On Dec 6, 2007 9:30 AM, Alistair Bayley <[EMAIL PROTECTED]> wrote:
> > 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
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