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Rank N type tutorial?
by Greg Buchholz 29 Oct '06

29 Oct '06

Anyone know of a good source for learning about higher ranked types? I'm not quite sure why this is illegal... > foo :: Integer -> (forall a. Show a => a) > foo 2 = ["foo"] > foo x = x ...while this is just fine... > bar :: Integer -> (forall a. Show a => a->b) -> b > bar 2 k = k ["bar"] > bar x k = k x Thanks, Greg Buchholz

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The Read Class
by Tiago Veloso 29 Oct '06

29 Oct '06

Hi, I am trying to find out how to work with the Read Class for a school project, i need to declare instances of Read for a few data types. My problem is that i do not know how to do it, i mean i do not know how to build a Read instance, i do know about it for the Show Class. Can some one help? Thanks Tiago Veloso

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Partially applied type synonyms in instance heads
by Mathieu Boespflug 29 Oct '06

29 Oct '06

Hi everyone, I'm running into trouble with type synonyms in instance heads and I can't figure out what the resulting error message means. The case I'm considering is as follows: -- hoist a and b to class variables so that instances declarations may -- constrain them. class Map m a b where toAssoc :: m a b -> [(a, b)] fromAssoc :: [(a, b)] -> m a b type Assoc a b = [(a, b)] instance Map Assoc a b where toAssoc = id fromAssoc = id The class Map is used to allow translation from one map type to another (FiniteMap, arrays, etc...) by means of expressing the map as an association list. Useful for defining isomorphisms and so on. Now I'd like to define an association list as itself a trivial instance of Map class, but I cannot do so without wrapping the type of an association list, [(a, b)], behind a type synonym, as I'm not aware of any way of writing a type constructor of kind (* -> * -> *) constructing the type [(a, b)]. But when compiling the above code with GHC I get the following error: Map.hs:9:0: Type synonym `Assoc' should have 2 arguments, but has been given 0 In the instance declaration for `Map Assoc a b' Any idea what this means? Also, is there any other way of declaring an instance for [(a, b)] without using type synonyms? Many thanks, Mathieu

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Re: [Haskell] Animal guessing game - critique my code
by David Roundy 29 Oct '06

29 Oct '06

I haven't made anything more elegant, but have a few improvements that I found made it more fun to play (and yes, I'm rather zoned out after spending most of the day solving electromagnetism problems). It's nice (and quite easy) to save the tree, so that the game is more exciting every time you play. With a bit of work and integration with darcs, you could probably interface with a public server, so the guesser can be collaboratively improved by people who have too much time on their hands or are too tired to do anything productive. Here's the improved code: \begin{code} module Main where import Data.Char ( isUpper ) data GuessTree = Answer String | GuessTreeQuestion {guessTreeQuestion :: String, guessTreeNo, guessTreeYes ::GuessTree} deriving (Show, Read) askQuestion :: String -> IO Bool askQuestion str = do putStrLn str response <- getLine return $ isYes response where isYes ('y':xs) = True isYes _ = False a :: String -> String a s@(c:_) | isUpper c = s | c `elem` "aeiou" = "an " ++ s | otherwise = "a " ++ s a "" = "nonexistent" runTree :: GuessTree -> IO GuessTree runTree (Answer name) = do response <- askQuestion $ "Is it " ++ a name ++ "?" if response then do putStrLn "Ha!" return $ Answer name else do putStrLn "What is it?" animal <- getLine putStrLn $ "Enter a question to help distinguish between " ++ a name ++ " and " ++ a animal question <- getLine a <- askQuestion $ "Is the answer yes for " ++ animal ++ "?" (if a then id else flip) (\n y -> return $ GuessTreeQuestion {guessTreeQuestion = question, guessTreeNo = n, guessTreeYes = y}) (Answer animal) (Answer name) runTree GuessTreeQuestion {guessTreeQuestion = ques, guessTreeYes = yesTree, guessTreeNo = noTree} = do response <- askQuestion $ ques if response then do a <- runTree noTree return $ GuessTreeQuestion {guessTreeQuestion = ques, guessTreeNo = a, guessTreeYes = yesTree} else do a <- runTree yesTree return $ GuessTreeQuestion {guessTreeQuestion = ques, guessTreeNo = noTree, guessTreeYes = a} run :: GuessTree -> IO GuessTree run tree = do putStrLn "Think of an animal and press enter." getLine a <- runTree tree writeFile "animaldata" $ show a r <- askQuestion "Play again?" if r then run a else return a read_animal :: IO GuessTree read_animal = (read `fmap` readFile "animaldata") `catch` \_ -> return $ Answer "bear" main = read_animal >>= run \end{code} As you can see, I also modified it so that it'll be a bit smarter about not saying things like "a David Roundy" or "a elephant". -- David Roundy Dept. of Physics Oregon State University

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Self Study with SOE
by Aditya Siram 28 Oct '06

28 Oct '06

Hi all, I have been steadily working through Haskell SOE. However, as the exercises become more involved, I would like to know, not only that the answer I come up with works, but that I am doing it the right way (the elegant way?). For instance, Chapter 8 Exercise 8.3 requires me to modify the "area" and "perimeter" functions to accept negative arguments. My solution would be to change the functions to take the absolute value of the arguments as they come in. This would work but it doesn't seem all that elegant. Additionally, SOE exercises do not come with test data so I can test correctness. Is it part of my responsibililty as a student to come up with test data (boundary conditions etc) as I try to answer the questions? Is there some kind of solutions manual available? I promise I am not doing this as homework for a course. Thanks... Deech _________________________________________________________________ Find a local pizza place, music store, museum and morethen map the best route! http://local.live.com?FORM=MGA001

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Re: [Haskell-cafe] Rank N type tutorial?
by rossberg＠ps.uni-sb.de 27 Oct '06

27 Oct '06

"Greg Buchholz" <haskell(a)sleepingsquirrel.org> wrote: > I'm not quite sure why this is illegal... > >> foo :: Integer -> (forall a. Show a => a) >> foo 2 = ["foo"] >> foo x = x The type signature promises that foo returns a function that has type a *for all a* (that are in Show). But neither a string list nor an integer can have all types. Rather, they are very specific. That is, what you have implemented actually could have type foo :: Integer -> (exists a. Show a => a) (if GHC supported such types). Note that only bottom can have a type like forall a.a. > ...while this is just fine... > >> bar :: Integer -> (forall a. Show a => a->b) -> b >> bar 2 k = k ["bar"] >> bar x k = k x Here, you declare bar to *take* a function that delivers some b for any a. That is, the burden of defining such a function now is on the caller, not bar itself. However, in this case, defining such an argument function actually is easy, for another reason: the signature says k returns a result of type b, but b is fixed. So it is always possible to come up with a trivial instance of such a function (which will just ignore its argument). For instance, bar 2 (\_ -> "boo") But try to call this for a different example: ;-) baz :: Integer -> (forall a b. Show a => a->b) -> c baz 2 k = k ["baz"] baz x k = k x Hope this helps, - Andreas

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UTF and Parsec
by John Ky 27 Oct '06

27 Oct '06

Hi, I'm interested in using Parsec to parse utf-8 files and do some processing and output a utf-8 document. Is there anything I should be aware of? Does GHC handle all the utf-8 stuff automatically? What does it mean when I find that parsec is picking up "ï»¿" at the beginning of the file. Thanks -John

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Solitaire cipher
by jim burton 26 Oct '06

26 Oct '06

I'm a beginner having a go at implementing the Solitaire cipher (http://www.rubyquiz.com/quiz1.html as mentioned in another post) and I'd be really grateful if you could help me improve the code to be neater & use more functions from the Prelude etc, or errors (eg at the moment I can't work out why padding accumulates after encrypting, decrypting?)...Thanks. *Main> decrypt $ encrypt "haskell is better by miles" "HASKE LLISB ETTER BYMIL ESAYP XXXXX " *Main> decrypt $ encrypt $ decrypt $ encrypt "haskell is better by miles" "HASKE LLISB ETTER BYMIL ESAYP XXXXX BFCRK XXXXX " *Main> ------------------------------------------- import Char import Random import List import Foreign import Maybe data Card = Clubs Int | Spades Int | Diamonds Int | Hearts Int | JokerA | JokerB deriving (Show, Eq) type Deck = [Card] --cardval - clubs are face value, diamonds plus 13, and so on - Jokers are both 53 cardval :: Card -> Int cardval (Clubs i) = i cardval (Diamonds i) = i+13 cardval (Hearts i) = i+26 cardval (Spades i) = i+39 cardval _ = 53 isJoker :: Card -> Bool isJoker JokerA = True isJoker JokerB = True isJoker _ = False --take a card to a letter card2char :: Card -> Char card2char c = case c of (Clubs i) -> int2alpha $ cardval c --can case fall through in haskell? (Diamonds i) -> int2alpha $ cardval c (Hearts i) -> int2alpha $ (cardval c-26) (Spades i) -> int2alpha $ (cardval c-26) _ -> error ("Can't make " ++ show c ++ "into alpha") --take a letter to int, A=1, Z=26 char2int :: Char -> Int char2int = (64 `subtract`) . (ord) --take a letter to int, 1=A, Z=26 int2alpha :: Int -> Char int2alpha = (chr) . (+64) splitAtMb n l = let p = splitAt n l in if null $ fst p then Nothing else Just p in_fives l = foldr (\x y -> x++" "++y) [] $ unfoldr (splitAtMb 5) (l ++ replicate (5 - length l `mod` 5) 'X') --get an ordered deck newdeck :: Deck newdeck = suit 'c' ++ suit 'd' ++ suit 'h' ++ suit 's' ++ JokerA : JokerB : [] where suit s = case s of 'c' -> [Clubs i | i <- [1..13]] 's' -> [Spades i | i <- [1..13]] 'd' -> [Diamonds i | i <- [1..13]] 'h' -> [Hearts i | i <- [1..13]] --key the deck ready to provide a keystream - move JokerA down one place, --JokerB down 2 places, perform a triplecut then a countcut keydeck :: Deck -> Deck keydeck = countcut. triplecut . (movedown JokerB) . (movedown JokerB) . (movedown JokerA) --bump a card down by one place in a deck, treating the deck as circular so if the card is -- last in the deck it becomes 2nd to front not 1st movedown :: Eq a => a -> [a] -> [a] movedown c d = if c == last d then head d : c : init (tail d) else top ++ bot!!1 : c : (tail (tail bot)) where splt = splitAt (locate c d) d top = fst splt bot = snd splt --substitute the cards above the first joker for those below the 2nd one triplecut :: Deck -> Deck triplecut d = afterLastJoker d ++ center d ++ beforeFirstJoker d where beforeFirstJoker = takeWhile (not . isJoker) afterLastJoker = reverse . beforeFirstJoker . reverse center = reverse . dropWhile (not . isJoker) . reverse . dropWhile (not . isJoker) --get the value of the last card and move that many cards from the top of deck to above the last card countcut :: Deck -> Deck countcut d = init (drop n d) ++ take n d ++ [last d] where n = cardval (last d) --key the deck, read the value of the top card as n, add the nth card to stream, repeat keystream :: Deck -> String keystream d = if isJoker c then keystream d' else card2char c : keystream d' where d' = keydeck d c = d'!!(cardval $ d'!!0) locate :: Eq a => a -> [a] -> Int locate x xs = fromJust (elemIndex x xs) clean :: String -> String clean = map toUpper . filter isAlpha encrypt, decrypt :: String -> String --encrypt a string using an unshuffled deck to start encrypt = process (\x y -> max26 (x+y)) where max26 x = if x > 26 then x-26 else x --decrypt a string using an unshuffled deck to start decrypt = process (\x y -> if x <= y then (x+26)-y else x-y) process :: (Int -> Int -> Int) -> String -> String process f s = in_fives $ map int2alpha $ zipWith f ints1 ints2 where str1 = clean s str2 = take (length str1) (keystream newdeck) ints1 = map char2int str1 ints2 = map char2int str2 -- View this message in context: http://www.nabble.com/Solitaire-cipher-tf2500700.html#a6971077 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Ruby quiz, Haskell wiki
by dons＠cse.unsw.edu.au 26 Oct '06

26 Oct '06

I've created a page to document haskell solutions to the ruby quiz puzzle series. http://haskell.org/haskellwiki/Haskell_Quiz Those of you working on them, please upload your solutions, and create sub pages for new puzzles as they appear. Cheers, Don

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Glade Gtk2Hs Getting Started
by Hans van Thiel 26 Oct '06

26 Oct '06

Hello, My adaptation, with the author's permission, of the 'Developing Gnome Apps with Glade' tutorial by Eddy Ahmed for Gtk2Hs and Haskell can now be accessed on: http://j-van-thiel.speedlinq.nl/EddyAhmed/GladeGtk2Hs.html It's for the absolute beginner in both Glade and Gtk2Hs and covers the basics of Glade and how to access a .glade file and widgets in Gtk2Hs. Estimated learning time: 2 hours. Thanks to Duncan Coutts and Eddy Ahmed for their help and cooperation. The tutorial will be made available on the Gtk2Hs web site and on Eddy's homepage as well. Regards, Hans van Thiel

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