Ufdjrw

Give the below code, it appears that the order you instatiate the objects matter. The below code will print the same list for both objects when i would expect a different list for each because list is an instance attribute that is created at BUILD time.

package t;

use Moo;
use Types::Standard qw(ArrayRef);

my @list = qw/foo bar baz/;

has list => (
 is => 'rw',
 isa => ArrayRef,
 default => sub @list
);

1;
---
package u;

use Moo;
use Types::Standard qw(ArrayRef);
extends 't';

sub BUILD 
 my ($self) = @_;

 push @$self->list(), qw/apple banana/;
 return $self;

1;
---
#!perl

use Data::Printer;
use t;
use u;

my $u = u->new();
p $u->list();

my $t = t->new();
p $t->list();

Current Output:

 [
 [0] "foo",
 [1] "bar",
 [2] "baz",
 [3] "apple",
 [4] "banana"
]
 [
 [0] "foo",
 [1] "bar",
 [2] "baz",
 [3] "apple",
 [4] "banana"
]

Expected output:

 [
 [0] "foo",
 [1] "bar",
 [2] "baz",
 [3] "apple",
 [4] "banana"
]
 [
 [0] "foo",
 [1] "bar",
 [2] "baz"
]

Since you mutate the array in question, you don't want a reference to the array that you use as the default @list, you want to take a shallow copy [@list].

package t;

use Moo;
use Types::Standard qw(ArrayRef);

my @list = qw/foo bar baz/;

has list => (
 is => 'rw',
 isa => ArrayRef,
 builder =>
 default => sub [@list] 
);

package u;

use Moo;
use Types::Standard qw(ArrayRef);
extends 't';

sub BUILD 
 my ($self) = @_;

 push @$self->list(), qw/apple banana/;
 return $self;


package main;

use Data::Printer;

my $u = u->new();
p $u->list();

my $t = t->new();
p $t->list();

While I'm at it, using BUILD to modify an attribute is possible but not necessarily the best. You can use something like a lazy attribute with a builder method, then overload that method in the subclass, ala

package t;

use Moo;
use Types::Standard qw(ArrayRef);

my @list = qw/foo bar baz/;

has list => (
 is => 'rw',
 isa => ArrayRef,
 builder => '_build_list',
 lazy => 1,
);

sub _build_list 
 my $self = shift;
 return [@list];


package u;

use Moo;
extends 't';

sub _build_list 
 my $self = shift;
 my $list = $self->SUPER::_build_list();
 push @$list, qw/apple banana/;
 return $list;


package main;

use Data::Printer;

my $u = u->new();
p $u->list();

my $t = t->new();
p $t->list();