November 2009 - Haskell-Cafe

Some help needed to start Haskell with Yi
by Jaco van Iterson 03 Dec '09

03 Dec '09

Hi there I'm new to Haskell and need some help to get started faster (busy busy busy). I like to adjust and extend an editor to my liking and I choose Yi. Only installation with 'cabal install yi' in a Cygwin shell under MS Windows XP ended in: Yi\Prelude.hs:182:9: Duplicate instance declarations: instance Category Accessor.T -- Defined at Yi\Prelude.hs:182:9-38 instance Category Accessor.T -- Defined in data-accessor-0.2.1:Data.Accessor.Private cabal.exe: Error: some packages failed to install: yi-0.6.1 failed during the building phase. The exception was: exit: ExitFailure 1 :( Seems easy to fix but I can't even find where on my drive I can find the source code. Where is the source? Cheers, Jaco

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O(n) algorithm for determining subset
by Michael Mossey 02 Dec '09

02 Dec '09

Can someone tell me if this is correct. I'm guessing that if I represent two sets of integers by Word32 (where ith bit set means i is in the set), then an algorithm to determine if x is a subset of y would go something like (y - x) == (y `exclusiveOr` x)

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Type synonym family inside type class
by Martijn van Steenbergen 02 Dec '09

02 Dec '09

Hello, I have a type family and a type class: > type family ErrorAlg (f :: (* -> *) -> * -> *) e ix :: * > class MkErrorAlg f where > mkErrorAlg :: ErrorAlg f e a -> f (K0 a) ix -> Either e a Instances for these two really go hand in hand, so I thought I would move the type family into the type class. However, this causes GHC to complain about the type variables that are bound on the LHS of the type synonym: > Not in scope: type variable `e' > Not in scope: type variable `ix' In function types, using new type variables (i.e. type variables not bound by the type class header) implicitly means universal quantification over these variables. Why is this disallowed in type families inside type classes? Thanks, Martijn.

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Building network package on Windows
by Iavor Diatchki 02 Dec '09

02 Dec '09

Hi, I have been trying to build the package "network" from hackage (version 2.2.1.3) on Windows Vista, and I could really use some help. Building on the command line, or under cygwin completely failed (command line due to cabal not being able to execute something---possibly configure---although it would not say; cygwin first due to lack of gcc, which is tested but, apparently, the outcome ignored, and after gcc was installed some incompatibility with the header files which were detected but reported unusable). I managed to build the library under MinGW with msys without serious obstacles. I can also build my package against the result and all is well. Unfortunately, if I try to use my package to build an executable application I get a linker error, reporting a missing symbol during linking: C:\Users\diatchki\AppData\Roaming\cabal\network-2.2.1.3\ghc-6.10.3/libHSnetwork-2.2.1.3.a(Socket.o):fake:(.text+0xb014): undefined reference to `getnameinfo' collect2: ld returned 1 exit status Now, "getnameinfo" is present in the header files, and it is also defined in the library ws2_32.a which is being passed to GHC so I am not sure what is going on. Any ideas? Searching the web suggests that the problem may be somehow related to the standard calling conventions but I don't really understand. Also, if I understand correctly, this functionality is related to IPv6 support, which I do not need at the moment, so it would be great if it could be easily disabled in some way. Any ideas, suggestion, workarounds, etc. would be greatly appreciated, -Iavor PS: I am using GHC 6.10.3

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Re: [Haskell] ANN: NoSlow - Microbenchmarks for array libraries
by Henning Thielemann 01 Dec '09

01 Dec '09

On Fri, 27 Nov 2009, Roman Leshchinskiy wrote: > You can get more information (including the ugly tables) from my blog > > http://unlines.wordpress.com/2009/11/27/noslow Btw. storablevector supports 'zip' using http://hackage.haskell.org/package/storable-tuple but you may also use 'zipWith' with an atomar result type for testing. I found that the dependency on Cairo and GTK originates from criterion. I have to build criterion with 'cabal install -f-chart criterion'. Unfortunately I cannot do 'cabal fetch criterion' in order to look into the cabal file, before the dependencies are resolved ...

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Great Programs to Read?
by Michael Lesniak 01 Dec '09

01 Dec '09

Hello, In terms of "to become a great programmer, you need to read great programs"[1] what are "great" programs written in Haskell (for your personal definition of great), which source code is freely available on hackage or somewhere else on the net? I'm personally also interested in your definitions of great; for me, a great programs is defined by one of * good and well-written documentation (literate Haskell helps a lot) * novel ideas to use functional programming * elegance * showing how functional programming can ease tasks that are difficult to achieve in an imperative style Maybe we should create a Page on haskell.org (which I would do if I had write-access) mirroring the pages [2,3]? Kind regards, Michael [1] http://c2.com/cgi/wiki/Wiki?ReadGreatPrograms [2] http://c2.com/cgi/wiki/Wiki?GreatProgramsToRead [3] http://c2.com/cgi/wiki/Wiki?ProgramsToRead -- Dipl.-Inf. Michael C. Lesniak University of Kassel Programming Languages / Methodologies Research Group Department of Computer Science and Electrical Engineering Wilhelmshöher Allee 73 34121 Kassel Phone: +49-(0)561-804-6269

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Help mixing pure and IO code
by papa.eric＠free.fr 01 Dec '09

01 Dec '09

Hi haskell helpers, Learning haskell, I wanted to explore how to write pure code and then add some IO on top of it, keeping the main code pure. The idea was to write a very simple two-player game, then define some strategies to play it that do not involve IO, and finally use strategies involving Random or IO ("ask the user"). I failed to reuse the pure code, and the only solution I found was to rewrite most things for IO. Here is my attempt in literate haskell, it is quite short, I hope someone will be kind enough to tell me what I have missed... Thanks for any answer! The game is: each player in turn chooses a number, and wins if this number has already been chosen twice (the third occurrence wins). > import Debug.Trace > import Control.Monad Choosing a number is a "move", a game is all moves played so far. > type Move = Int > type Game = [Move] > > newGame :: Game > newGame = [] Game updating rule, Nothing for a winning move (the game stops) else Just the ongoing game. > update :: Game -> Move -> Maybe Game > update game move = if move `wins` game then Nothing else Just (move:game) > where move `wins` game = length (filter (== move) game) == 2 Let's give a name to both players, mainly for tracing purposes. > data Player = Player1 | Player2 deriving Show > > myTrace :: Player -> Move -> (a -> a) > myTrace player move = trace ((show player) ++ " plays " ++ (show move)) A strategy looks at the game and proposes the next move, and may involve state. > class Strategy a where > proposeNext :: Game -> a -> (Move, a) The "game engine" takes two strategies and returns the winner, tracing what is going on. runGame starts from a new (empty) game, runGame' updates an ongoing game and keeps track of who is Player1 or Player2. > runGame :: (Strategy a1, Strategy a2) => a1 -> a2 -> Player > runGame x y = runGame' (Player1, x) (Player2, y) newGame > > runGame' :: (Strategy a1, Strategy a2) => > (Player, a1) -> (Player, a2) -> Game -> Player > runGame' (px, x) (py, y) game = -- x and y are strategies > let (move, x') = proposeNext game x > follows = case update game move of > Nothing -> px -- winning move > Just nextgame -> runGame' (py, y) (px, x') nextgame > in myTrace px move $ follows An example of a pure strategy that plays a fixed list of numbers, then zeroes. > data Fixed = Fixed [Move] deriving Show > > instance Strategy Fixed where > proposeNext game s = case s of > Fixed [] -> (0, Fixed []) > Fixed (x:xs) -> (x, Fixed xs) Now you may run runGame (Fixed [1,2,3]) (Fixed [3,2,1,3]) and Player2 will win. Now I want the user to be asked for moves. This one works quite well, asking *once* the user for a list of moves. > askFixed :: String -> IO Fixed > askFixed name = liftM Fixed askio > where askio = putStr (name ++ ", pick a list of numbers: ") >> readLn I could easily reuse runGame: > runGameSingleIO :: (Strategy a1, Strategy a2) => IO a1 -> IO a2 -> IO Player > runGameSingleIO = liftM2 runGame And this works: runGameSingleIO (askFixed "Joe") (askFixed "Jack") Now I want each user to be asked for moves repeatedly until there is a win. This was my first try, using an infinite list and hoping lazyness would work. > askUntil :: String -> IO Fixed > askUntil name = liftM Fixed (sequence $ repeat askio) > where askio = putStr (name ++ ", pick a number: ") >> readLn However it does not work, if I evaluate: liftM2 runGame (askUntil "Joe") (askUntil "Jack") then Joe is indefinitely asked for his move... Now, I supposed that IO [Move] was maybe (when triggered) a "unitary action" returning the whole list with no lazyness, and that I would rather need to have IO attached to individual moves [IO Move], so that they could be triggered independently. Am I right, or is it still subtler? Anyway, I imagined I needed to start from > data FixedIO = FixedIO [IO Move] And that's where I failed. I could not define any instance of Strategy that could be turned to a (IO Strategy) and reuse runGame, and use [IO Move] somewhere. I could not bring the inner IOs of [IO Move] to be syhthetised in front of IO Strategy. Was it possible? All I was able to contrive is a new definition of Strategy and runGame for the IO case. This seems very awkward. Here it is: > class StrategyIO a where > proposeNextIO :: Game -> a -> IO (Move, a) > > runGameIO :: (StrategyIO a1, StrategyIO a2) => a1 -> a2 -> IO Player > runGameIO x y = runGameIO' (Player1, x) (Player2, y) newGame > > runGameIO' :: (StrategyIO a1, StrategyIO a2) => > (Player, a1) -> (Player, a2) -> Game -> IO Player > runGameIO' (px, x) (py, y) game = > do (move, x') <- proposeNextIO game x > case update game move of > Nothing -> return px > Just nextgame -> runGameIO' (py, y) (px, x') nextgame Then FixedIO could be made a strategy: > instance StrategyIO FixedIO where > proposeNextIO game s = case s of > FixedIO [] -> return (0, FixedIO []) > FixedIO (x:xs) -> liftM2 (,) x (return $ FixedIO xs) > > askFixedIO :: String -> FixedIO > askFixedIO name = FixedIO (repeat askio) > where askio = putStr (name ++ ", pick a number: ") >> readLn and this works: runGameIO (askFixedIO "Joe") (askFixedIO "Jack") However, I wonder how to do it reusing the "pure" versions, runGame and Strategy? Thanks for anyone that has followed so far! Eric

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ANNOUNCE: haskell-mode 2.7.0
by Svein Ove Aas 01 Dec '09

01 Dec '09

Haskell-mode, a major mode for writing Haskell code in Emacs, has been updated to version 2.7.0. This release contains mostly bugfixes, but also adds support for hlint highlighting via flymake. Additionally, haskell-mode now has a bug-tracker and mailing list. Mailing list: http://projects.haskell.org/cgi-bin/mailman/listinfo/haskellmode-emacs Bug tracker: http://trac.haskell.org/haskellmode-emacs/ Unless I am asked not to, minor (N.M.x) releases will only be announced on the mailing list from now on. Download location is, as always, http://projects.haskell.org/haskellmode-emacs/ The changelog is as follows: === Changes since 2.6.4 * fill-paragraph (M-q) now only affects comments, and correctly handles Haddock commentary. adaptive-fill-mode is turned off, as it was interfering. * Yet more unicode symbols * Better support for unicode encoding of haskell source files * mdo correctly indented * Indentation fixes, fixes to the fixes, and fixes to the fixes to the fixes * New command: M-x haskell-check, calls (by default) hlint on the current file. Also bound to C-c C-v. You can also use the flymake minor mode with this. -- Svein Ove Aas

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Tips for a FFI-wrapped C library?
by Casey Hawthorne 30 Nov '09

30 Nov '09

I would suspect that the IO Monad is to unsafe, maybe a more stringent monad would be more appropriate. -- Regards, Casey

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Re: [Haskell-cafe] Function composition questions from a newbie
by muad 30 Nov '09

30 Nov '09

newbie2009 wrote: > > I am a newbie. Consider this code: > > square x = x * x > add3 x y z = x + y + z > add x y = x + y > composition5 x = (square . add3) x > composition6 x y = (square . add3) x y > > 1) What arguments can i pass to composition5? Please give an example of > calling it. > 2) What arguments can i pass to composition6? Please give an example of > calling it. > 3) How can I write the function "add" to take any number of arguments? I > would like to call it either as > "add 1 2" or "add 1 2 3" etc > > Thank you. > > for (1) and (2) you can ask haskell what the types are, Prelude> :t composition5 composition5 :: (Num (a -> a -> a), Num a) => a -> a -> a -> a Prelude> :t composition6 composition6 :: (Num (a -> a -> a), Num a) => a -> a -> a -> a This is a worry, it wants binary operation to be a number but it is not! There must be an error in the program. Let us try and fix by changing them to composition5 x = (square . add x) composition6 x y = (square . add3 x y) y now we see: Prelude> :t composition5 composition5 :: (Num a) => a -> a -> a Prelude> :t composition6 composition6 :: (Num a) => a -> a -> a which is reasonable, Prelude> composition5 1 2 9 Prelude> composition6 3 4 121 As for question 3, one would be better to take a list of numbers and to add [1,2,3] instead of add 1 2 3. (Something similar to add 1 2 3 is possible in a couple different way (have a go without using any typeclasses!) but more advanced than one should like to bother with at this point). add [] = 0 add (x:xs) = x + add xs would be fine anyway. -- View this message in context: http://old.nabble.com/Function-composition-questions-from-a-newbie-tp265702… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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