pkg = shift; my $caller = caller; no strict 'vars'; push @{"$caller\::ISA"}, @_; } 42;
        Et voilà:
        foo is a Bar
        

        Of course, with base doing the same and more, I don't know if there's much need for this here module.

      I'm on Perl 5.6.1, and it gives me:
      Base class package "Bar" is empty. (Perhaps you need to 'use' the module which defines that packa +ge first.) at test.pl line 10 BEGIN failed--compilation aborted at test.pl line 10.
      If I don't put the sub null {} in, but that wouldn't generally be a problem for a real situation because you'd almost always have something in the package.

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