Paulo J. Matos wrote:
Hello all,
Hi.
findAllPath :: (a -> Bool) -> (BTree a) -> Maybe [[a]]
findAllPath pred (Leaf l) | pred l = Just [[l]]
| otherwise = Nothing
findAllPath pred (Branch lf r rt) | pred r = let lfpaths = findAllPath pred lf
rtpaths = findAllPath pred rt
in
if isNothing lfpaths &&
isNothing rtpaths
then Nothing
else
if isNothing lfpaths
then Just (map (r:)
$ fromJust rtpaths)
else
if isNothing rtpaths
then Just (map
(r:) $ fromJust lfpaths)
else Just (map
(r:) $ fromJust rtpaths ++ fromJust lfpaths)
| otherwise = Nothing
Ignoring the fact that you found a better way to write this entirely, a
style point.
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.
Hope someone finds that useful,
Jules
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