November 2009 - Beginners

Functor question
by Matthew Wong 17 Nov '09

17 Nov '09

Hi, When I define my data as fellow, data Moo a = Moo a deriving (Show) instance Functor Moo where fmap f (Moo a) = Moo (f a) GHC gives me no problem. Then I add something, data (Num a) => Moo a = Moo a deriving (Show) instance Functor Moo where fmap f (Moo a) = Moo (f a) Now GHC gives me the follow error - see bellow. What is the reason behind this? What should I do to correct this? thx, // matFun_v2.hs:16:12: Could not deduce (Num a) from the context () arising from a use of `Moo' at matFun_v2.hs:16:12-14 Possible fix: add (Num a) to the context of the type signature for `fmap' In the pattern: Moo a In the definition of `fmap': fmap f (Moo a) = Moo (f a) In the instance declaration for `Functor Moo' matFun_v2.hs:16:21: Could not deduce (Num b) from the context () arising from a use of `Moo' at matFun_v2.hs:16:21-29 Possible fix: add (Num b) to the context of the type signature for `fmap' In the expression: Moo (f a) In the definition of `fmap': fmap f (Moo a) = Moo (f a) In the instance declaration for `Functor Moo'

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Functor question.
by Phillip Pirrip 16 Nov '09

16 Nov '09

Hi, When I define my data as fellow, data Moo a = Moo a deriving (Show) instance Functor Moo where fmap f (Moo a) = Moo (f a) GHC gives me no problem. Then I add something, data (Num a) => Moo a = Moo a deriving (Show) instance Functor Moo where fmap f (Moo a) = Moo (f a) Now GHC gives me the follow error - see bellow. What is the reason behind this? What should I do to correct this? thx, // matFun_v2.hs:16:12: Could not deduce (Num a) from the context () arising from a use of `Moo' at matFun_v2.hs:16:12-14 Possible fix: add (Num a) to the context of the type signature for `fmap' In the pattern: Moo a In the definition of `fmap': fmap f (Moo a) = Moo (f a) In the instance declaration for `Functor Moo' matFun_v2.hs:16:21: Could not deduce (Num b) from the context () arising from a use of `Moo' at matFun_v2.hs:16:21-29 Possible fix: add (Num b) to the context of the type signature for `fmap' In the expression: Moo (f a) In the definition of `fmap': fmap f (Moo a) = Moo (f a) In the instance declaration for `Functor Moo'

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Dynamic libraries not of required architecture when installing via cabal (Snow Leopard)
by Malthe Jørgensen 15 Nov '09

15 Nov '09

I'm on Snow Leopard, so I've read numerous posts on the subject of GHC and it's Snow Leopard (non-)compatibility. I've got most things working - I can do a 'cabal update' - something that gave errors before. GHC itself seems to be working fine. I want to install the SDL package from Hackage, which I first tried doing like this: 'cabal install sdl' which gives a bunch of warnings, all looking like this 'ld: warning: in /opt/local/lib/libSDL.dylib, file is not of required architecture' with 'libSDL.dylib' replaced with various *.a and *.dylib files I then found this in a Usenet post about Snow Leopard breaking GHC: -- Once cabal works, options --ld-option=-m32 (and also --gcc-option=- m32) may be used. These options may also be passed to "./Setup configure" And tried this: (I think maybe my problem is that I'm not passing these flags correctly to cabal) 'cabal install sdl --ld-options="-arch i386" --gcc-option=-m32 -- reinstall' Well I get the same errors, well warnings actually. Can I just ignore these warnings? When compiling my small sample code: import Prelude import Graphics.UI.SDL as SDL main = do SDL.init [InitEverything] setVideoMode 640 480 32 [] I again get the warnings: ld: warning: in /opt/local/lib/libSDL.dylib, file is not of required architecture ld: warning: in /opt/local/lib/libSDLmain.a, file is not of required architecture and then a bunch of undefined symbols such as: _SDL_Init _SDL_SetClipRect Well this post basically based on the assumption that the warnings are breaking the SDL library and thereby keeping my code from compiling - so how do I fix those warnings? - Sorry for the long mail Malthe

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index too large
by Nathan M. Holden 13 Nov '09

13 Nov '09

I've been working on a quick program that acts (vaguely) like a console, lets me type notes and outputs them in a .tex file. I figure it's not that complicated, but it uses a vaguely n-ary structure, so that I can have headers and sub-notes, which loos like: data Note = N { nTitle :: {Char}, nBody :: {Char}, nFormat :: Format, subs :: [Note] | E [Char] Format's just a collection of preset formats, so I don't have to dynamically handle that (since I didn't need to for my uses). Anyways, for whatever reason, sometimes while adding notes, it sends me an error: Prelude.(!!): index too large My only 2 uses of that are in getFullName :: [Note] -> [Int] -> [Char] getFullName d [] = "/" getFullName d (x:xs) = "/" ++ getPartName (d!!x) ++ getFullName (subs (d!!x)) xs and moveIn :: [Note] -> [Int] -> [Char] -> Int -> [Int] moveIn [] i m c = reverse i moveIn ((N dt db df du):ds) [] m c = if matchNote (N dt db df du) m then c:[] else moveIn ds [] m (c+1) moveIn n i m c = last i : ((moveIn (subs (n!!(last i)))) (allButLast i) m 0) where the [Int] is the variable I use to track where in the tree I am. Since I'm not moving into a Note that doesn't exist, and the display shouldn't be changing, what could be the problem? This is probably too in-depth a question, now that I think about it...

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maybe this could be improved?
by Michael Mossey 12 Nov '09

12 Nov '09

I've got some code which could be made simpler, I hope. the problem is this: I am implementing a software sampling synthesizer. For a given musical instrument, like piano, there are sound samples in memory. One purchases or creates sample sets. To save time money & resources, most sample sets are not complete---they have samples for only some of the pitches. Perhaps every third pitch has a sample. For the software to produce the sound for a non-included pitch, the software finds the closest included sample and plays it back slightly slower/faster to get the target pitch. That leads to the following code. Any ideas for improvement are welcome. The problem is that there are many cases to check: an empty map? the requested pitch less than all available pitches, greater than all available, or somewhere between? I am specifically writing this to run in O( log n) time. (It would be simpler as O(n).) This particular algorithm probably doesn't need to run in O(log n) time, but I want to do it as an educational experience---I will have other applications that need to use Map in O(log n) time. import Control.Monad.Identity import Control.Monad.Error import Control.Monad import qualified Data.Map as M type Pitch = Int type Sample = String type SampleMap = M.Map Pitch Sample -- Given a SampleMap and a Pitch, find the Pitch in the SampleMap -- which is closest to the supplied Pitch and return that. Also -- handle case of null map by throwing an error. findClosestPitch :: SampleMap -> Pitch -> ErrorT String Identity Pitch findClosestPitch samples inPitch = do when (M.null samples) $ throwError "Was given empty sample table." case M.splitLookup inPitch samples of (_,Just _,_ ) -> return inPitch (m1,_ ,m2) | (M.null m1) && not (M.null m2) -> case1 | not (M.null m1) && (M.null m2) -> case2 | otherwise -> case3 where case1 = return . fst . M.findMin $ m2 case2 = return . fst . M.findMax $ m1 case3 = return $ closest (fst . M.findMax $ m1) (fst . M.findMin $ m2) closest a b = if abs (a - inPitch) < abs (b - inPitch) then a else b

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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 10 Nov '09

10 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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