It seems reasonable that filter should be generic. The underlying container need only support some kind of traversal, a zero element and a concatenation that eliminates zeroes. A `Monad` provides traversal via `join` and `fmap`; the remaining qualities are satisfied by `MonadPlus`. I've put together an implementation of `filter` for `MonadPlus` that works well enough for lists (though it presents somewhat greater obstacles to efficient compilation). I'm not sure whether `filter` is important enough to warrant its own generics package -- it's one of many collection operations that need to be generified. I'd like to know what folks think about the use of `MonadPlus` in this case. -- Jason Dusek |...an implementation of `filter`...| http://github.com/jsnx/genfil/blob/246026b975ec13587186681b7b346ae1e440d0c9/...