---------- Forwarded message ---------- From: Rafael Gustavo da Cunha Pereira Pinto Date: Mon, Sep 22, 2008 at 15:46 Subject: Re: [Haskell-cafe] hGetContents and lazyness To: Max Vasin 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 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. -- Rafael Gustavo da Cunha Pereira Pinto Electronic Engineer, MSc.