August 2006 - Haskell-Cafe

rand* why not of type State g a
by Marc Weber 20 Aug '06

20 Aug '06

the rand* function are examples for a typical state usage, arent' they? Is there any reasoon why they are not defined RandomGen g => State g a rather than RandomGen g => (a,a) -> g -> (a,g) ? Marc

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automatic instance derivation
by Greg Fitzgerald 19 Aug '06

19 Aug '06

How do you automatically derive an instance for a data type defined in an imported module? -- automatic instance derivation directly after type declaration data Val1 = V1 Int deriving Show -- manual instance derivation can be in separate file data Val2 = V2 Int instance Show Val2 where show (V2 i) = show i -- Is something like this possible? data Val1 = V1 Int ... derive Show Val2 Thanks, Greg

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Problem getting started with graphics library in GHC on Windows
by Tomi Maila 19 Aug '06

19 Aug '06

Hi, I'm a Haskell newbie. I've been studying Haskell by reading the book The Haskell School of Expression. Most of the example in the book use graphics library SOEGraphics. Windows GHC didn't find the library, but I assumed that the new name for the library is Graphics.SOE which seemed to properly loaded. The problem is that I fail to get the Graphics library to work properly. Instead GHC freezes if I use the graphics library in interactive mode and if I compile into a binary the binary freezes. The problem is not only with Graphics.SOE but also with more general Graphics.HGL. If I execute the Hello World program presented in Graphics.HGL documentation front page, GHC interactive mode freezes after opening the window but before drawing any text on the window. Compiled binary never even opens a window but freezes before that. Examples in HSOE (the book) behave the exactly the same way. I get not error messages what so ever. The example I refer to above is below. module Main where import Graphics.HGL main :: IO () main = runGraphics $ withWindow_ "Hello World Window" (300, 200) $ \ w -> do drawInWindow w $ text (100, 100) "Hello World" drawInWindow w $ ellipse (100, 80) (200, 180) getKey w The question is what I could be doing wrong or is Graphics library just incompatible with my environment. I'm using GHC 6.4.2 on Windows XP SP2. I've been developing using Eclipse 3.2 with EclipseFP plugin. However I've also tried to compile on command line. Could somebody help me over (no I'm not moving to Linux...) Cheers, Tomi -- Tomi Maila Research scientist Bio- ja ympäristötieteiden laitos Department of Biological Biotieteellinen tiedekunta and Environmental Sciences PL 65 (Viikinkaari 1) P.O. Box 65 (Viikinkaari 1) 00014 HELSINGIN YLIOPISTO 00014 UNIVERSITY OF HELSINKI puhelin (09) 191 59837 tel. +358-9-191 59837 matkapuh. 040 554 9024 mobile. +358-40-554 9024 http://www.helsinki.fi/~tmaila/

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Re: [Haskell-cafe] HTTPS in Haskell
by Adam Peacock 19 Aug '06

19 Aug '06

On 8/18/06, Pasqualino 'Titto' Assini <tittoassini(a)gmail.com> wrote: > > Is there any implementation of HTTPS/SSL in Haskell? > > > > This would seem to be critical to develop commercial web applications. WASH has a run time function ( runWithOptions ) that has type: runWithOptions :: CGIOptions -> CGI () -> IO () with... type CGIOptions = [CGIOption] data CGIOption = .... AutoHttps -- autodetect Https by checking for port number 443 and env var HTTPS ... Taken from: http://www.informatik.uni-freiburg.de/~thiemann/WASH/doc/WASH-CGI-CGI.html Adam.

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Haskell & LLVM
by Ryan Cleven 19 Aug '06

19 Aug '06

Has anyone ever investigated using LLVM as a target for a haskell compiler? Perhaps a backend for GHC? Thanks, Ryan.

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Haskell genetic algorithm
by Maurício 18 Aug '06

18 Aug '06

Hi, Does anyone knows of nice genetic algorithms implementations in Haskell? Thanks, Maurício

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how to generate XHTML 1.1?
by Tamas K Papp 18 Aug '06

18 Aug '06

Hi, I would like to use Haskell to generate static HTML webpages, preferably using XHTML 1.1. How can I do this? I found libraries to generate XHTML 1.0 transitional, but not 1.1. Scheme has LAML [1], which parses the DTD, uses S-expressions and validates the XHTML, I am looking for something similarly elegant (even if without the S-expressions, which I haven't seen in Haskell yet). Thanks, Tamas [1] http://www.cs.aau.dk/~normark/laml/

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More threading confusion
by Creighton Hogg 17 Aug '06

17 Aug '06

Good afternoon Haskellers, So I'm trying to understand how STM works, and wrote a quick 'eating philosophers' example to see if I understood how it's supposed to work. The problem is that while it executes, it doesn't appear to *do* anything. Did I completely write things wrongheadedly or am I being bitten by something more subtle? Thanks. import Control.Concurrent.STM import Control.Concurrent import Data.Array import System.Random think :: IO () think = do ms <- randomRIO (20,1000) threadDelay ms data Philosopher = Philosopher {left::Bool,right::Bool,neighbors::(Int,Int)} deriving Show makeInitPhilosopher a = Philosopher {left=False,right=False,neighbors=a} initPhilosophers = listArray (0,4) (map makeInitPhilosopher [(1,4),(2,0),(3,1),(4,2),(0,3)]) main = do z <- atomically $ newTVar initPhilosophers mapM_ (\x -> forkIO (loop x z 0 10000)) [0,1,2,3,4] loop n tps c l | c > l = (atomically (readTVar tps)) >>= (\x -> print x) | otherwise = do think atomically $ eat n tps loop n tps (c+1) l eat :: Int -> TVar (Array Int Philosopher) -> STM () eat n tps = do takeLeft n tps takeRight n tps releaseLeft n tps releaseRight n tps takeLeft :: Int -> TVar (Array Int Philosopher) -> STM () takeLeft n tps = do ps <- readTVar tps let p = ps ! n if right (ps ! (fst $ neighbors p)) == False then (writeTVar tps $ ps // [(n,p{left=True})]) else retry takeRight :: Int -> TVar (Array Int Philosopher) -> STM () takeRight n tps = do ps <- readTVar tps let p = ps ! n if left (ps ! (snd $ neighbors p)) == False then (writeTVar tps $ ps // [(n,p{right=True})]) else retry releaseLeft n tps = do ps <- readTVar tps let p = ps ! n writeTVar tps $ ps // [(n,p{left=False})] releaseRight n tps = do ps <- readTVar tps let p = ps ! n writeTVar tps $ ps // [(n,p{right=False})]

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Samba/FTP bindings
by Ivan Tarasov 17 Aug '06

17 Aug '06

Is there some Haskell library which provides Samba bindings and some FTP client library bindings (e.g. ftplib3)? I started writing the bindings for some of the functions myself for my project but it looks like it is a lot of work and probably someone has done it already. -- Best regards, Ivan Tarasov

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Last statement in 'do' must be an expression error.
by Szymon Ząbkiewicz 17 Aug '06

17 Aug '06

Hi. When trying to compilke this code: {...} 8.if (a == 0) && (b == 0) 9. then do 10. nr1 <- read (prompt "enter 1. number: ") 11. nr2 <- read (prompt "enter 2. number: ") 12. else do 13. let nr1 = a 14. nr2 = b {...} The compiler tells me thats there's an error on line 10: "The last statement in a 'do' construct mesy be an expression" Could you tell me how to change it so that the "declaration" of the first nr1 and nr2 is still in the "then" block.

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