- Haskell-Cafe - Haskell.org

fedora-devshell and Haskell
by Yaakov Nemoy 31 Dec '08

31 Dec '08

Hi Folks, I spent the better part of yesterday working on some tools for doing Fedora packages for Cabal. I've put up a blog report with some preliminary explanations on how to use it, but it might be of value to regular Haskell programmers. The post can be found below. http://loupgaroublond.blogspot.com/2008/12/lazyness-and-cabal-packages.html fedora-devshell's going to be aimed primarily on managing and automating the development of haskell packages for an RPM based distro. It might be useful to other developers too, and i would like to hear if anyone finds it useful. Cheers, and Happy New Year, Yaakov Nemoy

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Reading group for Programming Collective Intelligence
by Creighton Hogg 31 Dec '08

31 Dec '08

Hello Haskellers, For those of your who aren't nose-deep in Real World Haskell at the moment, I'd like to start a small group for the O'Reilly book Programming Collective Intelligence*. I think it might be very instructive to convert the examples & exercises to Haskell. I think it would be good experience for doing neat applications in Haskell & may provide opportunities for adding to Hackage if the book relies on Python libraries that we don't have an equivalent for. If anyone is interested, just e-mail me & I can work on the organization. Cheers, Creighton * http://tinyurl.com/7x4rga

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instance Enum [Char] where ...
by JustinGoguen 31 Dec '08

31 Dec '08

I am having difficulty making [Char] an instance of Enum. fromEnum is easy enough: map fromEnum to each char in the string and take the sum. However, toEnum has no way of knowing what the original string was. For example, running fromEnum on the string "d" will result in 100. But when we pass 100 to toEnum, it does not know if it should treat 100 as "d" or "22" (fromEnum '2' == 50). Source so far: instance Enum [Char] where succ = undefined pred = undefined toEnum n = undefined -- what to do? fromEnum xs = sum $ map fromEnum xs

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Function composition
by Oscar Picasso 30 Dec '08

30 Dec '08

Hi, I can write: *Main> let yes = not . not *Main> :t yes yes :: Bool -> Bool But not: *Main> let isNotEqual = not . (==) <interactive>:1:23: Couldn't match expected type `Bool' against inferred type `a -> Bool' Probable cause: `==' is applied to too few arguments In the second argument of `(.)', namely `(==)' In the expression: not . (==) Why? Oscar

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Re: [Haskell-cafe] represent data sturcture using function
by Ryan Ingram 30 Dec '08

30 Dec '08

On Mon, Dec 29, 2008 at 4:29 AM, <raeck(a)msn.com> wrote: > Would you please give me a complete example of code that I could have more > information > on the idea? Sure, I put up an example at http://ryani.freeshell.org/haskell/gmap.hs class MapKey k where data (:->) k :: * -> * newMap :: (k -> v) -> (k :-> v) fetch :: (k :-> v) -> (k -> v) update :: k -> (v -> v) -> (k :-> v) -> (k :-> v) assign :: k -> v -> (k :-> v) -> (k :-> v) assign k v m = update k (const v) m empty :: v -> (k :-> v) empty = newMap . const with instances & associated data types: instance MapKey () where -- A single value newtype () :-> v = UMap v instance MapKey Bool where -- A value for False and True data Bool :-> v = BMap v v instance (MapKey k1, MapKey k2) => MapKey (k1,k2) where -- A "curried" map newtype (k1,k2) :-> v = PMap (k1 :-> k2 :-> v) instance (MapKey k1, MapKey k2) => MapKey (Either k1 k2) where -- sub-maps for Left k1 and Right k2 data (Either k1 k2 :-> v) = EMap (k1 :-> v) (k2 :-> v) instance MapKey k => MapKey (Maybe k) where -- Now we can build up from existing structures! newtype (Maybe k) :-> v = MaybeM (Either () k :-> v) instance MapKey k => MapKey [k] where -- Value for [] and map for (head:tail) -- -- Note that this includes a recursive ([k] :-> v) map -- in the pair map (k,[k]) :-> v data [k] :-> v = ListM v ((k,[k]) :-> v) instance MapKey Positive where -- We just convert a positive number into -- a list of Bools, then make a map of those newtype Positive :-> v = PosMap ([Bool] :-> v) instance MapKey Integer where -- Now an integer is either negative, zero, or positive. -- So we store a map for negative numbers, a zero value, -- and a map for positive numbers. data Integer :-> v = IntMap (Positive :-> v) v (Positive :-> v) The rest of the class functions are reasonably easy to derive from their type and these data types. -- ryan

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Strange error with HDBC-odbc and MySQL
by Justin Bailey 30 Dec '08

30 Dec '08

I have run into a weird bug with HDBC-odbc that only occurs on my unix system. When I execute a select which returns more than 435 rows twice in the same process, the second execution fails with this error message: SQL error: SqlError {seState = "[\"HYT00\"]", seNativeError = -1, seErrorMsg = "connectODBC/sqlDriverConnect: [\"1045: [unixODBC][MySQL][ODBC 3.51 Driver]Access denied for user 'dummy'@'localhost' (using password: NO)\"]"} I think HDBC-odbc is at fault because the same query works fine from ruby. In both cases the unixODBC library is used. isql (the unixODBC command line utility) also has no problem. I have these software packages installed: ghc 6.8.3 HDBC 1.1.5 HDBC-odbc 1.1.4.4 unixODBC - 2.2.11 MySQL - 5.0.38 Linux - 2.6.16-xenU The program below demonstrates the problem. It does require a table with 500+ rows. On most production servers, information_schema.columns will usually do the trick. \begin{code} import System.Environment import Database.HDBC import Database.HDBC.ODBC import System.IO (hPutStrLn, stderr) import qualified Data.ByteString.Lazy.Char8 as L -- Execute with DNS name as only argument. E.g.: -- ./TestMain localdb main = do [dsn] <- getArgs testMatches dsn testMatches dsn testMatches dsn = do let handler e = do hPutStrLn stderr ("SQL error: " ++ show e) -- Execute a statement and return all the rows getAllRows conn sql = do stmt <- prepare conn sql _ <- execute stmt [] fetchAllRows' stmt handleSql handler $ do conn <- connectODBC $ "dsn=" ++ dsn -- 435 rows works fine, but not 445. rows <- getAllRows conn "select 1 from information_schema.columns limit 445" let evaledRows = filter (\[id] -> (fromSql id) /= (-1 :: Int)) rows putStrLn $ "Got " ++ show (length evaledRows) \end{code}

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Re: [Haskell] hsc2hs can't find gcc under windows
by Duncan Coutts 30 Dec '08

30 Dec '08

On Thu, 2008-12-25 at 15:17 +0900, Curt Sampson wrote: > I'm trying to upgrade from ghc 6.8.3 to 6.10.1. Unfortunately, there > seem to be problems with the Windows version. (I've installed it on > two different machines, both Windows XP, and they've both had the same > problem. Both worked fine with 6.8.3.) > > When I run hsc2hs, on an hsc file, I get the error, "Can't find gcc". > > Just as an experiment, I tried adding c:\ghc\ghc-6.10.1 to my path. That > lets it find gcc, but then it can't find cc1. If I go even further down > this road and add c:\ghc\ghc-6.10.1\gcc-lib to my path, it can find cc1, > but then I get a stream of errors about not being able to find header > files, such as HsFFI.h. > > Does anybody have any clues here to help me? I can provide code, or even > access to the machine, if needed. So here's the story: Up until ghc-6.10, ghc shipped a fork of hsc2hs. The forked version used ghc as the C compiler rather than gcc. This had the advantage that it would know about ghc packages and the include files shipped with packages. The disadvantage was that it was a fork and behaved differently than using hsc2hs with hugs. It also meant that real C compiler options had to be double-escaped to get hsc2hs to pass them to ghc and get ghc to pass them to gcc. In ghc-6.10, ghc ships the normal hsc2hs. What we did not notice before 6.10.1 was released was that calling hsc2hs on its own now does not work without passing additional flags. We never noticed because few people actually do call hsc2hs directly, it is typically used via Cabal. Cabal tells it where gcc lives and what include dirs to use. So that's why at the moment in the Windows installer, hsc2hs cannot even find gcc, because gcc is not on the %PATH% and hsc2hs does not know about ghc's special bundled version of gcc. It also does not know about ghc's include directories. It's clear we'll need to think a bit more about how to get hsc2hs to be both a compiler neutral tool where we can share a single instance between multiple compilers and also have it work "out of the box". > We're also happy to pay for support, if someone out there is offering > commercial support for ghc. We offer commercial support through Well-Typed: http://www.well-typed.com/ Duncan

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IORef vs TVar performance: 6 seconds versus 4 minutes
by Jim Snow 30 Dec '08

30 Dec '08

I decided to try to implement a graph algorithm using STM. Each node in the graph has a set of TVar-protected lists of the nodes it links to and the nodes that link to it. Also, there is a global TVar-protected Data.Map that contains all the nodes in the graph, indexed by name (which is polymorphic): data Node k r = Node { fwdPos :: TVar [Node k r], -- forward links (nodes we like) fwdNeg :: TVar [Node k r], -- we allow "negative" links, too (nodes we don't like) revPos :: TVar [Node k r], -- backlinks (nodes that like us) revNeg :: TVar [Node k r], -- negative back links (nodes that don't like us) currRep :: r, -- extra user-defined data name :: k -- node's unique identifier } deriving Show data Network k r = Network { node :: TVar (M.Map k (Node k r)), -- map of nodes by name trusted :: TVar [Node k r] -- a list of nodes we need to iterate over occasionally } deriving Show I tried loading a datafile of about 20,000 nodes into the graph in one big transaction, and found that it takes about 4 minutes. This seemed rather slow, so I replaced all the TVars with IORefs (and substituted STM with IO in the type signatures), and the same operation with the new version took about 6 seconds! This is all with one thread, so there should be no contention for the TVars. Is there something about STM that makes it scale worse than linearly wrt the number of mutations in a transaction? Above performance numbers are for ghc-6.10.1. With ghc-6.8.3, the STM version takes more than 9 minutes. According to profiling, one of my trouble spots is this function, which just adds an entry onto a TVar [a]: stmcons :: k -> TVar [k] -> STM () stmcons x tv = do xs <- readTVar tv writeTVar tv (x:xs) This seems like it ought to be pretty innocuous, unless the whole list is getting evaluated each time I cons a new entry, or if readTVar or writeTVar are much more expensive than they appear. -jim

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Timing difference
by Aai 30 Dec '08

30 Dec '08

Programming for Rosetta Codes task http://www.rosettacode.org/wiki/Probabilistic_Choice I observed a remarkable (that's to say: for me) difference in timing using lambda function in the one case and point free in the other. Timing was measured in GHCi. Compiled there's no difference! Using lambda ============ ... labels = ["aleph", "beth", "gimel", "daleth", "he", "waw", "zayin", "heth" ] piv n = take n . flip (++) (repeat ' ') main = do g <- newStdGen let rs :: [Float] rs = take 1000000 $ randomRs(0,1) g ps, sps :: [Float] ps = ap (++) (return. (-) 1 .sum) $ map recip [5.0..11.0] sps = scanl1 (+) ps ix = map ((/1000000.0).fromIntegral.length). group.sort $ map (\x -> fromJust $ findIndex not $ map (x>) sps) rs mapM_ (\(l,s,c)-> putStrLn $ (piv 6 l) ++ " " ++ (piv 12 $ show $ s) ++ " " ++ ((piv 12 $ show $ c) )) $ zip3 labels ps ix Using point free ================ replace $ map (\x -> fromJust $ findIndex not $ map (x>) sps) rs by $ map (fromJust. findIndex not. flip map sps. (>)) rs lambda: ======= *Main> main ... (18.24 secs, 2524489600 bytes) point free: =========== *Main> main ... (12.15 secs, 2470893260 bytes) Should I program using point free as much as possible? :-) Thanks =@@i

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ANN: Control.Monad.IfElse 0.85
by Jeff Heard 30 Dec '08

30 Dec '08

Updated Control.Monad.IfElse to include Wren Thornton's Control.Monad.Extras, unlessM, ncond (unless-else chaining), ncondM, and more binary conditional operators.

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