- Haskell-Cafe - Haskell.org

haskeem -- (almost) scheme in haskell
by Uwe Hollerbach 23 Jan '08

23 Jan '08

Hello, haskellers, I am a total newbie in haskell, but I've been making some mud pies that are coming out kinda pleasingly to my admittedly biased eye. I found Jonathan Tang's excellent tutorial "Write yourself a scheme in 48 hours" a few weeks ago, went through and beyond that, and now have something that's starting to look moderately like a real scheme interpreter. I've got much, though not all, of the scheme number tower implemented, I have most of the core syntactic forms (and, apply, begin, case cond, define, if, lambda, let, let*, letrec, letrec*, or, quote, set!) and am working on a few more (delay and force are well along, guard is also in progress but less so), and I have implemented a small trace facility. It's still a toy, but it's starting to be a toy with some nice dance moves. I think it could do the majority of the code in SICP by now. If you'd like to have a look, please surf over to http://www.korgwal.com/software/haskeem/index.html. Feedback welcome! best regards, Uwe Hollerbach

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Irrefutable pattern love and new regex engine.
by Michael Speer 23 Jan '08

23 Jan '08

I've been using the creation of a regular expression engine as an ongoing project to learn Haskell. Last night I created the newest iteration. My love for the irrefutable pattern can be found in the definition of the rexn ( apply repetition to extracted nodes ) function below. /snip rexn ns pps = let ( ~( xs , rps ) , ~( ~( nxs ) , ~( rxs , rrps ) ) ) = ( exn nxs pps , case rps of ('?':'?':rr) -> ( ( ns ) , ( ns ++ xs , rr ) ) ('?':rr) -> ( ( ns ) , ( xs ++ ns , rr ) ) /snip Beautiful, beautiful lazy execution and irrefutable pattern matching. The data flow is able to easily twist in and out of functions, even as an argument to the function call that creates it. Is this kind of usage normal? Any and all comments, suggestions, refutations, and criticisms are welcome. -- regular expression engine -- (c) 2008 michael speer import Char ( isSpace ) -- import Debug.Trace ( trace ) xor :: Bool -> Bool -> Bool xor True a = not a xor False a = a data RxToken = RxStart -- matchable start of target string | RxChar Char -- a literal character to match | RxBound -- inserted wherever alphanums touch whitespace | RxEnd -- matchable end of target string | RxEOF -- an additional token to push through to catch anything trying for RxEnd -- RxEOF is never matched. deriving ( Show ) rxTokenize tts = RxStart : case tts of [] -> RxEnd : RxEOF : [] tts@(t:_) -> case not $ isSpace t of True -> RxBound : rxt tts False -> rxt tts where rxt (t:[]) | not $ isSpace t = RxChar t : RxBound : RxEnd : RxEOF : [] | otherwise = RxChar t : RxEnd : RxEOF : [] rxt (t:ts@(t':_)) | isSpace t `xor` isSpace t' = RxChar t : RxBound : rxt ts | otherwise = RxChar t : rxt ts data RxTransform = RxTransform ( RxNode -> RxToken -> [ RxNode ] ) | RxNullTransform data RxNode = RxActive { rxTransforms :: [RxTransform] , rxMatched :: String , rxNumSubs :: Integer , rxSubExprs :: [ String ] } | RxComplete { rxMatched :: String , rxNumSubs :: Integer , rxSubExprs :: [ String ] } instance Show RxNode where show (RxComplete matched _ _) = "<rx|matched:[" ++ matched ++ "]>" data RxDepth = RxTop | RxSub deriving ( Show ) rxCompile pps = let ( xs , rps ) = oexn [success] RxTop pps in case length rps of 0 -> RxActive { rxTransforms = xs , rxMatched = [] , rxNumSubs = 0 , rxSubExprs = [] } _ -> error $ "Not all of pattern consumed : remains : " ++ rps where -- or together different expression sections -- (a|b|c) oexn ns RxTop [] = ( ns , [] ) oexn _ RxSub [] = error "Pattern ended while still in sub expression" oexn ns d pps = let ( ~( xs , rps ) , ~( nxs , nrps ) ) = ( aexn ns pps , case rps of ('|':rr) -> let ( inxs , irps ) = oexn ns d rr in ( xs ++ inxs , irps ) (')':rr) -> case d of RxTop -> error "Erroneous close parenthesis in pattern " RxSub -> ( xs , rr ) [] -> case d of RxTop -> ( xs , [] ) RxSub -> error "End of pattern while still in sub expression" ) in ( nxs , nrps ) -- and together extracted nodes in a given expression segment -- abd?dfs aexn ns pps = let ( ~( xs , rps ) , ~( nxs , nrps ) ) = ( rexn nxs pps , case rps of ('|':_) -> ( ns , rps ) (')':_) -> ( ns , rps ) [] -> ( ns , rps ) _ -> aexn ns rps ) in ( xs , nrps ) -- replication application - weee! rexn ns pps = let ( ~( xs , rps ) , ~( ~( nxs ) , ~( rxs , rrps ) ) ) = ( exn nxs pps , case rps of ('?':'?':rr) -> ( ( ns ) , ( ns ++ xs , rr ) ) ('?':rr) -> ( ( ns ) , ( xs ++ ns , rr ) ) ('*':'?':rr) -> ( ( ns ++ xs ) , ( ns ++ xs , rr ) ) ('*':rr) -> ( ( xs ++ ns ) , ( xs ++ ns , rr ) ) ('+':'?':rr) -> ( ( ns ++ xs ) , ( xs , rr ) ) ('+':rr) -> ( ( xs ++ ns ) , ( xs , rr ) ) _ -> ( ( ns ) , ( xs , rps ) ) ) in ( rxs , rrps ) -- extract node ( including an entire subexpression as a single node ) exn _ ('?':_) = error "Bad question mark operator" exn _ ('*':_) = error "Bad splat operator" exn _ ('+':_) = error "Bad plus sign operator" exn ns ('(':ps) = oexn ns RxSub ps exn ns (p:ps) = ( [ RxTransform ( \rxn k -> case k of (RxChar c) -> if c == p then [ RxActive { rxTransforms = ns , rxMatched = c : rxMatched rxn , rxNumSubs = rxNumSubs rxn , rxSubExprs = rxSubExprs rxn } ] else [] (RxStart) -> [rxn] (RxBound) -> [rxn] _ -> [] ) ] , ps ) exn ns [] = error "can this be reached?" success = RxTransform ( \ rxn k -> [ RxComplete { rxMatched = reverse $ rxMatched rxn , rxNumSubs = rxNumSubs rxn , rxSubExprs = map reverse $ rxSubExprs rxn } ] ) rxExec n tts = iexec [n] (rxTokenize tts) where iexec (win@(RxComplete _ _ _ ):_) _ = Just win iexec [] _ = Nothing iexec nns (k:ks) = iexec ( concatMap ( \n -> case n of (RxComplete _ _ _) -> [n] a@(RxActive _ _ _ _) -> concatMap (\xf -> case xf of (RxTransform f) -> f a k (RxNullTransform) -> [] ) (rxTransforms a) ) nns ) ks main = do print $ rxTokenize "this is a test" print $ rxExec (rxCompile "hello|world") "hello" print $ rxExec (rxCompile "hello|world") "world" print $ rxExec (rxCompile "abcde|ab") "abcd" print $ rxExec (rxCompile "ab?c") "ac" print $ rxExec (rxCompile "ab?c") "abc" print $ rxExec (rxCompile "ab*c") "ac" print $ rxExec (rxCompile "ab*c") "abc" print $ rxExec (rxCompile "ab*c") "abbbc" print $ rxExec (rxCompile "ab+c") "ac" print $ rxExec (rxCompile "ab+c") "abc" print $ rxExec (rxCompile "ab+c") "abbbbbc" print $ rxExec (rxCompile "(a|b)+") "aaabbbabaaababaaabbbabbaba" print $ rxExec (rxCompile "abc|") "zyx"

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ghc6.8: hiding uninstalled package?
by Magnus Therning 23 Jan '08

23 Jan '08

This might seem like a silly question, but what's the reasoning behind the following behaviour? % ghc-pkg list dataenc /usr/lib/ghc-6.8.2/package.conf: % ghc --make -hide-package dataenc -isrc UT.hs ghc-6.8.2: unknown package: dataenc Hiding an uninstalled package doesn't seem to warrant failing compilation! /M

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Announce: mersenne-random 0.1, very fast pseudo-random number generation for Haskell
by Don Stewart 23 Jan '08

23 Jan '08

Hey all, I'm pleased to announce the creation of the mersenne-random package, The Mersenne twister is a pseudorandom number generator developed by Makoto Matsumoto and Takuji Nishimura, that provides for fast generation of very high quality pseudorandom numbers, suitable for simulators and monte carlo modelling. This library binds to 'SFMT', the SIMD-oriented Fast Mersenne Twister. SFMT is a variant of Mersenne Twister that is much faster than the original. It is designed to be fast when it runs on 128-bit SIMD. To compile in the vector instructions use Cabal, and pass '-f use_sse2' or '-f use_altivec' to configure, depend on which set your cpu supports (or none if it doesn't). The use of sse2 instructions will typically halve the generation time. The library is very fast (generation of Ints and Doubles will be some 50x faster than System.Random), but the implementation we bind to is heavily impure, so only a single generator is possible per-process (splitting generators is also not supported). Some light abstractions are implemented over the raw C interface, including a type class for SFMT-generated Haskell values, and lists of those values. Full 64 bit Ints and Doubles, and 53-bit precision Doubles, are supported on 64 bit machines. As usual, the stable branch is on hackage, so you can install it with cabal: $ cabal install mersenne-random Note that the cpu isn't detected automatically, so to turn on SIMD flags for your cpu, you'll need to build it by hand from: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/mersenne-random And the darcs repo is at code.haskell.org, darcs get http://code.haskell.org/~dons/code/mersenne-random A test suite and example programs are provided. Enjoy, Don

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[Fwd: Invitation for MathematiKa '08 : projectEuler format]
by anshuman＠students.iiit.ac.in 23 Jan '08

23 Jan '08

---------------------------- Original Message ---------------------------- Subject: Invitation for MathematiKa '08 : projectEuler format From: anshuman(a)students.iiit.ac.in Date: Mon, January 21, 2008 11:30 am To: haskell-cafe(a)haskell.org -------------------------------------------------------------------------- Hello Math Enthusiast, IIIT-Hyderabad, India cordially invites you to be a part of Mathematika '08, an online competition that emphasizes on fusion of mathematics and computing. Where: http://felicity.iiit.ac.in/~math When: 4th February, 2008 This Online Math Contest will consist of 10-12 problems made randomly from different domains of existing Math. All problem statements will be visible to the contestant from the start of contest spanning over a 10 hour period. The solution to each problem would be an integer or a floating point number which is to be submitted in real-time. One Minute Rule: The problems are designed in a way that support 1 minute rule which means that an efficient implementation will allow a solution to be obtained on a modestly powered computer in less than one minute. Also, make sure to bring your appetite for math, programming and fun. There will be plenty of all three to go around. Regards Mathematika Team http://felicity.iiit.ac.in/~math

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Hangman game
by Ronald Guida 22 Jan '08

22 Jan '08

Hi, I'm interested in learning how to program games. Since I have to start somewhere, I decided to write a simple Hangman game. I'm wondering if anyone can look at my code and give me some feedback. Thank you. -- Hangman game module Main where import Data.Char import Data.List import System.IO import System.Random import Control.Monad.State data GameState = GameState { gsAnswer :: String, -- the answer gsKnown :: [Maybe Char], -- partial answer known to the user gsGuesses :: [Char], -- incorrect letters guessed so far gsWrong :: Int, -- number of incorrect guesses gsWonLost :: Maybe Bool -- Just true = won, Just false = lost } deriving (Show) newGameState :: String -> GameState newGameState answer = GameState{ gsAnswer = map toUpper answer, gsKnown = (map (filt $ not . isAlpha) answer), gsGuesses = [], gsWrong = 0, gsWonLost = Nothing} data UserInput = UIGuess Char | UIQuit | UINewGame | UIRefresh deriving (Show) main :: IO () main = do hSetBuffering stdout NoBuffering putStr "Welcome to Hangman!\n\n" putStr instructions runStateT startNewGame undefined return () startNewGame :: StateT GameState IO () startNewGame = do nWord <- liftIO $ getStdRandom (randomR (0,length wordList - 1)) let word = wordList !! nWord let gs = newGameState word put gs liftIO $ putStrLn $ renderGameState gs gameLoop gameLoop :: StateT GameState IO () gameLoop = do ui <- liftIO getUserInput case ui of UIGuess c -> do modify $ handleGuess c gs <- get liftIO $ putStrLn $ renderGameState gs case (gsWonLost gs) of Nothing -> gameLoop Just True -> do liftIO $ putStrLn "Congratulations, you won!" startNewGame Just False -> do liftIO $ putStrLn "You've been hanged!" liftIO $ putStrLn $ "The word was \'" ++ (gsAnswer gs) ++ "\'." startNewGame UIQuit -> do gs <- get liftIO $ putStrLn $ "The word was \'" ++ (gsAnswer gs) ++ "\'." liftIO $ putStrLn "Thank you for playing!" UINewGame -> do gs <- get liftIO $ putStrLn $ "The word was \'" ++ (gsAnswer gs) ++ "\'." startNewGame UIRefresh -> do gs <- get liftIO $ putStrLn $ renderGameState gs gameLoop getUserInput :: IO UserInput getUserInput = do putStr "Hangman> " response <- getLine if null response then getUserInput else do let c:cs = response if isAlpha c then return $ UIGuess $ toUpper c else if c == ':' && not (null cs) then case toLower (head cs) of 'q' -> return UIQuit 'n' -> return UINewGame 'r' -> return UIRefresh '?' -> do putStr instructions getUserInput otherwise -> do putStrLn $ "Unknown command \'" ++ cs ++ "\'" putStrLn $ "Use \':?\' for help." getUserInput else do putStrLn $ "Invalid input \'" ++ response ++ "\'" putStrLn $ "Use \':?\' for help." getUserInput instructions :: String instructions = "Instructions:\n" ++ "To guess a letter, type the letter and press enter.\n" ++ "To quit or restart the game, use the following commands:\n" ++ " :q = quit\n" ++ " :n = new game\n" ++ " :r = re-display the game state\n" ++ " :? = show instructions\n" ++ "\n" filt :: (a -> Bool) -> a -> Maybe a filt pred x = if pred x then Just x else Nothing handleGuess :: Char -> GameState -> GameState handleGuess ch state = if (elem ch $ gsGuesses state) then state else if (elem ch $ gsAnswer state) then let revealed = map (filt (== ch)) (gsAnswer state) known = zipWith mplus (gsKnown state) revealed won = all (maybe False (const True)) known in state{gsKnown = known, gsWonLost = filt id won} else let wrong = 1 + (gsWrong state) in state{gsGuesses = ch:(gsGuesses state), gsWrong = wrong, gsWonLost = filt not (wrong < 7)} wordList :: [String] wordList = ["alligator", "angelfish", "ant", "bear", "buffalo", "butterfly", "canary", "chameleon", "crab", "dinosaur", "dog", "dolphin", "eel", "elephant", "flamingo", "frog", "giraffe", "goldfish", "grasshopper", "hedgehog", "hippopotamus", "horse", "iguana", "jaguar", "jellyfish", "kangaroo", "kinkajou", "lemur", "lizard", "llama", "meerkat", "moose", "mouse", "narwhal", "nautilus", "nuthatch", "ostrich", "owl", "panda", "pelican", "quail", "quokka", "raccoon", "rhinoceros", "salamander", "sea horse", "sea urchin", "snail", "tiger", "toucan", "uakari", "unicorn", "vampire bat", "vulture", "walrus", "wildebeest", "worm", "xenops", "yak", "yellow jacket", "zebra"] renderGameState :: GameState -> String renderGameState gs = let noose = renderNoose $ gsWrong gs report = ["","The Word:","",word,"","Your Guesses:","",guessed] word = intersperse ' ' $ map (maybe '_' id) (gsKnown gs) guessed = gsGuesses gs in (concat $ zipWith (++) noose $ map (++ "\n") report) renderNoose :: Int -> [String] renderNoose n | n <= 0 = [ " ___ ", " / | ", " | ", " | ", " | ", " | ", " | ", " -+- "] renderNoose 1 = [ " ___ ", " / | ", " | O ", " | ", " | ", " | ", " | ", " -+- "] renderNoose 2 = [ " ___ ", " / | ", " | O ", " | | ", " | ", " | ", " | ", " -+- "] renderNoose 3 = [ " ___ ", " / | ", " | O ", " | --| ", " | ", " | ", " | ", " -+- "] renderNoose 4 = [ " ___ ", " / | ", " | O ", " | --|-- ", " | ", " | ", " | ", " -+- "] renderNoose 5 = [ " ___ ", " / | ", " | O ", " | --|-- ", " | | ", " | ", " | ", " -+- "] renderNoose 6 = [ " ___ ", " / | ", " | O ", " | --|-- ", " | | ", " | / ", " | ", " -+- "] renderNoose n | n >= 7 = [ " ___ ", " / | ", " | O ", " | --|-- ", " | | ", " | / \\ ", " | ", " -+- "]

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Newbie question
by Alexander Seliverstov 22 Jan '08

22 Jan '08

Hi, I try to undestand why this code dosen't work f :: (Num a)=>Integer->a f i = i Integer is an instance of Num, so why does this code produce error: "Couldn't match expected type 'a' againsta inferred type 'Integer' ..."

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Re: [Haskell-cafe] Has character changed in GHC 6.8?
by Miguel Mitrofanov 22 Jan '08

22 Jan '08

> > chr . ord $ 'å' > '\229' > What would I have to do to get an 'å' from '229'? It seems you already have it; 'å' is the same as '\229'. But IO output is still 8-bit, so when you ask ghci to print 'å', you get '\229'. You can use utf-string library (from hackage).

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non-alphabetical mathematical symbols as non-infix function names
by Cetin Sert 22 Jan '08

22 Jan '08

(¬) :: Bool → Bool (¬) q = not q q = True ¬ q : parser error on input q ¬ : parser error (possibly incorrect indentation) (¬ q) : Couldn't match expected type `Bool -> t' against inferred type `Bool' In the expression: (� True) In the definition of `it': it = (� True) * (q ¬) : False (Why) is it not possible to define a (non-infix) function whose name consists of a single non-alphabetical mathematical symbol? ¬ :: Bool → Bool -- parser error on input ** ¬ q = not q -- parser error on input ** Cetin Sert

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Hamming's Problem
by Jose Luis Reyes F. 22 Jan '08

22 Jan '08

Hi, In exercise 2 of http://www.walenz.org/Dijkstra/page0145.html we need to write a function that holds (1) The value 1 is in the sequence (2) If x and y are in the sequence, so is f(x,y), where f has the properties a. f(x,y) > x b. (y1 > y2) => (f(x,y1)>f(x,y2)) This is a solution for this problem, but an inefficient one hammingff :: [Integer] hammingff = 1 : merge [ h x y | x <- hammingff, y <- hammingff ] [ h x y | y <- hammingff, x <- hammingff ] h x y = 2*x+3*y merge :: (Ord a) => [a] -> [a] -> [a] merge [] xs = xs merge ys [] = ys merge (x:xs) (y:ys) = case compare x y of LT -> x : merge xs (y:ys) GT -> y : merge (x:xs) ys EQ -> x : merge xs ys anybody has a better solution?. Thanks Jose

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Yi and Data.ByteString
by Cetin Sert 22 Jan '08

22 Jan '08

1) Can anyone tell me how I can build Yi or point me to a binary release of that editor? I tried to follow the instructions on http://www.nobugs.org/developer/yi/building.html but got a missing component error each time. 2) When if ever is Data.ByteString going to be the default string representation in GHC? I study computational linguistics and plan to switch to Haskell in the near future, that is once I get to grips with the language and the whole new thought model one has to develop as an imperative programmer. Best Regards, Cetin Sert www.corsis.de

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Haskell web dev: Using Haskell and HAppS for Openomy API v2.0
by Don Stewart 22 Jan '08

22 Jan '08

A deployment of HAppS, AlexJ et al's web framework for Haskell, at "openomy", > The latest release of our API is written in Haskell using the new HAppS > framework. The good news, > Since being in production, we've so far found very few issues and > everything runs quickly and smoothly. Haskell and HAppS have turned > out to be a very nice addition to our stable of tools and services. See the review here, http://blog.openomy.com/2008/01/case-study-using-haskell-and-happs-for.html Congratulations to Alex, Musasabi, Shapr, Igloo, Lemmih et al who've worked on getting HAppS into the real world! :) -- Don

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Control.Concurrent.STM.old?
by Thomas DuBuisson 22 Jan '08

22 Jan '08

Does the 'old' function referenced in the SPJ and Tim Harris data invariants paper stil exist? It is of type STM a -> STM a and allowed invariants to compare old TVar values with new ones. I can't find it in any of the Haddocks or the code (and the 'check' funciton is not commented in the Haddock). Tom

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need help for cabal-install
by Steve Lihn 22 Jan '08

22 Jan '08

Hi, I just got cabal-install working. However, I have a problem using it. My GHC is installed in a user-specific location, so when I do the Cabal dance, I have to specfiy "Setup configure --prefix=$HOME/ghc". Now with "cabal install", I can not figure out where to specify the --prefix. Cabal always complains "failed to install package". Help is appreciated. Steve

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Data.Binary questions
by Lauri Pesonen 22 Jan '08

22 Jan '08

Hi, I'm relatively new to Haskell so please bear with me. I'm trying to parse Java class files with Data.Binary and I'm having a few problems: (The class file format is described here: http://java.sun.com/docs/books/jvms/second_edition/html/ClassFile.doc.html and the bytecode instructions are described here: http://java.sun.com/docs/books/jvms/second_edition/html/Instructions.doc.ht… ) 1. The class file format contains a number of tables. The table definitions start with the length of the list and carry on with that many table entries. Lists would be a good representation for them in Haskell, because there is not need to index them directly (except with the constants table). I've created my own list type so that I can redefine the serialisation functions for it so that the serialisation matches the format defined in the class file format: newtype MyList e = MkList ([e]) deriving Show instance (Binary e) => Binary (MyList e) where put (MkList es) = do put (fromIntegral (length es) :: Word16) mapM_ put es get = do n <- get :: Get Word16 xs <- replicateM (fromIntegral n) get return (MkList xs) The problem is that one of the tables, namely the attribute_info structures, use a u32 length field whereas all the other tables use a u16 length field. My implementation uses u16, but it would be nice to be able to use the same data type for both types of tables. I think I can do it by adding a lenght field to MyList and specifying the type when I use MyList in some other data structure, but that would also mean that I have to keep track of the length of the list manually? I'm basically copy-pasting the same code just to use a u32 length field in the serialised form: data Info = MkInfo [Word8] deriving Show instance Binary Info where put (MkInfo xs) = do put (fromIntegral (length xs) :: Word32) mapM_ put xs get = do n <- get :: Get Word32 xs <- replicateM (fromIntegral n) get return (MkInfo xs) 2. This is the bigger problem. The Java class file contains a constants table in the beginning of the file. The other fields later on in the class file contain indexes that reference entries in that constants table. So in order to be able to replace an index in a data structure with the actual string, I need to be able to look up the string from the constants table while I'm deserialising the field. My guess is that I should be able to put the constants table into a state monad. On the other hand Data.Binary already uses the state monad for holding onto the binary data being deserialised. So it's not clear to me if I can use StateT with Data.Binary.Get? And if not, can I implement my own state monad and do it that way? I'm not very comfortable with Monads yet, so I might be missing something very obvious. This is what the get function looks like in my top-level Data.Binary instance: get = do magic <- get :: Get Word32 case magic == magicNumber of -- class files start with 0xCAFEBABE True -> do min <- get -- minor version number u16 maj <- get -- major version number u16 c <- get -- constants table a <- get -- access flags (public, abstract, ...) u16 t <- get -- a u16 index pointing to the name of this class in the constants table s <- get -- a u16 index pointing to the name of the super class in the constants table i <- get -- a table of interfaces f <- get -- a table of fields m <- get -- a table of methods attrs <- get -- a table of class level attributes return (ClassFile (min, maj) c a t s i f m attrs) False -> error "Invalid magic number" Thanks for all the help! -- ! Lauri

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Re: [Haskell-cafe] functional graphs
by Christian Maeder 22 Jan '08

22 Jan '08

Thomas Hartman wrote: > I don't think this will work. > > From > > http://www.haskell.org/ghc/docs/latest/html/libraries/fgl/src/Data-Graph-In… > > the minimal implementatin for Graph is > > -- | Minimum implementation: 'empty', 'isEmpty', 'match', 'mkGraph', 'labNodes' > .... > -- | Decompose a 'Graph' into the 'MContext' found for the given node and the > -- remaining 'Graph'. > match :: Node -> gr a b -> Decomp gr a b > > Basically, match given a node returns the graph minus the node, and a > "context for the node which has ingoing edges/labels, outgoing > edges/labels, the node itself and the node label. With the & operator > you can compose these two things and get back your original graph. > > With the implementation you have described I can't see any way to > implement this match function, unless per my above comment you're > doing something weird like having no graph edges, or all possible > graph edges. And then why use a graph? It's a _complete_ graph, i.e. there is an edge between every two nodes. I want to compute the minimum spanning tree using http://www.haskell.org/ghc/docs/latest/html/libraries/fgl/Data-Graph-Induct… without rewriting the FGL code and without generating the many edges explicitly. Eventually I want to have a "proper" tree (Data.Tree.Tree) for pre-order traversal. Preorder traversal of a MST gives a sub-optimal solution (not worse than twice as long as the optimum) for the travelling salesman problem (TSP). I may be on the wrong track, though. Thanks Christian > Unless I'm missing something... > > Thomas. > > > 2008/1/18, Christian Maeder <Christian.Maeder(a)dfki.de>: >> Hi, >> >> Given a complete graph as a list of nodes whose edge labels are given by >> a function over two nodes: >> >> data CGraph a b = CGraph [a] (a -> a -> b) >> >> Can I define an instance for the fgl Graph class? >> >> import Data.Graph.Inductive.Graph >> >> instance Graph CGraph where >> empty = CGraph [] -- and now? >> >> I had no idea how to define empty (except using undefined). >> >> I thought of requiring a context for the node labels of type a, but this >> type is not mentioned in the class header. So it looked to me like the >> impossibility to define sets (requiring an Ord) as monads. (i.e. >> instance Monad Data.Set.Set) >> >> Any working proposals for my graph problem? >> >> Cheers Christian

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ICFP2008 Call for Papers
by Matthew Fluet (ICFP Publicity Chair) 21 Jan '08

21 Jan '08

Call for Papers ICFP 2008: International Conference on Functional Programming Victoria, BC, Canada, 22-24 September 2008 http://www.icfpconference.org/icfp2008 Submission deadline: 2 April 2008 ICFP 2008 seeks original papers on the art and science of functional programming. Submissions are invited on all topics from principles to practice, from foundations to features, from abstraction to application. The scope includes all languages that encourage functional programming, including both purely applicative and imperative languages, as well as languages with objects and concurrency. Particular topics of interest include * Applications and Domain-Specific Languages: systems programming; scientific and numerical computing; symbolic computing; artificial intelligence; databases; graphical user interfaces; multimedia programming; scripting; system administration; distributed-systems and web programming; XML processing; security * Foundations: formal semantics; lambda calculus; type theory; monads; continuations; control; state; effects * Design: algorithms and data structures; modules; type systems; concurrency and distribution; components and composition; relations to object-oriented or logic programming * Implementation: abstract machines; compile-time and run-time optimization; just-in-time compilers; memory management; parallel hardware; interfaces to foreign functions, services, components or low-level machine resources * Transformation and Analysis: abstract interpretation; partial evaluation; program transformation * Software-development Techniques: design patterns; specification; verification; validation; debugging; test generation; tracing; profiling * Functional Pearls: elegant, instructive, and fun essays on functional programming * Practice and Experience: novel results drawn from experience in education or industry. Experience Reports are also solicited, which are short papers (2-4 pages) that provide evidence that functional programming really works or describe obstacles that have kept it from working in a particular application. Functional Pearls and Experience Reports are separate categories of papers that need not report original research results and must be marked as such at the time of submission. Detailed guidelines on both categories are below. What's different this year? ~~~~~~~~~~~~~~~~~~~~~~~~~~~ * No double blind reviewing. Instructions for authors ~~~~~~~~~~~~~~~~~~~~~~~~ By Wednesday, 2 April 2008, 09:00 AM Apia time, submit an abstract of at most 300 words and a full paper of at most 12 pages (4 pages for an Experience Report), including bibliography and figures. The deadline will be strictly enforced and papers not meeting the page limits are summarily rejected. Authors have the option to attach supplementary material to a submission, on the understanding that reviewers are not expected to read it. A submission will be evaluated according to its relevance, correctness, significance, originality, and clarity. It should explain its contributions in both general and technical terms, clearly identifying what has been accomplished, explaining why it is significant, and comparing it with previous work. The technical content should be accessible to a broad audience. Each submission must adhere to SIGPLAN's republication policy, as explained on the web. Violation risks summary rejection of the offending submission. Proceedings will be published by ACM Press. Authors of accepted submissions are expected to transfer the copyright to ACM. They may have the option to have their presentation videotaped and published along with the conference proceedings in the ACM Digital Library. Video recordings will only be released at the consent of the presenter, which is expressed by signing an additional copyright release/permission form. Formatting ~~~~~~~~~~ Submissions must be in PDF format printable in black and white on US Letter sized paper and interpretable by Ghostscript. If this requirement is a hardship, make contact with the program chair at least one week before the deadline. ICFP proceedings are printed in black and white. It is permissible to include color in a submission, but you risk annoying reviewers who will have to decide if your final paper will be understandable without it. Papers must adhere to the standard ACM conference format: two columns, nine-point font on a ten-point baseline, with columns 20pc (3.33in) wide and 54pc (9in) tall, with a column gutter of 2pc (0.33in). A suitable document template for LaTeX is available from SIGPLAN. Submission ~~~~~~~~~~ Submissions will be accepted electronically; the submission page is not yet ready. The deadline is set at Samoan time, so if your submission is in by 09:00 AM Wednesday according to your local time, wherever you are, the submission will be on time. The world clock (http://www.timeanddate.com/worldclock/fixedtime.html?month=4&day=2&year=200…) can give you the equivalent in your local time, e.g.,1:00 PM Wednesday in Seattle, 4:00 PM Wednesday in New York, and 9:00 PM Wednesday in London. Citation ~~~~~~~~ We recommend (but do not require) that you put your citations into author-date form. This procedure makes your paper easier to review. For example, if you cite a result on testing as ``(Claessen and Hughes 2000)'', many reviewers will recognize the result instantly. On the other hand, if you cite it as ``[4]'', even the best-informed reviewer has to page through your paper to find the reference. By using author-date form, you enable a knowledgeable reviewer to focus on content, not arbitrary numbering of references. LaTeX users can simply use the natbib package along with the plainnat bibliography style. Author response ~~~~~~~~~~~~~~~ You will have a 48-hour period, starting at 09:00 on 21 May 2008 Apia time, to read and respond to reviews. Special categories of papers ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ In addition to research papers, ICFP solicits two kinds of papers that do not require original research contributions: Functional Pearls, which are full papers, and Experience Reports, which are limited to four pages. Authors submitting such papers may wish to consider the following advice. Functional Pearls ~~~~~~~~~~~~~~~~~ A Functional Pearl is an elegant essay about something related to functional programming. It might offer: * a new and thought-provoking way of looking at an old idea * an instructive example of program calculation or proof * a nifty presentation of an old or new data structure * an interesting application of functional programming techniques * a novel use or exposition of functional programming in the classroom Functional Pearls are not restricted to the above varieties, however. While pearls often demonstrate an idea through the development of a short program, there is no requirement or expectation that they do so. Thus, they encompass the notions of theoretical and educational pearls. A pearl should be concise, instructive, and entertaining. Your pearl is likely to be rejected if your readers get bored, if the material gets too complicated, if too much specialized knowledge is needed, or if the writing is inelegant. The key to writing a good pearl is polishing. Some advice from Richard Bird: * Throw away the rule book for writing research papers. * Get in quick; get out quick. * Be self-contained; don't go deep into related work, with lengthy references. * You are telling a story, so some element of surprise is welcome. * Above all, be engaging. * Give a talk on the pearl to non-specialists, your students, or your department. If you changed the order of presentation for the talk, consider using the new order in the next draft. * Put the pearl away for a while, then take it out and polish it again. Experience Reports ~~~~~~~~~~~~~~~~~~ ICFP has long solicited submissions on the practice and experience of functional programming. But reports of experience are inherently different from research papers, and when judged by the criteria of scientific merit, novelty, or research contribution, they have not competed well against traditional ICFP submissions. Yet we believe that the functional-programming community would benefit from being able to draw on and cite the experience of others. For this reason, we have introduced the ICFP Experience Report. Unlike a normal ICFP paper, the purpose of an Experience Report is not to add to the body of knowledge of the functional-programming community. Rather, the purpose of an Experience Report is to help create a body of published, refereed, citable evidence that functional programming really works---or to describe what obstacles prevent it from working. An Experience Report is distinguished from a normal ICFP paper by its title, by its length, and by the criteria used to evaluate it. * Both in the proceedings and in any citations, the title of each accepted Experience Report must begin with the words ``Experience Report'' followed by a colon. * An Experience Report is at most 4 pages long. Each accepted Experience Report will be presented at the conference, but depending on the number of Experience Reports and regular papers accepted, authors of Experience reports may be asked to give shorter talks. * Because the purpose of Experience Reports is to enable our community to accumulate a body of evidence about the efficacy of functional programming, an acceptable Experience Report need not present novel results or conclusions. It is sufficient if the Report states a clear thesis and provides supporting evidence. The thesis must be relevant to ICFP, but it need not be novel. The program committee will accept or reject Experience Reports based on whether they judge the evidence to be convincing. Anecdotal evidence will be acceptable provided it is well argued and the author explains what efforts were made to gather as much evidence as possible. Typically, more convincing evidence is obtained from papers which show how functional programming was used than from papers which only say that functional programming was used. The most convincing evidence often includes comparisons of situations before and after the introduction or discontinuation of functional programming. Evidence drawn from a single person's experience may be sufficient, but more weight will be given to evidence drawn from the experience of groups of people. Possible topics for an Experience Report include, but are not limited to * Insights gained from real-world projects using functional programming * Comparison of functional programming with conventional programming in the context of an industrial project or a university curriculum * Project-management, business, or legal issues encountered when using functional programming in a real-world project * Curricular issues encountered when using functional programming in education * Real-world constraints that created special challenges for an implementation of a functional language or for functional programming in general An Experience Report should be short and to the point: make a claim about how well functional programming worked on your project and why, and produce evidence to substantiate your claim. If functional programming worked for you in the same ways it has worked for others, you need only to summarize the results---the main part of your paper should discuss how well it worked and in what context. Most readers will not want to know all the details of your project and its implementation, but please characterize your project and its context well enough so that readers can judge to what degree your experience is relevant to their own projects. Be especially careful to highlight any unusual aspects of your project. Also keep in mind that specifics about your project are more valuable than generalities about functional programming; for example, it is more valuable to say that your team delivered its software a month ahead of schedule than it is to say that functional programming made your team more productive. If your paper not only describes experience but also presents new technical results, or if your experience refutes cherished beliefs of the functional-programming community, you may be better off submitting it as a full paper, which will be judged by the usual criteria of novelty, originality, and relevance. If you are unsure in which category to submit, the program chair will be happy to help you decide. Other information ~~~~~~~~~~~~~~~~~ Conference Chair ~~~~~~~~~~~~~~~~ James Hook (Portland State University) Program Chair ~~~~~~~~~~~~~ Peter Thiemann Albert-Ludwigs-Universität Georges-Köhler-Allee 079 79110 Freiburg, Germany Email: icfp08(a)informatik.uni-freiburg.de Phone: +49 761 203 8051 Fax: +49 761 203 8052 Mail sent to the address above is filtered for spam. If you send mail and do not receive a prompt response, particularly if the deadline is looming, feel free to telephone and reverse the charges. Program Committee ~~~~~~~~~~~~~~~~~ Derek Dreyer (Max Planck Institute for Software Systems) Robert Ennals (Intel Research) Kathleen Fisher (AT&T Research) Zhenjiang Hu (University of Tokyo) Frank Huch (University of Kiel) Andrew Kennedy (Microsoft Research) Kevin Millikin (Google) Henrik Nilsson (University of Nottingham) Chris Okasaki (United States Military Academy) Brigitte Pientka (McGill University) Rinus Plasmeijer (University of Nijmegen) Alan Schmitt (INRIA) Chung-chieh Shan (Rutgers) Mitchell Wand (Northeastern University) Stephen Weeks (Jane Street Capital) Important Dates (at 09:00 Apia time, UTC-11) ~~~~~~~~~~~~~~~ Submission: 2 April 2008 Author response: 21 May 2008 Notification: 16 June 2008 Final papers due: 7 July 2008 ICFP 2008 Web Site ~~~~~~~~~~~~~~~~~~ http://www.icfpconference.org/icfp2008/ Special Journal Issue ~~~~~~~~~~~~~~~~~~~~~ There will be a special journal issue with papers from ICFP 2008. The program committee will invite the authors of select accepted papers to submit a journal version to this issue.

1 0

Parsec benchmarks
by Jon Harrop 21 Jan '08

21 Jan '08

I'd like to compare the performance of Parsec to other parsers but the only reference to a benchmark I have found is a dead link from one of the papers about Parsec: http://research.microsoft.com/users/daan/download/parsec/parsec.pdf Are there any surviving Parsec benchmarks? -- Dr Jon D Harrop, Flying Frog Consultancy Ltd. http://www.ffconsultancy.com/products/?e

3 2

functional graphs
by Christian Maeder 21 Jan '08

21 Jan '08

Hi, Given a complete graph as a list of nodes whose edge labels are given by a function over two nodes: data CGraph a b = CGraph [a] (a -> a -> b) Can I define an instance for the fgl Graph class? import Data.Graph.Inductive.Graph instance Graph CGraph where empty = CGraph [] -- and now? I had no idea how to define empty (except using undefined). I thought of requiring a context for the node labels of type a, but this type is not mentioned in the class header. So it looked to me like the impossibility to define sets (requiring an Ord) as monads. (i.e. instance Monad Data.Set.Set) Any working proposals for my graph problem? Cheers Christian

2 3

Throwback of inferred types
by Jon Harrop 21 Jan '08

21 Jan '08

Is it possible to get throwback of inferred types into Emacs or an IDE for Haskell? -- Dr Jon D Harrop, Flying Frog Consultancy Ltd. http://www.ffconsultancy.com/products/?e

8 13

Invitation for MathematiKa '08 : projectEuler format
by anshuman＠students.iiit.ac.in 21 Jan '08

21 Jan '08

Hello Math Enthusiast, IIIT-Hyderabad, India cordially invites you to be a part of Mathematika '08, an online competition that emphasizes on fusion of mathematics and computing. Where: http://felicity.iiit.ac.in/~math When: 4th February, 2008 This Online Math Contest will consist of 10-12 problems made randomly from different domains of existing Math. All problem statements will be visible to the contestant from the start of contest spanning over a 10 hour period. The solution to each problem would be an integer or a floating point number which is to be submitted in real-time. One Minute Rule: The problems are designed in a way that support 1 minute rule which means that an efficient implementation will allow a solution to be obtained on a modestly powered computer in less than one minute. Also, make sure to bring your appetite for math, programming and fun. There will be plenty of all three to go around. Regards Mathematika Team http://felicity.iiit.ac.in/~math

1 0

bug in all about monads tutorial
by Peter Hercek 21 Jan '08

21 Jan '08

Hi, About 3 weeks ago I reported this bug to Jeff Newbern. But I got no response - maybe I got filtered out as spam :) Since it was not fixed I'm trying once more here. Maybe there is somebody here who has access to the web site http://www.haskell.org/all_about_monads and cares enough to fix it. On page http://www.haskell.org/all_about_monads/html/writermonad.html there is "listens" defined like this: listens f m = do (a,w) <- m; return (a,f w) ... but it should be like this: listens f m = do (a,w) <- listen m; return (a,f w) ... or maybe a less strict option (as ghc libs have it): listens f m = do ~(a,w) <- listen m; return (a,f w) Peter.

2 1

Re: [Haskell-cafe] First go at reactive programming
by Steve Lihn 21 Jan '08

21 Jan '08

This fixed the second example. Thanks. > > I think handleConnection should be > > handleConnection :: RequestHandler -> Handle -> IO () > > handleConnection r h = > handleToRequest h >>= responseSend h . runRequestHandler r > > > Levi > > >

1 0

CPP and INLINE pragmas
by Adam Langley 21 Jan '08

21 Jan '08

Since CPP mode removes newlines in the out macro expansion. It appears to be impossible to have a macro expand to a function with an INLINE pragma since it appears to need to be in its own line. For example: #define GETHOSTWORD(name, m, type) \ {-# INLINE name #-} \ name :: m type ; \ name = getPtr (sizeOf (undefined :: type)) \ Does work (since you can't follow the INLINE pragma with anything else on the same line. Likewise: #define GETHOSTWORD(name, m, type) \ name :: m type ; \ name = getPtr (sizeOf (undefined :: type)) \ {-# INLINE name #-} Also doesn't work since the INLINE needs to start at the beginning of a line. Has anyone a workaround for this, or a way to get the preprocessor to output a newline? Cheers AGL -- Adam Langley agl(a)imperialviolet.org http://www.imperialviolet.org 650-283-9641

3 3

ANN: A triple of new packages for talking to the outside world
by Adam Langley 21 Jan '08

21 Jan '08

Over the Xmas break I made some headway on writing an RPC package since many of the ideas that I want to play with involve such a thing as a basic building block. However, some might find some of the spin-off packages that I wrote along the way useful. * binary-strict: This is mostly a cut-n-paste job from the excellent binary package which provides Data.Binary.Strict.Get - a monad which is a drop in replacement for Get, but which parses strict ByteStrings and returns an Either, thus one can parse binary data without triggering an exception on failure. * codec-libevent: This parses the "tagging" IDL files introduced in libevent[1] 1.4.0-beta and later and can generate the Haskell code for them. These are platform-independent "structs" (in the C sense) and libevent provides the complementary C infrastructure for them. You should be able to understand almost everything these is to know about them from a quick example: struct msg { string from_name = 1; string to_name = 2; optional struct[kill] attack = 3; array struct[run] run = 4; } [1] http://monkey.org/~provos/libevent/ * control-timeout: This module provides simple timeouts - e.g. actions which occur after a given number of seconds and which can be canceled. It does it in a not-totally-stupid fashion so that you don't need to worry about setting hundreds of them. AGL -- Adam Langley agl(a)imperialviolet.org http://www.imperialviolet.org 650-283-9641

9 26

bytea decoding
by Bryan Green 21 Jan '08

21 Jan '08

Does anyone know of a library that will handle bytea encodings from postgres? The bytea field that I need to access contains a jpg file. I want to retrieve it from the database and write it out for an image display program. bytea: Bytea octets are also escaped in the output. In general, each "non-printable" octet is converted into its equivalent three-digit octal value and preceded by one backslash. Most "printable" octets are represented by their standard representation in the client character set. The octet with decimal value 92 (backslash) has a special alternative output representation. Details are in Table 8-8<http://www.postgresql.org/docs/7.4/interactive/datatype-binary.html#DATATYP…>. *Table 8-8. bytea Output Escaped Octets* Decimal Octet ValueDescriptionEscaped Output Representation ExampleOutput Result92backslash\\SELECT '\\134'::bytea;\\ 0 to 31 and 127 to 255 "non-printable" octets\*xxx* (octal value) SELECT '\\001'::bytea;\00132 to 126"printable" octetsclient character set representation SELECT '\\176'::bytea;~ Depending on the front end to PostgreSQL you use, you may have additional work to do in terms of escaping and unescaping bytea strings. For example, you may also have to escape line feeds and carriage returns if your interface automatically translates these. So, here is part of the file from the database: \377\330\377\340\000\020JFIF\000\001\001\000\000\001\000\001\000\000\377\333\000C\000\012\007\007\010\007\006\012\010\010\010\013\012\012\013\016\030\020\016\015\015\016\035\025\026\021\030#\037%$"\037"!&+7/&)4)!"0A149;>>>%.DIC<H7=>;\377 I may have to write a converter myself but I don't like re-inventing the wheel if I don't need to do so. Any pointers would be greatly appreciated. Bryan Green

2 1

Need help with Parsec
by Nicu Ionita 21 Jan '08

21 Jan '08

Hi, I'm playing since a few hours with Parsec and trying to write a small html (fragment) parser, but I'm stuck in a point which I really can't understand. The problem seem to be either in "parseProperCont" or in "closing" (see code below). It looks like closing does not work (but it is a very simple function!) or (also hard to believe) function "try" from Parsec has some problems. Anyway I get this answer: Prelude ParseSHtml> pf parseHtmlFrg "ptest.txt" Left "ptest.txt" (line 5, column 2): unexpected "/" expecting element name when I'm parsing this file: <div id="normtext"> one line with break<br /> another line <br /><br /> Mail: <a href="mailto:user@dom.at">user(a)dom.at</a> </div> with this code (sorry for the longer mail): import Text.ParserCombinators.Parsec hiding (label) import Text.XHtml.Strict -- Helper function: parse a string up to one of the given chars upTo :: [Char] -> Parser [Char] upTo ds = many1 (noneOf ds) parseHtmlFrg :: Parser Html parseHtmlFrg = do many space choice [parseElem, parseText] <?> "html fragment" parseElem :: Parser Html parseElem = do en <- parseElTag many1 space (ats, cnt) <- restElem en return $ compElem en cnt ! ats <?> "html element" -- Compose a html element from tag name and content compElem en cnt = if isNoHtml cnt then itag en else tag en cnt parseElTag :: Parser String parseElTag = do char '<' en <- elemName return en <?> "element tag" elemName :: Parser String elemName = many1 lower <?> "element name" restElem :: String -> Parser ([HtmlAttr], Html) restElem nm = do ats <- parseAttList ht <- (restElNoCont <|> restElCont nm) return (ats, ht) <?> ("> or /> to close the tag " ++ nm) -- Rest element with no content restElNoCont = do char '/' char '>' return noHtml <?> "/>" -- Rest element with content restElCont nm = do char '>' many space els <- parseProperCont nm return $ concatHtml els <?> "element with content" -- Parse closing tag or proper content(s) parseProperCont :: String -> Parser [Html] parseProperCont nm = try (do closing nm return [] ) <|> (do h <- parseHtmlFrg hs <- parseProperCont nm return (h:hs) ) -- <|> return [] <?> "proper element content" closing nm = do char '<' char '/' nm1 <- elemName char '>' if nm1 == nm then return () else fail $ nm ++ ", encountered " ++ nm1 <?> ("closing of " ++ nm) -- Parse a html attribute parseAttr :: Parser HtmlAttr parseAttr = do at <- many1 lower char '=' va <- parseQuote many space return $ strAttr at va <?> "Attribut" parseAttList = many1 parseAttr <|> return [] <?> "attribute list" -- Parse a quoted string parseQuote :: Parser String parseQuote = do char '"' cs <- upTo ['"'] char '"' return cs -- Parse a text element parseText :: Parser Html parseText = do s <- upTo "<" return (stringToHtml s) <?> "some text" -- For tests: pf p file = parseFromFile p file Nicu

2 1

Type issues for a foldable trie in Haskell
by Brad Larsen 21 Jan '08

21 Jan '08

Hello there, I have written a trie in Haskell generalized to Eq a => [a] rather than simply String. I want to make this type an instance of Foldable, but I've run into a type dilemma. My datatypes look like this: data TrieElem a = Elem a | Start | End deriving (Read, Show, Eq, Ord) data Trie a = Trie {label :: TrieElem a ,children :: [Trie a]} deriving (Read, Show, Eq, Ord) The signature of Data.Foldable.foldr is (Data.Foldable.Foldable t) => (a -> b -> b) -> b -> t a -> b. However, I want the functions in Foldable to operate on the _list type_ that Trie stores rather than the _elements_ of that list type---Trie a stores lists of type a. For example, a Trie storing strings would have type Trie Char, and I want Trie Char to be Foldable, but where the functions operate on String rather than Char. So, with the datatype definitions of Trie and TrieElem as they are above, to define a fold function that operates the way I want would have signature ([a] -> b -> b) -> b -> Trie a -> b, which is no good for making Trie a an instance of Foldable. Hopefully this doesn't just seem like rambling :-). How might I rewrite my datatypes to do what I want, preferably without using ghc extensions? Thanks! Brad Larsen

1 0

Basic binary IO
by Jamie Love 20 Jan '08

20 Jan '08

Hello all, I'm wondering if anyone has a reference to any binary IO and data conversion tutorials. I'm playing around with generating a BMP file in haskell, and am a little stuck on the "best" way to go about the simple task of creating the BMP header. The header is "BM" + 4 bytes for file size + 4 bytes reserved + 4 bytes for offset where data begins. I have the basis starting off at: bmpHeader = B.pack $ [ 0x42, 0x4D ] ++ [0 , 0, 0, 0] ++ [0 , 0, 0, 0] ++ [14 :: Int32] (where B is Data.ByteString) I'm wondering how I can: 1/ convert a 32 bit number (Int32, Char32) to 4 Char8 elements 2/ rotate bits/bytes in a 32 bit Char32 (or Int32) so they are explicitly little-endian (I work on a mac powerbook, and it is big-endian) 3/ convert an Integer or Int type to an Int32 type Any pointers or suggestions would be helpful. Thanks -- Jamie Love Senior Consultant Aviarc Australia Mobile: +61 400 548 048 ------------------------------------------------------------ This message has been scanned for viruses and dangerous content by MailScanner and is believed to be clean.

3 3

One-line haskell program with GHC
by Sukit Tretriluxana 20 Jan '08

20 Jan '08

Hi, I don't know if it's been asked before. I just wonder if GHC supports some sort of one-liner program that can be specify right as the argument to either ghci or runghc program. In perl, they have something like perl *-e* 'print "Hello"' Do we have similar thing with GHC? Thanks, Ed

4 5

overlapping instance error -- need help using instance Monoid (Sum a) where a is a map of "money" values
by Thomas Hartman 20 Jan '08

20 Jan '08

The code below compiles as given, however if I uncomment the tSM function I get the overlapping instance error mentioned in the subject line. Can someone give me advice on how to do what I want to do? basically I want to add, for example, (USD,1) and (USD,2) and (Euro,3) and get result fromList [(USD,3), (Euro,3)] the datatypes below are more verbose but the above is the basic idea thanks! thomas. ***** module TransactionRows {- ( mkTransactionRows,TransactionRows ) -} where import Data.Monoid import Data.List import qualified Data.Map as M data Currency a = Currency a deriving ( Show, Eq, Ord ) data Money c a = Money ( M.Map c a ) deriving Show instance (Num a, Ord c)=> Monoid ( Sum (Money c a) ) where mempty = Sum ( Money M.empty ) Sum ( Money m1 ) `mappend` Sum ( Money m2 ) = Sum (Money m1 `plusmoney` Money m2) plusmoney (Money m1s) (Money m2s) = Money msum where msum = foldl' f m1s (M.toList m2s) f m (k,v) = M.insertWith (+) k v m mkMoney1 :: Currency String -> Float -> Money (Currency String) Float mkMoney1 c a = Money $ M.singleton c a --sumMoney :: [Money (Currency String) Float] -> Money (Currency String) Float sumMoney ms = getSum $ mconcat $ map Sum ms -- if I uncomment this, get Overlapping instances for Monoid error --tSM = sumMoney [mkMoney1 (Currency "usd") 1 :: Money (Currency String) Float]

2 1

Re: [Haskell-cafe] Re: 0/0 > 1 == False
by Jonathan Cast 20 Jan '08

20 Jan '08

On 19 Jan 2008, at 2:52 AM, Kalman Noel wrote: > Jonathan Cast wrote: >> On 12 Jan 2008, at 3:23 AM, Kalman Noel wrote: >>> (2) lim a_n = ∞ > [...] >>> (2) means that the sequence does not converge, >> >> To a value in R. Again, inf is a perfectly well defined extended >> real number, and behaves like any other element of R u {-inf, inf}. >> (Although that structure isn't quite a field --- 0 * inf isn't >> defined, nor is inf - inf). > > Out of curiosity, is there some typical application domain for > extended real > numbers? In calculus and elementary analysis, the notion of limits at/to infinity, improper Riemann integrals, etc., are introduced by a succession of `special notations'. Taking the extended real numbers as the underlying space permits these notations to be defined more compositionally, because ∞ is now an ordinary mathematical object. For example, if f is a nonnegative measurable function, ∫f on a measurable set is /always/ defined (as an extended real number) and the special case of an `integrable' function is simply one where the integral (which is an actual mathematical value) is an element of R. So, when we say ∫f ∈ R, that notation is compositional --- that's real set membership there. Similarly, ∫_{-∞}^∞ f is defined (as a Lebesgue integral) the same way any other integral is, because the interval [-∞, ∞] is a perfectly good mathematical object. jcc

1 0

0/0 > 1 == False
by Mitar 20 Jan '08

20 Jan '08

Hi! Why is 0/0 (which is NaN) > 1 == False and at the same time 0/0 < 1 == False. This means that 0/0 == 1? No, because also 0/0 == 1 == False. I understand that proper mathematical behavior would be that as 0/0 is mathematically undefined that 0/0 cannot be even compared to 1. There is probably an implementation reason behind it, but do we really want such "hidden" behavior? Would not it be better to throw some kind of an error? Mitar

20 58

functional graphs
by Thomas Hartman 20 Jan '08

20 Jan '08

I don't get where the graph edges (Node,Node) are specified. Do you just assume that every two nodes have an edge between them and calculate the edge label from the function? Also, is there a real world / motivating use for a graph defined this way? thomas. 2008/1/18, Christian Maeder <Christian.Maeder(a)dfki.de>: > Hi, > > Given a complete graph as a list of nodes whose edge labels are given by > a function over two nodes: > > data CGraph a b = CGraph [a] (a -> a -> b) > > Can I define an instance for the fgl Graph class? > > import Data.Graph.Inductive.Graph > > instance Graph CGraph where > empty = CGraph [] -- and now? > > I had no idea how to define empty (except using undefined). > > I thought of requiring a context for the node labels of type a, but this > type is not mentioned in the class header. So it looked to me like the > impossibility to define sets (requiring an Ord) as monads. (i.e. > instance Monad Data.Set.Set) > > Any working proposals for my graph problem? > > Cheers Christian > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

2 1

First go at reactive programming
by Levi Stephen 20 Jan '08

20 Jan '08

Hi, Listed below is my first experiment with reactive programming. It is a simple web server written using the Data.Reactive[1] library. The intended interface is given by the runHttpServer function, so the remainder is intended to be internal. I'd be happy to hear comments on any parts of this, but am particularly interested in the following: 1. Is this kind of code what is intended from reactive programming? 2a. I'm not sure about passing the (Handle,...) tuple around. Is there a way to avoid this? 2b. I'm not sure of the best place to handle possible socket exceptions 2c. I'd like to be able to pass a function of type Event Request -> Event Response to runHttpServer, so that reactive programming could be used throughout client code also, but the (Handle,...) tuples seem to be getting in the way. 3. I have a feeling there's a clearer way to write responseSend. Thanks, Levi [1] http://www.haskell.org/haskellwiki/Reactive module Main where import Control.Applicative import Control.Arrow ((&&&),(>>>)) import Control.Concurrent import Control.Monad import Data.Reactive import Network.BSD import Network.HTTP import Network import System.IO import Text.XHtml.Strict type RequestHandler = Request -> Response main = runHttpServer helloWorldHandler helloWorldHandler :: RequestHandler helloWorldHandler _ = Response (2,0,0) "" [] $ prettyHtml helloWorldDoc helloWorldDoc = header << thetitle << "Hello World" +++ body << h1 << "Hello World" runHttpServer r = socketServer >>= runE . handleConnection r socketServer :: IO (Event Handle) socketServer = withSocketsDo $ do (e,snk) <- mkEventShow "Server" sock <- listenOn (PortNumber 8080) forkIO $ forever $ acceptConnection sock $ snk return e handleConnection :: RequestHandler -> Event Handle -> Event (IO ()) handleConnection r = handleToRequest >>> runRequestHandler r >>> responseSend handleToRequest :: Event Handle -> Event (Handle, IO (Result Request)) handleToRequest e = fmap (id &&& receiveHTTP) e responseSend :: Event (Handle, IO (Result Response)) -> Event (IO ()) responseSend e = fmap (\(h,rsp) -> rsp >>= either (putStrLn . show) (respondHTTP h) >> close h) e runRequestHandler :: RequestHandler -> Event (Handle, IO (Result Request)) -> Event (Handle, IO (Result Response)) runRequestHandler r e = fmap hrToHr e where rqhdl :: Result Request -> Result Response rqhdl rq = bindE rq (Right . r) hrToHr :: (Handle, IO (Result Request)) -> (Handle, IO (Result Response)) hrToHr (h,req) = (h, liftA rqhdl req) acceptConnection s k = accept s >>= \(h,_,_) -> forkIO $ k h instance Stream Handle where readLine h = hGetLine h >>= \l -> return $ Right $ l ++ "\n" readBlock h n = replicateM n (hGetChar h) >>= return . Right writeBlock h s = mapM_ (hPutChar h) s >>= return . Right close = hClose

4 8

Haskell-Support in Ohloh
by Joachim Breitner 20 Jan '08

20 Jan '08

Hi, I’m only margianlly involved or up to date there, but still some might have missed this: Ohloh has begun to release their tools, starting with ohcount, their tool to measure code and comments lines: http://labs.ohloh.net/ohcount/ They explicitly write that they want haskell support, and the oldest open bug report on their page is about this: http://labs.ohloh.net/ohcount/ticket/205 So if anyone feels like programming some ruby (I guess they want it to be in that language as well) and wants to give the haskell community a chance for wider audience, give it a shot. Greetings, Joachim -- Joachim "nomeata" Breitner mail: mail(a)joachim-breitner.de | ICQ# 74513189 | GPG-Key: 4743206C JID: nomeata(a)joachim-breitner.de | http://www.joachim-breitner.de/ Debian Developer: nomeata(a)debian.org

4 3

How best to make GHC 6.6 and 6.8 co-exist on one server/account
by Steve Lihn 20 Jan '08

20 Jan '08

Hi, It appears some of the latest hackages are moving towards 6.8 to take advantage of the new features, while quite a few remains at 6.6. The compatibility between the two versions has been problematic. I only have 6.6 installed, but now thinking to add 6.8 to my account (so I can switch between them, depending what works where). What is the best practice to make 6.8 co-exist with 6.6 ? Specific questions are: 1) I installed 6.6 under prefix=$HOME/ghc. If I install 6.8 to the same prefix, it will overwrite 6.6. Correct? How do I get around it? 2) I installed additional tools and packages under $HOME/htools using 6.6. (Which I begin to wonder if that is necessary.) Do I have to use a different directory for 6.8? I noticed that, for each package, the path is $HOME/htools/lib/<package-ver>/ghc-6.6.1/ etc etc. It seems that 6.8 will create subdirectories, so they can co-exist peacefully. Correct? Thanks, Steve

2 1

shootout using 6.6?
by Isaac Gouy 19 Jan '08

19 Jan '08

Duncan Coutts wrote > Note that ghc-6.8.2 is in gentoo now and has been for a few weeks. > There's no reason not to use it. Cool! It shall be done! Mea culpa - when I was considering building ghc from source I seem to have unmerged ghc, so ghc-6.8.2 didn't show up in the portage updates. ____________________________________________________________________________________ Never miss a thing. Make Yahoo your home page. http://www.yahoo.com/r/hs

3 4

problem building array package
by Jim Burton 19 Jan '08

19 Jan '08

Building array from cabal with ghc6.6: ~/array-0.1.0.0$ runhaskell Setup.hs configure Configuring array-0.1.0.0... ~/array-0.1.0.0$ runhaskell Setup.hs build Preprocessing library array-0.1.0.0... Building array-0.1.0.0... [ 1 of 10] Compiling Data.Array.Base ( Data/Array/Base.hs, dist/build/Data/Array/Base.o ) Data/Array/Base.hs:391:23: Not in scope: `Arr.numElements' Data/Array/Base.hs:1067:35: Not in scope: `ArrST.numElementsSTArray' Data/Array/Base.hs:1079:51: Not in scope: `ArrST.numElementsSTArray' Any ideas why? Thanks, Jim

3 2

Why purely in haskell?
by Yu-Teh Shen 19 Jan '08

19 Jan '08

I got question about why haskell insist to be a purely FL. I mean is there any feature which is only support by pure? I mean maybe the program is much easier to prove? Or maybe we can cache some value for laziness. Could anyone give me some more information about why haskell needs to be pure. Thanks a lot. Shen, Yu-Teh

35 94

library to read/write audio files
by John Lato 19 Jan '08

19 Jan '08

Hello, I've been working on a library to encode/decode audio files (wave, aiff, etc.) to and from lazy bytestrings, and it's finally in a form where I'm willing to share. It's available at http://mml.music.utexas.edu/jwlato/HSoundFile/, lightly cabalized and haddock-ified. The basic item is a Data.SoundFile datatype and a SndFileCls class. Each file format (wave, etc.) has a datatype with instances of SndFileCls and Data.Binary (thanks to Don S. for suggesting Data.Binary). At least that's the idea, I've only implemented the Wave format so far, but I wanted to make it easy to add new formats. I originally intended to use Erik de Castro Lopo's excellent libsndfile (and there are some remnants of that in darcs), but after realizing I wanted a more functional interface I decided to roll my own. It's likely possible to wrap libsndfile in a functional interface, but that is beyond my current abilities. This library is far from complete. Firstly, only the wave format is implemented (and incompletely). There are also only minimal HUnit tests; I plan to add QuickCheck tests when I have some more time. Secondly there are performance issues. For relatively short clips it's fine, but there seems to be a space leak. Attempting to copy large files while just decoding and encoding the data will quickly exhaust available memory. Other than the space leak; performance is good. Profiling indicates that reading is extremely fast, and the slowest operations during write are the multiple calls to Data.Binary.Put.putWord16le (for 16-bit files). Also, sound data is currently read into a list. I will probably change this at some point in the future, most likely copying the lazy bytestring implementation and using a list of CFloat arrays. If anyone would care to offer suggestions (or patches), they would be greatly appreciated. This is my first real project in Haskell, so please be kind. I've successfully build it on ubuntu linux with GHC 6.6.1 and 6.8.1, and I don't see any reason it won't work on other platforms. Thank you, John Lato PS I'd appreciate knowing if this project is reinventing the wheel. I didn't see anything similar in hoogle or google, but you never know.

7 8

Re: [Haskell] Ancient, but still having fun
by Felipe Lessa 19 Jan '08

19 Jan '08

[redirecting to haskell-cafe] On Jan 18, 2008 9:07 PM, Simon Peyton-Jones <simonpj(a)microsoft.com> wrote: > It's Friday 18 January 2008, which makes it my 50th birthday. Happy birthday, Simon! =) Also, you're not ancient at all, and I don't doubt that you'll continue to provide essential contributions to all the Haskell community. > Suddenly Haskell is cool. It seems it's difficult to avoid success at all costs hehe... -- Felipe.

1 0

STM in F#
by Don Stewart 19 Jan '08

19 Jan '08

http://cs.hubfs.net/blogs/hell_is_other_languages/archive/2008/01/16/4565.a… I imagine this can only ease the process of learning Haskell, and broaden the base of possible Haskellers, as more people on using .NET stuff become familiar with modern typed FP. -- Don

9 9

MonadPrompt + Gtk2Hs = ?
by Felipe Lessa 18 Jan '08

18 Jan '08

(This e-mail is a literate Haskell file.) Ryan Ingram enlightened us with MonadPrompt as a very nice abstraction for turn-based games, allowing easy programming and testing. http://www.mail-archive.com/haskell-cafe@haskell.org/msg33040.html http://ryani.freeshell.org/haskell/ I wonder how nicely it fits on a Gtk2Hs application. =) > {-# OPTIONS_GHC -fglasgow-exts -Wall #-} > import Control.Concurrent > import Control.Concurrent.MVar > import Control.Monad > import Control.Monad.Fix > import Graphics.UI.Gtk > import System.IO > import System.Random Needed for the GADTs and some type signatures. (yes, I'm stuck with GHC 6.6.1 for now =/ ) While you read the e-mail, choose the appropriate main function > main :: IO () > main = consoleMain > -- main = gtkApp forkAttempt > -- main = gtkApp subLoopAttempt > -- main = gtkApp lastAttempt For the purposes of this e-mail, I'll present here a simplified version of his MonadPrompt: > class Monad m => MonadPrompt p m | m -> p where > prompt :: p a -> m a > > data Prompt (p :: * -> *) r where > PromptDone :: r -> Prompt p r > Prompt :: p a -> (a -> Prompt p r) -> Prompt p r > > instance Monad (Prompt p) where > return = PromptDone > PromptDone x >>= f = f x > Prompt p cont >>= f = Prompt p ((>>= f) . cont) > > instance MonadPrompt p (Prompt p) where > prompt p = Prompt p return With the monad above, we may program a simple guessing game. This is based on http://ryani.freeshell.org/haskell/Main.lhs and you can do every analysis done here with his sources, but I'm again trying to keep everything as simple as possible. > guessGame :: MonadPrompt GuessP m => Int -> m Int > guessGame answer = guessMe 1 where > guessMe tries = do > prompt (Print $ "Guess #" ++ show tries ++ ":") > guess <- prompt Guess > if guess == answer > then do prompt (Print "Right!") > return tries > else do prompt (Print $ "You guessed too " ++ > if guess < answer then "low" else "high" ++ > "! Try again.") > guessMe (tries + 1) Okay, so our game do prompts over the GuessP data type, > data GuessP a where where you may ask the user for a guess > Guess :: GuessP Int or you may show him some info about the game. > Print :: String -> GuessP () To play our little game, we have to execute the Prompt. We have > runPromptM :: Monad m => (forall a. p a -> m a) -> Prompt p r -> m r > runPromptM _ (PromptDone result) = return result > runPromptM f (Prompt p cont) = f p >>= runPromptM f . cont which basically maps prompts to actions on some monad -- in particular, IO. Doing a console interface isn't hard at all: > consolePrompt :: forall a. GuessP a -> IO a > consolePrompt (Print s) = putStrLn s > consolePrompt Guess = fmap read getLine > guessGameNew :: MonadPrompt GuessP m => IO (m Int) > guessGameNew = randomRIO (1, 10) >>= return . guessGame > consoleMain :: IO () > consoleMain = do > hSetBuffering stdout NoBuffering > game <- guessGameNew > attempts <- runPromptM consolePrompt game > putStrLn $ "You took " ++ show attempts ++ " attempts." That's really cool =). However, mapping runPromptM into a Gtk2Hs application isn't easy at all (we'll see why shortly). Before that, let's take the common blocks of code away. All of our attempts will develop a function that takes a function for showing info, an entry for reading guesses and a button that will be clicked to make the guess. > type AttemptCode = (String -> IO ()) -> Entry -> Button -> IO () > > gtkApp :: AttemptCode -> IO () > gtkApp run = do > unsafeInitGUIForThreadedRTS -- allows runghc > w <- windowNew > w `onDestroy` mainQuit > container <- vBoxNew False 7 > w `containerAdd` container > > let showInfo info = do m <- messageDialogNew (Just w) [] MessageInfo > ButtonsOk info > dialogRun m >> widgetDestroy m > > entry <- entryNew > boxPackStart container entry PackNatural 0 > > button <- buttonNewWithLabel "Guess" > boxPackStart container button PackNatural 0 > > widgetShowAll w > timeoutAdd (run showInfo entry button >> return False) 0 > mainGUI With everything in place, let's start! The first naïve attempt would be to just connect every point > {- > naïveAttempt :: AttemptCode > naïveAttempt showInfo entry button = do > game <- guessGameNew > let mapping (Print s) = showInfo s > mapping Guess = -- Oops > -} It's easy to see that we can't get the guess from our user using a function like 'getLine'. The problem lies in the fact that Gtk is event-driven, so every time we ask the user for something, we have to wait for the corresponding event that will bring us his answer. 'runPromptM' basically creates one big monolithic monad that will run from the beginning to the end of the game -- exactly the same thing 'mainGUI' does! The standard way of solving the problem of running two sequential things at once is using threads, and this solution is specially appealing since Control.Concurrent simplifies the matters *a lot*. So let's try this instead > forkAttempt :: AttemptCode > forkAttempt showInfo entry button = do > game <- guessGameNew > forkIO $ do attempts <- runPromptM forkedMapping game > postGUIAsync (showInfo $ "You took " ++ show attempts ++ > " attempts.") > return () > where > forkedMapping :: forall a. GuessP a -> IO a > forkedMapping (Print s) = postGUIAsync (showInfo s) > forkedMapping Guess = do > v <- newEmptyMVar > cid <- postGUISync $ onClicked button $ entryGetText entry > >>= putMVar v > guess <- takeMVar v -- f1 > postGUISync (signalDisconnect cid) -- f2 > return (read guess) Problem solved? Not really: - This kind of implementation hides lots of subtle bugs. For example, because of postGUIAsync being used in Print case, the user will see multiple dialog boxes at once and -- strangely enough -- he'll see first the last message printed. It isn't always easy to see this kind of bug at first sight, and it can be very hard to track it down. - Another problem may happen with scheduling. For some reason, there are times in which it takes some time for the control to pass from the Gtk thread to the forkIO one, effectively 'freezing' the game for some time. Unfortunately this problem doesn't show up above, but I have experienced it on a larger game I'm currently programming using Prompt. - It is possible that the user clicks on the button between f1 and f2. Again, on this very simple example nothing seems to go wrong, but there shouldn't be anything between f1 and f2 as the GUI is on an inconsistent state. It should be noted that the scheduling problem can be mitigated using 'yield' on some key spots. This not only feels hackish, but also doesn't scale very well. Another approach that is sometimes adopted to solve this kind of problem is creating a main "sub-loop" with 'mainIteration'. This essentially removes the need for those nasty evil threads =). > subLoopAttempt :: AttemptCode > subLoopAttempt showInfo entry button = do > game <- guessGameNew > attempts <- runPromptM subLoopMapping game > showInfo $ "You took " ++ show attempts ++ " attempts." > where > subLoopMapping :: forall a. GuessP a -> IO a > subLoopMapping (Print s) = showInfo s > subLoopMapping Guess = do > v <- newEmptyMVar > cid <- onClicked button $ entryGetText entry >>= putMVar v > guess <- subLoopGetMVar v > signalDisconnect cid > return (read guess) Here comes the magic! > subLoopGetMVar :: MVar a -> IO a > subLoopGetMVar v = do > m <- tryTakeMVar v > case m of > Just r -> return r > Nothing -> do > quitting <- mainIteration > when quitting (fail "quitting") > subLoopGetMVar v There are a couple of pitfalls in this approach as well: - The quitting code doesn't work very well anymore. Try to close the window before guessing right and you'll see an "user error" on the console. Some 'bracket' magic is needed to get out of the subloop without throwing errors at the user's face. - Every time we get into a subloop we add an overhead for every event the application receives. I don't know if this is important at all, maybe with lots of nested subloop. - The real problem is: what if the MVar gets full but does not generate an event? While 'subLoopGetMVar' waits for 'mainIteration', the game code should be executing already for a long time! Unfortunately I couldn't come up with a simple example that exposes the last problem listed above. But, for example, some network code could fill that MVar in a multiplayer internet game. In this case the game wouldn't proceed until an event was generated. It's known that to partially solve this problem it is possible to create a signal generator that runs every X milliseconds, giving an upper bound to the amount of time between putMVar being called and subLoopGetMVar finishing. This feels hackish and fragile as well, specially because it is difficult to hunt down bugs. But Prompt has a very *very* nice property we're missing to take advantage of here: it's is *pure*. In fact, I think that is one of the reasons why Ryan bothered sending us an e-mail -- you can not only plug different interfaces codes, but also plug no interface at all to make tests, with purity making matters very simple. Ryan also notes that "If you wanted to add undo, all you have to do is save off the current Prompt in the middle of runPromptM; you can return to the old state at any time." Eventually this feature rang some bells: you can save not only when you want to undo, but also when you want to ask something to the user. Unfortunately, I still haven't come up with a nice higher order function that generalizes this work without reinventing Prompt on an isomorphic type. Enough said, let's see how: > lastAttempt :: AttemptCode > lastAttempt showInfo entry button = do > game <- guessGameNew > runPromptM' game > where > -- Signature required > runPromptM' :: Prompt GuessP Int -> IO () -- note: not (IO Int)! > runPromptM' (Prompt (Print s) c) = showInfo s >> runPromptM' (c ()) > runPromptM' (Prompt Guess c) = do > mfix $ \cid -> do > let cont guess = do {signalDisconnect cid; > runPromptM' (c $ read guess)} > onClicked button $ entryGetText entry >>= cont > return () > runPromptM' (PromptDone attempts) = do > showInfo $ "You took " ++ show attempts ++ " attempts." After so many attempts looking with some much similar code, it may be hard to see exactly how our last attempt work. No threads, no MVars, no nothing. So I took out features, and now everything is better? =) Basically, we first assume that every time 'runPromptM'' is called, we're on Gtk's thread (which is easy in this case since there are no threads at all). Next, we see if we need to *wait* for something. In the Print case, we just call 'showInfo', so the code is the same as expanding 'subLoopMapping (Print s)' inside 'runPromptM'. This means that the Gtk event that called 'runPromptM'' will continue to execute the the next call to 'runPromptM'' as well. This is very nice, since it introduces no delays to the user (imagine a non-blocking showInfo -- e.g. printing on a textview). The PromptDone case is also very similar to what has been done before. However, in the Guess case we connect a signal to the button and return! This is where things get very different from the other approaches. When using 'forkIO', the forked thread would run from the game start until its end. When using 'mainIteration', the event that called 'subLoopAttempt' (in our case, the timeout) would execute until the end of the game. Instead, 'lastAttempt' will run only until the first Guess. Okay, so how do we proceed from here? The continuation of the 'Prompt' constructor goes inside 'cont's closure. When the user clicks on the button, it disconnects the signal and calls 'runPromptM'' again. Everything happens sequentially as I'm saying because there aren't any other threads playing with our continuation, so this not only removes the need for MVars but also fixes the whole problem of "will this run in the middle of that?". Even if the user could click twice at the same time, the Gtk main loop would execute only one event callback "concurrently" and by the time the other event gets its chance to execute we'll have disconnected its signal handler already. It's true that *another* callback would be connected if the guess was not right, but that doesn't cause any inconsistencies at all. Note also that we could have, for example, two buttons sharing the same continuation if their callbacks disconnected both 'ConnectID's before going on. No freezes, no races, no exceptions, no overheads. No generic 'runPromptM' abstraction as well, but I think the price is worth paying, specially because the other approaches have nasty subtle bugs. And we continue to have the possibility of using 'runPromptM' in our tests, for example. -- Oh, well. This e-mail got longer than I initially imagined, thanks God I started typing it on emacs already =). I'm looking forward suggestions on improvements and critics about problems in this last approach. Unfortunately the game I'm developing is still in an early stage, but I promise to release it under the GPL sometime soon =). Also, I welcome any meaningful comparison between 'lastAttempt' and 'callCC'. Thanks for reading until here, -- Felipe.

5 10

Reflection.Emit in Haskell
by Cetin Sert 18 Jan '08

18 Jan '08

As a .NET (C#/F#) programmer learning Haskell, I would love to know the best online sources about run-time compilation etc. like Reflection.Emit in .NET. I am making heavy use of this .NET API to compile customized (regular-expressions-) FSAs at run-time and want to learn how I might achieve the same in Haskell. Book or online article references specific to this issue will be highly appreciated ^_^ Best Regards, Cetin Sert INF 521, 4-6-2 69120 Heidelberg Germany http://www.corsis.de

5 6

shootout using 6.6?
by Greg Fitzgerald 18 Jan '08

18 Jan '08

http://shootout.alioth.debian.org/gp4/haskell.php Anyone know if the Language Shootout is actually using GHC 6.6 or is that a typo? Thanks, Greg

3 2

Haskell and GUI
by Torsten Otto 18 Jan '08

18 Jan '08

Seeing my woes with FranTk - what else is out there that people use if a (simple) GUI is desired for a Haskell app? Just a few textboxes and a button or two would do me. Thanks in advance! Regards, Torsten

10 11

Data constructors versus types
by Peter Verswyvelen 18 Jan '08

18 Jan '08

I know nothing about theoretical computer science, but I was wondering if it possible to forget about types, and just keep the concept of data constructors, and have an analyzer determine correctness of the code and "staticness" of the data? Basically this is what SCHEME does no? Doesn't SCHEME have static whole program analyzers to remove the overhead of the "symbol tags" and check correctness of a program (Stalin, Petit-Scheme, ...)? What are to pros/contras? Thank you, Peter

8 18

Compiling Blobs
by Peter Verswyvelen 18 Jan '08

18 Jan '08

I'm trying to build http://www.cs.york.ac.uk/fp/darcs/Blobs using GHC 6.8.2. It looks like a good Haskell program to learn from. So far I managed to modify the source code so it makes use of the new HaXML libraries, and after a lot of hacking I could build and link to wxHaskell, but my app crashes (I do get a window however, woohoo) Maybe someone else managed already? Thanks, Peter

3 4

libmpd-haskell RFC
by Ben Sinclair 18 Jan '08

18 Jan '08

Hello all, If anybody has already used libmpd-haskell (the darcs repo version) or would like to look over it I would appreciate their comments. Thanks, Ben http://turing.une.edu.au/~bsinclai/code/libmpd-haskell/

4 4