November 2007 - Haskell-Cafe

Dynamically find out instances of classes (plugin system for haskell)
by Radosław Grzanka 22 Nov '07

22 Nov '07

Hi, Is there any plugin system for haskell? For example, in Java, I can load all compiled classes from given directory, check their interfaces and run some methods through reflection etc. Is it possible in haskell, to load modules from given directory, and if in module there is instance of class then return instantiation of that class? I hope this is understandable what I'm trying to achieve here. Thank you, Radek. -- Codeside: http://codeside.org/ Przedszkole Miejskie nr 86 w Lodzi: http://www.pm86.pl/

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question about ghci on windows ...
by Galchin Vasili 22 Nov '07

22 Nov '07

Hello, I am reading through http://www.haskell.org/ghc/docs/latest/html/users_guide/index.html in particular the description of the boot libraries. I don't see how I can display function signatures from compiled code (i.e. .hi). ??? Kind regards, Vasya

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How to abort a computation within Continuation Monad?
by Dimitry Golubovsky 22 Nov '07

22 Nov '07

Hi, I have been using plain non-monadic CPS for a while in my web-browser related stuff. Now I am tempted to switch from plain CPS to syntactically sweetened monadic style based on Continuation Monad, but I feel stuck with one important issue that I need an advice on. In plain CPS, I may write: type CPS x y = (y -> x) -> x a :: x a = f1 x $ \r -> case r of foo1 -> bar -- of type x foo2 -> f2 r $ \p -> ... -- something finally evaluating to a value of type x So, if at any time I return a value of a final type (x) instead of doing something with given continuation, I abort/suspend the whole computation. This can happen at any function call depth. I can also save the continuation reference in some persistent place, to resume the remainder of computation later. Now, I am trying to do the same with a Continuation Monad. But does anything similar exist in this monad? callCC is not exactly what I need because it only helps abort the inner computation, and what is returned, is a monadic value, not a final value. Besides, callCC defines a name of the function corresponding to the current continuation, and in order to use it, this name should be visible. If I have callCC $ \exit -> do foo ... I cannot jump to `exit' from within foo unless `exit' is given to foo as an argument. Any suggestions? Thanks. -- Dimitry Golubovsky Anywhere on the Web

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The Yampa Arcade: source code available?
by Peter Verswyvelen 22 Nov '07

22 Nov '07

I can find the paper, but is the source code for that Space Invaders alike game also available somewhere? Thanks, Peter

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Problems with do notation
by Peter Verswyvelen 22 Nov '07

22 Nov '07

worksFine = if True then putStrLn "True" else putStrLn "False" worksNOT = do if True then putStrLn "True" else putStrLn "False" worksAgain = do if True then putStrLn "True" else putStrLn "False" Of course the worksFine function returns an IO action, so has different behavior, but I mean the indentation is different. Is this by design? Also the following is rather strange: doubleDefError = let x = 1 x = 2 in x * x alsoDoubleDefError = do let x = 1 x = 2 return (x*x) doubleDefNOERROR = do let x = 1 let x = 2 return (x*x) Now I understand why this is (the second let starts a new invisible scope no?), but for newbies, this is all very strange :-) Cheers, Peter

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Scheme in Haskell, Parsec Example, how to add scheme comments
by Berlin Brown 22 Nov '07

22 Nov '07

I am sure many of you have looked at the scheme in haskell example that is on the web by Jonathan Tang. If you are familiar with the code, I need a little help trying to add scheme style comments: "; This is my comment" I added this code here and I think it works (I replaced the name parseSpaces with his "spaces" function). But, if I start a line with a comment, it errors out with unbound variable. Anybody have any ideas? -- Added the ';' symbol :: Parser Char symbol = oneOf ";!$%&|*+-/:<=>?@^_~#\n" -- -- Handle whitespace and comments parseSpaces :: Parser () parseSpaces = (try oneLineComment) <|> whiteSpace <|> return () where whiteSpace = do skipMany1 space parseSpaces oneLineComment = do { try (string ";") ; skipMany (satisfy (/= '\n')) ; parseSpaces } http://halogen.note.amherst.edu/~jdtang/scheme_in_48/tutorial/overview.html

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Re: [Haskell-cafe] expanded standard lib
by Magnus Therning 21 Nov '07

21 Nov '07

On Wed, Nov 21, 2007 at 20:40:01 +0000, Alex Young wrote: > Magnus Therning wrote: >> On Tue, Nov 20, 2007 at 12:33:21 +0000, Vladimir Zlatanov wrote: >>> Yes, those are good points. Maybe adding functionality similar to plt's >>> planet http://planet.plt-scheme.org and >>> http://download.plt-scheme.org/doc/371/html/mzscheme/mzscheme-Z-H-5.html#no… >>> >>> In plt scheme including a module, not present in the local repository , >>> but included via planet, resolves the module, including version, >>> etc..., downloads it from planet, and uses it appropriately. It makes >>> following various dependencies extremely easy. Updating with a new >>> version is updating the appropriate local module definitions. >>> >>> I have no clue how it would be best to implement this for haskell, but >>> it is a very user friendly no hassle way to work, so I reckon worth >>> investigating. >> >> Many other programming languages have packaging strategies that sound >> very similar. Several of them have managed to have a negative impact on >> platforms that already have good packaging technologies (i.e. almost >> every platform apart from Windows ;-). I'd hate to see Haskell go in a >> direction where packaging for e.g. Debian is made more difficult than it >> is at the moment. >> >> See [1] for the Debian Ruby packagers' opinion of RubyGems. IIRC >> similar concerns have been raised for Python's eggs. >> >> /M >> >> [1]: http://pkg-ruby-extras.alioth.debian.org/rubygems.html > Much of that's either outdated or just plain wrong, as I understand > it. In the interest of balance, note the following thread on > ruby-talk, which devolved fairly rapidly into a bunfight over Debian's > policies (and a comparison with Apple's approach to the same problem): > > http://www.nabble.com/-ANN--RubyGems-0.9.5-tf4840470.html > > There are arguments on both sides, but the utility of having RubyGems > available far outweighs the minor inconvenience of having to install > RubyGems outside apt. All I was trying to say is that if discussions like the one you link to can be avoided altogether then that would be good. I have never been heavy into Ruby programming, but I thought that the location for gems was the least of the problems they list. The major one, IFAIU is: “Rubygems is source-intrusive. The require instruction is replaced by a require_gem instruction to allow for versioned dependencies. Debian and most other systems think that dealing with versioned dependencies outside of the source is a better idea.” And it's not only a problem for Ruby in Debian: http://people.debian.org/~terpstra/message/20070904.152810.4f84c924.en.html “There are currently no plans to improve RubyGems to ease the work of Debian and RPM packagers.” I think that making it easy to package Haskell libraries, and programs, for any OS/distribution (Debian, Gentoo, PC-BSD, windows, etc) is /extremely/ important. I for one would like to see programs written in Haskell make it into as many places as possible. I'd hate to see a Haskell-specific packaging system prevent that in a similar way that RubyGems does. From the Debian Ruby team again: “[Users can] continue to install their apps the way they are used to (using apt-get), since most of them do not care about the language their apps are written in or about other ways this application/library is made available.” /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus．therning＠gmail．com http://therning.org/magnus

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"Using GHC as a library"?
by Galchin Vasili 21 Nov '07

21 Nov '07

Hello, I am reading 1) http://hackage.haskell.org/trac/ghc/wiki/Commentary/Compiler/API 2) http://haskell.org/haskellwiki/GHC/As_a_library Conceptually by allowing importation of GHC itself into a problem is this some kind of reification? Kind regards, vasya

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Best Linux for Haskell?
by Peter Verswyvelen 21 Nov '07

21 Nov '07

Each year I give Linux a try. And usually I kick it off my harddrive after a month, and stick to Windows. However, it does get better each year, so. So which kind Linux works best for running GHC (6.8.1) and related tools? (I want to give Yi a go, I can't get it to work on Windows). Debian? Fedora? Ubuntu? This is most likely a very stupid question, but please understand I'm a Windows user, so I'm supposed to be stupid J Thanks, Peter

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gtk2hs problem
by Gregory Propf 21 Nov '07

21 Nov '07

I've written a small program using the gtk2hs library and it crashes at unpredictable times with X windows errors like the one below. I looked for the messages online and found various people talking about buggy gtk libraries but no clear solutions. I don't know a lot about X windows or what these errors mean. Has anyone else had this problem or know what to do? The program is a simple implementation of the Conway Life game. It doesn't have very much going on except that the main window is an animation showing the evolution of the game. I tried running with the --sync option as it said and you get an almost immediate error like "Xlib: unexpected async reply (sequence 0x1652)!". Even these are not predictable and often come after several screen updates. I'm using the Gtk.timeoutAddFull function to do the animation. Note that the error_code, request_code, etc.. changes with each crash. It's as though it picks a random error code each time. I've compiled with both ghc 6.4.2 and 6.6.1 and gtk2hs 0.9.12 and 0.9.12.1. I've tried various combinations of -O flags as well. Same problem every time. A typical crash message ----------------------------------------------------------------------- The program 'hlh' received an X Window System error. This probably reflects a bug in the program. The error was 'BadDrawable (invalid Pixmap or Window parameter)'. (Details: serial 236321 error_code 9 request_code 62 minor_code 0) (Note to programmers: normally, X errors are reported asynchronously; that is, you will receive the error a while after causing it. To debug your program, run it with the --sync command line option to change this behavior. You can then get a meaningful backtrace from your debugger if you break on the gdk_x_error() function.) --------------------------------- Never miss a thing. Make Yahoo your homepage.

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