gistfile1.txt

use nqp;

my class WriteOnceAttribute is Attribute {
    has Mu $!type;

    method compose(Mu $package) {
        # Generate accessor method, if we're meant to have one.
        if self.has_accessor {
            my str $name   = nqp::unbox_s(self.name);
            my $meth_name := nqp::substr($name, 2);
            unless $package.^declares_method($meth_name) {
                my $dcpkg := nqp::decont($package);
                my $meth;
                my int $attr_type = nqp::objprimspec($!type);
                if self.rw {
                    $meth  := nqp::p6bool(nqp::iseq_i($attr_type, 0))
                        ??
                        method (Mu:D \fles:) is raw {
                            state $written = False;
                            if !$written {
                                $written = True;
                                nqp::getattr(nqp::decont(fles), $dcpkg, $name)
                            }
                            else {
                                return nqp::getattr(nqp::decont(fles), $dcpkg, $name)
                            }
                        }
                        !!
                        nqp::p6bool(nqp::iseq_i($attr_type, 1))
                        ??
                        method (Mu:D \fles:) is raw {
                            state $written = False;
                            if !$written {
                                $written = True;
                                nqp::getattrref_i(nqp::decont(fles), $dcpkg, $name)
                            }
                            else {
                                return nqp::getattr(nqp::decont(fles), $dcpkg, $name)
                            }
                        }
                        !!
                        nqp::p6bool(nqp::iseq_i($attr_type, 2))
                        ??
                        method (Mu:D \fles:) is raw {
                            state $written = False;
                            if !$written {
                                $written = True;
                                nqp::getattrref_n(nqp::decont(fles), $dcpkg, $name)
                            }
                            else {
                                return nqp::getattr(nqp::decont(fles), $dcpkg, $name)
                            }
                        }
                        !!
                        method (Mu:D \fles:) is raw {
                            state $written = False;
                            if !$written {
                                $written = True;
                                nqp::getattrref_s(nqp::decont(fles), $dcpkg, $name)
                            }
                            else {
                                return nqp::getattr(nqp::decont(fles), $dcpkg, $name)
                            }
                        }
                } else {
                    nextsame;
                }
                $meth.set_name($meth_name);
                $package.^add_method($meth_name, $meth);
            }
        }

        # Apply any handles trait we may have.
        self.apply_handles($package);
    }
}

my module EXPORTHOW { }
EXPORTHOW.WHO<class-attr> = WriteOnceAttribute;
######
$ cat test.pl
use WriteOnceAttribute;

class A {
    has $.foo is rw;
}

my A $a .= new;
$a.foo = 5;
say $a.foo;
$a.foo = 10;
say $a.foo;
######
$ ./perl6-m -I. test.pl
5
Cannot assign to a readonly variable or a value
  in block <unit> at test.pl:10