November 2006 - Haskell-Cafe

Separate a string into a list of strings
by Sara Kenedy 18 Apr '08

18 Apr '08

Hi all, I want to write a function to separate a string into a list of strings separated by commas. Example: separate :: String -> [String] separate "Haskell, Haskell, and Haskell" = ["Haskell", "Haskell", "and Haskell"] If anyone has some ideas, please share with me. Thanks. S.

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Creating static libraries using GHC
by Tom Hawkins 16 Mar '07

16 Mar '07

I have a chunk of Haskell code I would like wrap up and distribute as a library. Is there a way to build a static library (*.a) that includes my code plus the Haskell runtime, which C programs can easily link against? Here is what I have tried so far... ghc --make -fffi MyLib # Builds MyLib.o and MyLib_stub.o. gcc -c -I/usr/local/lib/ghc-6.4.1/include MyLibWrapper.c ar -r libMyLib.a MyLib.o MyLib_stub.o MyLibWrapper.o This works fine when I use ghc to compile and link a C program... ghc main.c libMyLib.a But if I use gcc, it throws a lot of unresolved references. I have no problem compiling with ghc, but the folks using my library probably won't have it installed. I added -v to ghc to see how it calls gcc; it seems to link in different libraries based on what Haskell libraries are being used, and it undefines a bunch is symbol references. Are there any switches to have ghc return a single archive with everything included? Please excuse my ignorance of the linking process. Thanks for any help! -Tom

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question about "How to Write a Haskell Program" tutorial
by Michael Vanier 09 Jan '07

09 Jan '07

First off, I apologize if this has come up before. As far as I can tell, the mailing list archives don't have a search function. I'm running ghc-6.6 and haddock-0.8, both compiled from source. I'm working my way through the "How to Write a Haskell Program" tutorial (which is a great idea, thanks guys!), and everything works fine until I get to the part about creating the haddock documentation. When I run: $ runhaskell Setup.hs haddock I get this: Preprocessing executables for haq-0.0... Running Haddock for haq-0.0... Warning: cannot use package haq-0.0: ghc-pkg failed With verbosity: $ runhaskell Setup.hs haddock -v I get this: Preprocessing executables for haq-0.0... Running Haddock for haq-0.0... /usr/local/packages/haskell-misc/bin/haddock --html --odir=dist/doc/html/haq --title=haq --use-package=haq-0.0 --use-package=base-2.0 dist/build/tmp/Haq.hs Warning: cannot use package haq-0.0: ghc-pkg failed Note that I'm installing into a non-standard location. I have no idea why haddock (called from cabal) is trying to use a package that is in the process of being created. Nevertheless, the documentation does get generated. Is this just something that should be ignored? Is it a Cabal bug? Mike

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Interoperating with Java
by mark＠ixod.org 14 Dec '06

14 Dec '06

I was wondering, what's the status of the various means of two-way interoperation with Java? Are things actively maintained? According to the WWW, HaskellDirect's (I have Lambada in mind) current version was released Jan 2004 and compiles with 'recent' versions of GHC, such as ghc-6.2, and the latest released JVM Bridge isn't much newer. We were looking at using the Standard Widget Toolkit (SWT) but don't want to rely on driving it from Haskell if the connectors are going to rot away. (We could still use FP, after all - e.g. Scheme's Kawa and Bigloo have good Java interoperation.) -- Mark

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Translating Haskell to VHDL. What approach to follow?
by Alfonso Acosta 12 Dec '06

12 Dec '06

Hi all, As I wrote in the previous HLADPSA announcement, during the following moths I'm going to write a translator from Haskell to VHDL in order to accomplish my masters thesis goal. The main requirement is managing to translate from a ForSyDe (http://www.imit.kth.se/info/FOFU/ForSyDe/ ) specification which is Haskell after all, to VHDL. In order to get part of the work for free I decided to design the translator as a compiler backend. The best option so far seems to be Yhc's Core API (http://haskell.org/haskellwiki/Yhc/API/Core ), which unfortunately lacks type information. I discarded GHC due to the current "bit-rotted" state of External Core (http://www.haskell.org/pipermail/haskell-cafe/2006-October/018786.html ) and higher complexity of its core representation (SystemFC). I might have overlooked other approaches and/or intermediate representations such as GHC's C-- and STG (is it still used?). I'd be pleased to hear any suggestions. Thanks in advance, Alfonso Acosta

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SMP support in GHC 6.6 on PPC Macs
by Ivan Tomac 09 Dec '06

09 Dec '06

I've noticed that code compiled with -threaded doesn't like +RTS - N<thread count> as an argument. This seems to be the case with both the binary distribution of GHC 6.6 at www.haskell.org/ghc as well as the version I built from sources. I got rid of all the old versions of GHC on my system and ghc -- version reports 6.6. So I'm guessing this is a bug? Ivan

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known, I know: class contexts and mutual recursion
by Conor McBride 08 Dec '06

08 Dec '06

Hi folks I just tripped over the "Contexts differ in length" error message. I know it's not a new problem, but I thought I'd enquire as to its status. For those of you who haven't seen it, here's an example, contrived but compact. > data Thing > = Val Int > | Grok Thing (Maybe Int -> Int) > eval :: Monad m => Thing -> m Int > eval (Val i) = return i > eval (Grok t f) = return (f (eval t)) My eval function compiles ok. See? The recursive call to eval targets the Maybe monad, so I get *Mmm> eval (Grok (Val 5) (maybe 0 (1 +))) :: Maybe Int Just 6 However, when I try to decompose eval as a pair of mutually recursive functions, namely > foo :: Monad m => Thing -> m Int > foo (Val i) = return i > foo (Grok t f) = return (goo t f) > goo :: Thing -> (Maybe Int -> Int) -> Int > goo t f = f (foo t) I get Mmm.lhs:15:1: Contexts differ in length When matching the contexts of the signatures for foo :: forall (m :: * -> *). (Monad m) => Thing -> m Int goo :: Thing -> (Maybe Int -> Int) -> Int The signature contexts in a mutually recursive group should all be identical Poking about on the web, I got the impression that this was a known infelicity in ghc 6.4 (which I'm using), due to be ironed out. However, an early-adopting colleague with 6.6 alleges that foo-goo is still poisonous. I'm wondering what the story is. I mean, is there some nasty problem lurking here which prevents the lifting of this peculiar restriction? I'm not in a panic about this. I have a workaround for the problem as I encountered it in practice. Mind you, it's the sort of thing that's likely to happen more often, the more you localise the effects you tend to use. In the above, goo doesn't throw exceptions; rather, because goo has a handler, it can offer a /local/ exception-throwing capability to foo. Curious Conor

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[Haskell] Defining Cg, HLSL style vectors in Haskell
by Slavomir Kaslev 06 Dec '06

06 Dec '06

Hello, I have to define a couple of float2, float3, float4 Cg, HLSL style vectors in Haskell. At first I was tempted to make them instances of Num, Floating, RealFrac, etc. but some of the functions defined in those classes have no sense for vectors. One such example is signum from class Num. There are several workarounds for this. One may come up with some meaning for vectors of such functions, for example: instance Num Float3 where ..... signum a | a == Float3 0 0 0 = 0 | otherwise = 1 This is silly. Other option, which I prefer, is to leave such functions undefined (that is signum=undefined, not just not defining them). Is this ok? Are there any other options? Another bugging thing is that some of the functions do have meaning for vectors but they need different signatures. For example (**) :: Floating a => a -> a -> a, for vectors should be (**) :: (Floating a, Vector v) => v -> a -> v, that is (**) applied for every component of the vector. Any workarounds for that? I know that I can scrap all those Num, Floating, RealFrac, etc. classes and define class Vector from scratch, but I really don't want to come up and use different names for +, -, etc. that will bloat the code. Last question: Does haskell have something like C++ templates? For example, some time in the future I may need types like int2, short3, etc., that behave just like float2, float3, but use different types for their components. I really, really wouldn't like to copy-paste the definitions of floatn and manually change their types to intn respectfully. Cheers. -- Slavomir Kaslev

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what GUI library should i select?
by Bulat Ziganshin 06 Dec '06

06 Dec '06

Hello haskell-cafe, afaik, there are just two good enough libs - wxHaskell and GtkHs. can anyone point (or write) detailed comparison of their features? i plan to write large GUI program in Haskell and want to select best one. the requirements that i can imagine at this moment is the following: * my main target is Windows but ability to compile the same code both for Windows and Linux would be a plus * the program developed is a sort of advanced file manager, so i need treeview, table view and tabbed view controls * user likes beauty, so various bells-and-whistles are welcomed. in particular, it would be great to have skinnable interface * use of resource file for all texts to make internalization easier -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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Re: help with threadDelay
by Neil Davies 04 Dec '06

04 Dec '06

Ian/Simon(s) Thanks - looking forward to the fix. It will help with the real time enviroment that I've got. Follow on query: Is there a way of seeing the value of this interval from within the Haskell program? Helps in the calibration loop. Neil

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