Lyndon Maydwell | 11 Jul 09:03 2013
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Problem with pipes interpretation of Lazy MapM program.

Hi café.

I've come up with a little version of 'uniq' that should take into account md5 sums of the file changes... In essence, this:


main :: IO ()
main = getContents
   >>= mapM check . lines -- PROBLEM!!!!
   >>= mapM_ (putStrLn . (" --> " ++ )) . strip

check :: String -> IO (String, ABCD)
check s = (s,) . md5 . Str <$> readFile s

strip :: (Ord a, Eq b) => [(a,b)] -> [a]
strip = concat . uncurry (zipWith look) . (id &&& maps)

look :: (Ord a, Eq b) => (a,b) -> M.Map a b -> [a]
look (k,v) m | M.lookup k m == Just v = []
             | otherwise              = [k]

maps :: Ord a => [(a,b)] -> [M.Map a b]
maps = scanl (flip (uncurry M.insert)) M.empty


Unfortunately mapM isn't lazy, so this doesn't work. I thought this would be a good opportunity to try out the Pipes library for a simple real-world task, but I've come up against some issues with using 'zip' and 'scan' like functions when translating the code.

This is what I've got so far, but I'm not sure how to proceed:


main :: IO ()
main = runProxy $ stdinS >-> pipe >-> stdoutD

pipe :: () -> ProxyFast () String () String IO ()
pipe = mapMD check
   >-> mapScan
  -- zip, check, output go here
   >-> mapD ((" --> " ++) . show)

mapScan :: () -> ProxyFast () (String, ABCD) () (M.Map String ABCD) IO b
mapScan = scanlp (uncurry M.insert) (M.empty)

check :: String -> IO (String, ABCD)
check s = (s,) . md5 . Str <$> readFile s

-- Utils

scanlp :: (Monad (p () t a b1 m), Monad m, Functor (p () t a b1 m), Proxy p) =>
          (t -> b1 -> b1) -> b1 -> () -> p () t a b1 m b
scanlp f a b = do
  void $ respond a
  v <- request ()
  scanlp f (f v a) b


There doesn't seem to be any easy zipLike functions, and having to write my own scan function seems odd. Can someone point me in the right direction for this?


Thanks!

 - Lyndon
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