August 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 debug GHC
by Monique Louise de Barros Monteiro 10 Sep '05

10 Sep '05

Hi, all, I'm developing a back end for GHC and I have the following problem: my program is throwing an "empty list exception" due to head [] and I need to compile GHC with -prof -auto-all in order to see the stack trace when running it with +RTS -xc -RTS. I changed the makefile but the option +RTS -xc -RTS was not recognized as an available RTS option Does anyone have any idea about how I can do that ? Thanks in advance, __________________________________________________ Monique Louise Monteiro Mestranda em Ciencia da Computacao Centro de Informatica - CIn - UFPE

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Practical introduction to monads
by Paul Moore 07 Sep '05

07 Sep '05

I've started learning Haskell, and I'm going through all the tutorial material I can find - there's a lot of good stuff available. One thing I haven't found a really good discussion of, is practical examples of building monads. There's plenty of discussion of the IO monad, and the state monad, and a lot of good theory on monads, but although I've seen tantalising statements about how powerful the ability to define your own monads can be, but no really concrete examples - something along the lines of - here is problem X - this might be our first cut at coding it - we can abstract out this stuff, as a monad - see how the code looks now, how much cleaner it is (I've seen this type of model developing a state monad, but I'm looking for a more application-specific approach). Also, a lot of (non-IO) monad code I've seen seems to revolve around higher order features like monad combinators, which, while certainly powerful, still make my head explode. On the other hand, monadic IO feels entirely comprehensible, revolving round do-notation, and looking reassuringly sequential and imperative :-) Surely there are other uses of monads which can feel familiar to non-functional programmers? Otherwise, I'm still left with the (inaccurate, and unhelpful) impression that "IO is a badly integrated exception"... I don't know if the above makes sense. I've struggled with this for a while now, and haven't managed to express my confusion any more clearly - for which I apologise. I'd appreciate any pointers or explanations. FWIW, I've read (among other papers) "Why Functional Programming Matters", "A Gentle Introduction to Haskell", Hal Daume's "Yet Another Haskell Tutorial", Simon Peyton Jones' "Tackling the Awkward Squad", and "Haskell: The Craft of Functional Programming". I don't claim to have digested all of this - indeed, there are probably many areas I've only skimmed - so pointers to areas in these documents I might have missed would be appreciated. On the other hand, pointers to papers I've missed altogether would also be gratefully received. Thanks in advance for any help, Paul. -- There is a theory which states that if ever anybody discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable. There is another theory which states that this has already happened. -- Douglas Adams

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Newbie question
by André Vargas Abs da Cruz 01 Sep '05

01 Sep '05

Hi everyone, I think this is a totally newbie question as i am a complete novice to Haskell. I am trying to write down a few programs using GHC in order to get used with the language. I am having some problems with a piece of code (that is supposed to return a list of lines from a text file) which I transcribe below: module Test where import IO readDataFromFile filename = do bracket (openFile filename ReadMode) hClose (\h -> do contents <- hGetContents h return (lines contents)) The question is: if i try to run this, it returns me nothing (just an empty list). Why does this happen ? When i remove the "return" and put a "print" instead, it prints everything as i expected. Thanks in advance André

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RE: [Haskell-cafe] Monadic vs "pure" style (was: pros and cons of sta tic typing and side effects)
by Bayley, Alistair 01 Sep '05

01 Sep '05

> From: Duncan Coutts [mailto:duncan.coutts@worcester.oxford.ac.uk] > > This is often a misconception, that just because you find you need to > 'do' something in the middle of your algorithm, that you need > to convert it wholly to monadic style. Yes. However, Wadler makes a convincing (at least to me) case that the monadic style is easier to extend. The code changes for the monadic style appear to be more localised. Something else I noticed about my non-monadic code was the way I was threading state through functions. I was tempted to introduce a State monad to make this easier to manage, but then I decided to try mutable arrays instead, so that experiment was not attempted. So it might well have been better in monadic style anyway, even with immutable arrays. I'm conscious that for most (?) monads, monadic code can be invoked from non-monadic code. I'm only aware of the IO monad as a one-way trap. So changing code from pure to monadic doesn't necessarily involve program-wide changes, unless the monad you're introducing happens to be IO. In my array example, I introduced STArrays, but the main interface remained pure (non-monadic), which was my goal. I was also wondering what the disadvantages of monadic style are? Are there compiler optimisations which are not possible with monadic code? Alistair. ----------------------------------------- ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

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Re: [Haskell] pros and cons of static typing and side effects ?
by Martin Vlk 01 Sep '05

01 Sep '05

Hi folks, have you read this paper: http://www-i2.informatik.rwth-aachen.de/Staff/Current/michaelw/sttt-ml-hask… It contains descriptions of lots of real-world problems and how easily they are solved with Haskell (and ML, because the paper compares the two languages). Among other things it touches on the static typing pros and cons in a way that anyone who ever did serious programming will understand. vlcak

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embedding prolog in haskell.
by Keean Schupke 01 Sep '05

01 Sep '05

Does anyone know if the source code for the embedded prolog (by Silvija Seres & Michael Spivey) is available for download from anywhere? I have read the paper and found some of the types are wrong, some critical definitions are missing, and the definition of unify is missing. Regards, Keean.

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Haskell thread preemption
by Dmitry Vyal 31 Aug '05

31 Aug '05

Hello, all. I'm experimenting with concurrent haskell using GHC 6.4. I wrote these fuctions as described in "Tackling the Awkward Squad": par_io :: IO a -> IO a -> IO a par_io t1 t2 = do c <- newEmptyMVar :: IO (MVar a) id1 <- forkIO $ wrapper c t1 id2 <- forkIO $ wrapper c t2 res <- takeMVar c killThread id1 killThread id2 return res where wrapper :: MVar a -> IO a -> IO () wrapper mvar io = do res <- io putMVar mvar res timeout :: Int -> IO a -> IO (Maybe a) timeout n t = do res <- par_io thr timer return res where thr = do res <- t return $ Just res timer = do threadDelay n return Nothing Now, then I try something like timeout 1000000 (print $ cycle "test") it behaves correctly: repeatingly prints "test" for one second and then returns Nothing. But then I try this: timeout 1000000 (print $ 2^2^2^2^2^2) it hangs for about eight seconds. Currently I'm playing with theorem-proving using resolution. So I need some technique to break a computation, if takes too long. It seems that using "timeout" there does nothing. I waited for several minutes for invalid theorem and timeout didn't expire. "Control.Concurrent" page of "Haskell Hierarchical Libraries" says, that a thread may be pre-empted whenever it allocates some memory, and that tight loops which do no allocation tend to lock out other threads. But resolution function, i wrote, hogs a *lot* of memory and (print $ 2^2^2^2^2^2) does either. So i'm curious about this behavior of timeout. So what is the correct way of running such computations in parallel? Maybe I'm missing something?

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RE: [Haskell-cafe] Haskell poker server
by Bayley, Alistair 31 Aug '05

31 Aug '05

> From: Cale Gibbard [mailto:cgibbard@gmail.com] > > Well, here's an attempt at a start on a similar mechanism for Haskell: > > ---------- (start Packet.hs) > module Packet where > > import Data.Bits > import Data.Word > > class Packet a where > readPacket :: [Word8] -> (a, [Word8]) ... There's a request on LtU for a similar ability (somewhat wider in scope, perhaps): http://lambda-the-ultimate.org/node/view/938 There must be a "gap in the market" :-) Alistair. ----------------------------------------- ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

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