Some sort of scoping problem

[Mintcake]

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 $self->{xyzzy}, ' ', scalar @{$self->{xyzzy}};
}

sub ini {
my ($self, $field) = @_;
eval "\$self->{$field} = \\\@$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.

[Ben Bullock]

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:

[xho...@gmail.com]

Mintcake <to...@skelding.co.uk> wrote:
> 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 $self->{xyzzy}, ' ', scalar @{$self->{xyzzy}};
> }
>
> sub ini {
> my ($self, $field) = @_;
> eval "\$self->{$field} = \\\@$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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[Ronny]

On 6 Mai, 03:04, Mintcake <t...@skelding.co.uk> wrote:
> package Foo;
>
> my @xyzzy = (1,2,3);
>
> sub new {
> my $self = bless {}, shift;
> $self->ini('xyzzy');
> print \@xyzzy, ' ', scalar @xyzzy;
> print $self->{xyzzy}, ' ', scalar @{$self->{xyzzy}};
>
> }
>
> sub ini {
> my ($self, $field) = @_;
> eval "\$self->{$field} = \\\@$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->{$field} = \\\@$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} = \\\@$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

[Dave Weaver]

On Mon, 5 May 2008 18:04:09 -0700 (PDT), Mintcake <to...@skelding.co.uk> wrote:
> 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 $self->{xyzzy}, ' ', scalar @{$self->{xyzzy}};
> }
>
> sub ini {
> my ($self, $field) = @_;
> eval "\$self->{$field} = \\\@$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 ) = @_;
$self->{$field} = $fields->{field};
}

[Dave Weaver]

On 09 May 2008 09:06:22 GMT, Dave Weaver <zen1...@zen.co.uk> wrote:
> sub ini {
> my ( $self, $field ) = @_;
> $self->{$field} = $fields->{field};

Oops! That should, of course, be:
$self->{$field} = $fields{$field};

[Ben Bullock]

On Fri, 09 May 2008 09:06:22 +0000, Dave Weaver wrote:

> Others have explained the problem and pointed out why you should "use
> warnings;".

It has nothing to do with "use warnings;".

> Here are a couple of suggestions to solve your problem:

I'm sorry but you have missed the point of the question and the answers
given.

[Dave Weaver]

Ben Bullock <benkasmi...@gmail.com> wrote:
>
> > Here are a couple of suggestions to solve your problem:
>
> I'm sorry but you have missed the point of the question and the answers
> given.
>

I admit that may be the case, but I am none the wiser for your reply.