May 2011 - Beginners - Haskell.org

Anyone have problems with cabal and SDL?
by Tom Murphy 17 Jul '11

17 Jul '11

When I use Cabal to get any program which depends on the SDL library, this is the error I get. " checking for sdl-config... no checking for sdl11-config... no configure: error: *** SDL not found! Get SDL from www.libsdl.org. If you already installed it, check it's in the path. If problem remains, please send a mail to the address that appears in ./configure --version indicating your platform, the version of configure script and the problem. cabal: Error: some packages failed to install: Raincat-1.1.1.2 depends on SDL-0.6.2 which failed to install. SDL-0.6.2 failed during the configure step. The exception was: exit: ExitFailure 1 SDL-image-0.6.1 depends on SDL-0.6.2 which failed to install. SDL-mixer-0.6.1 depends on SDL-0.6.2 which failed to install. " I have SDL 1.2.14 (correctly) installed on my system. I'd really appreciate any help with this. Tom

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I don't understand mapM in mapM id (Just 1, Nothing, Just 3)
by Kees Bleijenberg 06 Jun '11

06 Jun '11

I was playing with winghci and I tried: mapM id [Just 1, Just 2, Just 3] result: Just [1,2,3] I don't understand this answer. >From <http://members.chello.nl/hjgtuyl/tourdemonad.html#mapM> http://members.chello.nl/hjgtuyl/tourdemonad.html#mapM mapM mf xs takes a monadic function mf (having type Monad m => (a -> m b)) and applies it to each element in list xs; the result is a list inside a monad. A few things I've found: mapM :: (a -> m b) -> [a] -> m [b] So in this case: a = Maybe Int (second arg in mapM id [Just1, Just 2, Just 3] and b = Int and m = Maybe. So id is :: Maybe Int -> Maybe Int mapM id [Just 1, Nothing, Just 3] result: Nothing. My first guess for the result: Just [Just 1, Nothing, Just 3] when I do: mapM id [1,2,3] I get an error (id has wrong type, which makes sense) Can somebody explain what is going on here? Kees

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Install on "foreign" server problem
by Lawrence Bottorff 01 Jun '11

01 Jun '11

I'm trying to install the binary of 7.0.3 on a Bluehost.com server where I have an account and ssh shell. I want to eventually run Yesod or happstack or some other sort of haskell on web. I've talked with their tech support and they're cool with it as long as it's in-place. Good. I try the ./configure and it errors saying things like: checking for path to top of build tree... utils/ghc-pwd/dist/build/tmp/ghc-pwd: /lib64/libc.so.6: version `GLIBC_2.9' not found (required by utils/ghc-pwd/dist/build/tmp/ghc-pwd) utils/ghc-pwd/dist/build/tmp/ghc-pwd: /lib64/libc.so.6: version `GLIBC_2.7' not found (required by utils/ghc-pwd/dist/build/tmp/ghc-pwd) configure: error: cannot determine current directory I'm not (nor ever was) much of a sysadmin type. Any advice? Actually, I'm just trying to do Web with Haskell on a server/cloud other than my own home box. I have a Web client and I'd like to get us off PHP. Any Haskell-friendly clouds/hosts to suggest? Olwe

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How to avoid repeating code
by Federico Mastellone 31 May '11

31 May '11

Hi, I created a Data.MultiMap module based on Data.Map and Data.Set like this: data MultiMap k v = MultiMap (Map k (Set v)) and a Data.IntMultiMap module based on Data.IntMap and data.IntSet like this: data IntMultiMap = IntMultiMap (IntMap IntSet) For example the functions to add a value I wrote are: For MultiMap: addValue :: k -> v -> MultiMap k v -> MultiMap k v addValue k v (MultiMap m) = MultiMap $ Map.insertWith (\new old -> Set.insert v old) k (Set.singleton v) m For IntMultiMap: addValue :: Int -> Int -> IntMultiMap -> IntMultiMap addValue k v (IntMultiMap m) = IntMultiMap $ IntMap.insertWith (\new old -> IntSet.insert v old) k (IntSet.singleton v) m Both modules look almost the same, with the same documentation, same behavior, same function names but with different type signatures. Is there a way to make this simpler? The same thing happens to the modules that are using MultiMap and IntMultiMap, I have to write two versions of each. Thanks! -- Federico Mastellone Computer Science Engineer - ITBA ".. there are two ways of constructing a software design: One way is to make it so simple that there are obviously no deficiencies, and the other way is to make it so complicated that there are no obvious deficiencies. The first method is far more difficult." Tony Hoare, 1980 ACM Turing Award Lecture.

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Please critique my first library
by Gideon 31 May '11

31 May '11

I've written a modest data binding library for haskell GUIs; the code is at http://code.accursoft.com/binding/. binding-core exposes functionality for binding IO objects to mutable variables and lists. bindnig-gtk and binding-wx are wrappers for Gtk2Hs and WxHaskell respectively. Examples of how to use these can be found in the tests directory of binding-core, and the demo directories of the GUI wrappers. Thank you!

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Handling a MonadError
by Christopher Howard 30 May '11

30 May '11

Hi. I'm trying to learn Data.ConfigFile. I want to use the function readfile: [DESCRIPTION] readfile :: MonadError CPError m => ConfigParser -> FilePath -> IO (m ConfigParser) Loads data from the specified file. It is then combined with the given ConfigParser using the semantics documented under merge with the new data taking precedence over the old. However, unlike merge, all the options as set in the old object are preserved since the on-disk representation does not convey those options. May return an error if there is a syntax error. May raise an exception if the file could not be accessed. [/DESCRIPTION] I understand that this function returns a monad, and I pull the value out of it with (<-). And I believe this is similar to a "Maybe" situation where I can do one thing if it provides a syntax error and another thing if it provides the ConfigParser. But I'm not sure what this would look like in actual code. Could somebody give me a small, simple example? -- frigidcode.com theologia.indicium.us

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Re: [Haskell-beginners] Install on "foreign" server problems
by Michael Snoyman 30 May '11

30 May '11

On Mon, May 30, 2011 at 11:53 PM, Lawrence Bottorff <galaxybeingplan9(a)gmail.com> wrote: >> Hi Lawrence >> >> I think a lot of us are finding that the easiest solution is to >> compile the binaries locally and upload to the server. In my case, >> I've tried having the compiler on a VPS, but compiling is very >> memory-intensive, and it can be a real pain whenever you need to >> upgrade libraries. >> >> My newest deployment is simply an EC2 micro instance running Ubuntu >> 32-bit, just like I have on my local system. I compile the app, copy >> the binary and the static files, and deployment is done. I'll likely >> be adding some notes about this to the Yesod book, and perhaps even >> put together some scripts to automate the process more. >> >> Michael > > So I would do an "in situ/in-place" binary install on a home machine and > simply move that whole installation directory onto the foreign machine. > Hmmm.... What about dependent libraries? If it can't find libc (or the > GL-mesa libraries) won't it break? I'm guessing from the instructions that I > would do a > # ./configure --prefix=/home/me/haskell/ > then a > # make in-place > Is this right? Then move that /haskell directory onto the foreign box? > Lb > What I meant was not to have GHC on the server at all. Build your web apps locally, and then upload the binaries to the server. You can even install a virtual machine locally with the same OS as what your server is using. Michael

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Install on "foreign" server problems
by Lawrence Bottorff 30 May '11

30 May '11

I'm trying to install the binary of 7.0.3 on a Bluehost.com server where I have an account and ssh shell for a web client of mine. I want to eventually run Yesod or happstack or some other sort of "haskell on the web." I've talked with their tech support and they're cool with it as long as it's an "in-place" install. Good. I try the ./configure and it errors saying things like: checking for path to top of build tree... utils/ghc-pwd/dist/build/tmp/ghc-pwd: /lib64/libc.so.6: version `GLIBC_2.9' not found (required by utils/ghc-pwd/dist/build/tmp/ghc-pwd) utils/ghc-pwd/dist/build/tmp/ghc-pwd: /lib64/libc.so.6: version `GLIBC_2.7' not found (required by utils/ghc-pwd/dist/build/tmp/ghc-pwd) configure: error: cannot determine current directory I'm not (nor ever was) much of a sysadmin type. Any advice? I also installed 7.0.3 on my home machine and got a lot of dependency problems I'm sure I'll face once I get past this GLIBC problem. Actually, I'm just trying to do Web with Haskell on a server/cloud other than my own home box. Again, I have a Web client and I'd like to get us off PHP. Any Haskell-friendly clouds/hosts to suggest? Somebody said they had Yesod running on Amazon EC2.... Olwe

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return IO () vs. ()
by Neil Jensen 29 May '11

29 May '11

I've been attempting to refactor some working code and running into confusion about returning IO values. The basic sequence is to query a database, calculate some values, and then store the results back in the database. The function which does the querying of the db and calculating results has the following type signature: calcCUVs :: AccountId -> IO [((ISODateInt, ISODateInt), CUV)] This function stores the results back into the database saveCUVs :: AccountId -> [((ISODateInt, ISODateInt), CUV)] -> IO () saveCUVs account cuvs = do r' <- mapM (\x -> storeCUV (snd $ fst x) account (snd x)) cuvs return () I had a working variation of the below using 'do' notation, but for some reason when I moved to using bind, I'm getting messed up with return values. processAccountCUVs :: AccountId -> ISODateInt -> ISODateInt -> IO () processAccountCUVs account prevMonthEnd monthEnd = -- do if (prevMonthEnd == 0 && monthEnd == 0) then calcCUVs account >>= (\cuvs -> saveCUVs account cuvs) >>= return () else calcCUVs account prevMonthEnd monthEnd >>= (\cuvs -> saveCUVs account cuvs) >>= return () The compiler gives the following error message: Couldn't match expected type `IO ()' against inferred type `()' In the first argument of `return', namely `()' In the second argument of `(>>=)', namely `return ()' In the expression: calcCUVs account >>= (\ cuvs -> saveCUVs account cuvs) >>= return () I thought the last return () would correctly return us IO () as we are in the IO monad... what am I missing? Thanks for any input you can provide. Neil

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question about round function
by Prashant Shah 29 May '11

29 May '11

Hi, This is my first time with Haskell. Here is what I found while trying out some examples : Prelude> round 3.499999999999999 3 it :: Integer Prelude> round 3.4999999999999999 4 it :: Integer --------------- Prelude> round 111111.49999999999 111111 it :: Integer Prelude> round 111111.499999999999 111112 it :: Integer Is it something to do with the CPU register size ?

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