Use of isNothing and fromJust and a cascade of ifs is generally a poor sign, much better to use case:
findAllPath pred (Branch lf r rt) | pred r = case (findAllPath pred lf,findAllPath pred rt) of (Nothing,Nothing) -> Nothing (Nothing,Just rtpaths) -> Just (map (r:) rtpaths) (Just lfpaths,Nothing) -> Just (map (r:) lfpaths) (Just lfpaths,Just rtpaths) -> Just (map (r:) $ rtpaths ++ lfpaths) | otherwise = Nothing
the general pattern is : replace isNothing with a case match on Nothing, replace fromJust with a case match on Just, don't be afraid to case two expressions at once.
Nested Maybe cases put me in mind of the Maybe monad. Although in this case it''s not trivial; we also need to involve the Maybe [a] instance of Data.Monoid too (for the mappend function). I do wonder if I'm abusing the monadic instances of Maybe though; is this really any clearer than Jules' code? (BTW, this has probably come up before, but wouldn't it be a little bit nicer if "when" returned mzero rather than () in the "do nothing" case?)
when' :: MonadPlus m => Bool -> m a -> m a when' pred action = if pred then action else mzero
findAllPath :: (a -> Bool) -> (BTree a) -> Maybe [[a]] findAllPath pred (Leaf l) = when' (pred l) (return [[l]]) findAllPath pred (Branch lf r rt) = when' (pred r) $ do x <- mappend (findAllPath pred lf) (findAllPath pred rt) return (map (r:) x)
Alistair