- Haskell-Cafe - Haskell.org

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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Re: Re: Read a single Char
by Hans van Thiel 26 Oct '06

26 Oct '06

>That is normal Unix behaviour. Try the same input with head -1c | od -c; head -1c | od -c The terminal buffer is only discarded if a program changes the terminal mode. Hugs and ghci do (because they use readline), but runhugs doesn't. > Ah, so that's it! Thanks, all. Hans van Thiel

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Haskell custom search engine
by dons＠cse.unsw.edu.au 26 Oct '06

26 Oct '06

Google now lets us create our own custom search engine pages, so I whipped one up for Haskell, http://www.google.com/coop/cse?cx=015832023690232952875%3Acunmubfghzq also, as a demo, embedded http://www.cse.unsw.edu.au/~dons/search.html Seems to do a reasonable job of targetting just Haskell sites. Feel free to add more Haskell material, or refine the search. In fact, search for mailing list items seems rather easy this way. Neil, I wonder if we could integrate this with Hoogle somehow? -- Don

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Read a single char
by Maurício 25 Oct '06

25 Oct '06

Hi, How can I read a single character from standard output? I would like the user to press a single key and the reading function return imediately after that key is pressed. Thanks, Maurício

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Re: Re: Read a single char
by Hans van Thiel 25 Oct '06

25 Oct '06

On Tue, Oct 24, 2006 at 06:14:14PM +0200, Hans van Thiel wrote: > I have a similar question. When I use getChar with Hugs the newline > (caused by pressing 'Enter') seems to be carried over into the next > call of main, where it is treated as a single input character. > The workaround is getLine and head, but it would be nice to drop the > newline right away. I suppose the solution offered here: >>that is the correct behavior, input is never discarded, it is just >>saved >>up until you press enter for the first time. if you type >>foo<enter>, >>your next four getChars will get 'f' 'o' 'o' and '\n'. it is not >>haskell >>that is treating enter as specially, it is your terminal, getChar >>just returns exactly what was passed to it. Yes, but the problem is with two subsequent runs of main. I'd have thought the buffer would flush automatically and the second run would start with an empty buffer. Now I enter 'b', then press 'Enter' and it works as expected. Then I 'main' again, and it terminates right away with a blank line. If I enter 'a', 'b' and then Enter, the second run takes the second character. If I'm not doing anything wrong (always a possibility :-), then it seems more like a (small) bug in Hugs, as Neil suggested. I'll have to check what chgi does. John -- John Meacham - ⑆repetae.net⑆john⑈ Regards, Hans

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Fwd: [Haskell-cafe] Haskell custom search engine
by Diego Navarro 25 Oct '06

25 Oct '06

> Google now lets us create our own custom search engine pages, so I > whipped one up for Haskell, I volunteered. Are you planning to add just sites for Haskell-related software, or are research papers included in the scope of this? (Dude, where's my english grammar.) -- syntaxfree

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