- Haskell-Cafe - Haskell.org

Where is "Typing Haskell in Haskell"?
by Ralf Lammel 14 Aug '06

14 Aug '06

Hi Mark, Cc Haskell-café, It seems the OGI links for THIH have gone dead. Is there any preferred download location for THIH these days? I couldn't spot anything. Thanks, Ralf

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Why Exotic Languages Are Not Mainstream
by Johan Tibell 14 Aug '06

14 Aug '06

Haskell was mentioned in an article called "Why Exotic Languages Are Not Mainstream" on the blog defmacro.org the other day and I thought maybe someone would be interested (i.e. is procrastinating at work and need an excuse to do something else). Any comments? http://www.defmacro.org/ramblings/not-ready.html

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Re: [Haskell-cafe] Compiling To C And Why Not Compiling To Java Or C#?
by Kaveh Shahbazian 14 Aug '06

14 Aug '06

I do not meant to compile Haskell to MSIL/JVM. I meant to compile Haskell to the Java or C# itself! And GHC will be there for a high performance language (but still O'Caml is better by the time). But Java proves that in enterprise solution, performance is a complex factor of many thing other than speed alone. I love high performance implementation like Haskell. But there must be a way to populize Haskell! Thanks

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Can someone explain this typing restriction?
by Tim Walkenhorst 14 Aug '06

14 Aug '06

I've stumbled over the following (code *extremely* simplified): f :: Bool f = g 1 g :: Num a => a -> Bool g _ = f This results in the following error message (on GHC): Contexts differ in length When matching the contexts of the signatures for f :: Bool g :: forall a. (Num a) => a -> Bool The signature contexts in a mutually recursive group should all be identical A similar problem is described in: darcs.haskell.org/haskore/docs/Tutorial.pdf on page 128. My first question is: 1) Is there a name for this restriction: I can't clearly identify it as some case of Let-Bound Polymorphism or the monomorphism restriction. [http://www.cs.auckland.ac.nz/references/haskell/haskell-intro-html/pitfalls…] 2) What exactly is the problem with it? 3) What's the best workaround? I've first tried to add an abundance of explicit type-annotations like f = (g::Int->Bool) 1 to it, but it doesn't work. Then, I found a solution, but it's awkward: f :: Bool f = g f 1 g :: Num a => Bool -> a -> Bool g f _ = f As you may have guessed, the actual code is a bit more complicated than this. Basically I interpret a symbol in a syntax-tree which can belong to different type-classes. When doing that, other more general symbols must be evaluated. An indirect recursion (potentially) occurs... I hope this hasn't been done to death yet. If it has, please just answer question one so that I can read up on it... Thank you, Tim

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Re: [Haskell] AJAX applications in Haskell
by Joel Björnson 13 Aug '06

13 Aug '06

2006/8/9, Pasqualino 'Titto' Assini <tittoassini(a)gmail.com>: > > Hi, > > Those among you who have an interest in AJAX-style web development - that > is > to say the development of web applications that run entirely into the > browser environment, calling back to the server back-end only to get raw > data -- will probably have noticed the recent appearance of the Google Web > Toolkit (http://code.google.com/webtoolkit) ... > Is anyone working on anything similar or that might be interested in such a > project? I am currently working on a Google Summer of Code project relating to some of this... The project is more or less bound to Haskell Server Pages ( http://www.cs.chalmers.se/~d00nibro/hsp/) enabling JavaScript code generation. The main difference compared to the Google Web Toolkit approach is that there are no special compiler magic added for compiling directly into JavaScript. Instead the compiled program, e.g. a cgi program generates the code (JavaScript/HTML/XML) at runtime. However, type safety is gained via an embedded language (HJScript) and a typed abstract representation of JavaScript in Haskell. For more info see http://www.dtek.chalmers.se/~bjornson/soc/ Regards, Joel

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Re: [Haskell-cafe] A lazy question
by Mario Blazevic 13 Aug '06

13 Aug '06

> > sourceToSinkTransducer :: SourceTransducer x y -> SinkTransducer x y > > > > I can't find any way to implement the sourceToSinkTransducer function > > Indeed that cannot be done because it breaks referential transparency. > Ok, thanks for clarifying the reason. > Food for thought: > > You need some sort of side effects (I'm not quite sure which), so > it'd be interesting to try > > newtype Source m x = Source (m (x, Source x)) > newtype Sink m x = Sink (x -> m (Sink x)) > > where m is an appropriate monad. Will the continuation monad work? I'm not sure. I'm trying to find the minimal data types that will let me implement the higher-level combinators. So far, the best I've found is the CPS and the following declarations: data Source i r = Source {get :: Sink i r -> r} data Monoid r => Sink i r = Sink {put :: i -> Source i r -> r, close :: r} newtype Filter x y = Filter { transduce :: forall r. Monoid r => Source x r -> Sink y r -> r } newtype Splitter i = Splitter { split :: forall r. Monoid r => Source i r -> Sink i r -> Sink i r -> r } The 'close' function and result type 'r' are there because the streams are usually finite. The 'Source' type should really be declared as data Source i = Source {get :: forall r. Sink i r -> r} because sources should not depend the result type, but that caused a lot of problems with type checking: the two 'r's in Source and Sink declaration are not unified. I don't see a way to express my intention. Anyway, thanks again for the explanation of why it has to be this complicated.

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A lazy question
by Mario Blazevic 13 Aug '06

13 Aug '06

I was hoping to use Haskell's laziness to implement some of the stuff from "Continuation and Transducer Composition" (http://www-static.cc.gatech.edu/~shivers/papers/fcoro.pdf) a bit more elegantly, without manually coding the coroutine logic in continuation-passing style. Unfortunately, I'm beginning to think there is no workaround. Here's the challenge in a simplified form: newtype Source x = Source (x, Source x) newtype Sink x = Sink (x -> Sink x) type SourceTransducer x y = Source x -> Source y type SinkTransducer x y = Sink y -> Sink x sourceToSinkTransducer :: SourceTransducer x y -> SinkTransducer x y I can't find any way to implement the sourceToSinkTransducer function and its inverse. This is disappointing, because SourceTransducer and SinkTransducer are obviously isomorphic and there are languages like Oz that can accomplish this. Tell me I'm missing something obvious?

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Nested Monads Questions
by Stefan Aeschbacher 12 Aug '06

12 Aug '06

Hi I'm trying to understand Monad Transformers. The code below works as expected but I have the following questions: - why can I use liftIO but not lift in the doSomething function? - why is there no liftSTM function? now to the code: module Main where import Control.Monad.Reader import Control.Monad.Writer import Control.Concurrent.STM type MyM a = WriterT [Entry] (ReaderT MyData IO) a data MyData = MyData {myData::TVar String} data Entry = Log String deriving (Eq, Show) logMsg :: String -> MyM () logMsg s = tell [Log s] doSomething :: MyM Int doSomething = do dataRef <- asks myData logMsg "Writing" liftIO $ do --lift $ do mv <- atomically $ readTVar dataRef putStrLn mv logMsg "Written" return 1 main :: IO () main = do i <- atomically $ newTVar "2" log <- runReaderT (runWriterT doSomething) (MyData i) print log regards and thanks Stefan

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Re: [Haskell] AJAX applications in Haskell
by Jared Updike 11 Aug '06

11 Aug '06

> This would require to: > > - Retarget one of the existing Haskell compilers to generate JavaScript > (other possible targets would be Flash or higher level UI languages such as > OpenLaszlo that in turn compiles down to either Flash or JavaScript/HTML) As I've noticed from experience, you can just write JavaScript itself! (You will end up doing this a lot anyway because any library is bound to be missing some feature you need, say, input textbox history w/ up/down keys, or, say, autoscroll to bottom of iframe when content added, etc. as here): http://www.ugcs.caltech.edu/~lordkaos/calc.cgi (source available here http://www.ugcs.caltech.edu/~lordkaos/calc.tar.gz) > Is anyone working on anything similar or that might be interested in such a > project? I'm definitely interested in this. I would love to be able to deploy a Haskell application in the web browser w/out having to drain server computing resources. Imagine reading a Haskell tutorial and providing interactive applications that run in the reader's browser, etc. Also, I really would love to see something in Haskell that can compete with the Web Services / Web Forms stuff from Visual Studio 2005/C# in terms of simplicity and power and feature-completeness: just write your app and it generates all the HTML/CSS/JavaScript on the client and the XML request stuff on the serve. Perhaps in Haskell we could have something fugdets-like but deployable in any (relatively recent) browser w/ no downloads. Jared. -- http://www.updike.org/~jared/ reverse ")-:"

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A sudoku solver in 707 bytes
by Marco Maggesi 11 Aug '06

11 Aug '06

Hi, just a short message to publicize my summer exercise in Haskell: a stupid sudoku solver (yes, again). Neither fast nor clever, but very short source code (707 bytes). Solves the "nefarious" test board cited in then Haskell Wiki in aprox 100s on my laptop (P4 2GHz). The code and an example session follows. -- Marco Code: (http://web.math.unifi.it/users/maggesi/haskell_sudoku_solver.html) ------------------------------------------------------------------------------- import List type T = (Int,Int) -> [Int] main = do s <- getContents putStr $ unlines $ map disp $ solve [input s] solve :: [T] -> [T] solve s = foldr search s idx where search p l = [mark (p,n) s | s <- l, n <- s p] mark :: ((Int,Int),Int) -> T -> T mark (p@(i,j),n) s q@(x,y) = if p==q then [n] else if x==i || y==j || e x i && e y j then delete n $ s q else s q where e a b = div (a-1) 3==div (b-1) 3 disp :: T -> String disp s = unlines [unwords [show $ head $ s (i,j) | j <- [1..9]] | i <- [1..9]] input :: String -> T input s = foldr mark (const [1..9]) $ [(p,n) | (p,n) <- zip idx $ map read $ lines s >>= words, n>0] idx :: [(Int,Int)] idx = [(i,j) | i <- [1..9], j <- [1..9]] ------------------------------------------------------------------------------- How to use it: Compile with $ ghc -o sudoku sudoku.hs then feed the board in the standard input as follows: $ cat schema 0 5 0 0 6 0 0 0 1 0 0 4 8 0 0 0 7 0 8 0 0 0 0 0 0 5 2 2 0 0 0 5 7 0 3 0 0 0 0 0 0 0 0 0 0 0 3 0 6 9 0 0 0 5 7 9 0 0 0 0 0 0 8 0 1 0 0 0 6 5 0 0 5 0 0 0 3 0 0 6 0 $ ./sudoku < schema 9 5 3 7 6 2 8 4 1 6 2 4 8 1 5 9 7 3 8 7 1 3 4 9 6 5 2 2 8 9 4 5 7 1 3 6 1 6 5 2 8 3 4 9 7 4 3 7 6 9 1 2 8 5 7 9 6 5 2 4 3 1 8 3 1 8 9 7 6 5 2 4 5 4 2 1 3 8 7 6 9

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