- Haskell-Cafe - Haskell.org

STM applications
by Nehir Sonmez 18 May '08

18 May '08

Good day all, We are Haskell STM researchers, looking for Haskell applications written using STM for profiling their STM behaviour. The benchmark currently consists of the ones in: www.cs.rochester.edu/meetings/TRANSACT07/papers/perfumo.pdf (3rd page) + binary trees + parallel sudoku and additional programs would be greatly greatly appreciated... Cheers, Nehir Sonmez Barcelona Supercomputing Center PS: We also know of Conjure and Happs.

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hmp3 compilation problem
by Levi Stephen 18 May '08

18 May '08

Hi, I am trying to compile hmp3. I have the version from darcs at http://code.haskell.org/~dons/code/hmp3 I am getting the following errors: Tree.hs:190:14: No instance for (Binary FilePathP) arising from a use of `get' at Tree.hs:190:14-16 Possible fix: add an instance declaration for (Binary FilePathP) In a 'do' expression: nm <- get In the expression: do nm <- get i <- get return (File nm i) In the definition of `get': get = do nm <- get i <- get return (File nm i) Tree.hs:236:31: Couldn't match expected type `L.ByteString' against inferred type `bytestring-0.9.0.1:Data.ByteString.Lazy.Internal.ByteString' In the second argument of `L.writeFile', namely `(compress (encode s))' In the expression: L.writeFile f (compress (encode s)) In the definition of `writeTree': writeTree f s = L.writeFile f (compress (encode s)) Tree.hs:240:46: Couldn't match expected type `bytestring-0.9.0.1:Data.ByteString.Lazy.Internal.ByteString' against inferred type `L.ByteString' In the first argument of `decompress', namely `s' In the first argument of `decode', namely `(decompress s)' In the first argument of `return', namely `(decode (decompress s))' Any help is much appreciated. Thanks, Levi

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Starting Haskell with a web application
by Jonathan Gardner 18 May '08

18 May '08

I am sure I am missing something obvious, but since the cafe is the place to ask stupid questions... I am getting more and more excited about Haskell as I learn more and more about it. Professionally, and as a hobby, I love working on web applications. Where do I get started in writing a web app with Haskell? I am looking more for a framework like Pylons and less like Apache, if that helps. I am sure someone has already done the homework on this, so a link will be fine. -- Jonathan Gardner jgardner(a)jonathangardner.net

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Another optimization question
by Jeroen 18 May '08

18 May '08

Hi, I know there's been quite some performance/optimization post lately, so I hope there still room for one more. While solving a ProjectEuler problem (27), I saw a performance issue I cannot explain. I narrowed it down to the following code (never mind that 'primes' is just [1..], the problem is the same or worse with real primes): primes :: [Int] primes = [1..] isPrime :: Int -> Bool isPrime x = isPrime' x primes where isPrime' x (p:ps) | x == p = True | x > p = isPrime' x ps | otherwise = False main = print $ length (filter (== True) (map isPrime [1..5000])) $ time ./experiment1 5000 real 0m4.037s user 0m3.378s sys 0m0.060s All good, but if I change isPrime to the simpeler isPrime x = elem x (takeWhile (<= x) primes) it takes twice as long: time ./experiment2 5000 real 0m7.837s user 0m6.532s sys 0m0.141s With real primes, it even takes 10 times as long. I tried looking at the output of ghc -ddump-simpl, as suggested in a previous post, but it's a bit over my head (newby here...). Any suggestions? Thanks a lot, Jeroen

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Std lib equivalent for liftM concat . sequence
by Alistair Bayley 18 May '08

18 May '08

A couple of days ago I had need for: > concatM :: Monad m => [m [a]] -> m [a] > concatM = liftM concat . sequence but found no such thing in the std libs, except perhaps for msum (I don't want to add instances for MonadPlus. Should I have to?). Have I missed something trivial? Alistair

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Random numbers and splitting
by Lauri Oksanen 18 May '08

18 May '08

Hi, Does there exists any random number generator in Haskell that is suitable for doing heavy simulations and that can be splitted? At least there exists some c implementations of such generators, see http://www.iro.umontreal.ca/~lecuyer/myftp/papers/streams00.pdf Also some new theory for Mersenne Twister and its relatives exists, see http://www.iro.umontreal.ca/~lecuyer/myftp/papers/jumpf2.pdf http://www.iro.umontreal.ca/~lecuyer/myftp/papers/jumpmt.pdf I'm trying to implement Metropolis algorithm with lazy mutations and a state space that is infinite dimensional in theory (although, of course, not in practice). Without good splitting I'm forced to write as ugly code as with non-lazy language. Thanks in advance, Lauri

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Multi threaded garbage collector
by Peter Verswyvelen 18 May '08

18 May '08

Hi all, I did some experiments with concurrent Haskell, and unfortunately I couldn't get my code run faster when using more cores, actually it ran a bit slower. Now I noticed after profiling that about 70% of the time my program was performing garbage collection (I had lists of 50000 tiny objects that got recreated 100 times per second). If I understood it correctly, the current garbage collector of GHC is single threaded, which really hinders SMP Any plans on improving this or workarounds? Thanks, Peter

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Short circuiting and the Maybe monad
by John Hamilton 18 May '08

18 May '08

I'm trying to understand how short circuiting works with the Maybe monad. Take the expression n >>= f >>= g >>= h, which can be written as (((n >>= f) >>= g) >>= h) because >>= is left associative. If n is Nothing, this implies that (n >>= f) is Nothing, and so on, each nested sub-expression easily evaluating to Nothing, but without there being a quick way to short circuit at the beginning. Now take the example do x <- xs y <- ys z <- zs return (x, y, z) which I believe desugars like xs >>= (\x -> ys >>= (\y -> zs >>= (\z -> return (x, y, z)))) Here the associativity of >>= no longer matters, and if xs is Nothing the whole expression can quickly be determined to be Nothing, because Nothing >>= _ = Nothing. Am I looking at this correctly? - John

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another Newbie performance question
by Philip Müller 18 May '08

18 May '08

Hi everybody, I was doing an assignment in Java for my university concerning a program that reads, modifies and writes CSV files, when I suddenly had the idea of implementing parts of this in Haskell for fun. When I finished the Haskell programm, I was disappointed by the performance: To parse a 200k lines CSV file, insert a line (yes I know i could insert a line without parsing the file, that's just an example) at pos. 199999 and write the file again, the Java program takes 1.1 seconds while the Haskell program takes 12.5 seconds. I have read Don's blog post but am unsure how to implement his tips into my program, as I am still kind of a Haskell beginner. The source code (40 lines incl. comments and empty lines) and the 200k CSV file I used for testing and a smaller CSV file demonstrating the special easy-to-parse CSV syntax are available on my ftp server, ftp://baah.servegame.org/public/haskell The call syntax is <program> <csv file> <line to insert> e.g. main lang.csv "test","this","line" I haven't posted the source code here directly because I thought it might be too long. If someone here finds the time to look at my code and give me some hints, that would really be nice. Regards Philip

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Mapping Haskell Concepts to C# 3
by Galchin, Vasili 18 May '08

18 May '08

Kaveh, Maybe I am misguessing why you are asking your question, but wouldn't it be better to ask how to map these Haskell concepts to CLR? If so, check out work on Mondrian. Vasili

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