Micah Cowan <[EMAIL PROTECTED]> writes: > 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. > > Combined with the fact that you're not evaluating its non-strict result > until after the file handle has been closed, yes. > > Your current set of IO actions is probably similar to: > . open file > . process file > . close file > . use results from processing the file. > where the first three steps are all handled by your getPackageList. To > avoid either getting incomplete (or empty) results, or having to > strictify everything with $!, it'd be better for you to use a process > more like: > . open file > . process file > . use results from processing the file. > . close file > probably by moving the withFile outside of getPackageList, to wrap a > function that prints the results after they've been obtained. The > function passed to withFile should generally include all the processing > related to the file and its results, I believe. Yes. Probably I should leave closing file to the GC and use readFile, this seems to be the simplest way.
>> I tried changing line (1) to >> >>> return $ map (drop 10) $ filter (startsWith "Filename:") $! lines c > > The $! forces strictness, but since it's deep in the result, it isn't > evaluated until it's too late. > >> Chaning it to >> >>> return $! map (drop 10) $ filter (startsWith "Filename:") $ lines c >> >> makes getPackageList function return several (but not all) filenames. > > I think we'd need to see the actual input and expected output, to > understand what's going wrong here. It worked fine for me, for small tests. The gzipped example file is here: ftp://ftp.debian.org/debian/dists/lenny/contrib/binary-i386/Packages.gz > > By the way, it's good policy to always post complete, runnable examples. > Requiring anyone who wants to help you to write additional code just to > get it to run decreases the chances that someone will bother to do so. Sorry. I've just omitted module imports: > import Control.Monad (filterM, mapM) > import System.IO (withFile, IOMode (ReadMode), hGetContents) > import qualified System.Posix.Files as SPF (isDirectory, getFileStatus) Running in GHCi: GHCi, version 6.8.2: http://www.haskell.org/ghc/ :? for help Loading package base ... linking ... done. Prelude> :load Foo.hs [1 of 1] Compiling Foo ( Foo.hs, interpreted ) Ok, modules loaded: Foo. *Foo> getPackageList "Packages" >>= mapM_ putStrLn Loading package old-locale-1.0.0.0 ... linking ... done. Loading package old-time-1.0.0.0 ... linking ... done. Loading package filepath-1.1.0.0 ... linking ... done. Loading package directory-1.0.0.0 ... linking ... done. Loading package unix-2.3.0.0 ... linking ... done. pool/contrib/a/acx100/acx100-source_20070101-3_all.deb pool/contrib/a/alien-arena/alien-arena_7.0-1_i386.deb pool/contrib/a/alien-arena/alien-arena-browser_7.0-1_all.deb pool/contrib/a/alien-arena/alien-arena-server_7.0-1_i386.deb pool/contrib/a/alsa-tools/alsa-firmware-loaders_1.0.16-2_i386.deb pool/contrib/a/amoeba/amoeba_1.1-19_i386.deb pool/contrib/a/apple2/apple2_0.7.4-5_i386 *Foo> Printed list of package files is incomplete. -- WBR, Max Vasin. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
