On Dec 6, 2007 9:30 AM, Alistair Bayley
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.
I have actually seen this pattern a lot recently. Recently I have started using a function: mergeMaybes :: (a -> a -> a) -> Maybe a -> Maybe a -> Maybe a mergeMaybes f Nothing y = y mergeMaybes f x Nothing = x mergeMaybes f (Just x) (Just y) = Just (f x y) With which findAllPath could be written: finaAllPath pred (Branch lf r rt) | pred r = fmap (map (r:)) $ mergeMaybes (++) (findAllPath lf) (findAllPath rt) | otherwise = Nothing Or this more search-like feel: findAllPath pred (Branch lf r rt) = do guard (pred r) subpaths <- mergeMaybes (++) (findAllPath lf) (findAllPath rt) return $ map (r:) subpaths Luke