Re: make_immutable bug?

On 1 Dec 2011, at 08:42, woosley. xu. wrote:

> Hi all,
>    Is this a Moose make_immutable bug?

No.

> package b;
> use Moose;
> around 'BUILDARGS' => sub {
>    my $orig = shift;
>    my $self = shift;

This happens at object construction time! Meaning you will apply the  
'a' role once for every object you ever construct. (Meaning method  
modifiers will apply N times, where N is number of objects you've  
constructed).

>    with 'a';
>    $self->$orig;
> };
> sub BUILD {
>    my $self  = shift;
>    $self->meta->make_immutable;
> }


So what happens is that the first object creation adds the role to the  
_class_, and then makes the _class_ immutable.

The second object you construct then tries to add the role to the  
class (again), and this fails - as the class is immutable.

You really really don't want to be having class-wide effects whenever  
you construct an instance of the class, and make immutable telling you  
that you're doing something insane is absolutely correct here.

Consider:

package Double;
use Moose::Role;

around foo => sub { my ($orig, $self, @args) = @_; $self->$orig(@args)  
* 2 };

package Class;
use Moose;

sub { with 'Double'; };

has foo => (
     is => 'ro'
);

package main;

use Test::More;

my $four = Class->new(foo => 2);
is $four->foo, 4, 'So far so good';
my $six = Class->new(foo =>  3);
is $six->foo, 6, 'Not so good - returns 12';
is $four->foo, 4, 'Really not so good, now returns 8!';

done_testing;

The last case being the key one - just by constructing a 2nd instance,  
you've had effects on what should be a totally un-related first  
instance - epic fail!

Cheers
t0m