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Posted by Mintcake on May 5, 2008, 9:04 pm
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This is *not* a trivial problem. If you know Perl well, please take a
bit of time to look at this.
I have the following code in a file Foo.pm
package Foo;
my @xyzzy = (1,2,3);
sub new {
my $self = bless {}, shift;
$self->ini('xyzzy');
print \@xyzzy, ' ', scalar @xyzzy;
print $self->, ' ', scalar @};
}
sub ini {
my ($self, $field) = @_;
eval "$self-> = \@$field";
}
1;
__END__
My main program is simply this:
#!/usr/local/bin/perl -l
use Foo;
new Foo;
__END__
The two lines of output are:
ARRAY(0x90edda4) 3
ARRAY(0x90edfcc) 0
It seems that there are two separate arrays, one of which is empty. I
was expecting the blessed hash to simply contain a reference to the
@xyzzy lexical declared with module scope.
If I include the package Foo code in the main program instead of a
separate module I get the expected result.
If I lose the ini() subroutime and put the eval directly in the
constructor I get the expected result.
If I don't declare @xyxxy with my or use our instead I get the
expected result.
If I add a use strict in Foo.pm and change $self->ini('xyzzy') to
$self->ini('plugh') I get the expeted error:
Can't use an undefined value as an ARRAY reference at /home/tony/lib/
Foo.pm line 11.
I'm using perl v5.8.8 and I get the some on i686-linux and Activstate
on Windoze.
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Posted by Ben Bullock on May 5, 2008, 10:08 pm
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On Mon, 05 May 2008 18:04:09 -0700, Mintcake wrote:
> This is *not* a trivial problem. If you know Perl well, please take a
> bit of time to look at this.
I don't know Perl that well, but in case this needs confirmation, I had a
look & confirmed the following odd behaviour:
> If I include the package Foo code in the main program instead of a
> separate module I get the expected result. If I lose the ini()
> subroutime and put the eval directly in the constructor I get the
> expected result. If I don't declare @xyxxy with my or use our instead I
> get the expected result.
> If I add a use strict in Foo.pm and change $self->ini('xyzzy') to
> $self->ini('plugh') I get the expeted error:
>
> Can't use an undefined value as an ARRAY reference at /home/tony/lib/
> Foo.pm line 11.
>
> I'm using perl v5.8.8 and I get the some on i686-linux and Activstate
> on Windoze.
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Posted by xhoster on May 5, 2008, 11:32 pm
Please log in for more thread options > This is *not* a trivial problem. If you know Perl well, please take a
> bit of time to look at this.
>
> I have the following code in a file Foo.pm
>
> package Foo;
>
> my @xyzzy = (1,2,3);
>
> sub new {
> my $self = bless {}, shift;
> $self->ini('xyzzy');
> print \@xyzzy, ' ', scalar @xyzzy;
> print $self->, ' ', scalar @};
> }
>
> sub ini {
> my ($self, $field) = @_;
> eval "$self-> = \@$field";
> }
ini never latches onto @xyzzy, because @xyzzy is not mentioned
in ini at compile time. Very similar to:
http://groups.google.com/group/comp.lang.perl.misc/browse_frm/thread/eaf48dac9f298e29
Xho
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Posted by Ronny on May 7, 2008, 7:45 am
Please log in for more thread options > package Foo;
>
> my @xyzzy = (1,2,3);
>
> sub new {
> my $self = bless {}, shift;
> $self->ini('xyzzy');
> print \@xyzzy, ' ', scalar @xyzzy;
> print $self->, ' ', scalar @};
>
> }
>
> sub ini {
> my ($self, $field) = @_;
> eval "$self-> = \@$field";
>
> }
>
> 1;
> My main program is simply this:
>
> #!/usr/local/bin/perl -l
> use Foo;
> new Foo;
>
> The two lines of output are:
>
> ARRAY(0x90edda4) 3
> ARRAY(0x90edfcc) 0
First I run your program with
use warnings;
enabled, and here I got the message:
Variable "@xyzzy" is not available at (eval 1) line 2.
Which means @xyzzy can't be seen from within eval. Things are
different if I "use" the variable inside the routine, so that
the compiler can see it - for example by writing
sub ini {
my ($self, $field) = @_;
print "ini: ", \@xyzzy,"\n";
eval "$self-> = \@$field";
}
You can also put the usage after the eval; it is only important
that the variable is used somewhere in the function:
sub ini {
my ($self, $field) = @_;
eval "$self-> = \@$field";
print "ini: ", \@xyzzy,"\n";
}
In both cases, Foo::new will print the same value for the hash.
We learn two things from this:
(1) Perl can be pretty bizarre in its details.
(2) If you do not "use warnings", you are automatically in a state of
sin.
Ronald
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Posted by Dave Weaver on May 9, 2008, 5:06 am
Please log in for more thread options > This is *not* a trivial problem. If you know Perl well, please take a
> bit of time to look at this.
>
> I have the following code in a file Foo.pm
>
> package Foo;
>
> my @xyzzy = (1,2,3);
>
> sub new {
> my $self = bless {}, shift;
> $self->ini('xyzzy');
> print \@xyzzy, ' ', scalar @xyzzy;
> print $self->, ' ', scalar @};
> }
>
> sub ini {
> my ($self, $field) = @_;
> eval "$self-> = \@$field";
> }
Others have explained the problem and pointed out why you
should "use warnings;".
Here are a couple of suggestions to solve your problem:
1. Use a package variable instead of a lexical:
our @xyzzy = ( 1, 2, 3 );
2. Use a lookup table:
my %fields = (
xyzzy => [ 1, 2, 3 ],
);
sub ini {
my ( $self, $field ) = @_;
$self-> = $fields->;
}
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