- Beginners - Haskell.org

how to define records containing finite length lists.
by Srikanth K 02 Dec '09

02 Dec '09

Hi, I want to define a record say "Student". The 'C' equvivalent for the same could be struct Person { char name[10] int age } The closest I can find for doing such a thing in haskell appears to be data Person = Person{ name :: [char] age :: Int } I have not yet been able to find a suitable way to specify the constraint on the length of list(name in this case). So can someone let me know how we can impose this length constratint on the list and derive a new type. If standard haskell doesn't give this flexibility, are there any extensions in ghc to achieve the same. - Srikanth

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Typeclass question
by Patrick LeBoutillier 01 Dec '09

01 Dec '09

Hi all, I want to make all types that are instances of Integral instances of another type class, i.e: {-# LANGUAGE FlexibleInstances #-} class Intable a where toInt :: a -> Int instance (Integral a) => Intable a where toInt l = fromIntegral l However this gives the following error that I don't understand: q.hs:6:0: Constraint is no smaller than the instance head in the constraint: Integral a (Use -XUndecidableInstances to permit this) In the instance declaration for `Intable a' Can anyone offer insight on thie error? Also, is there perhaps a different way to do what I want to do? Thanks, Patrick -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

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Tips for a FFI-wrapped C library?
by Patrick LeBoutillier 01 Dec '09

01 Dec '09

Hi all, I have wrapped, using FFI, a C-library I am writing that calls into different programming languages. I put everything in the IO monad. So far it works as expected but I find the resulting usage code somewhat ugly (especially the bits musing 'sequence'). I tried using finalizers to delete the resources but it couldn't respect my dependencies. Can anyone offer any advice on making it feel more natural in Haskell? Note: The FFI code is available here: http://svn.solucorp.qc.ca/repos/solucorp/mlp/trunk/lib/haskell/ import qualified MLP import qualified MLP.Error import qualified MLP.Context import qualified MLP.Language import qualified MLP.Runtime import qualified MLP.Value import qualified MLP.Object main = do ctx <- MLP.Context.new -- create a context MLP.Context.with ctx $ do lang <- MLP.Language.init ctx "perl" -- init a language, i.e. perl rt <- MLP.Runtime.new ctx lang -- create a runtime for the language MLP.Runtime.loadLibrary ctx rt "calc" -- load a library, i.e. calc.pm val <- MLP.Object.new ctx rt "calc" [] "" -- create a calc object, i.e. my $obj = new calc() ; obj <- MLP.Value.takeObject val -- extract the object from the value MLP.Value.delete ctx val args_add <- sequence [MLP.Value.newLong 2, MLP.Value.newLong 3] args_mul <- sequence [MLP.Value.newDouble 3.5, MLP.Value.newDouble 4.6] val_add <- MLP.Object.callMethod ctx rt obj "add" args_add "" sequence $ map (MLP.Value.delete ctx) args_add val_mul <- MLP.Object.callMethod ctx rt obj "mul" args_mul "" sequence $ map (MLP.Value.delete ctx) args_mul MLP.Object.delete ctx obj MLP.Value.getLong val_add >>= putStrLn . show >> MLP.Value.delete ctx val_add MLP.Value.getDouble val_mul >>= putStrLn . show >> MLP.Value.delete ctx val_mul MLP.Runtime.delete ctx rt MLP.Language.delete ctx lang Thanks, Patrick -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

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recommended way to get haskell 6.10 installed on ubuntu hardy?
by Joe Van Dyk 30 Nov '09

30 Nov '09

There's no ubuntu package for 6.10 (only 6.8), and the haskell platform says you need 6.10 installed to build the source package. -- Joe Van Dyk http://fixieconsulting.com

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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.

4 3

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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