package Foo {
    use Moo;
    has 'attr_A' => ( is => 'ro', default => sub { 'A' } );
    has 'attr_B' => ( is => 'ro', default => sub { 'B' } );
    has 'attr_C' => ( is => 'ro', isa => sub { die unless $_[0] > 0 } );
}