prototype for an "is write-once" trait for Attributes

gistfile1.txt

use MONKEY;

sub trait_mod:<is>(Attribute:D $attr, :$write-once!) {
    my $class := nqp::getattr($attr,Attribute,'$!package');
    my str $name = nqp::getattr($attr,Attribute,'$!name');
    my $type := nqp::getattr($attr,Attribute,'$!container_descriptor').default;
    $class.^add_method(
      nqp::substr($name,2),
      my method () is rw {
          nqp::if(
            nqp::eqaddr(nqp::decont(nqp::getattr(self,$class,$name)),$type),
            nqp::getattr(self,$class,$name),
            nqp::decont(nqp::getattr(self,$class,$name))
          )
      }
    )
}

class A {
    has Int $!a is write-once;
}

my $a = A.new;
dd $a.a;
$a.a = 42;
dd $a.a;
$a.a = 666;