December 2006 - Haskell-Cafe

howto c2hs marshall in out argument by reference
by Marc Weber 19 Dec '06

19 Dec '06

The function takes a pointer to an integer, prints its value and returns the increment. void cl_inc(int *i){ printf("changing int from %d to %d", *i, (++i)); } I've managed it by using this code: {#fun pure cl_inc {alloca `CInt' peek*} -> `()'#} -} and calling it this way: alloca $ \mem -> do poke mem 3; print_int_ptr_value mem then its a matter of reading mem by using peak again. Is there a better way letting {#func do the marshalling to be able to do directly? print $ cl_inc 3 4 Marc

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Data.Generics question
by Grote Collective 19 Dec '06

19 Dec '06

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

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Class annotation on datatype ineffective in function
by Reto Kramer 19 Dec '06

19 Dec '06

The code below does not compile unless the "bar" function is annotated with a suitable constraint on the class of the formal parameter. > module Main where > > class (C a) > data (C foo) => XY foo = X foo | Y foo > > bar :: a -> XY a > bar aFoo = X aFoo > > main = return () I get: > $ ghc Test.hs > > Test.hs:8:8: > No instance for (C a) > arising from use of `X' at Test.hs:8:8-10 > Possible fix: add (C a) to the type signature(s) for `foo' > In the expression: X a > In the definition of `foo': foo a = X a As suggested, this works: > bar :: (C a) => a -> XY a Can someone explain to me why the compiler can not infer that "a" (in bar) must be (C a) from the bar result type "XY a" (by way of the "C class" provided for the datatype)? Thanks, - Reto

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On the various faces of Composition
by Jacques Carette 19 Dec '06

19 Dec '06

[As recently seen on LtU - http://lambda-the-ultimate.org/node/1919] An amazing paper http://www.dbmcintyre.pwp.blueyonder.co.uk/index_f/menu_f/j_f/apl95.pdf on "composition" as seen from APL and J. The terminology (using verbs, nouns, adverbs, etc) is _so_ much more evocative than the terminology used for similar ideas here, I hope that some of it can rub off. Also, if anyone thought that "point free" programming was something mostly-recent, mostly-Haskell, they _really_ need to read this paper! Also, I am fairly sure that some of the high-powered operators in J may stress Haskell's type system quite severely, if not to the breaking point. It would be a fascinating exercise to try to reproduce the various examples in that paper in as-idiomatic-as-possible Haskell. While J's syntax is somewhat on the terse side, its power regarding practical mathematics is superb -- especially when it comes to polymorphism. Jacques

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exception output
by Ferenc Wagner 19 Dec '06

19 Dec '06

Hi, the following program main = readFile "nonexistent" >>= putStr is meant to produce an exception. When compiled with GHC 6.4, it outputs the "*** Exception..." string to stderr, which makes me happy. However, when run with runghc6 or runhugs, the message is sent to stdout, which makes me sad. Is this inconsistency intentional? -- Thanks, Feri.

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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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