Why don't you use OpenFile?
> getPackageList packageFile = do
> h <- OpenFile packageFile ReadMode
> c <- hGetContents h
> return $ map (drop 10) $ filter (startsWith "Filename:") $ lines c
I am not at home so I don't have GHC around here, but this usually works for me!
Anyway, it also depends on who is consuming the IO [FilePath] data returned. It will not be evaluated unless asked for!
On Mon, Sep 22, 2008 at 14:52, Max Vasin
<max.vasin@gmail.com> wrote:
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.
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--
Rafael Gustavo da Cunha Pereira Pinto
Electronic Engineer, MSc.