A discussion on p5p the past day, lead me to a new insight that I would like to share with you. An insight that attempting to reduce a problem to a single set of lines, may sometimes backfire on you.

The problem can be shown by running the following lines: 

my $foo = Foo->new;
print "foo is".($foo->isa( 'Bar' ) ? '' : ' _not_')." a Bar\n";

package Bar;

package Foo;
@Foo::ISA = qw(Bar);
sub new { bless {},shift }
__END__
$foo is _not_ a Bar

[download]

You might wonder why does not produce the expected "$foo is a Bar" string.

It actually was Hugo van der Sanden on p5p who gave the solution, so I can't take any credit for that. The reason why: @Foo::ISA = qw(Bar) doesn't get executed until after the print with the ->isa check is executed. So at that time, the Foo package was indeed not yet inheriting from Bar.

As I said on p5p, this is when you realize that objects are bolted onto Perl 5.

And thus ends my meditation for today.

Liz

In reply to Foo is not a Bar, why? by liz

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