- Beginners - Haskell.org

Re: [Haskell-beginners] Why is type "Integer -> Integer" and not "(Num a) => a -> a"?
by Nathan M. Holden 12 Nov '09

12 Nov '09

This is just a theory, but in my (limited) experience, GHCi is willing to guess at the type of values, where at functions (explicitly or implicitly typed) it can't guess. In your original function, it multiplied a number (Num a) => a by an Int, therefor it must be Int -> Int (because you can't multiply a Double by an Int, don't be crazy. In the new function, though, it can infer the type of the 2, and therefor it can infer it to whatever numerical type you send it. I've run into this problem a few times, but for whatever reason I can't get myself to fall into any of the usual ways I walk blindly into these sorts of problems this time.

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Why is type "Integer -> Integer" and not "(Num a) => a -> a"?
by Dag Hovland 12 Nov '09

12 Nov '09

Hi! I have a problem with understanding some types given by ghc and hugs. The file loaded is: f1 = \x -> x * 2 f2 x = x * 2 After they are loaded I get the following from ghci *Main> :t f1 f1 :: Integer -> Integer *Main> :t f2 f2 :: (Num a) => a -> a *Main> :t \x -> x * 2 \x -> x * 2 :: (Num a) => a -> a I do not understand why f1 is given Integer -> Integer as a type and not the polymorphic (Num a) => a -> a. I believed that f1, f2 and the lambda expression should all have the same type. Similar output to that above is given by Hugs. Thanks, Dag Hovland

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What is the best practice for code]
by legajid 11 Nov '09

11 Nov '09

Hello, i wanted to write a program that searches for all combinations of some numbers, the sum of which is a given value. So, i started writing my program, creating a function for each separate phase : creating list of triples, selecting valuable ones, filtering the result. Looking at my code, i've reduced it several ways; the last version holds on one single line of code. Please, from the 3 versions i established, which one is " better"? What are the criterias of a "good" code ? What about using many anonymous functions? I think there are other solutions than those i propose. Following is my code {- First solution -} nombres=[9,8..1] -- all combinations ftoutes xx = [(x, y, z) | x <- xx, y <- xx, z <- xx] -- keep valuable ones futiles xyz = [(x, y, z) | (x,y,z) <- xyz, y < x, z < y ] -- filter f_flt (x,y, z) = (x+y+z) == 19 -- final result f = filter (f_flt) (futiles (ftoutes nombres )) {- Second solution -} futiles2 xx = [(x, y, z) | x <- xx, y <- xx, z <- xx, y < x, z < y] f2 = filter (\(x,y,z) -> (x+y+z)==19) (futiles2 nombres ) {- Third solution -} f3 = filter (\(x,y,z) -> (x+y+z)==19) ((\ xx -> [(x, y, z) | x <- xx, y <- xx, z <- xx, y < x, z < y]) nombres ) Thanks for your advice Didier.

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Complex list manipulation
by legajid 10 Nov '09

10 Nov '09

Hello, i'm trying to manipulate lists with a "complex" (two-level) structure : a list containing tuples, containing a list containing tuples. The object is to find a specific value according to 2 criterias. Here is my code tabmul= [ ( a, [ ( n, n*a ) | n <- [1..9] ] ) | a <- [2,3,4] ] {- gives the following list [ (2, [(1,2), (2,4), ....(9,18)]), (3, [(1,3), (2,6), ....(9,27)]), (4, [(1,4), (2,8), ....(9,36)]) ] -} {- Get the result for a=3 and n=5 -} -- select level 1 tuple for a=3 s1_flt (x,_) = (x==3) s1=filter (s1_flt) tabmul -- get the list [(3, [(), () .......])] s1_liste =head(s1) -- get the tuple (3, [.....]) -- extract the level 2 list of tuples s1_tup (x,y)=y -- then tuple for n=5 s1_flt2 (x, y) = x==5 s1_soltup = filter (s1_flt2) (s1_tup s1_liste) -- [(3,15)] --finally result for 3 * 5 s1_sol1 (x, y)= y s1_sol = s1_sol1 (head s1_soltup) Perhaps the structure is not the most efficient for this example, but it may simulate records in a database. Getting the result seems really hard. Do you know a shorter way to implement this search? It probably would be simpler when i had triples (a, n ,a*n) ? Another question : the values of criterias are hard-coded. What if i would like to type in s1_sol 3 5; how to put these parameters in the expressions for filters; the filters must get the parameters of the function, in other words the function should return or generate filters ? Thanks for helping Didier.

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Either Monadic Trouble
by iæfai 10 Nov '09

10 Nov '09

With the below code, I am getting an error that I cannot resolve… Chess.hs:52:82: Couldn't match expected type `Map [Char] [Char]' against inferred type `Either ParseError ConfigMap' In the third argument of `findWithDefault', namely `c' In the `documentRoot' field of a record In the first argument of `return', namely `Config {documentRoot = (findWithDefault "web" "Document- Root" c)}' The specific code is: getConf :: FilePath -> IO (Either ParseError Config) getConf filePath = return $ do c <- readConfig filePath -- (Either ParseError ConfigMap) return Config { documentRoot = Map.findWithDefault "web" "Document-Root" c } The type of c should be Either ParseError ConfigMap, which by my understanding of the Either monad would cause the c to be the Right side stripped, or skipped if Left. Full source for the module is below, and full project is hosted at http://patch-tag.com/r/iaefai/chess For some general information, I am replacing ConfigFile dependancy with a Parsec based config parser (I call it SimpleConfig) that suits my needs - it came from http://www.serpentine.com/blog/2007/01/31/parsing-a-simple-config-file-in-h… originally and I modified it. On windows ConfigFile's dependancy on a posix regex library was causing trouble, so this is the effort to get rid of that dependancy. Any thoughts would be useful. There is one associated thought… The original function used to get configuration back to the program is -- Mostly from Chris Done's Blog getConf :: FilePath -> IO (Either (C.CPErrorData, String) Config) getConf filePath = runErrorT $ do let cp = C.emptyCP { optionxform = id } contents <- liftIO $ readFile filePath config <- C.readstring cp contents let get = C.get config "DEFAULT" Config <$> get "Document-Root" I noted it used <$> and in the code that I retrieved originally from Chris Done's blog (no longer able to find it) used <*> for additional items. I would like some easy method of constructing the new Config structure in my new code, especially if it can be done without the record syntax like this thing gets away with. I am not sure how this thing associated "Document-Root" with documentRoot mind you. Thank you again. iæfai. -- import Network.Shed.Httpd import Network.URI import Data.List.Split import Data.Either import Data.Map as Map import Text.ParserCombinators.Parsec import Control.Monad.Error import Control.Applicative import System.Directory import ChessBoard import SimpleConfig data Config = Config { documentRoot :: String } deriving (Read, Show) main :: IO () main = do let docPath = "" let config = Config { documentRoot = "" } putStrLn $ "Using document root: " ++ docPath putStrLn "Starting up httpd on port 6666" server <- initServer 6666 (request config) return () request :: Config -> Request -> IO Response request config req = do putStrLn $ "Recieved " ++ (show $ uriPath $ reqURI req) case url of "ajax" : _ -> return $ Response 404 [] "Not found." _ -> do str <- readFile ((documentRoot config) ++ uri) return $ Response 200 [] str where url = drop 1 $ splitOn "/" uri uri = uriPath $ reqURI req getConf :: FilePath -> IO (Either ParseError Config) getConf filePath = return $ do c <- readConfig filePath -- (Either ParseError ConfigMap) return Config { documentRoot = Map.findWithDefault "web" "Document-Root" c } -- **** ERROR

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Double Trouble
by Philip Scott 10 Nov '09

10 Nov '09

Hi Folks, I'd just like to say thank you very much for your patient answers to my questions so far; it has been a real help on my Haskell adventure. Once I am become a Haskell god like y'all I will endeavour to repay the debt. My current puzzle is doubles. I've extensively scoured the net and found various mentions of the same problem, but as far as I can see no answers; which I can't quite believe as it must be something people do all the time. I need to interface my Haskell program with an existing C++ one over a TCP socket. I need to feed the C++ program doubles in standard 64-bit network order IEEE 765-1985 format, but if I serialize a double using Data.Binary I get something which has altogeather too many bytes (I read somewhere it is an int and a long for the exponent and mantissa). Any advice (or pointers to old threads, it isn't very easy to search the list archives) will be compensated for by a credit note for milk and cookies next time you are in the Cambridge, UK area :) - Philip

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turning a value into an expression
by John Moore 10 Nov '09

10 Nov '09

Hi, How do I turn a value into an expression I want to do for e.g. 8 - 1 turn it into (subtract (Val8) (Val1) Any ideas J

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Simple haskell problem ! Help please
by Denis Firsov 09 Nov '09

09 Nov '09

Hi ! I am beginner in Haskell and have problems with this problem: compress :: Eq a => [a] -> [(a, Int)] If you have string "AAABCCC" it transforms it to : {A, 3} {B,1} {C,3} Could you help me with it ? Thank you in advance !

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Installing packages in Ubuntu
by Glurk 08 Nov '09

08 Nov '09

Hi, I'm a beginner to both Cabal and Linux. I'm a bit confused as to whether I need to be using Cabal when installng packages. I'm running Ubuntu 9.10, and from the Synaptic Package Manager, I can select various Haskell packages to install - will this install the package correctly, or is there something I need to do in Cabal as well ? Thanks ! :)

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Re: Installing packages in Ubuntu
by Nathan M. Holden 08 Nov '09

08 Nov '09

I can't say that you NEED to be using Cabal. I have it, and I'm also an Ubuntu newbie. You can download the bootstrap installer, you'll need to install (via runghc Setup configure, runghc Setup build, runghc Install) the most recent HTML package, and parsec 2.x to get it to run. Then just add an alias (or, I did) so you don't have to write a super-long command to run Cabal. Running Kubuntu 9.10 On Sunday 08 November 2009 08:28:24 am beginners-request(a)haskell.org wrote: > Message: 6 > Date: Sat, 7 Nov 2009 22:05:34 +0000 (UTC) > From: Glurk <streborg(a)hotmail.com> > Subject: [Haskell-beginners] Installing packages in Ubuntu > To: beginners(a)haskell.org > Message-ID: <loom.20091107T230222-873(a)post.gmane.org> > Content-Type: text/plain; charset=us-ascii > > Hi, > > I'm a beginner to both Cabal and Linux. > I'm a bit confused as to whether I need to be using Cabal when installng > packages. > > I'm running Ubuntu 9.10, and from the Synaptic Package Manager, I can > select various Haskell packages to install - will this install the package > correctly, or is there something I need to do in Cabal as well ? > > Thanks ! :)

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