March 2011 - Haskell-Cafe

hxt validation outside of IO?
by cafe-haskell＠coreland.ath.cx 26 Mar '11

26 Mar '11

Hello. Is it possible to validate XML trees against a RELAX NG schema outside of IO, using hxt? All of the functions I can see use an IOSArrow. I'm not sure what I'm missing.

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ANNOUNCE: version 0.7 of monad-parallel, monad-coroutine and SCC
by Mario Blažević 26 Mar '11

26 Mar '11

Packages monad-parallel [1], monad-coroutine [2] and SCC [3] have been upgraded on Hackage to version 0.7. The monad-parallel library defines two Monad subclasses, MonadParallel and MonadFork, that enable some monadic computations to be executed in parallel and their results combined. The library also exports a subset of the Control.Monad interface (ap, sequence, and related functions), adjusted to exploit the parallelism. The new version of the library adds several more instances of these classes. The monad-coroutine library exports a generic monad transformer Coroutine: Functor s => MonadTrans (Coroutine s). A Coroutine-transformed monad can suspend at any point, returning its resumption wrapped in the functor s. The new release of monad-coroutine makes Coroutine an instance of Functor and Applicative classes. It also adds support for inter-coroutine communication through chunks parsed by the incremental-parser library [4]. The new version of Streaming Component Combinators (a.k.a. SCC) relies the aforementioned incremental-parser support to some good effect. Many coroutine components have been simplified and sped up by ~30% on average. [1] http://hackage.haskell.org/package/monad-parallel-0.7 [2] http://hackage.haskell.org/package/monad-coroutine-0.7 [3] http://hackage.haskell.org/package/scc-0.7 [4] http://hackage.haskell.org/package/incremental-parser-0.1

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ANN: clash-0.1.3.0 (Functional Hardware Descriptions)
by Christiaan Baaij 25 Mar '11

25 Mar '11

Hello, I am pleased to announce an incremental update to CLaSH, version 0.1.3.0. CLaSH can translate a (semantic) subset of Haskell to RTL-style VHDL. Instead of being an embedded DSL like Lava or ForSyDe, CLaSH takes are more 'traditional' approach to synthesis/compilation. It uses the GHC API as a front-end for parsing, type-checking and desugaring the Haskell source. It then exhaustively applies a list of meaning-preserving transformations to the intermediate GHC Core representation until it is in the desired normal form. The normalized Core/System Fc is then 'trivially' translated to RTL-style VHDL. The new version of CLaSH has the following updates: - Support for simulation and synthesis of multi-clock hardware [6] - Significant synthesis speed-up (4x to 10x) CLaSH already supported synthesis of: - User-defined Higher-Order functions [1] - User-defined ADTs (GADTs are not tested, but *might* work) - All of Haskell's choice constructs (Guards, Pattern Matching, etc.) - Lambda-abstraction / Anonymous Functions You can use CLaSH as a library, but the use of the adapted GHC interpreter (added the :vhdl command) is recommended. The interpreter can be found on the CLaSH website [2], where you will also find examples, papers, tutorials, etc. The library can be downloaded from Hackage [3]. I recently gave a demo of CLaSH at the DATE'11 conference in grenoble, this demo already made use of the multi-clock feature (and is actually the only reference if you want to experiment with multi-clock hardware yourself). The source for the demo (audio spectrum analyzer programmed on an Altera Cyclone II FPGA board) can be downloaded from my github page [4]. Do note that CLaSH only works when you have the 6.12.* branch of GHC installed! I am currently analyzing the impact of the GHC API changes made in the 7.0.* brach, and hope to make the transition to the new API within the next month. Although the compiler is already used by 2 other phd's in our group [5], it is not a thoroughly tested product, so your coding style might not be anticipated by the current version of CLaSH ;-) -- Christiaan Baaij [1] There is hard-coded support for a set of recursively defined higher-order functions such as map, fold, zipWith, etc. [2] http://clash.ewi.utwente.nl [3] http://hackage.haskell.org/package/clash-0.1.3.0 [4] http://github.com/christiaanb/DE1-Cyclone-II-FPGA-Board-Support-Package [5] http://caes.ewi.utwente.nl [6] Simulation does not show meta-stability, you will have to take care of synchronization (dual flipflop) yourself

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package cache being out of date
by Wolfgang Jeltsch 25 Mar '11

25 Mar '11

Hi, I have a freshly installed Haskell Platform 2010.2.0.0 on Windows 7. When I run ghc-pkg list, I get the following message (besides the expected package info): WARNING: cache is out of date: C:/Program Files (x86)/Haskell Platform/2010.2.0.0\lib\package.conf.d\package.cache use 'ghc-pkg recache' to fix. What does this mean? Is this harmful? (In principal, I could just run ghc-pkg recache, but the installation is replicated on about 20 machines, and I don’t have admin rights on these.) Best wishes, Wolfgang

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ANNOUNCE: Multiplate 0.0.1
by roconnor＠theorem.ca 25 Mar '11

25 Mar '11

Multiplate is a lightweight generic library for mutually recursive data types that won't make Conor lose his lunch. Multiplate is an alternative extension of the Uniplate/Compos core design to support mutually recursive datatypes in a way that is as powerful as Compos, almost as easy to use as Uniplate, and more portable than both of them. Multiplate does not require you to rewrite your data type, does not require run-time reflection, does not require GADTs, and does not even require multi-parameter type classes. It only requires rank 3 polymorphism. <http://hackage.haskell.org/package/multiplate-0.0.1> A more detailed paper is forthcoming, but the library is available to try right now. -- Russell O'Connor <http://r6.ca/> ``All talk about `theft,''' the general counsel of the American Graphophone Company wrote, ``is the merest claptrap, for there exists no property in ideas musical, literary or artistic, except as defined by statute.''

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threading question for ghc 7.0.2
by Oliver Mueller 25 Mar '11

25 Mar '11

hello, i'm a bit lost here so i hope s.o. can help me here: seems i have a problem in my code when compiled with ghc-7.0.2: what i try to do: serverside: -listen on a socket and reply to requests client-side: -send a request to that server -wait for a response (with a timeout) any ideas on how i can make this work again without using the -threaded flag? is there is a better way for what i'm trying to achieve? many thanks, oliver the simplified code is available at https://gist.github.com/882770 explanation: the code forks of a lightweight thread (not an OS thread) that will listen for incoming responses. the main thread will go on to push out a message and then wait for the receiver thread to finish. i used to compile my application with the -threaded option but ran into some obscure FFI problems after upgrading from ghc-6.12.3 to ghc-7.0.2. so i tried to remove the -threaded option. but now the message does not get pushed out until the receiver-thread has finished. what is strange to me is that this worked without problems on 6.12.3 but not on 7.0.2. also the behavior is a little different on mac os X with 7.0.2: here i get a "resource vanished (Connection reset by peer)" see the output of the program below ([S] marks output from the main thread, [R] from the receiver thread) compiled on ghc-7.0.2/ubuntu 64bit, without -threaded "running all things..." [R]listening for response... [R]wait for data with timeout:2000 ms ......................................................................................................................................................................................................... [R]no message available [S]pushing out the message [S]sending --> "\EOT\ETX" "[S]going on..." Nothing compiled on ghc-7.0.2/ubuntu 64bit, using -threaded "running all things..." [S]pushing out the message [R]listening for response... [S]sending --> "\EOT\ETX" [R]wait for data with timeout:2000 ms "[S]going on..." ..................................................Just "\ETX\EOT" and...also very strange: when i introduce a putStrLn in the main thread (not a print !) it will work again...only a lot (see code snipped line38: putStrLn "[S]hickup" "running all things..." [S]hickup [R]listening for response... [S]pushing out the message [S]sending --> "\EOT\ETX" [R]wait for data with timeout:2000 ms "[S]going on..." ..................................................Just "\ETX\EOT"

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STM, newArray, and a stack overflow?
by Ketil Malde 25 Mar '11

25 Mar '11

Hi, I'm seeing some weirdness here. My code does this: omap <- atomically $ (newArray_ (0,n-1) :: STM (TArray Int Int)) This gives a stack overflow when n is one million. The equivalent code in the ST monad doesn't show this behavior, and works as expected. I'm not sure what is going on here, so I thought I'd ask in case anybody can put me on the right track. -k -- If I haven't seen further, it is by standing in the footprints of giants

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Haskell programming 3 month internship in Tokyo
by employment 25 Mar '11

25 Mar '11

Tsuru Capital has an opening for an intern for three months starting next month in Tokyo. We are looking for people with moderate experience with Haskell or another functional programming language. Evidence of work written in Haskell is a big plus. More details can be found here. There is a small coding task on the site. Please send this code with your application. http://www.tsurucapital.com/en/jobs.html Any who have applied before but would like to be considered again, please feel free to send a reminder of your availability along with the code sample. Interested candidates please send resumes with code to jobs(a)tsurucapital.com . The package includes flights, and accommodation plus some salary while here.

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Tying the recursive knot
by Joshua Ball 24 Mar '11

24 Mar '11

{- - Hi all, - - I'm having trouble tying the recursive knot in one of my programs. - - Suppose I have the following data structures and functions: -} module Recursion where import Control.Monad.Fix import Data.Map ((!)) import qualified Data.Map as M import Debug.Trace newtype Key = Key { unKey :: String } deriving (Eq, Ord, Show) data Chain = Link Int Chain | Trace String Chain | Ref Key deriving (Show) reduce :: M.Map Key Chain -> Key -> [Int] reduce env k = follow (env ! k) where follow (Link i c) = i : follow c follow (Ref k) = reduce env k follow (Trace message c) = trace message (follow c) -- Now I want a "force" function that expands all of the chains into int sequences. force1, force2 :: M.Map Key Chain -> M.Map Key [Int] -- This is pretty easy to do: force1 mp = M.fromList (map (\k -> (k, reduce mp k)) (M.keys mp)) -- But I want the int sequences to be lazy. The following example illustrates that they are not: example = M.fromList [(Key "ones", Link 1 . Trace "expensive computation here" . Ref . Key $ "ones")] -- Run "force1 example" in ghci, and you will see the "expensive computation here" messages interleaved with an infinite -- list of ones. I would prefer for the "expensive computation" to happen only once. -- Here was my first attempt at regaining laziness: fixpointee :: M.Map Key Chain -> M.Map Key [Int] -> M.Map Key [Int] fixpointee env mp = M.fromList (map (\k -> (k, reduce env k)) (M.keys mp)) force2 env = fix (fixpointee env) main = print $ force2 example {- - However, this gets stuck in an infinite loop and doesn't make it past printing "fromList ". - (It was not difficult for me to see why, once I thought about it.) - - How do I recover laziness? A pure solution would be nice, but in the actual program - I am working on, I am in the IO monad, so I am ok with an impure solution. - It's also perfectly ok for me to modify the reduce function. - - Thanks in advance for you help, - Josh "Ua" Ball -}

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GHC 7/Cabal request for help
by John Goerzen 24 Mar '11

24 Mar '11

Hi folks, I don't have a GHC 7 environment running yet (it's on my list...) but I received a bug report pointing me at this build failure: http://hackage.haskell.org/packages/archive/testpack/2.0.1/logs/failure/ghc… Among other things, this noted: Dependency QuickCheck >=2.1.0.3: using QuickCheck-2.4.0.1 and the errors were: [1 of 3] Compiling Test.QuickCheck.Instances ( src/Test/QuickCheck/Instances.hs, dist/build/Test/QuickCheck/Instances.o ) src/Test/QuickCheck/Instances.hs:39:10: Duplicate instance declarations: instance Arbitrary Word8 -- Defined at src/Test/QuickCheck/Instances.hs:39:10-24 instance Arbitrary Word8 -- Defined in Test.QuickCheck.Arbitrary src/Test/QuickCheck/Instances.hs:42:10: Duplicate instance declarations: instance CoArbitrary Word8 -- Defined at src/Test/QuickCheck/Instances.hs:42:10-26 instance CoArbitrary Word8 -- Defined in Test.QuickCheck.Arbitrary Now, that's fairly standard, and in fact, in my code, is wrapped with: #if MIN_VERSION_QuickCheck(2,3,0) -- we have Word8 instances here #else instance Arbitrary Word8 where arbitrary = sized $ \n -> choose (0, min (fromIntegral n) maxBound) instance CoArbitrary Word8 where coarbitrary n = variant (if n >= 0 then 2 * x else 2 * x + 1) where x = abs . fromIntegral $ n #endif And that code has been working to support modern QuickCheck versions for some time. It would appear that something in Cabal, GHC 7, or QuickCheck is breaking this check. Ideas? -- John

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