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Fucntion composing
by Adam Krauze 13 Apr '11

13 Apr '11

Hello, as I am newbie to Haskell and my introductory question is: given functions say f and g with type signatures f :: (Num a) => [a] -> [a] -> [(a,a)] // f takes two lists and zips them into one in some special way g :: (Num a) => a -> [(a,a)] -> [a] // g using some Num value calculates list of singletons from list of pairs of course g 0 :: (Num a) => [(a,a)] ->[a] now I want to create function h :: (Num a) => [a] -> [a] -> [a] in such way that (g 0) consumes output of f. But when I try Prelude> :t (g 0).f I get an error: <interactive>:1:9: Couldn't match expected type `[(a0, a0)]' with actual type `[a1] -> [(a1, a1)]' Expected type: [a1] -> [(a0, a0)] Actual type: [a1] -> [a1] -> [(a1, a1)] In the second argument of `(.)', namely `f' In the expression: (g 0) . f In pointfull representation it works well Prelude> let h x y = (g 0 (f x y)) How to do pointfree definition of h? Ajschylos.

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Service Component Architecture and Distributed Haskell
by James Keats 13 Apr '11

13 Apr '11

Hi all. There's recently been some talk about distributed haskell, but what I wish to see is interest from an experienced member of the Haskell community in implementing Haskell components with SCA/Tuscany. Please see this and consider it, it'd be great to be able to use Haskell in the same project with Java/Scala. Thanks. Link: http://www.haskell.org/pipermail/haskell/2010-April/022004.html

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Higher-kinded Quantification
by Leon Smith 13 Apr '11

13 Apr '11

I have a type constructor (Iterator i o m a) of kind (* -> * -> (* -> *) -> *), which is a monad transformer, and I'd like to use the type system to express the fact that some computations must be "pure", by writing the impredicative type (Iterator i o (forall m. m) a). However I've run into a bit of difficulty expressing this, due to the kind of m. I've attached a minimal-ish example. Is there a way to express this in GHC?

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Generating random graph
by Mitar 13 Apr '11

13 Apr '11

Hi! I have made this function to generate a random graph for Data.Graph.Inductive library: generateGraph :: Int -> IO (Gr String Double) generateGraph graphSize = do when (graphSize < 1) $ throwIO $ AssertionFailed $ "Graph size out of bounds " ++ show graphSize let ns = map (\n -> (n, show n)) [1..graphSize] es <- fmap concat $ forM [1..graphSize] $ \node -> do nedges <- randomRIO (0, graphSize) others <- fmap (filter (node /=) . nub) $ forM [1..nedges] $ \_ -> randomRIO (1, graphSize) gen <- getStdGen let weights = randomRs (1, 10) gen return $ zip3 (repeat node) others weights return $ mkGraph ns es But I noticed that graph has sometimes same weights on different edges. This is very unlikely to happen so probably I have some error using random generators. Could somebody tell me where? Mitar

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Syntastic support for cabal
by Luke Randall 13 Apr '11

13 Apr '11

Hi all I'm using Haskell mode for Vim, as well as syntastic. I've found syntastic particularly useful as I'm quite new to Haskell and sometimes get the types of my functions wrong. When working on Cabal packages however, syntastic marks import statements which import modules local to the cabal package as being incorrect because the module cannot be found. Has anyone solved this, or do you have pointers on how I can? Thanks -- Luke Randall Sent with Sparrow

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Debugging with gdb?
by Svante Signell 13 Apr '11

13 Apr '11

Hi, As I don't know anything about Haskell, can I make a stupid question: Is there any method to create debug symbols for a Haskell program, and is it possible to debug with gdb? Thanks!

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RFC: Extendable and Easy-To-Use Logger (HLogger)
by Jon Kristensen 13 Apr '11

13 Apr '11

Hello Haskellers! I have developed a very simple logging library to be extendable and easy-to-use. You can see the current release at http://hackage.haskell.org/package/hlogger/. The SimpleHLogger module is an example of what a HLogger implementation could look like. It could be used like this: import System.Log.SimpleHLogger main = do logger<- simpleLogger "test" loggerLog logger (Just ("LoggerTest", "main")) Debug "Test" loggerStop logger As I have now implemented everything that I could think of, I would like to ask the Haskell community to give some feedback on HLogger so that I can continuing improving it. Some questions: 1. Do you have any general/architectural thoughts on things I might be doing wrong? 2. Is the ease-of-use goal contradicting making this library useful in more complex applications? 3. Do I want to redesign HLogger to allow loggers to have a mutable state, and if so, how can I do that? 4. Is there a nice way to provide the module and the function than using (Just ("LoggerTest, "main)) (shortening it somehow, I guess)? Thanks! Warm regards, Jon Kristensen

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Using DPH
by Wilfried Kirschenmann 13 Apr '11

13 Apr '11

Hi, In order to do a performance comparison beetween different approaches for our application, I make different implementation of a simple example (computing the norm of a vector expression. I rely on Repa to do this. However, when I tried to build the parallel version (-threaded -fvectorise -rtsopts), I got an error specifying that dph-par was not available. Indeed, It wasn't. As explained on the dph webpage, I installed a developpment version of ghc (ghc-7.1.20110331). However, when I try to build my application, I get the folowing error: user error (Pattern match failure in do expression at compiler/vectorise/Vectorise/Monad.hs:57:10-20) My source code is attached and compiled with the following line: ghc haskell.hs -O3 -XBangPatterns -msse2 -fforce-recomp -rtsopts -threaded -fvectorise So my question is : is the problem concerning the pattern matching known ? If yes, is there a workaround ? is there an other version of dph that would work with Repa ? Thank you ! ----- Wilfried Kirschenmann "An expert is a person who has made all the mistakes that can be made in a very narrow field."* *Niels Bohr - *Danish physicist (1885 - 1962)*

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ANNOUNCE: TCache 0.8
by Alberto G. Corona 13 Apr '11

13 Apr '11

Tired of your persistence layer? Take persistence the haskelll way. Besides the backward compatible stuff, TCache now defines persistent STM variables (DBRef's) that leverages the traditional haskell reference syntax to perform fast in-memory database transactions and inter-object references. It includes indexing, triggers and an straightforward pure-haskell query language based on record field relations. http://hackage.haskell.org/package/TCache-0.8.0.2 This version includes a lot of changes and must be considered beta. Please submit any bugs, comments and so on. Cheers!

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Request for library recommendations
by Erik de Castro Lopo 13 Apr '11

13 Apr '11

Hi all, I'm currently scoping out a project for work that I'd like to do in Haskell. Basically its a HTTP proxy (ie like the Squid proxy) but with a few wrinkles: - Since the connecting clients are untrusted, the incoming connection is HTTP, the header is modified by the proxy to add BasicAuth (credentials known only to the proxy, not the client) and then forwared upstream with responses pass back to the client. - Needs to work via upstream Squid proxy (so this proxy needs to be able to requirest a CONNECT from the upstream Squid proxy). - In some cases, the client will POST via the proxy and immediately exit and the proxy will be required to re-try the POST if it fails. - Other manipulation of headers and data. I am aware that Squid can satisfy the first two requirements above but not all the others (without actually hacking the Squid code base). I have done stuff like this before in C++ and Ocaml, so the mechanics are not a mystery. However I would like as much as possible to make use of existing Haskell libraries to cut down the development time. Performance is also not critical, so as long as the Haskell version can beat a hypothetical Python implementation I'd be happy. Thanks in advance. Cheers, Erik -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/

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