Conal Elliott

For quite a while, I’ve been using a handy operation for filtering functional events:

justE :: Event (Maybe a) -> Event a

The idea of justE is to drop the Nothing-valued occurrences and strip off the Just constructors from the remaining occurrences. Recently I finally noticed the similarity with a standard function (in Data.Maybe):

catMaybes :: [Maybe a] -> [a]

I asked on #haskell about a common generalization, and oerjan offered the following:

joinMaybes :: MonadPlus m => m (Maybe a) -> m a
joinMaybes = (>>= maybe mzero return)

I like this name and definition very much, including the use of mzero and return for empty and singleton monadic values, respectively. I particularly like how it works out for lists and events. Each element is passed into maybe mzero return, so that a Nothing becomes empty (mzero), while a Just x becomes a singleton (return x).

I also like how joinMaybes specializes easily into traditional, predicate-based filtering, generalizing the standard filter function on lists:

filterMP :: MonadPlus m => (a -> Bool) -> m a -> m a
filterMP p m = joinMaybes (liftM f m)
 where
   f a | p a       = Just a
       | otherwise = Nothing

So it’s easy to define filterMP in terms of joinMaybes (and hence filter in terms of catMaybes). What about the reverse? Here’s one go at it:

filterMP' :: MonadPlus m => (a -> Bool) -> m a -> m a
filterMP' p = (>>= f)
 where
   f a | p a       = return a
       | otherwise = mzero

joinMaybes' :: MonadPlus m => m (Maybe a) -> m a
joinMaybes' = liftM fromJust . filterMP' isJust

Comparable simplicity, but I don’t like the isJust and fromJust. Not only is there some redundant checking, but the fromJust function is partial, and so this definition is not so obviously total to an automated checker like Catch.

Edit 2008-02-08: added a “Continue reading …” token, to keep my blog’s front page uncluttered.

For quite a while, I’ve been using a handy operation for filtering functional events: justE :: Event (Maybe a) -> Event a The idea of justE is to drop the...