December 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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First steps in Haskell
by Daniel Carrera 22 Jan '06

22 Jan '06

Hello all, I'm trying to write the simplest possible Haskell program, and I'm not getting anywhere. I have installed Hugs, GHC and GHCI. I want to run the following program: fac :: Integer -> Integer fac 0 = 1 fac n | n > 0 = n * fac (n-1) This is what I see: $ hugs Hugs.Base> fac :: Integer -> Integer ERROR - Undefined variable "fac" Hugs.Base> fac 0 = 1 ERROR - Syntax error in input (unexpected `=') $ ghci Prelude> fac :: Integer -> Integer <interactive>:1:0: Not in scope: `fac' Prelude> fac 0 = 1 <interactive>:1:6: parse error on input `=' $ # Write the program to fac.hs $ ghc fac.hs fac.hs:1:0: The main function `main' is not defined in module `Main' When checking the type of the main function `main' This is a real problem for Haskell. I expect that a lot of people try Haskell and give up because they can't even write the simplest function. It's hard not to be put off by this. I love the theory behind Haskell, but the practice of it seems to be a real problem. I hope someone will show me how to make this program work. Even better, I hope someone will fix the compilers and interpreters if they need fixing, or fix the documentation if that's what needs fixing. Best, Daniel. -- /\/`) http://oooauthors.org /\/_/ http://opendocumentfellowship.org /\/_/ \/_/ I am not over-weight, I am under-tall. /

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RE: [Haskell-cafe] Optimizing a high-traffic network architecture
by Simon Marlow 05 Jan '06

05 Jan '06

On 15 December 2005 10:21, Joel Reymont wrote: > Here are statistics that I gathered. I'm almost done modifying the > program to use 1 timer thread instead of 1 per bot as well as writing > to the socket from the writer thread. This should reduce the number > of threads from 6k (2k x 3) to 2k plus change. > > It appears that +RTS -k3k does make a difference. As per Simon, 2-4k > avoids the thread being garbage collected because each thread gets > its own block in the storage manager. Simon, did I get that right? The 3k threads are still GC'd, but they are not actually *copied* during GC. It'll increase the memory overhead per thread from 2k (1k * 2 for copying) to 4k (4k block, no overhead for copying). Cheers, Simon

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Joels Time Leak
by Adrian Hey 03 Jan '06

03 Jan '06

Hello, I haven't followed everything that's happened on the Binary IO thread, but has anybody else actually tried Joels code? .. http://wagerlabs.com/timeleak.tgz I can reproduce the problem (ghc/Linux), but can't explain it. It seems very strange that friggin about with an otherwise irrelevant (AFAICT) MVar fixes the problem. Regards -- Adrian Hey

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Help with "shootout"
by Chris Kuklewicz 03 Jan '06

03 Jan '06

Hello all, I decided to put together an entry for one of the shooutout categories: http://haskell.org/hawiki/ChameneosEntry It involves 4 threads. There is no current Haskell entry. Anyone have comments? Alterations? I can't have created the best code yet. I don't have the compilers for the other entries, so I can't compare speed on my powerbook (which is the wrong kind of CPU anyway -- they test on an AMD system). -- Chris

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Bug in "Haskell for C programmers" tutorial?
by Erik de Castro Lopo 02 Jan '06

02 Jan '06

Hi all, I'm working through the "Haskell for C programmers" tutorial and in this section": http://www.haskell.org/~pairwise/intro/section2.html#part3 it has: fib :: Num a, Num b => a -> b fib n = fibGen 0 1 n fibGen :: Num a, Num b => b -> b -> a -> b fibGen a b n = case n of 0 -> a n -> fibGen b (a + b) (n - 1) which doesn't work with GHC, but it does if I add parentheses around both instances of "Num a, Num b". Is this a bug in the tutorial or is there another explanation? Cheers, Erik -- +-----------------------------------------------------------+ Erik de Castro Lopo +-----------------------------------------------------------+ "Anyone who considers arithmetical methods of producing random digits is, of course, in a state of sin." - John Von Neumann (1951)

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expanding do notation / StateMonad example from tutorial all about monads
by Marc Weber 31 Dec '05

31 Dec '05

Hi.. I'm still struggling with monads ;-) In the tutorial "All about monads" there is the function getOne used: getOne:: (Random a) => (a,a) -> State StdGen a getOne bound = do g <-get (x,g')= return $ randomR bounds g put g' return x In the haskell "Yet another Haskell tutorial" by Hal Daume it is said that do {fp <- e; es} is translated to: let ok p = do {es} ; ok _ = fail "..." in e >>= ok Using this rule I was able to expand the first "<-" of getOne: getOne2 :: (Random a) => (a,a) -> State StdGen a getOne2 bounds = let ok g = do (x,g') <- return $ randomR bounds g put g' return x ok _ = fail "asd" in get >>= ok but I couldn't manage to also expand the second. My attempt is: getOne3 :: (Random a) => (a,a) -> State StdGen a getOne3 bounds = let ok g = do let ok2 (x,g') = do { put g'; return x } -- ok2 _ = fail "2" in (return $ randomR bounds g >>= ok2 ok _ = fail "1" in get >>= ok But I get this compile error in at the line marked by a trailing --: "The last statement in a 'do' construct must be an expression" Can you help me? Sincerly Marc

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OpenAL bindings / compiling ghc 6.5
by Michael Benfield 31 Dec '05

31 Dec '05

I see here: http://www.haskell.org/HOpenGL/newAPI/ OpenAL bindings listed as part of the Hierachical Libraries. And when I download the source to a development snapshot of GHC, there they are. Is there a way to install this on GHC 6.4? Alternatively... I can't get GHC 6.5 to compile. I do ./configure && make and it gets to this step: ------------------------------------------------------------------------ ==fptools== make all -r; in /Users/mike/source/ghc-6.5.20051221/ghc/compiler ------------------------------------------------------------------------ and sits forever (well, I only let it sit a day and a half, actually...). And the only relevant process that seems to be running is gmake (no ghc or gcc or anything). I've tried a few different versions of 6.5, including HEAD and STABLE branch. I'm on Mac OS 10.3.9. Thanks. Mike Benfield

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Testing serialization/deserialization.
by Pupeno 31 Dec '05

31 Dec '05

Hello, I am working with NewBinary, I have made a lot of datatypes which are Binary instances. Now I want to write some testing for the serialization/de-serialization. I've started with QuickCheck but it seems I can't do IO inside a property of QuickCheck, am I right ? If not, how is it done ? Not being able to work with QuickCheck, I tried HUnit only to find another problem (which I would have found with QuickCheck anyway). How do I generate a BinHandle where I can write and read what I written ? Tring to do that I wrote a test for Int: intSerialization = TestCase (do bh <- openBinMem 1024 [] let i :: Int i = 3 put_ bh i i2 <- (get bh) :: IO Int assertEqual "i == i2" i i2) it didn't work: *Network.Server.DNS> runTestTT tests ### Error in: 1 user error (HUnit:i == i2 expected: 3 but got: 209944584) In general, what method/way do you recommend to test serialization/de-serialization (note: I'd like to use QuickCheck if possible)? Thank you. -- Pupeno <pupeno(a)pupeno.com> (http://pupeno.com)

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