- Haskell-Cafe - Haskell.org

HList darcs repo
by oleg＠okmij.org 04 Feb '10

04 Feb '10

HList (and OOHaskell, for that matter) are now hosted on community.haskell.org, in the directories /srv/code/HList /srv/code/OOHaskell The repositories are also available at these URLs: http://code.haskell.org/HList/ http://code.haskell.org/OOHaskell/ Many thanks to Ian Lynagh for moving the repositories. Ian has kindly set up the redirection so that the old, used in many places URL http://darcs.haskell.org/HList/ still works. It now redirects to code.haskell.org.

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Re: [Haskell-cafe] matrix question
by John Lato 03 Feb '10

03 Feb '10

> From: Ivan Lazar Miljenovic <ivan.miljenovic(a)gmail.com> > > man <manuel.a.castro(a)gmail.com> writes: >> http://hackage.haskell.org/package/hmatrix-0.8.1.1 >> >> it wraps gls, blas and lapack (so you need to install the libraries). > > There's also the blas package if you just want blas support. I've been using blas recently with very good results. It's great if all you need are simple matrix operations. I just wanted to point out that it doesn't install on ghc-6.12.1 because the ieee dependency provides a type class instance for CLDouble, which was omitted from ghc-6.12.1. I got around this by commenting out the instance in CLDouble in the ieee package, then everything installed and runs properly (using OSX 10.6, compiled with -faccelerate). John

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The errorCalls and ioErrors in extensible exceptions way
by Gracjan Polak 03 Feb '10

03 Feb '10

Hi all, I have base==3.* code that uses errorCalls and ioErrors to intercept either ErrorCall or IOError that may arise in deeper code. I'd like to convert this code to base==4.* new exceptions. -- | Evaluate the argument and catch error call exceptions errorToErr :: Monad m => a -> Err m a errorToErr x = let e = unsafePerformIO (tryJust errorCalls (evaluate x)) in ErrorT (return e) -- | Catch IO errors in the error monad. ioErrorToErr :: IO a -> Err IO a ioErrorToErr = ErrorT . liftM (either (Left . show) Right) . tryJust ioErrors Look here for more context: http://hackage.haskell.org/packages/archive/haxr/3000.5/doc/html/Network-Xm… I know that just importing OldException will do the trick for now. But I'd like to know how to do such a trick in extensible exceptions way. Also that unsafePerformIO looks a bit scary. Isn't there a better way to achieve purpose? -- Gracjan

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Multi-Class monadic type?
by Alexander Treptow 03 Feb '10

03 Feb '10

Hi, i got a little problem and don't know how to solve that. Hope you can help me. code: ---------- module Test where {-# LANGUAGE Rank2Types, RankNTypes #-} import Control.Monad.Trans data TestType = TestType {tst :: (Conf a, MonadIO m) => m a} class Conf a where get :: MonadIO m => m a testFunc :: (forall a. Conf a, MonadIO m => m a) -> TestType testFunc = TestType -------------- error: -------------- Test.hs:11:23 Class `Conf' used as a type In the type signature for `testFunc': testFunc :: (forall a. Conf a, (MonadIO m) => m a) -> TestType -------------- explanation: I need a data type that creates a record with a member that has no fixed type, because its not known at compile-time. The Rank2Types language extension fits that need, but i figured out that i ll need to make that time monadic to avoid the use of unsafePerformIO in the program that uses this lib. Thanks and greetings, Alex

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Hackage Package version ranges
by Neil Mitchell 03 Feb '10

03 Feb '10

Hi, I maintain the Haskell package HLint. HLint depends on haskell-src-exts, cpphs, hscolour and uniplate, plus things which are shipped with GHC. For each of the external library dependencies, I have to specify a version constraint. For example, I developed HLint against cpphs-1.10 so I can write one of: 1) cpphs == 1.10.* 2) cpphs >= 1.10 I know (1) works, since I've tested it. However, every time I use (1) some distro (usually Arch) emails me to say they need wider ranges so they can get all their packages consistent. I then usually flip to (2). Then, sometime later, someone releases a new version of cpphs that changes the interface in a way that breaks HLint, and I get emails from users, and tend to go back to (1). Flip flopping between version constraints isn't a great idea. What should I do? Alternatively, if I could sign up to be emailed when something went wrong, I'd happily fix it. i.e. I'd like an email either when my package fails to compile against the latest version of all packages but within my constrained range, or when the latest version falls outside my constraint range. Thanks, Neil

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GHC/base API documentation has been generated incorrectly
by John Millikin 03 Feb '10

03 Feb '10

Looking at (for example) the docs for 'bracket'[1], the parameter documentation has been "shifted" up by one. The source code[2] looks correct, so perhaps Haddock is parsing it incorrectly? Docs for other functions, such as 'showSigned'[3], exhibit the same error. [1] http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control-Exceptio… [2] http://www.haskell.org/ghc/docs/latest/html/libraries/base/src/Control-Exce… [3] http://www.haskell.org/ghc/docs/latest/html/libraries/base/Numeric.html#v%3…

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Lectureships and Chair at St Andrews
by Kevin Hammond 03 Feb '10

03 Feb '10

We have vacancies for one chair (full professor) and two lectureships (assistant professor) in computer science at the University of St Andrews. It would be good to have some applications from our community. http://www.jobs.ac.uk/job/AAK643/2-lectureships-/ http://www.jobs.ac.uk/job/AAK633/chair-in-human-computer-interaction-/ Please do not be put off that the chair advert specifies "Human-Computer Interaction". Candidates with an excellent research background will always be considered. Best Wishes, Kevin -------- Kevin Hammond, Professor of Computer Science, University of St Andrews T: +44-1334 463241 F: +44-1334-463278 W: http://www.cs.st-andrews.ac.uk/~kh In accordance with University policy on electronic mail, this email reflects the opinions of the individual concerned, may contain confidential or copyright information that should not be copied or distributed without permission, may be of a private or personal nature unless explicitly indicated otherwise, and should not under any circumstances be taken as an official statement of University policy or procedure (see http://www.st-and.ac.uk) The University of St Andrews is a charity registered in Scotland : No SC013532

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Game of life in haskell.
by Lyndon Maydwell 03 Feb '10

03 Feb '10

Hi Cafe. I've made a basic game of life implementation with Haskell and OpenGL: https://github.com/sordina/Life/ The basic premise is to generate a random snapshot, then iterate the successor function on it to create an infinite list of snapshots, then output them using OpenGL. I haven't really run into any major issues aside from not being able to figure out how to automatically rerun the display function, however I believe this is an OpenGL problem, and not related to the Haskell side (press 'n' to move to the next snapshot for now). To run the game: cabal configure && cabal build && ./dist/build/life/life <size of square life matrix> I'm intending to improve the performance, and add more advanced features, but I thought I'd get some feedback first. Can anyone see a way to make this code more idiomatic, or any optimizations I might have missed? I'm still fairly new to Haskell and I haven't really come to grips with monad-transformers and the like yet, so if any advanced techniques are applicable here it would really help me link what I think I understand to reality :-) Also, is something like this worth uploading to Hackage, or should I leave it on github? Thanks guys.

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Re: [Haskell-cafe] Game of life in haskell.
by Jon Harrop 03 Feb '10

03 Feb '10

On Wednesday 03 February 2010 01:08:33 Richard O'Keefe wrote: > On Feb 3, 2010, at 3:04 PM, Jon Harrop wrote: > > Fair enough. If you want to model either an infinite board or a very > > sparse one then a sparse data structure will be asymptotically faster. But > > they won't be simpler and asymptotically efficient mutable data > > structures will be faster again. > > Did I say a word about mutability? The whole point of my message > was that the original message WASN'T about mutable or immutable > data structures but about dense (2d array) or sparse data structures. When Serguey said that arrays were not idiomatic Haskell because they are "hard to update functionally" I assumed he was talking about mutable vs immutable data structures. > >> A simple Haskell analogue, which has the right asymptotic cost, > > > > That will be a lot faster than using Map or Set but you're still > > paying a lot for the indirections between cons cells. Mutating an array > > in-place would be significantly faster and no more difficult to code > > correctly because this is such a trivial algorithm. > > Again, you seem to be responding to something that wasn't said. > No claim was made that the list of (row,[col...]) pairs approach > was *FAST*, Were you not talking about performance when you said "the right asymptotic cost"? > or indeed that the code to manipulate it was notable for simplicity, only > that the data structure in itself is simple. Ok. > I once had Prolog code running faster than the Fortran code it was > based on. Mutable arrays are not in themselves a guarantee of > efficiency, and while I don't imagine that you believe that, the > paragraph I quoted came uncomfortably close to saying it. My statements were specifically about Conway's Game of Life. > In fact mutating an array in place probably would be a bit trickier > because you _can't_ mutate the row you are just calculating; you will > need the old version for the next row. Yes, of course. You would read from and write to different mutable data structures, swapping over their roles. Like double buffering or Cheney's to and from spaces. > And in a sparse-compressed representation, the new and old rows will seldom > be the same size. Just use a conventional resizeable array and it will amortize all those costly allocations for you without complicating the code at all. > For what it's worth, the sparse-compressed code that someone pointed > to, and which I alluded to, is written in C. Remember, the original > message didn't say *arrays* were a poor choice, He said: "Arrays are not fully "idiomatic" for Haskell as they are hard to update functionally." Is that not saying that arrays were a poor choice? > but that the "obvious" *2d array of boolean* was a poor choice. And that's > the (only) claim I defended. The OP was solving a dense problem on a finite board. Is a 2d array of booleans really such a bad choice in that context? Even if you change the problem to a sparse one, how large must it be before a solution like Serguey's becomes competitively efficient in practice? -- Dr Jon Harrop, Flying Frog Consultancy Ltd. http://www.ffconsultancy.com/?e

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matrix question
by 조광래 03 Feb '10

03 Feb '10

define functions type Matrix=[[Double]] multMM :: Matrix -> Matrix -> Matrix --multiplies two matrices det :: Matrix -> Double --computes the determinant of a matrix inv :: Matrix -> Matrix --inverts a matrix i stuck on those problems can any one help me out?

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