When it comes to maintaining state I don't see much difference between runtime trait and statically applied trait. As far as I remember the original paper the traits are stateless by nature. So IMHO what you try to do is breaking the traits :-)

But your need is also very natural. I often use Class::Trait and many of my traits would like to have their state variables (usually class or instance accessors).

I solve their wish by a subroutine in the trait (currently named build_class) which I call immediately after the trait is applied. The build_class then modifies the class according to trait's needs.

Class::Trait->apply( $some_class, $some_trait );
my $build_class_sub = $some_trait->can('build_class');
$build_class_sub->($some_class);
[download]

Inside the trait I have, for example:

sub build_class {
   my ($class) = @_;

   $class->mk_accessors( qw(smb_hostname) );
   $class->mk_accessors( qw(smb_username) );
   ...
}
[download]

I guess that with your inside out approach you would probably need something like:

sub build_class {
    my ($class) = @_;

    my $destructor = sub {
        my ($this) = @_;
        delete $is_selected{ get_obj_id($this) };
    };
    $class->register_destructor($destructor);
}
[download]

I know that this approach is not very traity, but my primary goal is to find a consistent and clear way to build my classes not to use pure traits.

There is also somehow complementary issue I need to solve with traits: fixing the name of applying class in the trait. In other words I need my __PACKAGE__ in the trait.

Imagine few logging functions - perfect task for traits. One of them is the logging path for the module. Implemented in class it would look like:

sub logpath {
    my ($this) = @_;

    return File::Spec->rel2abs( join( '-', split /::/, __PACKAGE__ ) .
+ '.log',
        $this->log_root );
}
[download]

All subclasses log into the same file as the class which defined the method. My Durian::Call::Statement logs into $ROOT/Durian-Call-Statement.log as does its Durian::Call::Statement::Incoming subclass.

But how to accomplish this task with trait? The only way I found is to modify the trait symbol table with respect to the class applying the trait immediately before the application. In other words apply a slightly modified trait to every class.

To call preapply subroutine right before the trait methods are collected.

sub preapply {
    my ($class) = @_;

    *logpath = sub {
        my ($this) = @_;

        return File::Spec->rel2abs( join( '-', split /::/, $class ) . 
+'.log',
            $this->log_root );
    };
}
[download]

With respect to preapply the better name for build_class would be postapply. It sounds good, doesn't it?

Unless someone explains to me that it is a very bad idea I'll try to make my own preapply/postapply Class::Trait subclass.

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