---------- Forwarded message ---------- From: Rafael Gustavo da Cunha Pereira Pinto <[EMAIL PROTECTED]> Date: Mon, Sep 22, 2008 at 15:46 Subject: Re: [Haskell-cafe] hGetContents and lazyness To: Max Vasin <[EMAIL PROTECTED]>
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 <[EMAIL PROTECTED]> 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. > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > -- Rafael Gustavo da Cunha Pereira Pinto Electronic Engineer, MSc. -- Rafael Gustavo da Cunha Pereira Pinto Electronic Engineer, MSc.
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