- Beginners - Haskell.org

Getting the Takusen example code to compile - failed import problem
by Daniel Everett 07 Aug '09

07 Aug '09

I have Takusen 0.8.5 installed and I'm trying to build the code found at http://darcs.haskell.org/takusen/doc/html/Database-Enumerator.html using ghc 6.10.3. This always fails with the error: Could not find module `Database.Oracle.Enumerator' The analogous errors are encountered if I substitute Oracle with Sqlite, Postgresql or ODBC. Any ideas how to resolve this? TIA, Daniel

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am I wording this right?
by Michael P Mossey 04 Aug '09

04 Aug '09

Is this the right way of saying what I'm trying to say? "Functor is a typeclass of type constructors which take one argument." Thanks, Mike

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Trying to read a bitmap with hOpenGL
by builes.adolfo＠googlemail.com 04 Aug '09

04 Aug '09

Hi All, I have been following an OpenGL book which has its examples in cpp, but using haskell instead. I have a problem when trying to read a bitmap, I haven't managed to render the file to the buffer, I did a little parser based on the one in RWH for the bmp file, and apparently everything seems to work well, but when I call drawPixels I don't get a clear image. The code that I'm using is here : http://gist.github.com/160787 What am I doing wrong ? Thanks in advance for any help. abuiles

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Transforming (State s a) to (StateT s m a)
by Toby Murray 04 Aug '09

04 Aug '09

Hi all, Am very new to Haskell, apologies in advance. Suppose I have a transformer A of type State s a. Suppose I have other state transformers of type StateT s m a. These are similar to the first kind but e.g. might throw errors or perform IO or whatever. I can manually convert A to be of the second kind as follows: -- | Used to invoke a "pure" state transformer -- from a context in which other effects may occur. pure :: (Monad m) => State s a -> StateT s m a pure func = StateT (\arg -> (return (runState func arg))) This seems like something one might want to do often. For instance, I might have a state type that I want to operate on, call it System. Then type PureSystemTransformer a = State System a defines a type for "pure" System transformers that return results of type a. However, other system transformers might need to throw errors. So I might define type SystemTransformer a e = StateT System (Either e) a for the type of system transformers that return results of type a or throw errors of type e. 'pure' is then used to convert a PureSystemTransformer to a SystemTransformer. I wonder whether there is something in the standard libraries that achieves the same thing as 'pure' above. i.e. Is there a more standard way to do this? Cheers Toby

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The Haskell IRC Channel
by Matthew J. Williams 03 Aug '09

03 Aug '09

Dear all, Does the list recommend a particular TUI IRC client for windows? Apologies for the off topic post. Sincerely, MJW

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All-in-one package?
by Matt f 02 Aug '09

02 Aug '09

Hey... I'm new to Haskell, been working in Java for 5 months and have decided to move up. Haskell is very much different than what I'm use to and I'm wondering if there is a all-in-one package I can download and just press install to get everything needed for Haskell. Something that includes compilers, libraries, wxHaskell, OpenGl for Haskell, ect... Like how Visual basic or Netbeans are. A few more questions: Is there an IDE? If not, is there some program that will open Haskell files, edit them, and be able to run them without any Terminal usage? Is there an official forum for Haskell? If Haskell had individual platform install packages installing everything needed for Haskell without any hassle with a beginner friendly forum and website, it'd become probably as popular as Java or Vb.net. Is anything like this planned? Thanks, -Matt

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Haskell OpenGL trouble
by Matt f 02 Aug '09

02 Aug '09

I am currently running Mac OS 10.5.7, and I have install many Haskell files, compiled a bouncing ball example, fixed the enableGUI bug, tried an IDE, and have started installing graphics libraries. I am using Cabal in the terminal to install what I need, though whenever I try to install opengl or a package of opengl; it comes up with an error. Macintosh-2:opengl matthew$ sudo cabal install Resolving dependencies... Configuring OpenGLRaw-1.0.1.0... cabal: Missing dependency on a foreign library: * Missing C library: GL This problem can usually be solved by installing the system package that provides this library (you may need the "-dev" version). If the library is already installed but in a non-standard location then you can use the flags --extra-include-dirs= and --extra-lib-dirs= to specify where it is. cabal: Error: some packages failed to install: GLURaw-1.0.0.0 depends on OpenGLRaw-1.0.1.0 which failed to install. OpenGL-2.3.0.0 depends on OpenGLRaw-1.0.1.0 which failed to install. OpenGLRaw-1.0.1.0 failed during the configure step. The exception was: exit: ExitFailure 1 Macintosh-2:opengl matthew$ It appears that I have zoned onto the bit that's causing all this mess, I'm missing a C library: GL. Unfortunately after googling a while I have found no answer. Am I doing something wrong? Is there another way? Could there be a fix? Thanks, -Matt

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type constructors
by Michael P Mossey 01 Aug '09

01 Aug '09

I was playing around with type constructors and I wrote this: data Foo a b = Foo1 [a] | Foo2 (a -> b) t3 = Foo1 [1, 2, 3] I wanted to see what ghci thought the type of t3 was. Essentially, it's data that doesn't use all of the type variables. So this ran fine, and *Main> :t t3 t3 :: Foo Integer b Wow! The data exists but it doesn't have a "complete type" so to speak. This worked, too: f3 (Foo1 xs) = length xs *Main> f3 t3 3 This is surprising to a conventional programmer. But does this naturally relate to other features of Haskell. Perhaps laziness? (I.e. data of type Foo doesn't always need a type b so it just doesn't have one until it needs one.) Thanks, Mike

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MonadRandom or Control.Monad.Random
by Michael P Mossey 01 Aug '09

01 Aug '09

Where can I find MonadRandom or Control.Monad.Random to install? It doesn't seem to be a system library, I can't find it with cabal or Hoogle. Thanks, Mike

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type class question from Ternary Trees
by MH 29 Jul '09

29 Jul '09

I am going over ternary tree library and it all looks clear to me with exception of the method below. Could you please explain to me how to read this: data TernarySet a = Node !a !(TernarySet a) !(TernarySet a) !(TernarySet a) | Null !(TernarySet a) | End deriving (Show, Eq) instance Binary a => Binary (TernarySet a) where ...... skipped get = do ---first call to get tag <- getWord8 case tag of _ | tag < 8 -> do ch <- get --- second call to get l <- if tag `testBit` 2 then get else return End ---third call to get e <- if tag `testBit` 1 then get else return End h <- if tag `testBit` 0 then get else return End return (Node ch l e h) 8 -> return (Null End) .......skipped What I don't understand here is recursive(?) call to 'get'. The first 'get' as I understand is an instance implementation of get from Binary module. What is 'ch <- get' (the second call) and 'get' inside 'if tag `testBit` 2 then get ...' (the third call)? tag <- getWord8 reads word8 from monad, then ch <- get reads what (just a copy of tag???)? Is get inside if ..then else statements is a look ahead get calls, to read next word8? Please provide more details if possible. Thanks a lot. Malik

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