You should use a default or builder for the attributes. Defaults work like this:

package LibraryTest;
use Moose;
use Book;

has unread_books => (
   is      =>'rw',
   isa     => 'ArrayRef[Book]',
   default => sub { [] },
);

has read_books => (
   is      =>'rw',
   isa     => 'ArrayRef[Book]',
   default => sub { [] },
);

[download]

Builders work like this:

package LibraryTest;
use Moose;
use Book;

has unread_books => (
   is      =>'rw',
   isa     => 'ArrayRef[Book]',
   builder => '_build_unread_books',
);

has read_books => (
   is      =>'rw',
   isa     => 'ArrayRef[Book]',
   builder => '_build_read_books',
);

sub _build_unread_books {
   return [];
}

sub _build_read_books {
   return [];
}

[download]

Defaults are very convenient to define, but builders are arguably a better way to do things: the builder is a regular method, so subclasses can easily override it, which is handy.

Builders are a very handy thing when you get used to them - especially when you use them in conjunction with Moose's lazy attribute initialisation feature.

package Cow { use Moo; has name => (is => 'lazy', default => sub { 'Mooington' }) } say Cow->new->name

In reply to Re: Can't call method "MoveBook" on an undefined value by tobyink
in thread Can't call method "MoveBook" on an undefined value by bgu

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