- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] What ever happened to Haskell 98 as a "stable branch"?
by Andrzej Jaworski 25 Mar '07

25 Mar '07

>If you answer "because H98 is obsolete", then file this away as a >must-read after H' is released Ideas always originate in a single mind. Good ideas are only footnotes to the best idea that determine them. Now: a team of people with different views on the same thing can achieve their best decorating a Christmas Tree. Unless, it is Haskell;-) Regards, -Andrzej

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newbie "concatenating" monad question
by Leandro Penz 25 Mar '07

25 Mar '07

Hi I'm new here and this is my first mail to the list, so be gentle :) I have some functions that build big strings by calling other functions and appending the result, like buildStuff = func1 ++ func2 ++ func3 ++ func4 My idea is to have a monad with a concatenating >>, so that I can: bulidStuff = do func1 func2 func3 func4 I am thinking about using the writer monad, but I would still have to use "tell". The list monad is not an option either, as it does not concatenate. Is there a proper way of doing something like this (efficiently) ? Leandro Penz

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Application of iterate
by Logesh Pillay 24 Mar '07

24 Mar '07

Ordinarily I would not post to a list when I've got a program running. But I've found learning Haskell so tough (this is my 4th try in the last two years) that I feel I have to bore everyone with my first non-trivial program. It is Newton's method adapted to give the square root of a number. And all in just one line which I think is an amazing demonstration of the power of Haskell. And it took no more than a couple of minutes to write (directly to the screen too as opposed to the usual many paper drafts with C). It works too, correct to 13 decimal points on my GHC system It is :- my_sqrt t = last (take 20 (iterate (\n -> n/2 + t/(2 * n)) t)) It is a bit crude though. 20 iterations is a bit arbitrary. I don't suppose there is a easy way to iterate until the results stop changing.

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"runST $ return ()" /= "runST (return ())" ??
by Marc A. Ziegert 24 Mar '07

24 Mar '07

hi! i've just discovered this strange behaviour of existential quantifiers with runST: --- Prelude Control.Monad.ST> :t runST (return ()) runST (return ()) :: () Prelude Control.Monad.ST> :t runST $ (return ()) <interactive>:1:9: Couldn't match expected type `forall s. ST s a' against inferred type `m ()' In the second argument of `($)', namely `(return ())' Prelude Control.Monad.ST> --- the same with "id runST undefined". is this a bug or an unsolved problem? i'm not sure wheather it is a part of the wanted feature of runST's type definition. i did not find any discussions about this. can anyone enlighten me, please? - marc

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OpenGL and GLUT in GHC
by Ruben Zilibowitz 24 Mar '07

24 Mar '07

Hi, I'm experimenting using OpenGL and GLUT in Haskell using GHC. There are modules Graphics.Rendering.OpenGL and Graphics.UI.GLUT. I am using these. I've encountered a strange bug which I'm having trouble with. If I render a sphere and a plane, then the plane is facing the wrong way and is shaded on the wrong side. If however I only render the plane then it appears to be facing the right way and is shaded on the correct side. I have made the source file available as a download here: http://www.cse.unsw.edu.au/~rubenz/stuff/test.hs It can be built by running: ghc --make test.hs I am using ghc 6.6 There is a comment in the source file saying: "Commenting out the line below here causes the plane to be rendered facing towards the camera..." The bug can be seen by commenting out the line of code that follows and recompiling. If anyone can help me by explaining why I am getting this bug or how to fix it that would be great. I'd be very appreciative. Cheers, Ruben

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Performance Help
by Dominic Steinitz 24 Mar '07

24 Mar '07

I have re-written the sha1 code so that it is (hopefully) easy to see that it faithfully implements the algorithm (see http://www.itl.nist.gov/fipspubs/fip180-1.htm) Having got rid of the space leak, I have been trying to improve performance. Currently, the haskell code is 2 orders of magnitude slower than the sha1sum that ships with my linux. > dom@heisenberg:~/sha1/testdist/sha1> time ./perfTest perfTest > c7eae62ddabb653bb9ce4eb18fa8b94264f92a76 > Success > > real 0m2.152s > user 0m2.112s > sys 0m0.028s > dom@heisenberg:~/sha1/testdist/sha1> time sha1sum perfTest > c7eae62ddabb653bb9ce4eb18fa8b94264f92a76 perfTest > > real 0m0.057s > user 0m0.008s > sys 0m0.004s I've played around with profiling and doubled the performance of the haskell code but I'm nowhere near the C performance. > Sun Mar 11 19:32 2007 Time and Allocation Profiling Report (Final) > > perfTest +RTS -p -RTS eg > > total time = 6.75 secs (135 ticks @ 50 ms) > total alloc = 1,483,413,752 bytes (excludes profiling overheads) > > COST CENTRE MODULE %time %alloc > > oneBlock Data.Digest.SHA1 39.3 40.1 > $& Data.Digest.SHA1 20.7 21.6 > f Data.Digest.SHA1 13.3 6.2 > getWord32s Data.Digest.SHA1 7.4 6.6 > test2 Main 5.9 8.7 > blockWord8sIn32 Data.Digest.SHA1 5.2 5.3 > blockWord8sIn512 Data.Digest.SHA1 3.0 4.4 > pad Data.Digest.SHA1 1.5 3.5 > k Data.Digest.SHA1 1.5 0.0 > fromBytes Data.Digest.SHA1 0.0 3.5 Here's the code that is taking the majority of the time. > ($&) :: [Word32] -> [Word32] -> [Word32] > a $& b = zipWith (+) a b > > -- Word128 -> Word512 -> Word128 > oneBlock ss xs = Word128 (as!!80) (bs!!80) (cs!!80) (ds!!80) (es!!80) > where > ws = xs ++ map (rotL 1) (zipWith4 xxxor wm3s wm8s wm14s ws) > where > xxxor a b c d = a `xor` b `xor` c `xor` d > wm3s = drop (16-3) ws > wm8s = drop (16-8) ws > wm14s = drop (16-14) ws > as = ai:ts > bs = bi:as > cs = ci:(map (rotL 30) bs) > ds = di:cs > es = ei:ds > ts = (map (rotL 5) as) $& (zipWith4 f [0..] bs cs ds) $& es $& (map k > [0..]) $& ws > Word128 ai bi ci di ei = ss Any help would be appreciated. I've put a copy of a working system here if anyone wants to experiment (http://www.haskell.org/crypto/downloads/sha1.tar.gz) Thanks, Dominic.

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Rank-2-polymorphism problem
by Martin Huschenbett 24 Mar '07

24 Mar '07

Hi, I'm writing some database code using HSQL and had to stop on a problem with rank-2-polymorphism that I can't solve. The essence of my code is: module Value where import Data.Maybe class SqlBind a where fromSqlValue :: String -> a data Field data Value emptyValue :: Field -> Value emptyValue _ = ... readValue :: Field -> (forall s. SqlBind s => s) -> Value readValue _ = ... That works just fine. But now I want a version of readValue that has a Maybe wrapped around the second parameter and that shall call readValue in the case of a Just and emptyValue in the case of Nothing. But I can't figure out how to write this function as I always get compiler errors. My trials were: -- The type I want to get. readValue' :: Field -> (forall s. SqlBind s => Maybe s) -> Value -- First trial: readValue' fld s = if isJust s then readValue fld (fromJust s) else emptyValue fld -- Second trial: readValue' fld s | isJust s = readValue fld (fromJust s) | otherwise = emptyValue fld -- Third trial: readValue' fld (Just s) = readValue fld s readValue' fld Nothing = emptyValue fld -- Fourth trial: readValue fld s = case s of Just s' -> readValue fld s' Nothing -> emptyValue fld But none of these trials worked. Is there any solution that works with GHC-6.6 for now? Thanks in advance, Martin.

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Re: Bouncing ball in Gtk2Hs
by Duncan Coutts 23 Mar '07

23 Mar '07

On Fri, 2007-03-23 at 20:24 +0300, Thach Si Lam wrote: > I'm trying to write Bouncing Ball demo in Gtk2hs. But i coudn't find > timer class (like in wxhaskell demo) in Gtk2hs. There is timeoutAdd: http://haskell.org/gtk2hs/docs/gtk2hs-docs-0.9.10/System-Glib-MainLoop.html… used like so: timeoutAdd action 1000 where action is an action with type IO Bool and the number is the time to wait in milliseconds, (ie 1000 is 1 second). If the action returns True then it will get called again at the next timeout, otherwise it will not be called again. > How to deal with it ? In fact i have 2 processes: p1 for computing > some variables and p2 for changing (applying those variables) in > canvas. How to let them communicate together and update to canvas ? > Please, help me. Thank you very much. I'm forwarding this to the Haskell Cafe mailing list where someone might be able to give you a pointer. Duncan

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Re: [Haskell-cafe] AI Strike Force!
by Andrzej Jaworski 23 Mar '07

23 Mar '07

Hi Andrew and all the involved, The idea is great. It will allow programming with a rewording feeling of depth to the exercise right from the start. However tackling some of the topics you mentioned, like GA, one first needs to develop new solid programming techniques that would circumvent Haskell's inbuilt reluctance to update variables distractively. I would myself wish to read a well documented study of using ST monad or finite maps to achieve this end. Perhaps then, your page should openly encourage the need to develop such "brute force" methods. Cheers, -Andrzej

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HDBC Windows binaries with SQLite 3 support
by Alex Queiroz 23 Mar '07

23 Mar '07

Hallo list, I guess the subject line says it all. Can anybody help me? -- -alex http://www.ventonegro.org/

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