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getting set up in ubuntu
by Martin DeMello 05 Jun '08

05 Jun '08

I finally ditched sabayon for ubuntu (one wireless problem too many), and now I'm slowly getting stuff set up on it. Any Ubuntu people care to share their experiences? I'm especially looking for guidelines on what to install via apt-get and what to install independently. Also, while I'm making Major Life Changes I might as well check out xmonad :) If anyone has made the switch from fluxbox, do share how it went. martin

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automatically deriving Map and Filter on datatypes etc.
by Cetin Sert 05 Jun '08

05 Jun '08

Hi ^_^, Let's say we have the following data type and functions: data Tab a = (:↺:) | a :↓: Tab a | Tab a :↙↘: (Tab a,Tab a) deriving (Eq, Show, Read) map f (:↺:) = (:↺:) map f (a :↓: t) = f a :↓: map f t map f (h :↙↘: (l,r)) = map f h :↙↘: (map f l, map f r) filter p (:↺:) = (:↺:) filter p (a :↓: t) | p a = filter p t | otherwise = a :↓: filter p t filter p (h :↙↘: (l,r)) = filter p h :↙↘: (filter p l, filter p r) is it possible to automatically derive map and filter? data Tab a = (:↺:) | a :↓: Tab a | Tab a :↙↘: (Tab a,Tab a) deriving (Eq, Show, Read, Map, Filter) If not, do you think it might be nice to have something like this in the future? Best Regards, Cetin Sert

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hmatrix
by Anatoly Yakovenko 05 Jun '08

05 Jun '08

What is the most efficient way to update a position in a matrix or a vector? I came up with this: updateVector :: Vector Double -> Int -> Double -> Vector Double updateVector vec pos val = vec `add` v2 where v2 = fromList $ (replicate (pos) 0.0) ++ ((val - (vec @> pos)):(replicate ((dim vec)- pos - 1) 0.0)) but this seems pretty inefficient to me. thanks, Anatoly

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More on performance
by Don Stewart 05 Jun '08

05 Jun '08

I wrote up the second part of the tour of understanding low level performance in GHC here, http://reddit.com/r/programming/info/6lx36/comments/ Follows on from the discussion last week about various performance related things. Enjoy. -- Don

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Ubuntu and ghc
by Galchin, Vasili 05 Jun '08

05 Jun '08

Hello, https://bugs.launchpad.net/ubuntu/+source/gtk2hs/+bug/229489 .... this is almost identical to my problem. I am just trying to help others on this list who are using Ubuntu Linux to avoid my predicament! Kind regards, Vasili

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re: very hard to build darcs with win32 ghc-6.8.2!
by Tim Newsham 05 Jun '08

05 Jun '08

Here's the wrapup, thanks to those who replied: - the haskell zlib package on hackage no longer requires that zlib (the C library) be prebuilt. It includes sources itself and builds and installs them using Cabal. - the haskell zlib package is not used by darcs. It links against libz.a directly. You do need to have libz.a somehow to build darcs. - the darcs link error with "SleepEx" is caused because it uses "foreign import ccall" when it should use "foreign import stdcall". Fixing the decl in src/win32/System/Posix.hs to stdcall fixes this issue. I still think its odd that ghc's "gcc" is not very useable outside of Cabal, but in this case it has no bearing on the zlib package. If you choose to build "libz.a" (the C library) from sources using this gcc it will be hard, but you could also get it from an external source (such as the prebuilt libcurl library package). Tim Newsham http://www.thenewsh.com/~newsham/

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Laziness leaks
by Ronald Guida 05 Jun '08

05 Jun '08

I was looking at the real time queues in [1] and I wanted to see what would happen if I tried to write one in Haskell. The easy part was translating the real time queue from [1], p43 into Haskell. The hard part is testing to see if the rotations really happen what they should. Basically, I decided to use Debug.Trace.trace to see when rotations were actually occurring. I pushed the numbers 1 to 10 into the queue, and then I popped the queue ten times. What I found is that none of the rotations would actually take place until the first time I actually tried to /display the value/ of a popped element. What I realized is that my test driver is lazy. I figured out that I could put a bunch of 'seq' functions in the test driver to get the rotations to happen. My demonstration code is in: http://hpaste.org/8080 This leads to two questions: 1. If I find a laziness leak, is 'seq' the appropriate way to plug it? 2. Is there any way to systematically search for or detect laziness leaks?

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GHCi gives type signature that won't compile.
by A A 05 Jun '08

05 Jun '08

Hi All I can successfully compile the following code using ghci import Data.Array.IArray makeArray (lower, upper) f = listArray (lower, upper) [(f i) | i <- [lower..upper] tridiagonal_solve (a, b, c, d, (lower, upper)) = x where gen_array f = (makeArray (lower, upper) f) x = gen_array x_f where x_f i | i == upper = (d_dash ! upper) | otherwise = (d_dash ! i) - (c_dash ! i) * (x ! (i + 1)) d_dash = gen_array d_dash_f where d_dash_f i | i == lower = (d ! lower) / (b ! lower) | otherwise = ((d ! i) - (d_dash ! (i - 1)) * (a ! i)) / ((b ! i) - (c_dash ! (i - 1)) * (a ! i)) c_dash = gen_array c_dash_f where c_dash_f i | i == lower = (c ! lower) / (b ! lower) | otherwise = (c ! i) / ((b ! i) - (c_dash ! (i - 1)) * (a ! i)) I then attempted to write a type signature for tridiagonal_solve. I didn't succeed at this however, all of my type signatures gave compile errors when attempting to load them in ghci. I asked in the #haskell channel, and they suggested that I try the type signature from ghci :type function. So I removed any type signatures, and reloaded the file in ghci, and typed ":type tridiagonal_solve" which produced the following output: tridiagonal_solve :: (IArray a3 e, IArray a6 e, IArray a7 e, Fractional e, Ix a, Num a, IArray a4 e, IArray a2 e, IArray a5 e, IArray a1 e, Enum a) => (a4 a e, a2 a e, a1 a e, a5 a e, (a, a)) -> a7 a e I then attempted to paste this into my code, the resulting code looking like this: import Data.Array.IArray makeArray (lower, upper) f = listArray (lower, upper) [(f i) | i <- [lower..upper]] tridiagonal_solve :: (IArray a3 e, IArray a6 e, IArray a7 e, Fractional e, Ix a, Num a, IArray a4 e, IArray a2 e, IArray a5 e, IArray a1 e, Enum a) => (a4 a e, a2 a e, a1 a e, a5 a e, (a, a)) -> a7 a e tridiagonal_solve (a, b, c, d, (lower, upper)) = x where gen_array f = (makeArray (lower, upper) f) x = gen_array x_f where x_f i | i == upper = (d_dash ! upper) | otherwise = (d_dash ! i) - (c_dash ! i) * (x ! (i + 1)) d_dash = gen_array d_dash_f where d_dash_f i | i == lower = (d ! lower) / (b ! lower) | otherwise = ((d ! i) - (d_dash ! (i - 1)) * (a ! i)) / ((b ! i) - (c_dash ! (i - 1)) * (a ! i)) c_dash = gen_array c_dash_f where c_dash_f i | i == lower = (c ! lower) / (b ! lower) | otherwise = (c ! i) / ((b ! i) - (c_dash ! (i - 1)) * (a ! i)) When I attempt to load this into ghci, I recieve the following error: [1 of 1] Compiling Main ( main.lhs, interpreted ) main.lhs:29:11: Could not deduce (IArray a1 e) from the context (IArray a3 e, IArray a6 e, IArray a7 e, Fractional e, Ix a2, Num a2, IArray a4 e, IArray a21 e, IArray a5 e, IArray a11 e, Enum a2) arising from a use of `gen_array' at main.lhs:29:11-28 Possible fix: add (IArray a1 e) to the context of the type signature for `tridiagonal_solve' In the expression: gen_array d_dash_f In the definition of `d_dash': d_dash = gen_array d_dash_f where d_dash_f i | i == lower = (d ! lower) / (b ! lower) | otherwise = ((d ! i) - (d_dash ! (i - 1)) * (a ! i)) / ((b ! i) - (c_dash ! (i - 1)) * (a ! i)) In the definition of `tridiagonal_solve': tridiagonal_solve (a, b, c, d, (lower, upper)) = x where gen_array f = (makeArray (lower, upper) f) x = gen_array x_f where x_f i | i == upper = (d_dash ! upper) | otherwise = (d_dash ! i) - (c_dash ! i) * (x ! (i + 1)) d_dash = gen_array d_dash_f where d_dash_f i | i == lower = (d ! lower) / (b ! lower) | otherwise = ((d ! i) - (d_dash ! (i - 1)) * (a ! i)) / ((b ! i) - (c_dash ! (i - 1)) * (a ! i)) c_dash = gen_array c_dash_f where c_dash_f i | i == lower = (c ! lower) / (b ! lower) | otherwise = (c ! i) / ((b ! i) - (c_dash ! (i - 1)) * (a ! i)) main.lhs:35:11: Could not deduce (IArray a e) from the context (IArray a3 e, IArray a6 e, IArray a7 e, Fractional e, Ix a2, Num a2, IArray a4 e, IArray a21 e, IArray a5 e, IArray a11 e, Enum a2) arising from a use of `gen_array' at main.lhs:35:11-28 Possible fix: add (IArray a e) to the context of the type signature for `tridiagonal_solve' In the expression: gen_array c_dash_f In the definition of `c_dash': c_dash = gen_array c_dash_f where c_dash_f i | i == lower = (c ! lower) / (b ! lower) | otherwise = (c ! i) / ((b ! i) - (c_dash ! (i - 1)) * (a ! i)) In the definition of `tridiagonal_solve': tridiagonal_solve (a, b, c, d, (lower, upper)) = x where gen_array f = (makeArray (lower, upper) f) x = gen_array x_f where x_f i | i == upper = (d_dash ! upper) | otherwise = (d_dash ! i) - (c_dash ! i) * (x ! (i + 1)) d_dash = gen_array d_dash_f where d_dash_f i | i == lower = (d ! lower) / (b ! lower) | otherwise = ((d ! i) - (d_dash ! (i - 1)) * (a ! i)) / ((b ! i) - (c_dash ! (i - 1)) * (a ! i)) c_dash = gen_array c_dash_f where c_dash_f i | i == lower = (c ! lower) / (b ! lower) | otherwise = (c ! i) / ((b ! i) - (c_dash ! (i - 1)) * (a ! i)) My question is, what should the type signature for tridiagonal_solve actually be? I would like one that doesn't limit the generality of tridiagonal_solve because I would like to pass different array types in the one function invocation, as long as they have the same index and element types. I'm happy if other functions need type signatures as well. I'm using ghc 6.8.2 on windows, though I get a similar issue with ghc 6.4.1. Thanks. Clinton

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example of FFI FunPtr
by Galchin, Vasili 05 Jun '08

05 Jun '08

Hello, I want to model a Haskell function that is a callback from C. I have only found one example in the unix package's Semaphore.hsc, which apparently is not used. I want to be able to marshall a Haskell function that is a first class citizen residing in a Haskell data type and pass to a C function via FFI. Are there examples of this? Thanks, Vasili

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HDBC-odbc 1.1.4.4 fixes Windows linking problems
by John Goerzen 05 Jun '08

05 Jun '08

Hi, HDBC-odbc version 1.1.4.4 has been uploaded to Hackage. It fixes the problems some here have encountered regarding ODBC crashes or other similar odd behavior on Windows. Thanks to Andrew Appleyard for tracking down the problem and sending me a patch. The detailed explanation, if you're interested, is a ccall vs. stdcall issue in FFI. Details here: http://git.complete.org/hdbc-odbc?a=commit;h=cca312d9891d227094494dcee518f4… People that are not using HDBC-odbc on Windows will not find any difference or other changes in this version. -- John Goerzen Author, Foundations of Python Network Programming http://www.amazon.com/exec/obidos/tg/detail/-/1590593715

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