gistfile1.txt

[jdv@wieldy ~]$ cat test_is_w1.p6 
use v6;

role WriteOnce {
    method compose($pkg) {
        callsame;
        $pkg.^find_method(self.name.substr(2)).wrap(-> $obj {
                  say "JDV1:",$obj.perl;
                  say "JDV2:",self.get_value($obj).perl;
                die "Cannot assign more than once to a writeonce attribute"
                  if self.get_value($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 }

say A.new.foo;
say A.new.foo = 123; # OK
say A.new.foo = 456; # should die
[jdv@wieldy ~]$