Re: [Haskell-cafe] Re: hGetContents and lazyness

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Max Vasin wrote:

Micah Cowan writes:

...

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)

Actually, I meant more the missing "main" function; but the example invocation you gave will do fine. As previously mentioned, the full list will be output if you ensure that the putStrLn calls take place within withFile, rather than without: getPackageList :: FilePath -> IO () getPackageList packageFile = withFile packageFile ReadMode $ \h -> do c <- hGetContents h mapM_ (putStrLn . drop 10) $ filter (startsWith "Filename:") $ lines c where startsWith [] _ = True startsWith _ [] = False startsWith (x:xs) (y:ys) | x == y = startsWith xs ys | otherwise = False Your experiments with $! don't work, I believe because seq only does a "shallow" evaluation: in particular, it doesn't evaluate successive elements in a list (RWH puts it: "seq stops as soon as it reaches a constructor" ($! is defined in terms of seq)). In order to work around this, you'd need to define your own versions of map and filter to manage the construction using seq. Not worth it, IMO, especially since accomplishing this forces the whole list to be held in memory, which isn't as nice as processing things sequentially before the file has been closed. - -- Micah J. Cowan Programmer, musician, typesetting enthusiast, gamer. GNU Maintainer: wget, screen, teseq http://micah.cowan.name/ -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.7 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iD8DBQFI2R9z7M8hyUobTrERAggSAJwIVUAEyeoeIRbNaljHIycTrub4UQCeM1j7 ARGXhGNl//o3SlipDuGhUyw= =KVlj -----END PGP SIGNATURE-----