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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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wxFruit with OpenGL and IO action
by shelarcy 17 Aug '06

17 Aug '06

Hello. Sometimes, a few people are interested in wxFruit. http://zoo.cs.yale.edu/classes/cs490/03-04b/bartholomew.robinson/ But nobady knows current status of wxFruit. Over one and half year ago, I heard that students working on that ... . http://www.haskell.org/pipermail/haskell-cafe/2005-January/008765.html http://www.haskell.org/pipermail/haskell-cafe/2006-March/014998.html So, I'm working on that for subproject of Kamiariduki ... but progress is very slowly. I uploaded current (initial) version on Kamiariduki's file space. File name is "YaPan-initial.tgz". Because this file includes YaPan - Yet another implementation, and reimplementation of Panic (http://www.cse.unsw.edu.au/~sseefried/pan/index.html) using Yampa (and wxFruit). http://sourceforge.net/project/showfiles.php?group_id=168626 I didn't think current verion is good enough to announse [Haskell] list, so I didn't announce that and repraced "YaPan.tgz" file sometimes ... but I know that a few people downloaded my file already. So, I decide to upload initial file on that space. Of cource, "YaPan-initial.tgz" has other wxFuit example. paddle.hs is almost same as original example, except that use menu istead of button. And CFK_GL* is a OpenGL sample code. But current version has some faults. Cabal file support to build and install only YaPan, doesn't support wxFruit. Code is very messy, and YaPan doesn't support all of Panic's features. And I should notice additonal things for older version's user. Now YaPan depends on AutoForms (not for wxFruit now). http://autoforms.sourceforge.net/ And YaPan can't save static one picture, YaPan can save animation by consecutive number files only. Current version, I tested on only Windows with my build of wxWidgets 2.4.2's wxHaskell (I uplaoded Kamiariduki's file space) and afrp-0.4 environment. So I don't know wxFruit and YaPan can works well other environments. Finally, I show current problem of wxFruit with IO action. I extend that button menu allow to regist extra IO action like this. pan pixels = hBox $ proc _ -> do let start = False rec v <- boxSF $ iPre 0 -< 1 + v ev <- getEvent -< () vTrans <- boxSF $ iPre (0,0,1) -< (keyboardEvent 0.1 vTrans) ev panedPixels <- boxSF $ pictureEvent -< (pixels, vTrans, (circle v), v, gameBox) _ <- glpicture (glpsizeWithAttributes gameBox [WXCore.GL_RGBA, WXCore.GL_DOUBLEBUFFER]) -< glppic $ paintGL gameBox panedPixels _ <- menu (mtextWithDialog "&SaveImage") -< mcommandWithDialog $ (\v f -> saveImageHandler (animToDisplayFun (circle)) circle (WX.sizeW gameBox, WX.sizeH gameBox) vTrans 1 v f) v returnA -< () This design is suitable for button and menu's command. But if I want to use IO action out of these control event, modualiry is broken. Because I want to do that action out of control's event, but I have to change control's action. And I must choose using what kind of event handler. (Is interval the best?) So I tried to implement command function that intend to use IO action on reative loop wihtout control in wxFruit.hs ... but action doesn't do registered action or program is not work on my apporoach. (I commented out current command function that occur action.) If anyone have good idea how to use IO action on wxFuit, please tell me that idea. Thanks. -- shelarcy <shelarcy capella.freemail.ne.jp> http://page.freett.com/shelarcy/

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Why does Haskell have the if-then-else syntax?
by Mike Gunter 16 Aug '06

16 Aug '06

I had hoped the "History of Haskell" paper would answer a question I've pondered for some time: why does Haskell have the if-then-else syntax? The paper doesn't address this. What's the story? thanks, -m

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Type hackery help needed!
by Niklas Broberg 16 Aug '06

16 Aug '06

Hi fellow Haskelleers, I have a tricky problem that I just can't seem to solve. The problem is one of unresolved overloading, much like the (show . read) issue, but unlike that particular problem I feel there should be a solution to the one I'm wrestling with. I've tried to strip away all the unnecessary bits so the following will be a bit abstract, but it shows the gist of the problem. I have two overloaded functions - one I call 'build' that creates values from subparts, and one I call 'embed' that turns values into a form appropriate to call 'build' on. What they really represent is creating values of some XML tree structure, with embed making sure that the children are of the right type. Conceptually, we could imagine that these two functions to be defined as > class Build x c | x -> c, c -> x where > build :: String -> [c] -> x > > class Embed a b where > embed :: a -> b They would be used together as in e.g. > p c = build "p" [embed c] This works pretty well, the fundep x -> c tells me what b should be, assuming I infer a proper type for the result of the composition. The type of p is then p :: (Embed a c, Build x c) => a -> x where the c in the middle is determined by x via the fundep. My problems arise because I want to start nesting these to form tree structures, as in > tree = p (p "foo") Expanding the definition of p, the argument to the outer call to build is now embed $ build "p" [embed "foo"] :: (Embed String c, Build x c, Embed x x1) => x1 Through the fundep on the outer build I can infer what x1 should be, but there's no way I can infer x (without inserting an explicit type signature which is out of the question). This problem is probably unsolvable in general, but for my particular problem there are some restrictions that makes me feel there should be a clever way of working with classes and fundeps to make this all work out. I just can't seem to find one. These are the rules that the setup must obey: * A value of any type should be embeddable inside a build expression of any result type, i.e. a -> b or b -> a cannot hold on Embed in general. * The exception to the above is that if an expression using 'build' is embedded inside an outer 'build', as in 'tree' above, the inner build should have the same result type as the outer build (in a sense forming b -> a only for types instantiating Build). In other words, children of an xml element must be of the same type as their parent, even before the call to embed. * However, it would be nice, but probably not crucial, if by using explicit type signatures where I want to, I could actually embed values of "other" xml types than the outer one I am embedding them in, and letting embed convert them to xml of the correct type. I suspect this cannot be. The types of build and embed are negotiable, and certainly the type classes, as long as the two can be used together in roughly the same way as indicated above. The calls to build and embed will be autogenerated by a preprocessor from a HSP-style XML syntax, e.g. <p><% c %></p> <==> build "p" [embed c] and for this reason any solution *must* be syntactically expressible as expressions on a form similar to the above, but those expressions can be arbitrarily convoluted. For instance in my failed attempts so far I have used let expressions around build to form self-recursive versions to "pass a type" down through the evaluation, as in p c = let x = build "p" [embed x c] in x This is an ok solution syntactically, if only it solved the problem, I still can't see how to propagate it to the next level of build. :-( Is there anyone out there with the proper type class fu who can see a way out for me? Is this even possible to do at all? If yes, tell me please, and if not, I would be most interested in seeing why it cannot work. Any and all comments are welcome. Thanks for reading this long :-) /Niklas

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Int->[Char] conversion
by Tamas K Papp 16 Aug '06

16 Aug '06

Hi, I am working through Hal Daume's tutorial, trying to do the exercises. I can't figure out how to output an integer with putStrLn (or any other way), I think I need an Int -> [Char] conversion but couldn't find it. Specifically, in Exercise 3.10, I have the product of numbers in pp, and would like to do putStrLn ("Product: " ++ convertnumbertostring(pp)) but I don't know which function does this... Sorry for the extremely dumb question. Thanks, Tamas

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Questions on threads and IO
by Creighton Hogg 16 Aug '06

16 Aug '06

Hello Haskell'rs, I've been playing with threads and I tried to do a toy example (that used java) from a class. When run, the program should print a prompt and accept commands just like a linux shell. It doesn't have to do anything fancy, just spawn new threads that make system calls when commands are entered at the prompt. The problem is that the UI doesn't work very well. It will seem fine at first, but in order to get back a prompt you have to hit enter one more time than you should. I've tried playing with the buffering settings but it seems to cause the same problem no matter what. The problem seems to be coming from calls of the form (forkIO . system_) "ls /usr/bin" Just entering this into ghci I get the same thing where I need to hit enter *again* in order to get back to the ghci prompt. I'm sure this is something silly on my part, but it is rather confusing. import Control.Concurrent import System import System.IO main = do putStr ">" z <- getLine runCommands z main genWords :: Char -> String -> [String] genWords c s = gwhelper c s [] [] gwhelper :: Char -> String -> [String] -> String -> [String] gwhelper c [] acc temp = acc ++ [(reverse temp)] gwhelper c (x:xs) acc temp | x /= c = gwhelper c xs acc (x:temp) | otherwise = gwhelper c xs (acc++[(reverse temp)]) [] runCommands s = mapM_ (forkIO .system_) (genWords '&' s) system_ :: String -> IO () system_ s = do system s return ()

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Genetic programming Haskell
by Maurício 16 Aug '06

16 Aug '06

Hi, Does anyone know of a package for genetic programming for Haskell? I tried some links from Haskell wiki and some I found on google, and found many interesting papers, but none of them pointed to downloadable code. Does anyone know where can I find a good implementation (even an experimental one)? Thanks, Maurício

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Re: map and fmap
by Duncan Coutts 15 Aug '06

15 Aug '06

On Tue, 2006-08-15 at 12:38 +0200, John Hughes wrote: > One thing I've observed repeatedly is that many students in later > years, who learned functional programming early, have a strong > impression that functional languages are only suitable for toy > programs. Of course, that's because in their first courses they only > SAW toy programs. But perhaps, they also moved on to Java as soon as > they started to build GUIs, and as a result have the impression that > you CAN'T build a GUI in the functional language they learned first. Yes, this was the same impression my colleagues had as undergraduates and it's the same thing I see in the students I now teach. > This is a very negative impression for students to have, and one that > is hard to combat because beginning undergraduates are sceptical to > functional languages from the word go: they "know" they really want to > learn C++ or Java, and wonder initially what we're playing at. > > I combat that--successfully--by showing them early on that Haskell > *is* a "real" programming language, in which one can build real > applications. I tell them about darcs, or whatever new application is > hot at the time. I show them how to build a GUI in wxHaskell--in the > fifth week of their university studies. Yay! I'm doing the same thing, the fifth week in their first term in their first year. Though I'm using Gtk2Hs of course :-) I don't make them build the GUI directly of course. I've got a practical where they do some list processing functions and we provide a GUI module which visualises what they've done. So while still teaching the basics I try to make it clear that that can do GUIs and do them easily and make them look attractive. This year I want to add an optional extension to go 3D, to visualise their stuff with singing dancing rotatey things. I managed to get the Gtk2Hs+HOpenGL stuff working by the end of the practical last year and had lots of students come over to say "wow!". So I'm looking forward to using it this year to motivate the students to do the optional sections of the practical. > I want them to be frustrated at how difficult GUI programming is when > they do it in Java (:-)! It works--some find it very hard, but many of > the good students go much further than my lectures and exercises, and > reappear in the third and fourth years as very expert and enthusiastic > Haskell programmers. This is the first year when people who did my Haskell GUI practical will go on to the OOP course which involves building a GUI in Java. I'll be interested to see how they think it compares. Duncan

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Re: [Haskell-cafe] Re: Why Haskell?
by Johan Tibell 15 Aug '06

15 Aug '06

For me library support, for networking in particular, has been the major hurdle. It gets problematic when too many libraries are still marked as experimental and only partially implements the specification (e.g. protocol) that they are supposed to handle. Also after a quick look at the source for some common (networking) libraries I found a few too many calls to error to feel comfortable. It's not okay for a library to bring down my application behind my back, error should be reported even if this uglifies the code and interfaces. Furthermore I think that Haskell would benefit from moving some commonly used functionality such as HTTP, SQL and XML support into the standard libraries. I haven't seen any news about the progress of HaskellNet but I guess it could solve my problems. I really don't have much problems with tool support, speed and the like. I'm convinced it's a library (and library documentation!) thing. > As for the 'popularity' of Haskell, someone made the very common sense > remark that all successful languages have been promoted by some large > corporation. (Please note 'successful' stands for 'spread of use', not > 'quality' here.) If fp is to reach the mainstream, it will probably be > because of some centralized support from outside the academic world. > Maybe something like a 'Haskell Consortium' or 'Haskell Initiative' to > promote Haskell should be set up. An example could be the promotion of > SystemC for hardware and software co-design by the Open SystemC > Initiative www.systemc.org . Availability of an IEEE standard for > Haskell would also be a good thing for its acceptance, I think. Having someone pay a group of people to hack on Haskell implementations would indeed be desirable. Without knowing the details Ubuntu looks like a promising model. If we could just find a willing billionaire out there... Cheers, Johan

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