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Pattern matching with one, two or more arguments
by legajid 29 Nov '09

29 Nov '09

Hello, I'm writing a little word game that consists of the following : between a consonant and a vowel, insert "AV" so "fabien" becomes "fAVabAVien". When i type : f "fabine", the result is ok ("fAVabAVinAVe") When i type : f "fabien", the result is fAVabAVie *** exception :prelude empty list Here's my program voyelles="aeiouy" consonnes="bcdfghjklmnpqrstvwxz" est_voyelle x = x `elem` voyelles est_consonne x = x `elem` consonnes --f []=[] ; unuseful - conflict with f a = a f (a:b:c) = case trait a b of True -> a : (" AV " ++ f (b:c)) False -> a : f (b:c) f (a:b) = case trait a (head b) of True -> a : (" EP " ++ f ( b) ) False -> a : f ( b) f a = a -- detect a vowel following a consonant trait a b = est_consonne a && est_voyelle b f (a:b) is never invoked (i wrote EP instead of AV to trace the problem). f(a:b:c) seems to always take precedence over the other patterns. I expect f(a:b:c) to treat words greater than or equal to 3 letters, f(a:b) the last two letters of the word (no third part) and f a the last letter. Another trick : how can i calculate consonnes with a method that makes the difference between the two lists [a..z] and voyelles, rather than writing each consonant. As we can append two lists (++), can we compute the difference between lists, i.e. delete from the first list the elements that are present in the second one ? Last, with Ghci, is it possible to debug my program, especially for recursive functions, because it's hard to follow the successive calls and the parameters that are passed each turn. Greetings, Didier.

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Questions on GHCi 6.10.4 + general questions
by Lev Broido 27 Nov '09

27 Nov '09

Hi I am getting the following message : parse error on input `= Code in question is attached . Another general question on haskell : I am not sure I understand correctly the 'where' clause usage , I'll happily accept comments on the code Thanks, Lev

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Re: [Haskell-beginners] Questions on GHCi 6.10.4 + general questions
by Lev Broido 27 Nov '09

27 Nov '09

On Fri, Nov 27, 2009 at 11:48 AM, Lev Broido <lev.broido(a)gmail.com> wrote: > Thanks for reply . > I used braces , since previous error was on indentation and I wasn't sure > what was wrong . > The intent of the *curr_bank_state = head . filter (\y -> y == > (wr_bank,_) state was > *get first entry (only entry) from list of tuples (Time,BankStateSIngle) > matching condition Time == wr_bank . > > After removing braces the message is > DdrController.hs:112:8: > parse error (possibly incorrect indentation) > Failed, modules loaded: none. > > Thanks, > Lev > > > On Fri, Nov 27, 2009 at 11:15 AM, Stephen Tetley <stephen.tetley(a)gmail.com > > wrote: > >> Hi Lev >> >> Where clauses don't use braces so that's first problem. >> >> Also this line near the end has a problem: >> >> curr_bank_state = head . filter (\y -> y == (wr_bank,_) state >> >> What should it be doing? >> >> Best wishes >> >> Stephen >> >> 2009/11/27 Lev Broido <lev.broido(a)gmail.com>: >> > Hi >> > I am getting the following message : >> > parse error on input `= >> > Code in question is attached . >> > >> > Another general question on haskell : >> > I am not sure I understand correctly the 'where' clause usage , I'll >> > happily accept comments on the code >> > >> > Thanks, >> > Lev >> > >> > >> > _______________________________________________ >> > Beginners mailing list >> > Beginners(a)haskell.org >> > http://www.haskell.org/mailman/listinfo/beginners >> > >> > >> > >

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Easier alternatives to existential types?
by Emile Melnicov 26 Nov '09

26 Nov '09

> Well, yes, I know that almost certanly that was not what you > were thinking when you wrote the question. Sorry, I should include my code snippet indeed. data Chunk a = Chunk { cHeader :: Header, cData :: a } data Header = Header { hID :: Word32, hSize :: Word32 } data Format = Format { ... } newtype Data = Data ByteString h :: Header ... formatChunk :: Chunk Format ... dataChunk :: Chunk Data ... data Container a = Container { ctHeader :: Header, ctType :: Word32, ctContent :: [Chunk a] } -- Very naive. containerChunk :: Container a containerChunk = Container h 42 [formatChunk, dataChunk] I'm interested in common pattern used in Haskell for this kind of problems. > If you really want existentials, though, I'd recommend using > GADT syntax OK, I'll try it out. I'm already know about GADT's, but can't catch the difference between regular Algebraic Data Types and Generalised ones (in terms of disjoint union etc.). Can you recommend some paper where GADT's are described, but without very deep mathematical entities?

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some help on do and preservingMatrix
by Zsolt Ero 26 Nov '09

26 Nov '09

Could someone point me to some help on how does "do" work and how does preservingMatrix? I am trying to write a program starting from the OpenGL tutorial on the wiki, but I never used do's. My very basic question is that how do I generally repeat a command many times? I understand "do" can do it, but it's not clear to me how to use it and how it works. So if I would like to write the following in one line, how could I write it?: spiral 0.7 spiral 0.8 spiral 0.9 spiral 1.0 I don't understand in which cases I can use "map something [0.7,0.8..1]" and in which cases I need do and how could I map using do. >From the tutorial, I could write this line, which works, but I don't really know what does mapM_ and preservingMatrix do. mapM_ (\x -> preservingMatrix $ do scale x x (1::GLfloat) rotate (x*(200)) $ Vector3 0 0 (1::GLfloat) spiral ((sin(a/50))^2/2+0.5) ) $ ([0.5,0.6..1::GLfloat]) Zsolt

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FFI: FinalizerPtr and freeHaskellFunPtr
by Patrick LeBoutillier 26 Nov '09

26 Nov '09

Hi, I'm trying to write a binding to a C library, and so far here is the code I have: {-# LANGUAGE ForeignFunctionInterface #-} import Foreign.Ptr import Foreign.ForeignPtr newtype C_mlp_context = C_mlp_context (Ptr C_mlp_context) data MLPContext = MLPContext !(ForeignPtr C_mlp_context) deriving (Show) foreign import ccall "wrapper" makeFinalizer :: (Ptr a -> IO ()) -> IO (FinalizerPtr a) foreign import ccall unsafe "mlp.h mlp_context_new" c_new :: IO (Ptr C_mlp_context) new :: IO MLPContext new = do mlp_context <- c_new fin <- mlp_context_finalizer fctx <- newForeignPtr fin mlp_context return $ MLPContext fctx mlp_context_finalizer :: IO (FinalizerPtr C_mlp_context) mlp_context_finalizer = do makeFinalizer $ \ctx -> do c_delete ctx foreign import ccall unsafe "mlp.h mlp_context_delete" c_delete :: Ptr C_mlp_context -> IO () main = do ctx <- new putStrLn $ show ctx This seems to work as expected, but I read that I'm supposed to call freeHaskellFunPtr on the finalizer when I'm done with it. However I don't know how I can do this since it is called by the GC... Can anyone offer any advice? Thanks, Patrick -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

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Easier alternatives to existential types?
by Emile Melnicov 26 Nov '09

26 Nov '09

Hello everyone. I'm trying to write RIFF parser and can't decide what to do with "container" type. "Container" chunk should hold a header (two Word32's), a "type" (Word32 like "WAVE" etc) and a "list" of other chunks. Of course lists can't be used because we have far more than one chunk types. What can I do with this? I discovered existentials for myself recently, but those could be too hard for me yet. It feels like I'm missing something very obvious here. Thanks for attention. -- Emile.

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combinatorial
by Michael P. Mossey 24 Nov '09

24 Nov '09

I'm trying to write a combinatorial search algorithm with evaluation, and kind of stuck. Not sure how to do this. I'm constructing a musical phrase, which is a list of MidiPitch: [MidiPitch] I have an evaluation function that determines the fitness of any given phrase: eval :: [MidiPitch] -> Maybe Float This returns Nothing if the phrase is completely unacceptable. The idea is to build up a phrase one midi pitch at a time, choosing all possible next pitches (notes) from a range: next pitch comes from: [10..90] Most of the pitches will result in a phrase that evaluates to Nothing, so the combinatoral "explosion" will be limited. I'd like to write a function that constructs a phrase of length n, and in fact will have to return a list of all phrases that have equal scores of the maximum. -- <length of output phrase> -> <first pitch> -> <eval func> -> -- <all tied phrases of best score> coolFunc :: Int -> MidiPitch -> ([MidiPitch] -> Maybe Float) -> [[MidiPitch]] I am stuck on how to write this. thanks, Mike

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how to install "import Foreign.C.Types( CFloat )" in Ubuntu
by Zsolt Ero 24 Nov '09

24 Nov '09

I am trying to run a file, which works well on the university system, but I don't know what do I need to do to make it work on my home Ubuntu system. The particular line which makes the error is: import Foreign.C.Types( CFloat ) And the error is: ----------------------------------------------------------- LSystem.hs:130:4: No instance for (ColorComponent CFloat) arising from a use of `color' at LSystem.hs:130:4-26 Possible fix: add an instance declaration for (ColorComponent CFloat) ..... ------------------------------------------------------------- If I write "import Foreign.C.Types" at the Prelude it says nothing. If I write "import Foreign.C.Types( CFloat )" at the Prelude, it says <interactive>:1:0: parse error on input `import' What can it be? Cheers, Zsolt

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questions about quickCheck
by Zsolt Ero 24 Nov '09

24 Nov '09

Hi, I have just started learning Haskell this semester, and I have some questions of things which puzzles me: 1. I would like to ask that how is it possible to set quickCheck test numbers to something else then the default 100 passes? I have found an article which says: deepCheck p = check (defaultConfig { configMaxTest = 10000}) p which I tried and it works at a home Ubuntu install but when I tried it at my university network, it says: Not in scope: `check' Not in scope: `defaultConfig' Not in scope: `configMaxTest' Isn't there a way of making quickCheck use a different number by using '==>' functions for example? Is quickCheck p = check (defaultConfig) p? 2. How could I print the result of a complex quickCheck generator? I mean if there is an error is says it, but what if I would like to generate just random expressions which quickCheck can do? Zsolt

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