# This is NOT a good way to validate for this use case.
# But this code works in my current Rakudo.

class Interval {
    has $.lb = die 'Lower bound is required';
    has $.ub = die 'Upper bound is required';

    method lb() is rw {
        return Proxy.new:

            # note `sub` declarator and ignored arg:
            FETCH => sub ($) { return $!lb; },

            # note `sub` declarator and ignored (first) arg
            STORE => sub ($, $lb) {
                die "Require lb <= ub" unless $lb <= $!ub;
                $!lb = $lb;
            };
    }

    method ub() is rw {
        return Proxy.new:
            FETCH => sub ($) { return $!ub; },
            STORE => sub ($, $ub) {
                die "Require lb <= ub" unless $!lb <= $ub;
                $!ub = $ub;
            };
    }
}