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Re: [Haskell-cafe] Data.Generics question
by Vyacheslav Akhmechet 19 Dec '06

19 Dec '06

Sorry, I sent the email to the wrong place and messed up the code (it's early :)). Take two: transform = everywhere (mkT transform') where transform = (\Term a->Flattened a) `extT` (Contstant a->Flattened a) On 12/19/06, Vyacheslav Akhmechet <coffeemug(a)gmail.com> wrote: > That should be pretty simple. This code probably won't work, but you > can use it as an example to look into this further. > > transform everywhere = (mkT transform') > where transform = (\Term a->Flattened a) > `extT` (Contstant a->Flattened a) > > Something like this should work. Look into the docs for more info. > > On 12/19/06, Grote Collective <grotecollective(a)gmail.com> wrote: > > Hello all, > > > > I have two datatypes, both representing the same language. However, one is a > > "flattened" version of the other. For instance: > > > > data Flattened = > > Const Flattened > > | Sum Flattened Flattened > > | ... > > | IntConst Int > > | RealConst Float > > | BoolConst Bool > > | Void > > > > ---- > > > > data Term = > > Const Constant > > | Sum Term Term > > | ... > > > > data Constant = > > IntConst Int > > | RealConst Float > > | BoolConst Bool > > | Void > > > > > > Now I want to create functions to convert between these two datatypes. This > > is easy to do but *very* tedious in its most naive approach. However, I > > think I can use Data.Generics to make a simple implementation of both > > functions. The question is: can I? I think that this conversion is a simple > > fold over the structure, changing merely the constructor name (the qualified > > name, because the unqualified name is the same), but I cannot seem to > > understand from the documentation how to do this... > > > > Thanks in advance, > > Mark Smith > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > > > > >

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maybeToM
by Joachim Breitner 19 Dec '06

19 Dec '06

Hi, dons mentions in his blog post that Data.Map’s lookup is generalized over the Monads, whereas Prelude.maybe isn’t. Are there good reasons not to do that to Prelude.maybe as well? Alternatively, how about adding this function to Data.Maybe, analogous to maybeToList > maybeToM Nothing = fail "Nothing" > maybeToM (Just x) = return x (Incidentially, this function is the same as maybeToList if run in the List Monad). This way, Prelude.lookup (or any function that returns a Maybe) can be generalized as (maybeToM . lookup). Greetings, Joachim -- Joachim "nomeata" Breitner mail: mail(a)joachim-breitner.de | ICQ# 74513189 | GPG-Key: 4743206C JID: joachimbreitner(a)amessage.de | http://www.joachim-breitner.de/ Debian Developer: nomeata(a)debian.org

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Newbie project and a question
by Jian Fan 19 Dec '06

19 Dec '06

Hi all, This is my first post. Inspired by Don's hmp3, I've started my pet project hxine: A Haskell music player using Xine engine. This was long time ago and I just picked this thing up again: darcs get http://lintao.org/repos/hxine To play CD: ./hxine cd:/ ./hxine cd://2- ./hxine cd://-5 to play files Xine understand: ./hxine *.ogg Key bindings: j : next track k : prev track p : pause/unpause d : toggle display mode q : quit Now my question: Yesterday I tried to compile (on Ubuntu Edgy) this again after couple months. First I had to hand install latest hsffig. Then the generated XINE_H.hsc give me some duplicated definitions at compile time and then some un-defined symbols at link time. I deleted those in XINE_H.hsc and everything working now. I'd like to know if this is a hsffig problem. I'd appreciate any comment on design, coding etc. Thanks. Jian

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Haskell ftplugin for vim
by Arthur van Leeuwen 18 Dec '06

18 Dec '06

Hiya folks, I got slightly annoyed at having to tab beyond .hi files when editing haskell code with vim, so I decided to wrap some stuff together in a haskell file type plugin. It has some preliminary (read: very stupid) folding support, and allows you to call out to ghci easily to get type signatures included in your text. Get it at http://www.cs.uu.nl/~arthurvl/haskell.vba Doei, Arthur. -- /\ / | arthurvl(a)cs.uu.nl | Work like you don't need the money /__\ / | A friend is someone with whom | Love like you have never been hurt / \/__ | you can dare to be yourself | Dance like there's nobody watching

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problem with Graphics.HGL
by asilovy 18 Dec '06

18 Dec '06

Hello, I posted this message to Haskell-Libraries but were advised to try Haskell-Cafe... I'm trying some stuffs with the Graphics.HGL library and have a question. I wonder if someone could help me with it. I'm trying to write some Win32 application using openWindowEx. I see it has a parameter to (eventually) define a timer. On the other hand, the maybeGetWindowEvent function is used to get "notified" of different events such as mouse button pressed, key pressed, resizing... I don't understand the special status of the timer. Why is it not an event like the others. It does'nt sound right to me. As I understand it, you have to wait for a timer tick rather than being informed by a timer event. For instance, I've written a small test program that uses the timer to draw some random "stars" on a window. Eventually, when the user presses the mouse button, the window is closed. But, as you can try, if you trigger a lot of events (by, for instance, moving the mouse a lot) BEFORE pressing the mouse button those events, although I don't treat them, will have to be processed with the delays of the timer before the mouse clic can be treated. And so, the window takes a long time to close. I'm I misunderstanding something ? And is there another way to treat the timer ? Thanks for your answer Alain {--------------------------------------------------------------------------------------- Draws randomly colored stars at random places Uses Events from : http://haskell.org/ghc/docs/latest/html/libraries/HGL/Graphics-HGL-Window.h… -} import Graphics.HGL import Hugs.Prelude import System.Random import Graphics.HGL.Win32.Types import Control.Monad import Data.Maybe main = runGraphics myWin myWin = do w <- openWindowEx "TestWindow" Nothing (400,400) DoubleBuffered (Just 100) let loop n = do x0 <- randomRIO (0, 39) y0 <- randomRIO (0, 39) couleur <- randomRIO (0, 0x00FFFFFF) let (x, y) = (x0 * 10, y0 * 10) c = if mod couleur 10 == 0 then couleur else 0 -- 1/10 will be colored drawInWindow w $ withRGB (toRGB (fromInt c)) $ ellipse (x, y) (x + 9, y + 9) e <- maybeGetWindowEvent w -- some Event ? when (isJust e) $ -- yes... case fromJust e of -- which ? Button _ _ _ -> closeWindow w -- mouse button _ -> return () -- something else -> nothing to do getWindowTick w -- wait and... loop (n + 1) -- loop loop 0 -------------------------------------------------------------------------------------- -- View this message in context: http://www.nabble.com/problem-with-Graphics.HGL-tf2834899.html#a7914596 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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I'd like start with Haskell, but...
by Waldemar Biernacki 18 Dec '06

18 Dec '06

Hello! I'd like to start programming in Haskell. But as an "industry programmer" I have a hope to use Haskell in my every-day work. Big part of my every-day work are GUI applications (in MS-Windows) working with SQL databases (PostgreSQL on Linux servers). My question: Is there a TRUE possibility to use Haskell for such applications? Is anybody there who have ANY experience in the field? If answer would be positive what GUI+database libraries could be used in such a case? I've seen some GUI libraries web pages, but they seem to be NOT maintenanced any more. cheers Waldemar

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type hackery question
by jeff p 18 Dec '06

18 Dec '06

Hello, The HList paper gives a type cast: class TypeCast a b | a -> b, b->a where typeCast :: a -> b class TypeCast' t a b | t a -> b, t b -> a where typeCast' :: t->a->b class TypeCast'' t a b | t a -> b, t b -> a where typeCast'' :: t->a->b instance TypeCast' () a b => TypeCast a b where typeCast x = typeCast' () x instance TypeCast'' t a b => TypeCast' t a b where typeCast' = typeCast'' instance TypeCast'' () a a where typeCast'' _ x = x and a type equality check: class TypeEq x y b | x y -> b instance TypeEq x x HTrue instance TypeCast HFalse b => TypeEq x y b The type cast succeeds causing its two argument types to be unified, or simply fails if the arguments are not unifiable. The type equality always succeeds "returning" HTrue if the arguments are syntactically equal, and HFalse if the arguments are not syntactically equal; thus the type equality will return false for two arguments which could be unified. If I understand correctly, these two classes do not provide a way to check for unifiability. Is it possible to write a class which checks to see if two given type arguments are unifiable? thanks, Jeff

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greencard and cabal, how to do it right?
by Marc Weber 18 Dec '06

18 Dec '06

I want to have a look at fruit which is using GCJNI, greencard, ... That's why I want to learn how to use greencard with cabal. If this is only a RTFM problem, can you point me to the docs? When using the cabal builtin preprocessor I get the error that --libdir has to be specified. So I tried adding this manually using a custom proprocessor beeing copied mostly from the already existing one: -- setup.hs ----------------------------------------------------------- import Distribution.PackageDescription import Distribution.Simple.Utils (rawSystemVerbose, dieWithLocation) main= do defaultMainWithHooks $ defaultUserHooks { hookedPreProcessors = [ myGreencardPP ] } where myGreencardPP :: (String, BuildInfo -> LocalBuildInfo -> PreProcessor) myGreencardPP = ("gc", ppGreenCard' ["--libdir ../c_lib"] []) -- << --libdir is added here -- copied from Distribution.PreProcess ppGreenCard' :: [String] -> [String] -> BuildInfo -> LocalBuildInfo -> PreProcessor ppGreenCard' args inputArgs bi lbi = maybe (ppNone "greencard") pp (withGreencard lbi) where pp greencard inFile outFile verbose = rawSystemVerbose verbose greencard (args ++ ["-tffi", "-o" ++ outFile, inFile] ++ inputArgs) ppNone :: String -> PreProcessor ppNone name inFile _ _ = dieWithLocation inFile Nothing $ "no " ++ name ++ " preprocessor available" ---------------------------------------------------------------------------- I don't understand why the command does work but not when beeing executed by cabal: The commandline shows the directory content (no Greencard.hs). (1>) Then it executes ./setup -v build which shows the commandline failing (2>) Then it executes this commandline from shell (using bash eval and getting the greencard line by grepping the output from ./setup -v build) The final ls (3>) shows that Greencard.hs is there. I've marked these parts using > and * * I've also replaced the greencard executable by a shell script printing pwd which is the same.. Whats going wrong here? How to do it right? > $ ls; ./setup -v build || echo -e "\n\n ===> build failed \n\n"; eval `./setup -v build 2>&1 | grep /usr/local/bin/greencard` && echo -e "\n\n =====> this works why?" ; ls 1> Greencard.cabal Greencard.gc Greencard_stub_ffi.c Greencard_stub_ffi.h dist greencard_stub_ffi.c greencard_stub_ffi.h setup setup.hi setup.hs Preprocessing executables for greencard-0.0... 2> */usr/local/bin/greencard --libdir ../c_lib -tffi -oGreencard.hs Greencard.gc* unrecognized option `--libdir ../c_lib' Usage: greencard [OPTION]... SOURCE Run Green Card, a foreign function interface preprocessor for Haskell, over SOURCE Green Card home page: http://www.haskell.org/greencard -?, -h --help print out this help message and exit -V --version output version information and exit -v --verbose print more information -d --debug output extra debug information --print-libdir output default library location --libdir=DIR use base library location -i DIRS --include-dir=DIRS Add DIRS (colon separated) to the include search path -s SUFS --suffix=SUFS colon separated list of source file suffixes -t TARGET --target=TARGET generate Haskell code for a particular system (ghc, ffi) -S --safe-code call C `safely' (GHC only) -n --no-inline put C code in a separate file (GHC only) -4 --haskell1.4 generate code compatible with a Haskell 1.4 system -m SCHEME --name-mangling-scheme=SCHEME controls mapping of external names into Haskell ones (std, classic) -D --stub-debug include debugging code in generated code -f DLL --dllname=DLL generate 'foreign imports' to dynamic library DLL (FFI backend only) -C CCONV --callconv=CCONV use specific calling convention (stdcall, ccall) -o FILE --output=FILE write Green Card Haskell output to FILE -c FILE --output-c=FILE write Green Card C output to FILE -p PRE --output-prefix=PRE write Green Card Haskell output to PRE.{hs,c} Building greencard-0.0... /usr/bin/ghc -Idist/build -o dist/build/greencard/greencard --make -hide-all-packages -i -idist/build/autogen -i. -odir dist/build/greencard/greencard-tmp -hidir dist/build/greencard/greencard-tmp -package haskell98-1.0 -package base-2.0 -package QuickCheck-1.0 Greencard.gc -lc_lib -L../c_lib/ ghc-6.5.20060917: no input files Usage: For basic information, try the `--help' option. ===> build failed =====> this works why? 3> Greencard.cabal Greencard.gc *Greencard.hs* Greencard_stub_ffi.c Greencard_stub_ffi.h dist greencard_stub_ffi.c greencard_stub_ffi.h setup setup.hi setup.hs Marc

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Re: A suggestion for the next high profile Haskell project
by Paul Johnson 17 Dec '06

17 Dec '06

Magnus Therning <magnus(a)therning.org> wrote: > When reading this[1] I couldn't help thinking that rewriting GPG is an > excellent opportunity for using Haskell to have an impact on the world. The Fedora project is planning to rewrite the foundations of RPM. I've noticed that the higher level RPM tools are written in Python, and are (1) very slow, (2) single threaded, so window redraws don't happen promptly. This would also be a cool place for Haskell to shine. Paul.

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Re: [Haskell-cafe] Aim Of Haskell
by Neil Bartlett 17 Dec '06

17 Dec '06

I think this hits the nail on the head. To be blunt, the presence of so many academics and scientists in the Haskell community is intimidating to those of us that work "in industry". Our brains are, after all, not as highly trained as yours, and we care about different things than you do. Now I don't mean to say that the academics and scientists should go away! Far from it. Just that it would be great to hear more about the mundane aspects of programming occasionally. Like, how exactly do I read from a relational database with Haskell? Or process an XML file? Or build an event-driven GUI? And crucially, why does Haskell do those things better than Java, or C#, or Ruby? If somebody could write some articles on those subjects, and get them up on popular websites like Digg or Reddit, this would be far more helpful than yet another monad tutorial. The Haskell web server that Simon Peyton-Jones et al described in their paper would be a great example. But where's the download? How do I get a copy to play with? In the "real world", things don't stop with the publication of a paper ;-) I think Haskell has huge potential to improve mainstream programming, if it could only catch on a bit. I don't know how to make that happen, unfortunately (if I did, I would do it, and hopefully get rich in the process). But whatever Haskell needs, it's not getting at the moment. Neil > Hello Kaveh, > > Sunday, December 10, 2006, 6:15:23 PM, you wrote: > >> chosen one. But Haskell seems to be buzz-full research platform. Now again to the top : what is the aim of Haskell project? If it is going to be used in real world applications it needs more attention to real world application developers and their needs. > > you are right - just now Haskell is a huge technology with non-obvious path to learn. there is some work to make Haskell more pragmatic, but it's an chicken-and-egg problem - we have a small number of pragmatic programmers > that use Haskell and therefore it's hard to change Haskell to suit their needs, on the other hand this means that pragmatic programmers can't grok > Haskell > > on the way to make Haskell more pragmatic i especially mention renewal of > Haskell standard to include modern language extensions, modern programming > environments such as WinHugs or BusinessObjects, development of web/db/gui > libraries, and definition of core (standard) libraries set > > one particular thing that we still lack is something like book "Haskell in > real world" > > -- > Best regards, > Bulat mailto:Bulat.Ziganshin@gmail.com > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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