Sieve of Eratosthenes in Perl 6, using streams

sieve.pl

class FilteringStream { ... }

role Stream {
    method next { ... }
    method value { ... }

    method filter {
        return FilteringStream.new(:factor($.value), :substream(self));
    }
}

class StreamOfIntegers does Stream {
    has $.value is rw = 1;

    method next { ++$.value; self }
}

class FilteringStream does Stream {
    has $.factor = die "Must provide factor";
    has $.substream = die "Must provide substream";
    has $.value is rw;

    method next {
        repeat until $.value % $.factor {
            $.value = $.substream.next.value;
        }
        return self;
    }
}

say .next.value for StreamOfIntegers.new, *.filter ... *;