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points-free problem
by I. J. Kennedy 20 Nov '09

20 Nov '09

I know I'm going to kick myself when the answer is revealed by one of you kind folks, but after staring at this a while I just can't figure out what's going on. The compiler (ghci) is trying so hard to tell me what's wrong, but I am failing to understand. I thought for the most part one could take a function definition like f x = (blah blah blah) x and turn it into points-free style by f = (blah blah blah) But from the dialog below, my assumption is incorrect. Help? I. J. Kennedy > sortBy (compare `on` fst) [(2,3),(0,1),(1,5)] -- test a little sort expression [(0,1),(1,5),(2,3)] > let sortpairs xss = sortBy (compare `on` fst) xss -- make it into a function > :t sortpairs sortpairs :: (Ord a) => [(a, b)] -> [(a, b)] > sortpairs [(2,3),(0,1),(1,5)] [(0,1),(1,5),(2,3)] > let sortpairs = sortBy (compare `on` fst) -- points-free style > sortpairs [(2,3),(0,1),(1,5)] <interactive>:1:24: No instance for (Num ()) arising from the literal `1' at <interactive>:1:24 Possible fix: add an instance declaration for (Num ()) In the expression: 1 In the expression: (1, 5) In the first argument of `sortpairs', namely `[(2, 3), (0, 1), (1, 5)]' > :t sortpairs sortpairs :: [((), b)] -> [((), b)] -- what?!

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Windows API and FFI
by iæfai 20 Nov '09

20 Nov '09

I have a specific function that I would like to get info to allow me to accept clicks into the cmd.exe window. The function seems to be here: http://msdn.microsoft.com/en-us/library/ms684961(VS.85).aspx One of the parameters is this: A pointer to an array of INPUT_RECORD structures that receives the input buffer data. The total size of the array required will be less than 64K. There are in and out parameters for this function. I have never done anything like this before, and was hoping this is done somewhere, or it is fairly easy to do. Any help would be appreciated. - iæfai.

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Named fields in data types
by legajid 18 Nov '09

18 Nov '09

Hello, i've some trouble with named fields in data types. data Carnet = Adresse { nom :: String, cp :: Integer, ville :: String } | Telephone {nom::String, telnum::String} deriving Show contact01 = Adresse "Didier" 51100 "Reims" contact02 = Adresse {nom="Laure", cp=0} -- line 1 --contact02 Adresse { ville="Nogent" } -- line 2 --contact02 { ville="Nogent" } -- line 3 contact03=Telephone "Didier" "0326..." -- line 4 When loading this source : line 1 says "warning : Fields not initialized :ville line 2 and 3 when uncommented give parse error (possibly indentation) I'm ok with line 1. Is it possible to write such things as line2 or 3 ? Which syntax ? Thanks for help. Didier.

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Question on data/type
by Phillip Pirrip 18 Nov '09

18 Nov '09

Hi, I have the following data defined. data TypeCon a = ValConA a | ValConB [a] | ValConC [[a]] So I can use functions with type like (a->a->a) -> TypeCon a -> TypeCon a -> TypeCon a for all 3 value types, and I think is easier to define one single typeclass for (+), (*) etc. If I want to express the following idea (the following won't compiler): data TypeCon a = ValConA a | ValConB [ValConA a] | ValConC [ValConB a] Is this even a good idea? If so how could I proceed? The closest thing I can get to compiler is like this: data TypeCon a = ValConA a | ValConB [TypeCon a] Which is a nightmare when I try to manipulate anything in this structure. The alternative I guess is to use 3 different type constructors, data TypeConA a = ValConA a data TypeConB a = ValConB [ValConA a] data TypeConC a = ValConC [ValConB a] but then I can't use one signal typeclass for (+) etc. Am I correct? thx, //pip

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Questions on data/type
by Matthew Wong 17 Nov '09

17 Nov '09

Hi, I have the following data defined. data TypeCon a = ValConA a | ValConB [a] | ValConC [[a]] So I can use functions with type like (a->a->a) -> TypeCon a -> TypeCon a -> TypeCon a for all 3 value types, and I think is easier to define one single typeclass for (+), (*) etc. If I want to express the following idea (the following won't compiler): data TypeCon a = ValConA a | ValConB [ValConA a] | ValConC [ValConB a] Is this even a good idea? If so how could I proceed? The closest thing I can get to compiler is like this: data TypeCon a = ValConA a | ValConB [TypeCon a] Which is a nightmare when I try to manipulate anything in this structure. The alternative is use 3 different type constructors, data TypeConA a = ValConA a data TypeConB a = ValConB a data TypeConC a = ValConC a but then I can't use one signal typeclass for (+) etc. thx, //pip

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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 16 Nov '09

16 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 13 Nov '09

13 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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