September 2005 - Haskell-Cafe

matrix computations based on the GSL
by Alberto Ruiz 21 Apr '06

21 Apr '06

Hello all! This is my first message to the list. In my work I often need linear algebra algorithms and other numeric computations. An option is using scientific computing systems like Matlab, Mathematica, Maple, etc. In Haskell there are several modules and bindings to matrix libraries; many of them are excellent. But I think that Haskell would be even more attractive if the basic matrix computations (what we find, for example, in the GNU Octave Quick Reference) were easily available in a standard library. We have all we need: an interactive environment, excellent graphics (HOpenGL), FFI and ForeignPtr with automatic garbage collection, QuickCheck, etc. Computations can be implemented by the GSL (GNU Scientific Library), a well designed numerical library written in C. Well... I know that I am trying to reinvent the wheel: I am working on a user friendly functional interface to matrix computations based on the GSL. I think that many numeric problems that can be solved using Octave or a similar system can be also solved more elegantly using Haskell. The library is very preliminary and incomplete but some simple problems can already be solved. The sources, haddock documentation and the draft of a tutorial can be found at: http://dis.um.es/~alberto/hmatrix/matrix.html I am not a Haskell expert, I only use the most basic programming techniques, so any suggestion will be greatly appreciated... Alberto

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RE: [Haskell-cafe] generics question, logical variables
by Ralf Lammel 03 Apr '06

03 Apr '06

Frederik, > As for your code example, it looks very interesting, but are you > saying that this could turn into an extension of the Data.Generics > library, or that this is something I could be implementing in terms of > what's already there? The posted code works with GHC 6.4 (SYB2) intentionally and actually. I have attached another attempt (again GHC 6.4, based on SYB2) which might be more useful for your purposes, and it may be useful in general, in fact. What I defined this time is a "certainty-improving" function: idify :: (Typeable1 f, Monad m, Data (a f), Data (a Id)) => (forall a. f a -> m a) -> a f -> m (a Id) That is, the function "idify get" takes a value whose type is parameterized in a type constructor f (such as Maybe or IORef), and the function attempts to establish Id instead of f on the basis of the function argument "get". > What is the 'a' parameter for in "force"? > > force :: ( Data (t Maybe a) > , Data (t Id a) > , Term t Maybe a > , Term t Id a > ) => t Maybe a -> t Id a The previous attempt was a more parameterized blow than required in your case. (I was guessing about what "typed logical variables" could mean. I was assuming that you would need some extra layer of embedding types on top of the Haskell term types. Looking at your code, this was not the case.) > For the part which I asked for help with, to get around my trouble > with generics, I defined a class GenFunctor and an example instance. > The intent is that generics should be able to provide this > functionality automatically later on, but you can see what the > functionality is. Let's look at the type of your GenFunctor: class GenFunctor f where gfmapM :: (Monad m, FunctorM b) => (forall x . a x -> m (b x)) -> f a -> m (f b) This type can be seen as a more relaxed version of the idify operation above. That is, idify fixes GenFunctor's b to Id. The particular encoding of idify (attached) takes advantage of this restriction. I wonder whether I should bother. (Exercise for reader :-)) > However, I am stuck on something else, the program doesn't typecheck > because of use of another function I defined, 'cast1'. Maybe you can > take a look. I had thought that I would be able to write a generic > 'unify' but I get the error: > > GenLogVar.hs:82:19: > Ambiguous type variable `a' in the constraint: > `Data a' arising from use of `cast1' at GenLogVar.hs:82:19-23 > Probable fix: add a type signature that fixes these type variable(s) > > This is because I need to do something special when I encounter a > "Var" variable in unification, but the compiler seems to not like the > fact that the type argument of the Var data type is not known. Please try to avoid new cast operations at all costs. :-) Your code can be arranged as follows: (i) Use extQ1 to dispatch to a special case for "Var x" for the first argument. (ii) In this special case, use again ext1Q to dispatch to a special case for "Var y" for the second argument. (iii) At this point, *cast* the variable value of *one* variable to the type of the other. So the problem with your code, as it stands, is that the target type of cast is ambiguous because you cast *both* arguments. The insight is to make the cast asymmetric. Then, not even polymorphism is in our way. Interesting stuff! Ralf

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how to break foldl' ?
by gary ng 30 Sep '05

30 Sep '05

Hi, say if I want to sum a list of numbers but only until it hits a max limit. Currently, I control it through the function and basically do nothing when the max is hit. However, if the list is very long, would this mean the same function would be called for the rest of the list which can be a waste of cycle ? In an imperative language, I just break/return in the middle of the loop. thanks for any help in advance. gary __________________________________ Yahoo! Mail - PC Magazine Editors' Choice 2005 http://mail.yahoo.com

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finally going on Solaris
by Brian McQueen 30 Sep '05

30 Sep '05

I've finally got a working ghc on Solaris. Its 6.4.1. I couldn't get the earlier, pre 6.4 versions going due to problems with my gcc. It seemed to be expecting a gcc shared library that my system didn't have, nor did any other gcc that I was able to compile. After loading a few prerequisites, and then reloading them with --enable-shared, its now going. I found a problem with a header file in the solaris distribution too - it expects stdint.h, which doesn't seem to be a part of Solaris anymore. I undef'd the HAVE_STDINT_H lines, and its working well now. Brian McQueen

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Problem with sparc distribution?
by Brian McQueen 29 Sep '05

29 Sep '05

I find there is a problem the binary distribution: http://www.haskell.org/ghc/dist/6.4.1/ghc-6.4.1-sparc-sun-solaris2.tar.bz2 But the older one seems to be OK. It seems that the tar file is corrupted or incomplete. If you untar it yu will find the ghc dir contains only the bin and lib dirs and none of the rest of the documentations and install scripts. Brian McQueen

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Neural networks and optimization in Haskell
by Joel Reymont 28 Sep '05

28 Sep '05

Folks, I got a project where I have a large number of variables and an outcome and I need to figure out which 20% of the variables has the largest effect on the outcome. Of course I also need to optimize the 20% of variables I end up with. This sounds like a job for a neural network to me, with arguments possibly optimized through genetic algorithms. I'm wondering, though, if I'm complicating things for myself and there's an easier approach to this. If not I'm wondering if there are ready-made NN or GA libraries for Haskell. Also, I'm curious if Haskell is really the best language for this type of problem and if lazy evaluation brings any specific advantages to the solution or would be a hindrance. I would welcome any pointers and feedback, yes, someone is actually paying me to do this :-). Thanks, Joel -- http://wagerlabs.com/

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newbe question
by feucht＠uni-koblenz.de 28 Sep '05

28 Sep '05

Hi i can not load program test1 into hugs, but test2 works. Am i missing some special syntax? greetings, Philip -------------- test1 ------------------ foo :: Maybe Int -> Int foo Nothing =-1 foo (Just a)= a -------------- test2 ------------------ foo :: Maybe Int -> Int foo Nothing = -1 -- ^ -- +-- note an extra space foo (Just a)= a

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Help wanted: Lazy multiway zipper with mismached intervals
by Rene de Visser 27 Sep '05

27 Sep '05

Hello, I need to zip together multiple lists. The lists are sorted by date, and each entry in the list represents data for a time interval. The time intervals between the lists may be missmatched from each other. This means that sometimes you don't need to move forward in list, while you move forward in other lists. It might also mean that you need to skip a number of entries foward. If the computation does not need part of the contents of one of the lists, then this list should be ignored until it is needed. As the lists are sorted you never need to go backwards. I found it fairly easy to write a zipper for two lists, where the items from both lists are needed (example binary operator on the payload of the list). However the combination of merging multiple lists together, and ignoring a list in the case it is not needed by the computation leads to very messy code. I though about if there was a away of encapsulating this in a monad, but haven't thought of any way to do this. Another idea is to have a mutable cursor for each list and move these forward as required. But I guess this might be worth avoiding? I think it would be good if somehow I could encapsulate each list, so on a per list basis, and can say given me the current head, or move one forward. But I haven't figured out how to pass the state of the other threads invisibly through. I guess the ony way might be to use the state monad? I guess there can be no simple recursive solution? Rene.

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XMLParse module
by Huong Nguyen 26 Sep '05

26 Sep '05

Hello all, I am reading XMLParse module http://www.germane-software.com/repositories/public/StatusReport/trunk/hxml… But, I am not clear about the purpose of this module and the function of two functions: parseInstance = unfoldr (pRun instanceItem) . pcdataMode parseDTD = foldl (\a b->b a) emptyDTD . unfoldr (pRun dtdItem) . markupMode parseDocument text = case pRun prolog (pcdataMode text) of Just (_, rest) -> unfoldr (pRun instanceItem) rest Nothing -> [ErrorEvent "Error parsing prolog"] -- can't happen. If any knows the meaning of those, please let me know. Thanks a lot. Best regards,

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Re: [Haskell-cafe] Typing problems with basic arithmetic - help!
by Marcin Tustin 26 Sep '05

26 Sep '05

For some reason the following code is producing an error message from ghci that the the patterns are non-exhaustive. Does anyone have any idea why that could be, given that the patterns are, at least in my meaning, provably exhaustive? choosenonuniqueset n (a:r) | (length r) > (n-1) = [ (sort (a:x)) | x <- (choosenonuniqueset (n-1) r)] `union` [ (sort (a:x)) | x <- (choosenonuniqueset n r)] | (length r) == (n-1) = [a:r] | (length r) < (n-1) = [] Error message is: *Main> :reload Compiling Main ( birthday.hs, interpreted ) Ok, modules loaded: Main. *Main> choosenonuniqueset 2 [1..5] *** Exception: birthday.hs:(22,0)-(27,29): Non-exhaustive patterns in function choosenonuniqueset *Main> choosenonuniqueset 5 [1..5] [[1,2,3,4,5]] *Main>

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