- Haskell-Cafe - Haskell.org

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 12 Aug '06

12 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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[Haskell] AJAX applications in Haskell
by Dimitry Golubovsky 10 Aug '06

10 Aug '06

Looks like the discussion is continuing in the Cafe, so I am reposting it here. Hi, Pasqualino 'Titto' Assini wrote: =================== Assuming that the balance is positive, this naturally raises the question: why not doing the same <generate Javascript/Ajax application code to run in browser - DMG> with our favourite language? 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) =================== Joel Björnson wrote: =================== 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. =================== Looks like the idea is picked up by more and more people ;) I saw some notes on this (generation of Javascript from Haskell source) in Jared Updike's blog, and I got this idea myself independently from Jared in about the same time. I am trying to add some code to nhc98 (contrary to GHC its code is much smaller and better observable) to generate Javascript at the same time when STG tree is converted into bytecodes*. I am in the very beginning though, so I cannot show any results. OTOH I do not feel like I have a lot of time to dedicate to this, so if someone else is working on this, could we team up (possibly for a different (if shown better) approach because one I chose is still questionable optimization-wise, but generation of Javascript from bytecodes does not seem better to me at the time)? -------------- * Work in progress may be observed at http://www.golubovsky.org/repos/nhc98/ in particular in http://www.golubovsky.org/repos/nhc98/src/compiler98/STGJcode.hs but at the moment there is virtually nothing done. -- Dimitry Golubovsky Anywhere on the Web

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Flags
by Maduser 10 Aug '06

10 Aug '06

I have started to program in Haskell. Now I want programm something like flags. It's a set of flags. It sould be possible to convert the Flag as Int/String (the bit combination). I have written this: import Data.Set as Set type Flags = Set Flag data Flag = Flag1 | Flag2 | Flag3 deriving(Eq, Ord, Enum, Show) flagToInt :: Flags -> Int flagToInt flags = flag1 + flag2 + flag3 where flag1 = if member Flag1 flags then 1 else 0 flag2 = if member Flag2 flags then 2 else 0 flag3 = if member Flag3 flags then 4 else 0 but this seems to me to be inelegant. Can somebody help me making it better, please. -- View this message in context: http://www.nabble.com/Flags-tf2083897.html#a5741741 Sent from the Haskell - Haskell-Cafe forum at Nabble.com.

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Strictness, order of IO operations: NewCGI & HDBC
by Tim Smith 10 Aug '06

10 Aug '06

Hello! I'm having a problem with using NewCGI & HDBC together. NewCGI: http://www.cs.chalmers.se/~bringert/darcs/haskell-cgi/doc/ HDBC: http://quux.org/devel/hdbc http://darcs.complete.org/hdbc/ I've distilled it down to a small test case: \begin{code} module Main where import Data.List (intersperse) import qualified Database.HDBC as DB import Database.HDBC.ODBC (connectODBC) import Network.NewCGI main :: IO () main = runCGI (handleErrors cgiMain) cgiMain :: CGI CGIResult cgiMain = do dbh <- liftIO $ connectODBC "DSN=test" res <- liftIO (DB.getTables dbh) -- Remove the disconnect call, and all works liftIO (DB.disconnect dbh) output ((concat . intersperse ", ") res) \end{code} 2:46 ~/m/tmp/hs$ ./cgidb Content-type: text/html; charset=ISO-8859-1 cgidb: unknown exception If I comment out the DB.disconnect call, then it works fine. I tried to work around it with the DeepSeq module, but couldn't find an application of $!! or deepSeq which would make it function. I made a similar test which uses file IO instead of HDBC, and it behaves the way I want: \begin{code} module Main where import Network.NewCGI import System.IO main :: IO () main = runCGI (handleErrors cgiMain) cgiMain :: CGI CGIResult cgiMain = do h <- liftIO $ openFile "t.hs" ReadMode res <- liftIO (hGetLine h) liftIO (hClose h) output res \end{code} 2:55 ~/m/tmp/hs$ ./t Content-type: text/html; charset=ISO-8859-1 module Main My guess is that I'm doing something wrong, but I'm not sure what it is. Or, is this a bug in HDBC, somehow? Thanks for any ideas you might have, Tim -- If you're not part of the solution, you're part of the precipitate.

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IA64 porting: spill code in the mangler
by C Rodrigues 10 Aug '06

10 Aug '06

Hi folks, I've been trying to compile a new ia64 port. I've cross-compiled an unregisterised compiler that generated working binaries the first time it was built, which was a pleasant experience. But I ran into issues with the registerised build. The mangler is choking on floating-point spill code: Prologue junk?: .global __divdf3# .global GHCziFloat_zdwlogBase_entry# .proc GHCziFloat_zdwlogBase_entry# GHCziFloat_zdwlogBase_entry: mov r16 = r12 .save.f 0x1 stf.spill [r16] = f2 .body This saves floating-point register 2 on the stack; it's loaded again later. I could change the mangler to remove this code, if that's the right approach. What should the mangler do about it?

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beginner's haskell question
by Jens Theisen 10 Aug '06

10 Aug '06

Hello, as a haskell newbie I'm wondering about the following question. Are there options to popular haskell implementations or other means (haskell lint?) to check for incomplete patterns at compile time for some? I can't see a reason why this shouldn't be possible or even a relatively simple thing to implement. Cheers, Jens

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Re: Haskell-Cafe Digest, Vol 36, Issue 29
by Mark Haniford 10 Aug '06

10 Aug '06

It seems irony gets lost so easily in these conversations. You have no > way of knowing what the state of haskell would have been had certain > key contributors to GHC and Haskell not taken jobs at Microsoft. > Therefore you statement is meaningless and only good for producing > approving nods among people who already agree with what you say. What's meaningless are GPL jihadists who are bitter that not everybody is joining their religion. Yeah...that's personal.

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Why Not Haskell?
by Hans van Thiel 09 Aug '06

09 Aug '06

Hello All, I'm wondering why I can't find any commercial Haskell applications on the Internet. Is there any reason for this? I can think of the following possibilities only: 1) Haskell is too slow for practical use, but the benchmarks I found appear to contradict this. 2) Input and output are not good enough, in particular for graphical user interfacing and/or data base interaction. But it seems there are several user interfaces and SQL and other data base interfaces for Haskell, even though the tutorials don't seem to cover this. 3) Haskell is not scaleable for commercial use. This looks unlikely to me, but could this be a factor? 4) Haskell is open source and licensing restrictions forbid commercial applications. I haven't seen any such restrictions, but is this a problem for the standard modules? Are there other reasons why there seem to be just a few thousand (hundred?) Haskell programmers in the world, compared to the 3 million Java programmers and x million C/C++ programmers? The reason I ask is that in the early nineties I wrote a data mining /machine learning program. This was not a commercial success and since then I've been working as a free lance IT journalist. As such I covered the O'Reilly European Open Source Convention in Amsterdam last year and there I heard of Haskell. I became interested and started learning from the available free tutorials and Hugs. As an aside, I'd like to thank all those people. I really appreciate their work and I'd like to single out Hal Daume III's 'Yet Another Haskell Tutorial', while not diminishing the other authors in any way. Meanwhile I kept working on a data mining algorithm in my spare time and finally I came up with something that appears to work. While I was learning Haskell, I thought, why not try this part of the algorithm as a Haskell exercise. I could easily test it in Hugs. To my own surprise, after only a few weeks of this, I had a working prototype of my algorithm, printed out on less than 3 pages, using only standard functions in the Prelude. So, I'm really impressed by the power of Haskell, and I'm just a beginner. Now I'm trying to come up with a business model for my algorithm and to avoid the mistakes I made 10 years ago. There is a lot of difference between a prototype and a working tool, and then there is a lot of difference between a working tool and a successful commercial application. Probably it doesn't make much sense to try and develop a tool in C++ or even Java, but if I have to go on my own on this, maybe Haskell could be feasible, both for fun and profit. So, that's the background of my question (while introducing myself to the Cafe at the same time). Why hasn't Haskell made it into the business world (yet), after being available for 15 years, or is this the wrong question? Many thanks for your comments, Hans van Thiel

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