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 {
            Proxy.new(
                FETCH => -> $ { self.get_value($obj) },
                STORE => -> $, $v {
                    die "Cannot re-set a set-once attribute"
                      if self.get_value($obj).defined;
                    self.set_value($obj,$v);
                }
            );
        });
   }
}

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

class A { has $.foo is set-once }

my $a = A.new;
say $a.foo;
say $a.foo = 123;
{ say $a.foo = 456; CATCH { default { say $_; } } }
say $a.foo;
[jdv@wieldy ~]$