February 2010 - Haskell-Cafe

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 02 Feb '10

02 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 02 Feb '10

02 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 02 Feb '10

02 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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About code style ?
by zaxis 02 Feb '10

02 Feb '10

For me i like C style instead of layout. For example, func1 a = do -- ... a * 2 -- ... I always write it as: func1 a = do { -- ...; a * 2; -- ...; } However, i donot know how to write pure function using C style. func1 a = { -- ...; a * 2; -- ...; } will not compile without `do`. Sincerely! ----- fac n = foldr (*) 1 [1..n] -- View this message in context: http://old.nabble.com/About-code-style---tp27414627p27414627.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Re: [Haskell-cafe] Game of life in haskell.
by Tony Finch 02 Feb '10

02 Feb '10

On Tue, 2 Feb 2010, edgar(a)ymonad.com wrote: > http://dotat.at/prog/life/hslife.hs Er, yes, I didn't link to that in my earlier message because it's a half-completed attempt. I think I got to the stage of realising that I needed to write a monad or a monad transformer to thread the hash cons state through the code without it getting in the way. For those interested in Hash Life whether or not it's written in Haskell, there's a description of how it works in http://dotat.at/prog/life/hashlife.c though that code is also incomplete. Tony. -- f.anthony.n.finch <dot(a)dotat.at> http://dotat.at/ GERMAN BIGHT HUMBER: SOUTHWEST 5 TO 7. MODERATE OR ROUGH. SQUALLY SHOWERS. MODERATE OR GOOD.

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SYB/Generics documentation inaccessible
by Arne D Halvorsen 02 Feb '10

02 Feb '10

I have tried to access a few of the documents on generics that are referred to from wikipedia, from the GHC documentation, from hackage and from the wiki. These documents (on vu.nl, subdirectory boilerplate and strafunski) are apparently forbidden to me, and have been for some weeks. I tried to mail a webmaster and some other mentioned mail addresses, but the mails bounced. Could somebody make the docs accessible, or should it be re-hosted on Hackage? Regards Arne D Halvorsen

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Failure to load hmatric
by dlb＠patriot.net 02 Feb '10

02 Feb '10

OK, I'm working on matrix stuff in Haskell now (I've been trying to get the professor to approve that) and when I use cabal install to install hmatrix, it fails at HUnit with: ----------- Configuring HUnit-1.2.2.1... Preprocessing library HUnit-1.2.2.1... Preprocessing executables for HUnit-1.2.2.1... Building HUnit-1.2.2.1... [1 of 5] Compiling Test.HUnit.Terminal ( Test/HUnit/Terminal.hs, dist/build/Test/HUnit/Terminal.o ) [2 of 5] Compiling Test.HUnit.Lang ( Test/HUnit/Lang.hs, dist/build/Test/HUnit/Lang.o ) [3 of 5] Compiling Test.HUnit.Base ( Test/HUnit/Base.hs, dist/build/Test/HUnit/Base.o ) [4 of 5] Compiling Test.HUnit.Text ( Test/HUnit/Text.hs, dist/build/Test/HUnit/Text.o ) [5 of 5] Compiling Test.HUnit ( Test/HUnit.hs, dist/build/Test/HUnit.o ) /usr/bin/ar: creating dist/build/libHSHUnit-1.2.2.1.a Test/HUnit/Lang.hs:22:1: lexical error at character 'i' cabal: Error: some packages failed to install: HUnit-1.2.2.1 failed during the building phase. The exception was: exit: ExitFailure 1 ---------------- Short term question: what the heck do I do with this? Long term question: how can I handle cabal failures of this type without bothering the list every time? Dave Barton University of Toronto

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Tracer for Haskell showing substitutions
by Ulrik Rasmussen 02 Feb '10

02 Feb '10

Hi all, I was wondering if someone has written a tracer/debugger that shows you how a given Haskell expression is evaluated, by generating all the intermediate states of the expression until it is in normal form? For instance, given the following code: > take' 0 xs = [] > take' n (x:xs) = x : take' (n-1) xs > exp = take' 2 [1,2,3,4,5,6] the trace of 'exp' would generate something like this: > exp = take' 2 [1,2,3,4,5,6] > exp = (\n (x:xs) -> x : take' (n-1) xs) 2 [1,2,3,4,5,6] > exp = 1 : take' (2-1) [2,3,4,5,6] > exp = 1 : take' 1 [2,3,4,5,6] > exp = 1 : (\n (x:xs) -> x : take' (n-1) xs) 1 [2,3,4,5,6] > exp = 1 : 2 : take' (1-1) [3,4,5,6] > exp = 1 : 2 : take' 0 [3,4,5,6] > exp = 1 : 2 : (\0 xs -> []) 0 [3,4,5,6] > exp = 1 : 2 : [] > exp = [1,2] That is, all the substitutions performed when evaluating 'exp' from left to right. I was thinking that something like this could be rather useful when teaching or learning Haskell. Thanks, Ulrik

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want to post haskell question
by 조광래 02 Feb '10

02 Feb '10

hi I have a question about haskell coding. Matrices One of many ways to define matrices in Haskell is the list of matrix rows, where a row is a list of double precision oating point numbers: type Matrix=[[Double]] Using this type you should define the following functions (definition of dim is given as an inspirational example). dim :: Matrix -> (Int,Int) --returns matrix dimension (number of rows and columns) dim m = if (not.isMatrix) m then error "Not a matrix" else (length m, length (head m)) isMatrix :: Matrix -> Bool --checks whether all rows have the same length isMatrix m = and [length (head m) == length a | a <- m] I have done upto here and I am stuck on multSM. multSM :: Double -> Matrix -> Matrix --multiplies a scalar and a matrix when I do: multSM d m = [[(b*a)| b<-[d], a<-(head m)]] or [map (*d) (head m)] ...I get (using Hugs): Main> multSM 3 [[2,3,4],[1,3,4],[6,3,0]] [[6.0,9.0,12.0]] So I solve 3 [2,3,4] but I do not know how to get the rest 3 [[1,3,4],[6,3,0]. Please help me on this problem.

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