October 2011 - Haskell-Cafe

Combining stream and list fusion
by Bas van Dijk 12 Oct '11

12 Oct '11

Hello, I'm trying to make the following faster: Data.Vector.Generic.fromList list where 'list' is some expression yielding a list. (For example: map (+1) $ map (*2) [1..1000000]) Note that Data.Vector.Generic.fromList is defined as: fromList :: Vector v a => [a] -> v a {-# INLINE fromList #-} fromList = unstream . Stream.fromList where Stream.fromList is defined in Data.Vector.Fusion.Stream as: fromList :: [a] -> Stream a {-# INLINE fromList #-} fromList = M.fromList where M.fromList is defined in Data.Vector.Fusion.Stream.Monadic as: fromList :: Monad m => [a] -> Stream m a {-# INLINE fromList #-} fromList xs = unsafeFromList Unknown xs where unsafeFromList is defined as: unsafeFromList :: Monad m => Size -> [a] -> Stream m a {-# INLINE_STREAM unsafeFromList #-} unsafeFromList sz xs = Stream step xs sz where step (x:xs) = return (Yield x xs) step [] = return Done I would like to fuse the construction of the list with the construction of the stream as in: import GHC.Base ( build ) {-# RULES "unsafeFromList/build" forall sz (g :: forall b. (a -> b -> b) -> b -> b). unsafeFromList sz (build g) = unsafeFromListF sz g #-} unsafeFromListF :: forall m a. Monad m => Size -> (forall b. (a -> b -> b) -> b -> b) -> Stream m a {-# INLINE unsafeFromListF #-} unsafeFromListF sz g = Stream step st sz where St step st = g c z c :: a -> St m a -> St m a c x st = St (\(St s st') -> s st') (St (\s -> return (Yield x s)) st) z :: St m a z = St (\_ -> return Done) undefined -- Ouch! data St m a = St ((St m a) -> m (Step (St m a) a)) (St m a) Unfortunately, some initial experiments show that this doesn't make it faster. Are there ways to improve this? (Bonus points for getting rid of the undefined!) Regards, Bas

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Re: [Haskell-cafe] The best way to call Java from Haskell?
by Janis Voigtländer 12 Oct '11

12 Oct '11

Ketil Malde <ketil(a)malde.org> writes: >Jerzy Karczmarczuk<jerzy.karczmarczuk(a)unicaen.fr> writes: >> Don't worry, my friend. Haskell is lazy, so there is no problem in >> "handling" those infinite modules. It will just take you an infinite >> amount of time before you get any money from such a work. But this is >> a general problem elsewhere as well. > > I guess you must be thinking of Haskell being increasingly used in > banks? It must have been some bank manager who, after hiring one too > many Haskell programmers, invented a scheme that would generate an > infinite amount of money. He didn't realize before it was too late that > the actual value of the scheme would be bottom... See also: http://www.haskell.org/haskellwiki/Humor/Enron -- Jun.-Prof. Dr. Janis Voigtländer http://www.iai.uni-bonn.de/~jv/ mailto:jv@iai.uni-bonn.de

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ANNOUNCE: simple-actors 0.1.0 - an eDSL library for actor model concurrency
by Brandon Simmons 11 Oct '11

11 Oct '11

I'm happy to announce the release of my library 'simple-actors', a DSL-style library for more structured concurrent programs based on the Actor Model. It offers an alternative to ad-hoc use of Chans that allows for tight control of side-effects and message passing, and is especially suited to applications such as simulations of communicating processes. You can try it with a $ cabal install simple-actors and view the documentation here: http://hackage.haskell.org/package/simple-actors or check out the repo here: https://github.com/jberryman/simple-actors Here is an example of a system of actors working as a binary tree, supporting insert and query operations: ---- EXAMPLE ---- module Main where import Control.Concurrent.Actors import Control.Applicative import Control.Concurrent.MVar type Node = Mailbox Operation -- operations supported by the network: data Operation = Insert { val :: Int } | Query { val :: Int , sigVar :: MVar Bool } -- the actor equivalent of a Nil leaf node: nil :: Behavior Operation nil = Receive $ do (Query _ var) <- received send var False -- signal that Int is not present in tree return nil -- await next message <|> do -- else, Insert received l <- spawn nil -- spawn child nodes r <- spawn nil branch l r . val <$> received -- create branch from inserted val -- a "branch" node with an Int value 'v' and two children branch :: Node -> Node -> Int -> Behavior Operation branch l r v = loop where loop = Receive $ do m <- received case compare (val m) v of LT -> send l m GT -> send r m EQ -> case m of -- signal Int present in tree: (Query _ var) -> send var True _ -> return () return loop insert :: Node -> Int -> IO () insert t = send t . Insert -- MVar is in the 'SplitChan' class so actors can 'send' to it: query :: Node -> Int -> IO Bool query t a = do v <- newEmptyMVar send t (Query a v) takeMVar v main = do t <- spawn nil mapM_ (insert t) [5,3,7,2,4,6,8] mapM (query t) [1,5,0,7] >>= print ---- END EXAMPLE ---- I need to do some work on the documentation and performance testing. If anyone has anyone questions or comments, I would love to hear them. Thanks, Brandon http://coder.bsimmons.name

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haskell i18n best practices
by Paulo Pocinho 11 Oct '11

11 Oct '11

Hello list. I've been trying to figure a nice method to provide localisation. An application is deployed using a conventional installer. The end-user is not required to have the Haskell runtimes, compiler or platform. The application should bundle ready to use translation data. What I am after is simple; an intuitive way that an interested translator, with little knowledge of Haskell, can look at and create valid translation data. This is what I've been looking at lately. The first thing I noticed was the GNU gettext implementation for Haskell. The wiki page [1] has a nice explanation by Aufheben. The hgettext package is found here [2]. I don't know if this is a bad habit, but I had already separated the dialogue text in the code with variables holding the respective strings. At this time, I thought there could be some other way than gettext. Then I figured how to import localisation data, that the program loads, from external files. The data type is basically a tuple with variable-names associated with strings. This is bit like the file-embed package [3]. Still uncomfortable with i18n, I learned about the article "I18N in Haskell" in yesod blog [4]. I'd like to hear more about it. What is considered the best practice for localisation? -- [1] http://www.haskell.org/haskellwiki/Internationalization_of_Haskell_programs [2] http://hackage.haskell.org/packages/archive/hgettext/ [3] http://hackage.haskell.org/package/file-embed [4] http://www.yesodweb.com/blog/2011/01/i18n-in-haskell

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Hunit Testing
by mukesh tiwari 11 Oct '11

11 Oct '11

Hello everyone I was going through this tutorial http://hunit.sourceforge.net/HUnit-1.0/Guide.html and just wrote some simple code but i am getting error ghci>let test1 = TestCase (assertEqual " equal " 3 ( id 3 )) ghci>let tests = [ TestLabel "test 1" test1 ]ghci>runTestTT tests <interactive>:0:11: Couldn't match expected type `Test' with actual type `[Test]' In the first argument of `runTestTT', namely `tests' In the expression: runTestTT tests In an equation for `it': it = runTestTT tests Could some one please tell me what is wrong with this code. Regards Mukesh Tiwari

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Best bit LIST data structure
by Ryan Newton 11 Oct '11

11 Oct '11

Hi Cafe, We are lucky to have a plethora of data structures out there. But it does make choosing one off hackage difficult at times. In this case I'm *not* looking for a O(1) access bit vector (Data.Vector.Unboxed seems to be the choice there), but an efficient representation for a list of bits (cons,head,tail). Let's say that you want to represent tree indices as you walk down a binary tree. [Bool] is a simple choice, you only add to the front of the list (0/1 = Left/Right), sharing the tails. But [Bool] is quite space inefficient. Something like [Int] would allow packing the bits more efficiently. A Lazy ByteString could amortize the space overhead even more... but in both cases there's a tiny bit of work to do in wrapping those structures for per-bit access. That's probably the right thing but I wanted to check to see if there's something else recommended, perhaps more off-the-shelf. What about just using the Data.Bits instance of Integer? Well, presently, the setBit instance for very large integers creates a whole new integer, shifts, and xors: http://haskell.org/ghc/docs/latest/html/libraries/base/src/Data-Bits.html#s… (I don't know if it's possible to do better. From quick googling GMP seems to use an array of "limbs" rather than a chunked list, so maybe there's no way to treat large Integers as a list and update only the front...) Advice appreciated! Thanks, -Ryan

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Trouble using State Monad.
by Captain Freako 11 Oct '11

11 Oct '11

Hi all, I'm trying to use the State Monad to help implement a digital filter: 17 newtype Filter e a = F { 18 runFilter :: a -> EitherT e (State FilterState) a 19 } deriving (Monad, MonadState FilterState) but I'm getting these compiler errors: Filter.hs:19:14: Can't make a derived instance of `Monad (Filter e)' (even with cunning newtype deriving): cannot eta-reduce the representation type enough In the newtype declaration for `Filter' Filter.hs:19:21: Can't make a derived instance of `MonadState FilterState (Filter e)' (even with cunning newtype deriving): cannot eta-reduce the representation type enough In the newtype declaration for `Filter' If I change the code to this: 17 newtype Filter e a = F { * 18 runFilter :: EitherT e (State FilterState) a ** * 19 } deriving (Monad, MonadState FilterState) it compiles, but I can't figure out how I'd feed the input to the filter, in that case. In comparing this to the tricks used in constructing the State Monad based version of the `Parser' type, I notice that Parser gets around this issue, by having the input (i.e. - input stream) be a part of the initial state, but I'm not sure that's appropriate for a digital filter. Thanks, -db

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hackage-cabal.tar.gz error
by diazepan 11 Oct '11

11 Oct '11

I've installed hoogle and when i type hoogle data, this comes out Extracting tarball... gzip: ../hackage-cabal.tar.gz: invalid compressed data--format violated hoogle: System command failed: gzip --decompress --force ../hackage-cabal.tar.gz What is going wrong? -- View this message in context: http://haskell.1045720.n5.nabble.com/hackage-cabal-tar-gz-error-tp4893434p4… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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The best way to call Java from Haskell?
by dokondr 11 Oct '11

11 Oct '11

Hi, I need to call Stanford NLP Parser from Haskell (unfortunately Haskell does not have a similar one): http://nlp.stanford.edu/software/lex-parser.shtml What would be the most reliable framework for this? Thanks! Dmitri

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wxhaskell : how to generate an event?
by Johannes Waldmann 11 Oct '11

11 Oct '11

Dear all, in wxHaskell (core) I can set and get event handlers http://hackage.haskell.org/packages/archive/wxcore/0.12.1.7/doc/html/Graphi… but how is it possible to create events (programmatically) and somehow feed them into the "main event handling loop"? I think I need this in an application that needs to handle concurrently events that arrive via the GUI (like mouse clicks) and events from an external source (specifically, alsa-midi input). Of course I could handle those "extra events" separately from wxcore, but they should ultimately result in changes to the GUI state, and it feels dangerous to do this without synchronisation. Any hints welcome. Thanks - J.W.

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