December 2007 - Haskell-Cafe

class default method proposal
by Duncan Coutts 17 Dec '07

17 Dec '07

I'd just like to float an idea that's related to the Class Alias proposal[1] but is perhaps somewhat simpler. We all know that Functor should have been a superclass of Monad, and indeed we now know that Applicative should be too. Making such a change would break lots of things however so the change does not happen. However in this case the Monad operations can be used to implement the Functor and Applicative class methods. So it would be nice if we could get them for free if the author did not choose to write the Functor and Applicative instances. So my suggestion is that we let classes declare default implementations of methods from super-classes. class Functor m => Monad m where {- the ordinary bits -} fmap f m = m >>= return . f So if there already is a Functor instance for m then the default implementation of fmap is not used. Does this proposal have any unintended consequences? I'm not sure. Please discuss :-) Duncan [1] http://repetae.net/recent/out/classalias.html

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Yampa / AFRPVectorSpace.hs
by Peter Verswyvelen 17 Dec '07

17 Dec '07

While studying the vector space class in AFRP, I encountered the following strange code: class Floating a => VectorSpace v a | v -> a where ... v1 ^-^ v2 = v1 ^+^ v1 -- (negateVector v2) I have no idea why the (negateVector v2) has been commented out, but surely this must be a typo? Cheers, Peter

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Problem with a Win32 API
by Eric 17 Dec '07

17 Dec '07

Dear all, When I run the program below a window appears for a split second, then disappears and the program exits. Can you help me figure out what's going wrong? *** import Graphics.Win32.Misc import Graphics.Win32.GDI.Types import Graphics.Win32.Window import System.Win32.DLL import Foreign.Marshal.Alloc main = do hInst <- loadLibrary "ole32.dll" res <- registerClass (style, hInst, Nothing, Nothing, Nothing, Nothing, classname) if res == Nothing then do{putStrLn "Unable to register." ;return()} else putStrLn "Registered." hwnd <- createWindow classname "Kleer 0" wS_VISIBLE Nothing Nothing Nothing Nothing Nothing Nothing hInst handler b <- showWindow hwnd sW_SHOWNORMAL if b then putStrLn "Window showing" else do{putStrLn "Unable to show window."; return()} updateWindow hwnd ptr <- mallocBytes 4 getMessage ptr Nothing loop ptr loop msg = do translateMessage msg dispatchMessage msg loop msg handler :: WindowClosure handler hwnd msg w l = case msg of wM_CLOSE -> do{destroyWindow hwnd; return 0} _ -> defWindowProc (Just hwnd) msg w l style :: ClassStyle style = 0 classname = mkClassName "KleerWnd" *** E.

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array documentation is missing
by Thomas Hartman 17 Dec '07

17 Dec '07

While trying to make Regex.TDFA 0.91 install on 6.8.2 I got [10 of 21] Compiling Text.Regex.TDFA.RunMutState ( Text/Regex/TDFA/RunMutState.\ hs, dist\build/Text/Regex/TDFA/RunMutState.o ) Text/Regex/TDFA/RunMutState.hs:601:9: Constructor `STUArray' should have 4 arguments, but has been given 3 In the pattern: STUArray _ _ msource In the definition of `copySTU': copySTU (STUArray _ _ msource) (STUArray _ _ mdest) = ST $ \ s1# -> case sizeofMutableByteArray# msource of n# ->\ ... ^[]0;~/hackageTarGzs/regexstuff/regex-tdfa-0.92^G ^[[32mhartthoma@dbnycws28816 ^[[33m~/hackageTarGzs/regexstuff/regex-tdfa-0.92^[\ [0m but I got stuck fixing it because the array documentation isn't there http://www.haskell.org/ghc/docs/latest/html/libraries/haskell98/Array.html assume I'm getting the above error because the array interface has changed thomas. --- This e-mail may contain confidential and/or privileged information. If you are not the intended recipient (or have received this e-mail in error) please notify the sender immediately and destroy this e-mail. Any unauthorized copying, disclosure or distribution of the material in this e-mail is strictly forbidden.

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Patter matching beginner question
by Adam Smyczek 17 Dec '07

17 Dec '07

Hi all, I assume the following behavior has a trivial explanation. When I write: case name of "a" -> ... "b" -> ... everything works fine. But when I extract a and b to constants: c_a = "a" :: String c_b = "b" :: String case name of c_a -> ... c_b -> ... I get Patterns match(es) are overlapped. Adam

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Implementing a MUD server in haskell
by Jack Kelly 16 Dec '07

16 Dec '07

Hi, I've been thinking about two things for some time now: 1. Sharpening my haskell 2. Implementing a MUD server I've been thinking about doing (1) and (2) at the same time but I've run into some problems with data structures. I need to be able to get a list of players from a room (for things like a 'look' command) and I need to be able to find which room contains a given player. I'm still looking at this problem through an imperative lens, so here's some sort-of-C: struct room{ ... struct player * players; ... }; struct player{ ... struct room * room; ... }; From what I can see, I'd use a record type for players and rooms, but I'm not sure how to replicate the pointer effects: keeping each player pointing to a single instance of a consistent world and maintaining the invariant that (given: p :: Player, r :: Room): p `elem` (Room.players r) => (Player.room p == r) This needs to stand up to concurrent modification of a shared world structure, but I think I'll set up the concurrency controls after I get my head around this. -- Jack

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Re[2]: [Haskell-cafe] Execution of external command
by Bulat Ziganshin 16 Dec '07

16 Dec '07

Hello gwern0, Thursday, December 13, 2007, 4:43:02 PM, you wrote: > to get at stdout. I don't think this would go through cmd/sh, but > honestly, specifying a particular interpreter breaks your > portability anyway these scripts will be written by users so they don't need to be the same for win&unix. i guess that users will prefer to use perl or something like this for these scripts and afaiu in both windows and linux this need to go through sh/cmd it seems that script may be not terminated if its output isn't read, so better code should be (_, h, g, _) <- runInteractiveCommand "script params" result <- hGetLine h hGetContents h >>= evaluate.length hGetContents g >>= evaluate.length am i right? -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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Cheap ByteString mmap
by Don Stewart 15 Dec '07

15 Dec '07

A little mmap binding for ByteStrings. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/bytestring-mmap-… Should be useful for managing file resources under pressure (cheaper than a readFile, and with different constraints). Posix only. the Win32 mmap for bytestrings is in the win32 package. -- Don

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JOB OFFER / Haskell for commercial projects?
by Peter Verswyvelen 15 Dec '07

15 Dec '07

Hello Happy Haskellers, I had to abandon improving my newbie Haskell skills the past weeks; I was busy creating a new startup company and finalizing financial funding for a cool project related to realtime "2D/3D animation and games". As I'm the CTO of this new company, and since I kinda like Haskell, I might consider creating a first prototype of the project using Haskell. However, this will be a commercial project. The software will be freely downloadable but not open source. However, if succesful, a highly priced professional version of the software might be created. Although I'm pretty sure GHC can be used for creating a private inhouse prototype, the prototype might evolve into the final product, and so my question is, which Haskell compilers & tools can: (1) be used to build commercial projects (2) be shipped with commercial projects (our project might call into the interpreter/compiler at runtime.) (3) be indirectly used by commercial projects (in the sense that the user will have to manually download and install the compiler) Furthermore, is the Haskell community willing to provide the excellent help (as in this forum and on IIRC) for commercial Haskell projects? That said, anybody who might be interested in getting a (paid J) Haskell job (parttime or fulltime) in the field of computer animation and games (but also (visual) programming languages, interpreters & compilers), please mailto:peter@nazooka.com. Preferable you will work onsite (in Belgium / Antwerp - the city of diamonds J), although working remote is also an option. You will make software that will be initially used by computer artists (that among other things made parts of <http://www.cgchannel.com/gallery/viewimage.jsp?imgID=547> "The Girl", Xyanide <http://www.gamespot.com/xbox/action/xyanide/index.html> , and many commercials <http://www.nazooka.com/site/html/index.php?p=movieplayer&m=showreel> ), that will be demonstrated at SIGGRAPH <http://www.siggraph.org/> , and will be released to the public in a later stage. Cheers to all, Peter Verswyvelen

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getMessage function in Win32 API
by Eric 15 Dec '07

15 Dec '07

Dear all, In the Graphics.Win32.Window package there is a function getMessage takes a value of type LPMSG. How can I construct a value of LPMSG so that I can call the function? E.

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