> 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 _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
