Is there some way in Moose to specify that I want an attribute to have a specific type, but also allow there to be a null value (undef?).
For example I am writing a simple implementation of a Linked List and have a Node class where the next
and prev
pointers are required to be of type Node
(this is probably what you would expect)
package Node;
{
use Moose;
has 'value' => (
is => 'rw',
isa => 'Any', # Nodes can be of any type
);
has 'prev' => (
is => 'rw',
isa => 'Node',
predicate => 'has_prev',
);
has 'next' => (
is => 'rw',
isa => 'Node',
predicate => 'has_next',
);
}
But I was hoping to use a sentinel, empty node stored at the head of the list to mark the head, instead of an actual element of the list. So a list of elements [1, 2, 3] would actually look like:
EMPTY -> 1 -> 2 -> 3
I was hoping to be able to specify a empty value (like undef) for the next
and prev
pointers, but when I create an empty Node
in my List class:
package List;
{
use Moose;
has 'head' => (
is => 'rw',
isa => 'Node',
# empty head node
default => sub {
Node->new( value => undef, next => undef, prev => undef );
},
);
Moose complains because undef
is not of type Node
.
Is there a way around this ?