choroba has asked for the wisdom of the Perl Monks concerning the following question:

Dear brethren and sistren!

Imagine a Moose class with an array attribute.

package My; use Moose; has list => ( is => 'rw', isa => 'ArrayRef[Str]', ); [download]

If we need to change all the elements of the attribute to uppercase, the simplest solution would be to use map directly:

my $obj = 'My'->new(list => [qw[ a b c ]]); $obj->list([ map uc, @{ $obj->list } ]); [download]

Now, suppose our attribute is more complex. We'll implement it with the Array trait and its map method:

package My; use Moose; has list => ( is => 'rw', isa => 'ArrayRef[Str]', traits => [qw[ Array ]], handles => { add_to_list => 'push', list_elements => 'elements', map_list => 'map' }, ); [download]

Which changes the code to

We can still imagine a simpler interface: the apply_to_list method that takes a code reference and applies it to each element of the list.

I was able to implement it as another trait:

#!/usr/bin/perl use warnings; use strict; use feature qw{ say }; { package Array::Apply; use Moose::Role; before install_accessors => sub { my $class = shift; my $name = $class->name; my $method = "apply_to_$name"; $class->associated_class->add_method( $method => sub { my ($self, $code) = @_; $_ = $code->($_) for @{ $self->$name }; } ); }; } { package My; use Moose; has list => ( is => 'rw', isa => 'ArrayRef[Str]', default => sub { [] }, traits => [qw[ Array Array::Apply ]], handles => { add_to_list => 'push', list_elements => 'elements', }, ); __PACKAGE__->meta->make_immutable; } my $obj = My->new; $obj->add_to_list(qw( a b c )); $obj->apply_to_list(sub { uc }); say for $obj->list_elements; [download]

But that's not exactly what we imagined. Is it possible to create a trait (or modify the Array trait) in a way that it adds a new apply method to the attribute which we can use in delegation, i.e. something like

I've been trying for several hours, but I just don't know how to do it. Do you?