gistfile1.txt

use v6;

role WriteOnce {
    method compose($pkg) {
        callsame;
        $pkg.^find_method(self.name.substr(2)).wrap(-> $obj {
                die "Cannot assign more than once to a writeonce attribute"
                  if self.get_value($obj).defined;
                callsame;
        });
   }
}

multi sub trait_mod:<is>(Attribute:D $attr, :$w1!) {
    $attr does WriteOnce;
    $attr.set_rw();
    warn "useless use of 'is w1' on $attr.name()" unless $attr.has_accessor;
}

class A { has $.foo is w1 }

my A $a .= new;
say $a.foo = 123; # OK
say $a.foo;