[Haskell-cafe] hGetContents and lazyness

22 Sep 2008

      Hello, haskellers!

Suppose we have function (it filters package filenames from apt Packages file):
...
getPackageList :: FilePath -> IO [FilePath]
getPackageList packageFile = withFile packageFile ReadMode $
                             \h -> do c <- hGetContents h
                                      return $ map (drop 10) $ filter (startsWith "Filename:") $ lines c -- (1)
    where startsWith [] _ = True
          startsWith _ [] = False
          startsWith (x:xs) (y:ys) | x == y    = startsWith xs ys
                                   | otherwise = False
When, I apply it to a Packages file I (surely) get an empty list. This is an expected result due to
lazyness of hGetContents. I tried changing line (1) to
...
return $ map (drop 10) $ filter (startsWith "Filename:") $! lines c
or
...
return $ map (drop 10) $! filter (startsWith "Filename:") $! lines c
with no success.

Chaning it to
...
return $! map (drop 10) $ filter (startsWith "Filename:") $ lines c
makes getPackageList function return several (but not all) filenames.

What I'm missing? And how can I fix my code?

--
WBR,
Max Vasin.