- Haskell-Cafe - Haskell.org

So how do people pronounce 'cabal' around here?
by Dan Piponi 30 May '08

30 May '08

In particular, which syllable gets the stress, and what are the lengths of the two vowels? Couldn't find anything in the FAQ (http://www.haskell.org/haskellwiki/Cabal/FAQ) -- Dan

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zlib, missing zlib.h
by Thomas Hartman 30 May '08

30 May '08

Tried to install http://hackage.haskell.org/cgi-bin/hackage-scripts/package/zlib which is required for http://darcs.haskell.org/cabal-branches/cabal-1.4 which is required for cabal-install. Got the following error. apt-get installed zlibc on a stab in the dark, but same result. advice? thartman@thartman-laptop:~/haskellInstalls/smallInstalls/cabal-install/zlib-0.4.0.4>runghc Setup.hs configure Configuring zlib-0.4.0.4... thartman@thartman-laptop:~/haskellInstalls/smallInstalls/cabal-install/zlib-0.4.0.4>runghc Setup.hs build &>out thartman@thartman-laptop:~/haskellInstalls/smallInstalls/cabal-install/zlib-0.4.0.4>head out Preprocessing library zlib-0.4.0.4... Stream.hsc:74:18: error: zlib.h: No such file or directory Stream.hsc: In function 'main': Stream.hsc:254:0: error: 'z_stream' undeclared (first use in this function) Stream.hsc:254:0: error: (Each undeclared identifier is reported only once thartman@thartman-laptop:~/haskellInstalls/smallInstalls/cabal-install/zlib-0.4.0.4>

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ANNOUNCE: Haskell Communities and Activities Report (14th ed., May 2008)
by Janis Voigtlaender 30 May '08

30 May '08

On behalf of the many, many contributors, we are pleased to announce that the Haskell Communities and Activities Report (14th edition, May 2008) http://www.haskell.org/communities/ is now available from the Haskell Communities home page in PDF and HTML formats. Many thanks go to all the people that contributed to this report, both directly, by sending in descriptions, and indirectly, by doing all the interesting things that are reported. We hope you will find it as interesting a read as we did. If you haven't encountered the Haskell Communities and Activities Reports before, you may like to know that the first of these reports was published in November 2001. Their goal is to improve the communication between the increasingly diverse groups, projects and individuals working on, with, or inspired by Haskell. The idea behind these reports is simple: Every six months, a call goes out to all of you enjoying Haskell to contribute brief summaries of your own area of work. Many of you respond (eagerly, unprompted, and well in time for the actual deadline ;) ) to the call. The editor collects all the contributions into a single report and feeds that back to the community. When we try for the next update, six months from now, you might want to report on your own work, project, research area or group as well. So, please put the following into your diaries now: ---------------------------------------- End of October 2008: target deadline for contributions to the November 2008 edition of the HC&A Report ---------------------------------------- Unfortunately, many Haskellers working on interesting projects are so busy with their work that they seem to have lost the time to follow the Haskell related mailing lists and newsgroups, and have trouble even finding time to report on their work. If you are a member, user or friend of a project so burdened, please find someone willing to make time to report and ask them to `register' with the editor for a simple e-mail reminder in the middle of November (you could point us to them as well, and we can then politely ask if they want to contribute, but it might work better if you do the initial asking). Of course, they will still have to find the ten to fifteen minutes to draw up their report, but maybe we can increase our coverage of all that is going on in the community. Feel free to circulate this announcement further in order to reach people who might otherwise not see it. Enjoy! Andres Loeh and Janis Voigtlaender <hcar at haskell.org> -- Dr. Janis Voigtlaender http://wwwtcs.inf.tu-dresden.de/~voigt/ mailto:voigt@tcs.inf.tu-dresden.de

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HDBC with SQL Server / OBDC
by Morten Holm Pedersen 30 May '08

30 May '08

am trying to do a simple DB connection from Haskell to a SQL Server 2005 (on Windows obviously). The DSN name ("Nylon") works from C++ but when running the below example (or any other I can think of) ghci crashes. Does anyone know a resolution for this or where the problem can possible be ? GHCi, version 6.8.2: http://www.haskell.org/ghc/ :? for help Loading package base ... linking ... done. Prelude> :m Database.HDBC Prelude Database.HDBC> :m + Database.HDBC.ODBC Prelude Database.HDBC Database.HDBC.ODBC> do { conn <- connectODBC "DSN=Nylon"; xs <- getTables conn; putStr $ head xs; } Loading package array-0.1.0.0 ... linking ... done. Loading package containers-0.1.0.1 ... linking ... done. Loading package bytestring-0.9.0.1 ... linking ... done. Loading package old-locale-1.0.0.0 ... linking ... done. Loading package old-time-1.0.0.0 ... linking ... done. Loading package mtl-1.1.0.0 ... linking ... done. Loading package HDBC-1.1.4 ... linking ... done. Loading package HDBC-odbc-1.1.4.3 ... linking ... done. CRASH !!! Thanks Morten

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happs install seems to have become very slow since upgrading cabal Re: [Haskell-cafe] test driving cabal install...
by Thomas Hartman 30 May '08

30 May '08

sudo cabal install --global HAppS-State went fine, and ghc-pkg had output on just the one (global) file. but sudo cabal install --global HAppS-Server is slooooooow. I finally canceled this and did sudo runghc Setup.hs build -v3 with the same result: extremely, extremely slow compile. It seems to get hung up one one line: src/HAppS/Server/Facebook.hs:416:2: Warning: Defined but not used: `a' *** Desugar: Result size = 52971 *** Simplify: Result size = 34350 Result size = 33980 Result size = 33972 Result size = 33972 *** Specialise: Result size = 52989 *** Float out (not lambdas, not constants): Result size = 55269 ... and more and more of same. for 15 minutes now. With verbose output on at least I can see there is something going on behind the scenes, but still. I don't remember things taking this long to compile before I upgraded to cabal v1.4... cabal bug? cabal --version cabal-install version 0.4.7 using version 1.3.11 of the Cabal library thomas. 2008/5/29 Thomas Hartman <tphyahoo(a)gmail.com>: > I see. I had done > > sudo cabal install HAppS-Server > > but this just installed with root ownership in my home directory. > > I'm now trying with > > sudo cabal install --global HAppS-Server > > > 2008/5/29 Daniel Patterson <daniel(a)dbpatterson.com>: >> I think you need to install HAppS-Data etc globally... I'm not sure why >> locally installed (ie, only your user) packages wouldn't be picked up by runghc >> Setup.hs'ing, but if they are install system wide it should definitely work. >> >> I think it is cabal --global install foo >> >> On Thu, 29 May 2008 18:11:04 -0700 >> "Thomas Hartman" <tphyahoo(a)gmail.com> wrote: >> >>> After a little drama with zlib, I managed to get cabal-install installed. >>> >>> I then attempted to do >>> >>> cabal install HAppS-Server >>> >>> since this is a module with a lot of dependencies, and in rapid >>> development flux, so perenially painful for me to install. >>> >>> The result is that I managed to install everything up to HAppS-State, >>> which I think is the last dependency, but then seemed to hang >>> indefinitely in the middle of installing HAppS-Server at the end. >>> >>> OK, I thought, then perhaps I can do normal runghc Setup.hs after >>> downloading and unzipping the tar from >>> >>> http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HAppS-Server-0.9… >>> >>> However, this resulted in error >>> >>> thartman@thartman-laptop:~/haskellInstalls/smallInstalls/HAppS-Server-0.9.2.1>runghc >>> Setup.hs configure >>> ... >>> Setup.hs: At least the following dependencies are missing: >>> HAppS-Data >=0.9.2... >>> >>> Strange, because I had just installed that module via cabal-install, >>> and I could load it in ghci with :m +HappS.Data. >>> >>> I then ran ghc-pkg and got this strange result that my packages were >>> broken into two different files. Is this by design? >>> >>> ghc-pkg list >>> /usr/local/lib/ghc-6.8.2/package.conf: >>> Cabal-1.2.3.0, Cabal-1.3.11, Cabal-1.5.2, DeepArrow-0.2, >>> .... >>> /home/thartman/.ghc/i386-linux-6.8.2/package.conf: >>> HAppS-Data-0.9.2.1, HAppS-IxSet-0.9.2.1, HAppS-State-0.9.2.1, >>> >>> >>> I am curious if anybody else is able to install HAppS-Server using >>> cabal install, and whether they can shed any light on the other isuses >>> I raised. >>> >>> Thomas. >>> _______________________________________________ >>> Haskell-Cafe mailing list >>> Haskell-Cafe(a)haskell.org >>> http://www.haskell.org/mailman/listinfo/haskell-cafe >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >

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WebSense doesn't like darcs.haskell.org
by Chad Scherrer 30 May '08

30 May '08

My work uses WebSense to filter viewable pages - I don't have an option. I used to be able to get to darcs.haskell.org just fine, but apparently a /hacking directory was added somewhere, so WebSense put it on the naughty list. I put in a request for WebSense to review the site; hopefully it will soon be allowed again. But if not, I'll have to go through lots of red tape, which I'd really like to avoid. I assume/hope this "hacking" is only in the sense of making adjustments to code. Is this right? If there's anything involving DoS attacks, etc, I might never be able to get to it from work again. I get the impression WebSense is relatively widely used. Is anyone else here having similar trouble? Chad

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reordering pure bindings in monads
by Tim Newsham 30 May '08

30 May '08

Intuitively it seems like the applicative expression: (++) <$> getLine <*> getLine should represent the same thing as the more traditional liftM2 expressions: do { x <- getLine; y <- getLine; return ((++) x y) } but it seems to me that you cant directly manipulate the first into the second. I get: do x2' <- getLine x1 <- return ((++) x2') x2 <- getLine return (x1 x2) the only way I can get from here to the liftM2 definition is if I treat "x1 <- return ((++) x2')" as "let x1 = (++) x2", and then allow it to be reordered after the second getLine. Then it is straightforward to reduce to the liftM2 expression above. It seems to me that this is a valid transformation if: - no side effects, including pattern match errors, can occur in the let (or x1 <- return ...). - movement doesnt change the dependencies - x1 isnt used between the original line and its new position - there are no new bindings for x2' introduced between the original line and the new line. Did I overlook anything? Do any haskell implementations allow rewrites like these to occur? Tim Newsham http://www.thenewsh.com/~newsham/

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ambiguous constraint errors
by Evan Laforge 30 May '08

30 May '08

I have two related questions: #1 I'm getting some annoying type errors that I don't fully understand, and wind up having to do a workaround that I don't totally like. Here's a simplified version of my situation: data Ambi m = Ambi { ambi_monad :: m Int , ambi_int :: Int } some_ambi :: Monad m => Ambi m some_ambi = Ambi (return 5) 10 ambi_table :: Monad m => [(String, Ambi m)] ambi_table = [("default", some_ambi)] get_int :: String -> Maybe Int get_int sym = fmap ambi_int (lookup sym ambi_table) ----------- get_int produces: Ambiguous type variable `m' in the constraint: `Monad m' arising from a use of `ambi_table' at ambi.hs:13:40-49 So I guess this means I'm not telling it which 'm', so it doesn't know how to resolve the 'return'... but the thing is, I'm not even using that value, so it doesn't matter what it resolves to. So it works if I pick some random monad: get_int sym = fmap ambi_int (lookup sym ambi_table :: Maybe (Ambi Maybe)) Note that I can't leave it as 'Monad m => Ambi m' because I still get an ambiguous type variable complaint. I'm a little disconcerted by having to pick some random dummy monad. Even worse, everything this type touches starts requiring explicit type declarations everywhere. Is there some easier way to do this? #2 This is somewhat related to another issue I've been having, which is that I have some kind of complicated type, e.g. '(SomeMonad some, Monad m) => some (SomethingM m Status)' that I use in a lot of places. It would be a lot less typing and easier to modify later if I wrote a type alias: type Command = (Monad some, Monad m) => some (State.StateT () m Status) but of course, this isn't allowed, since the type variables don't appear on the lhs, and if I put a context there, it's a syntax error. While I can write it with data: data (Monad some, Monad m) => Command some m = Command (some (State.StateT () m Status)) I've been told this doesn't mean what I expect it to, which is that the context constraints propagate up to and unify with the containing type (out of curiosity, since it's accepted, what *does* this do? I think I read it somewhere once, but now I forget and can't find it). And sure enough, using this type doesn't make my type declarations have the right contexts. So the first problem means that I have to declare types in various inconvenient places, and the second one means that I have to type out all the various class constraints (I can still alias away the non-polymorphic bits), and all my type declarations start looking much more complicated than they are. The "solution" I've been using for some of this is just to remove the polymorphism, so I can write a simple alias like type Command = SomethingM (State.StateT () Identity Status) and now I can think of "a command" and have various functions that take and return Commands, without caring that it's some kind of monad with context constraints. But of course, this isn't always possible since sometimes I need the type to remain polymorphic (i.e. while most of these I don't *think* will run in some other monad, some of them definitely get called in multiple contexts). Is there any nicer way around this? And what's the underlying issue that makes this necessary? I can live with all the context hair everywhere, but it sure would be nicer to be able to define it once and for all in one place.

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Question on type synonym definition and language extensions
by Olivier Boudry 30 May '08

30 May '08

Hi all, I'm trying to define a type synonym for 2D MArray instances. The goal is to keep the function signature simple and readable using type `Matrix` instead of something like `(Ix i, MArray a e m) => m (a i e)`. After some "read, guess, try, error" cycles I came up with this: type Matrix = forall m. forall a. forall i. forall n. (Ix i, MArray a n m, Num i, Num n) => m (a (i,i) n) it requires option -XRank2Types to work. But then I can write my function as: test :: Matrix test = do { a <- newArray ((0,0),(5,8)) 0; writeArray a (0,0) 1; return a } Then I wanted to be able to give the Index and Value types in the type synonym but keep it flexible in terms of which MArray instance is used. I changed it to: type Matrix i n = forall m. forall a. (MArray a n m) => m (a (i,i) n) and the type signature of the test function becomes: test :: Matrix Int Double For this one I had to add an extra -XFlexibleContexts option to build it. It works and I'm quite happy with the look of my new function type signatures, but I'm just wondering if I'm doing something bad or if there is a cleaner/simpler way to define the same type synonym without requiring language extensions. What are the risks associated with using these two language extensions, non compatibility with other compilers or more? Thanks, Olivier.

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Bug in parallel GHC runtime?
by Dusan Kolar 29 May '08

29 May '08

Hello all, The attached file was compiled by the following command: ghc -O2 --make -threaded ltest1pl.hs -o alall When run in a sequential mode, I get this result: ./alall Starting ... Lst1: 41666666650000 Lst2: 41669166700000 T1: 0m 1.0e-6s 36 End! On the other hand, when run in a threaded mode, I get the following error: ./alall +RTS -N2 Starting ... Lst1: 41666666650000 Lst2: 41669166700000 T1: 0m 0.0s Segmentation fault Is it fault of the GHC runtime, or is it something on my side? My machine: uname -a Linux pc 2.6.24-ARCH #1 SMP PREEMPT Sun Mar 30 10:50:22 CEST 2008 x86_64 Intel(R) Core(TM)2 CPU 6600 @ 2.40GHz GenuineIntel GNU/Linux My ghc: ghc --version The Glorious Glasgow Haskell Compilation System, version 6.8.2 Thanks and regards Dusan --import Control.Concurrent --import Control.Concurrent.MVar import System.Time import Control.Parallel.Strategies --import Data.List (foldl') import qualified Data.ByteString as B sumAllSums [] = 0 sumAllSums l@(_:xs) = sumlist 0 l + sumAllSums xs where sumlist res [] = res sumlist sr (v:vs) = sumlist (sr+v) vs wlist2wbs [] = B.pack [] wlist2wbs l@(_:_) = B.pack $ encode l where encode :: Integral a => [Int] -> [a] encode [] = [] encode (x:xs) = if x==0 then 0:0:encode xs else fromIntegral (x `mod` 256) : fromIntegral (x `div` 256) : encode xs main = do putStrLn $ "Starting ..." let lst1 = [0..49999] let lst2 = [0..50000] let bs1 = wlist2wbs lst1 let bs2 = wlist2wbs lst2 tm1 <- getClockTime let (v1:v2:_) = parMap rnf sumAllSums [lst1,lst2] tm1' <- getClockTime putStrLn ("Lst1: " ++ show v1) putStrLn ("Lst2: " ++ show v2) let tdiff1 = diffClockTimes tm1' tm1 --let tdiff2 = diffClockTimes tm2' tm2 putStrLn $ "T1: " ++ show (tdMin tdiff1) ++ "m " ++ show (fromIntegral(tdSec tdiff1) + fromIntegral(tdPicosec tdiff1)/1e12) ++ "s" --putStrLn $ "T2: " ++ show (tdMin tdiff2) ++ "m " ++ show (fromIntegral(tdSec tdiff2) + fromIntegral(tdPicosec tdiff2)/1e12) ++ "s" putStrLn $ show $ {-ibs1 +-} B.index bs1 99999 + B.index bs2 49999 {-((bs1 + fromIntegral (B.index bs2 99999)) :: Integer)-} putStrLn $ "End!" {- main = do tm1 <- getClockTime putStrLn $ "Starting ... " mv1 <- newEmptyMVar mv2 <- newEmptyMVar t1 <- forkIO (putMVar mv1 $! sumAllSums [0..49999]) t2 <- forkIO (putMVar mv2 $! sumAllSums [1..50000]) v1 <- takeMVar mv1 v2 <- takeMVar mv2 killThread t1 killThread t2 putStrLn $ "Result: " ++ show (v1+v2) tm2 <- getClockTime let tdiff = diffClockTimes tm2 tm1 putStrLn $ "End! " ++ show (tdMin tdiff) ++ "m " ++ show (fromIntegral(tdSec tdiff) + fromIntegral(tdPicosec tdiff)/1e12) ++ "s" -}

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