November 2009 - Beginners

Circular Linked Lists
by Matthew J. Williams 04 Feb '10

04 Feb '10

Dear All How would one mimic, in Haskell, a C++ circular linked list i.e., where the last element precedes (points to) the first? Sincerely Matthew J. Williams

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fail: wxhaskell with cabal
by Josh Bronson 03 Dec '09

03 Dec '09

Greetings Haskellers, Happy Thanksgiving! I'm using Ubuntu 9.10. I've performed the following steps: # apt-get install ghc6 I then successfully did a local install of the Haskell platform in ~/mine/install Now I am trying to get wxhaskell to install: $ cabal install wx [snip] Configuring wxcore-0.12.1.2... setup: Missing dependencies on foreign libraries: * Missing C libraries: wx_baseu-2.8, wx_baseu_net-2.8, wx_baseu_xml-2.8 This problem can usually be solved by installing the system packages that provide these libraries (you may need the "-dev" versions). If the libraries are already installed but in a non-standard location then you can use the flags --extra-include-dirs= and --extra-lib-dirs= to specify where they are. cabal: Error: some packages failed to install: wx-0.12.1.2 depends on wxcore-0.12.1.2 which failed to install. wxcore-0.12.1.2 failed during the configure step. The exception was: exit: ExitFailure 1 $ ls /usr/lib/libwx* /usr/lib/libwx_baseu-2.8.so /usr/lib/libwx_baseu_net-2.8.so.0.6.0 /usr/lib/libwx_baseu-2.8.so.0 /usr/lib/libwx_baseu_xml-2.8.so /usr/lib/libwx_baseu-2.8.so.0.6.0 /usr/lib/libwx_baseu_xml-2.8.so.0 /usr/lib/libwx_baseu_net-2.8.so /usr/lib/libwx_baseu_xml-2.8.so.0.6.0 /usr/lib/libwx_baseu_net-2.8.so.0 It looks like the libraries are there to me! any ideas?

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From Functional Dependencies to Type Families
by Adolfo Builes 03 Dec '09

03 Dec '09

> {-# OPTIONS -fglasgow-exts #-} > > module VarArg where > import Data.FiniteMap -- for an example below > > class BuildList a r | r-> a where > build' :: [a] -> a -> r > > instance BuildList a [a] where > build' l x = reverse$ x:l > > instance BuildList a r => BuildList a (a->r) where > build' l x y = build'(x:l) y > > --build :: forall r a. (BuildList a r) => a -> r > build x = build' [] x I'm trying to replace the code below to work with type families, I started out replacing the definition of class with : class BuildList r where type Build r build' :: [Build r] -> Build r -> r follow by the instance for [a] resulting in instance BuildList [a] where type Build [a] = a build' l x = reverse $ x:l Until here, everything is working, and I'm able to do > build' [2,3,4] 1 :: [Integer] > [4,3,2,1] then I move on to the next instance (a -> r) with instance BuildList r => BuildList (a-> r) where type Build (a -> r) = a build' l x = \ y -> build'(x:l) y But I get the following error Couldn't match expected type `Build r' against inferred type `a' `a' is a rigid type variable bound by the instance declaration at /home/adolfo/foo.hs:347:35 In the first argument of `(:)', namely `x' In the first argument of `build'', namely `(x : l)' In the expression: build' (x : l) y then I try with : instance BuildList r => BuildList (a-> r) where type Build (a -> r) = Build r build' l x = \ y -> build'(x:l) y And I get Couldn't match expected type `a' against inferred type `Build r' `a' is a rigid type variable bound by the instance declaration at /home/adolfo/foo.hs:347:35 Expected type: [Build r] Inferred type: [Build (a -> r)] In the second argument of `(:)', namely `l' In the first argument of `build'', namely `(x : l)' I have been trying to figure out, which type should it be, but I haven't found the correct one, any ideas ? Thanks - Adolfo Builes

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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

5 10

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?

3 2

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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