Can I make the Moose attribute “write once”?

Question

I would like to have an optional Moose attribute that can only be set once.

If I use is => 'ro' , I have to set the attribute when creating the object, but I want it to be possible to add after it (while it is not set yet).

Answer 1

MooseX :: SetOnce

Answer 2

Use the method modifier:

 has 'attr' => ( is => 'rw', predicate => 'is_set', ... }; before 'attr' => sub { my $self = shift; die 'attr already set' if $self->is_set; };