October 2004 - Haskell-Cafe

Integrating Haskell into a J2EE environment
by Bhinderwala, Shoeb 07 Oct '04

07 Oct '04

Hi All, I am new to Haskell and this mailing list. We have a system that uses a custom high-level language to express high-level business rules. Expressions in the high-level language get compiled to Java bytecode. We express the grammar using BNF notation as required by the javacc parser tool. This is then converted to an AST using jjtree and from there we build the final Java code. Our language could be considered a domain-specific language (DSL) and is used by our business users to express very high-level business logic. The language currently is very limited - we support boolean logic, function invocations and if-then statements. We want to convert it into a more powerful scripting language so that even lower level business logic can be expressed in it. I came across a few papers that talk about writing a DSL with Haskell as the underlying support language. How is this done. Is it possible to create a sort of domain specific business scripting language easily. How does that then compile to Haskell code. And how can the Haskell code be invoked from Java. Essentially, I am thinking if I could use a Haskell like DSL language to express our business rule logic and then be able to integrate into and invoke the logic from a J2EE app server environment. Has anybody done anything like this with Haskell. -- Shoeb

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Asteroids game, (was Space leaks)
by Ben Lippmeier 07 Oct '04

07 Oct '04

Andy, For the asteroids game, I just call GHC's System.Mem.performGC every frame and it seems to run ok. It's a very simple game, my 2.6Ghz P4 system walks all over it and the FPS counter seems to bounce around from about 150 to 300. I would imagine it's spending all it's time in the big XFillRectangle call I'm using to blank the screen, and without double buffering it flickers pretty bad. I wrote the game as a way to get some non-tree-processing experience with Haskell, and to test out a C struct marshalling FFI preprocessor I was messing with. All the graphics stuff is written using straight Xlib, called from Haskell. You're more than welcome to the code if you think you can do something with it. You can get it from my homepage at http://cs.anu.edu.au/people/Ben.Lippmeier There's also a mandelbrot set plotter, written in Haskell, which I wrote to try out various unboxing schemes. The only other (reasonably current) real-time projects i've heard of are: * The HOpenGL effort and Haskell Quake. HOpenGL seems to be somewhat active, you might like to try out some of their sample programs. I tried to get Haskell Quake to work once, but from memory they were using an ancient version of Greencard/HOpenGL for their FFI, which I couldn't get to work. The Haskell Quake home page says it hasn't been updated since 1997. * Alastair Reid's real-time computer computer vision project, FVision. http://www.reid-consulting-uk.ltd.uk/alastair/ I think they solved their Garbage Collection stall problem the same way I did, just invoked the garbage collector some fixed number of times per second / frame.. Don't know much about it, best ask him directly. * The Haskell DSP libraries http://haskelldsp.sourceforge.net/ I think its mostly an off-line, don't-care-too-much-about-runtime -performance kind of thing. I think it consists of a collection of (typically) list to list functions, so its not really designed for real time operation. However, a while back I had the idea that if you wanted to burn some CPU cycles in a real-time way, you might try piping to/from /dev/dsp with a full duplex souncard to make yourself some sort of poor-mans effect processor. Thats it. Not much code deployed out in the 'world' it seems. My PhD project is also aligned with addressing this issue. I think it's one of those catch 22 situations where no-one has written real-time programs in Haskell cause they knew it probably wouldn't work.. So when someone comes up with a new language feature which tries to solve the problem, there's no programs to try it on cause no-one's written any. Do you know of any other real-time type projects besides these? Perhaps the two of us, and who ever else is interested, should choose some real-time program (perhaps the asteroids game, or revive Haskell Quake) and spend some small, fixed amount of time per week working on it so we'll all have some application to showcase our glorious new language features with? Ben. Andrew Cheadle wrote: > Hi Ben > > >>-> An astroids game I wrote for X. >>The state: position, velocities of ship, astroids, missiles. >>The [x] list: ship control data, key codes. >>The [y] list: a list of graphics prims which get rendered to the screen. > > > I wonder if you could help me out. I'm trying to investigate the > practical use of my incremental collector for GHC over the standard > generational collector. I'm looking for applications that have soft-real > time constraints / interactive nature where garbage collector pauses are > noticable. Do you know of any such applciations, did your asteroids game > suffer any such problems? Although graphical programs would be great, > when written in Haskell the rendering engine is usually in C so they're > not usually appropriate! > > Any help greatly appreciated! > > Cheers > > Andy >

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500 great lines of Haskell code?
by Isaac Jones 06 Oct '04

06 Oct '04

This might be an interesting way to highlight the beauty and brevity of Haskell. Has anyone written a great 500-line Haskell program they want to submit? peace, isaac http://developers.slashdot.org/article.pl?sid=04/10/06/1530218&tid=156&tid=8 "Be part of the Open Source Annual 2005 and enter our hacker contest for the best 500-line open source program. The best program will be printed in next year's issue of the book. Following lasts year's huge success with our Open Source Annual, a mostly German reader concerned with the various aspects of open source, we are currently busy compiling the second edition of the annual which will be released next March for the CeBIT 2005 in Hannover. Aside from articles on subjects like economics, law and open innovation, to name but a few, we plan to print the source code of an open source software program."

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Re: [Haskell-cafe] Integrating Haskell into a J2EE environment
by Paul Hudak 06 Oct '04

06 Oct '04

Ok, I understand. I don't know much at all about J2EE, in fact! I would just hate to see an interesting project be abandoned if all that is needed is a simple way to invoke the Haskell code with a string argument, say. Perhaps Shoeb can tell us more about what he needs. -Paul Doug Kirk wrote: > Yes, I agree, and didn't mean to write off Haskell (at which, I'm > completely a newbie, trying to learn, and thankful for your book!). > > However, I'm a Java pro, and there are many technical issues on the Java > side that scream at me to keep out of the native arena, especially in a > J2EE container environment, where funny things can happen with hot > reloads (dumping old ClassLoaders for new ones), clustering, and the like. > > So it wasn't out of denigration of Haskell that I made my > recommendation; far from it...from what I've seen Haskell is perfect for > implementing DSL's. Rather, from the Java side is where it becomes > problematic. There have been many problems integrating with native > libraries from within a J2EE container, and I try to seek the most > cost-effective way (I'm an independent consultant) to get the problem > solved for my customers. > > --doug > > On Oct 6, 2004, at 2:59 PM, Paul Hudak wrote: > > I wouldn't write off Haskell so quickly. All of what Shoeb describes > concerning DSL issues might be much more easily solved in Haskell, and > will certainly be more flexible than a hard-wired approach. The J2EE > interface might be ugly, but if the functionality needed is not too > great it might not be too bad. Generally speaking, these kinds of apps > -- in this case "a DSL for high-level business rules" -- sounds like > just the sort of thing that Haskell is good for. > > -Paul

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What Functions are Standard?
by John Goerzen 06 Oct '04

06 Oct '04

Hello, I have been writing code using the docs over at http://www.haskell.org/ghc/docs/latest/html/libraries/index.html, which is the only comprehensive library reference I could find. I am using some code from System.IO, supposedly from base. When I try to build this with nhc98, it doesn't know about hGetBuf, hPutBuf, or openBinaryFile from there or about mallocForeignPtrArray from the Foreign.* area. But all these look standard to me. What am I missing here? Does nhc98 really completely lack the ability to read binary data from a file? Or where should I be finding it, and how could I have known for myself that those particular ghc functions were unsupported elsewhere? -- John Goerzen Author, Foundations of Python Network Programming http://www.amazon.com/exec/obidos/tg/detail/-/1590593715

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How do I get a long iteration to run in constant space
by W M 06 Oct '04

06 Oct '04

Hi, I'm starting to learn haskell, and I was considering it for a simulation I've been designing, but I ran into this problem with a simple numerical calculation. The attached program is supposed to solve a stiff differential equation, and you have to use a very small stepsize and lots of steps to resolve the part of the solution near a singularity. It looks to me like this program should run in constant space because the iterate2 function is tail-recursive, but when I run it on hugs or ghc 2.6.1, I get an error that it runs out of stack space on the third calculation. (And the quick fix is to give it a flag that increases the stack, but if I'm to use haskell for a potentially large and long simulation, I can't afford to let it run for three days only to get an error and figure out that it needs more stack space than the computer can give.) I suppose in my ODE example it's building up expressions somewhere for lazy evaluation, so I've tried putting seq and $! in various places. I have another example where using "else seq x (iterate2 ...)" helps, but that doesn't solve my problem here. I've also tried -O but it doesn't help. Can anyone give me some idea as to what to do? Thanks, -wgm iterate2 f x n = if n == 0 then x else iterate2 f (f x) (n - 1) integrate step f xInit xFinal yInit nSteps = let h = (xFinal - xInit) / fromIntegral nSteps next = step f h in iterate2 next (xInit, yInit) nSteps rk4Next f h (x, y) = let a1 = h * f x y a2 = h * f (x + h/2) (y + a1/2) a3 = h * f (x + h/2) (y + a2/2) a4 = h * f (x + h) (y + a3) in (x + h, y + a1/6 + a2/3 + a3/3 + a4/6) rk4 = integrate rk4Next stiff x y = -( ((exp (x - 1)) + (x - 1) * y) / (1 - x - 0.0001 * y) ) main = do putStr "Begin\n" putStr (show (rk4 stiff 0 1 (exp (-1)) 1000)) putStr "\n" putStr (show (rk4 stiff 0 1 (exp (-1)) 10000)) putStr "\n" putStr (show (rk4 stiff 0 1 (exp (-1)) 100000)) putStr "\n" __________________________________ Do you Yahoo!? Yahoo! Mail - You care about security. So do we. http://promotions.yahoo.com/new_mail

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Including registered packages for GHC using Eclipse
by Geoff C 05 Oct '04

05 Oct '04

Howdy, This list says that rank amateurs can post here. One has arrived :-! I am just getting going with Haskell and Eclipse both, and cannot figure out how to include a package with Eclipse. I am on Win2kPro using GHC 6.2.1, wxHaskell 0.8, and Eclipse 3.0.1. Since I have just recently downloaded all of these, they should all be the latest of the stable versions. The package I am using is wxHaskell. I can go to the samples directory and type: ghc helloworld.hs -package wx -o test ...and I get a successful compilation with an executable named test.exe that runs just fine. In Eclipse, I have no problem generating executables for short Haskell programs from my tutorial (YAHT) that use the console for output, but it cannot find the wx package. I have rename the file HelloWorld.hs to main.hs in my attempts to compile. The module tries to import Graphics.UI.WX but fails. I tried going to Window->Preferences->'FunctionalProgramming-> Haskell->Compiler->GHCCompiler, and in the "ghc Executable" field, changing "ghc" to "ghc -package wx", but as soon as I start typing, the grayed out text box below says: java.io.IOException: CreateProcess: "ghc " --version error=2 java.io.IOException: CreateProcess: "ghc " --print-libdir error=2 ...and the compiler still cannot find module Graphics.UI.WX. (Not surprising, eh :-?) I believe I successfully added the directory "c:\program files\wxhaskell-0.8\bin" in Projects->Properties->ImportLibraries, but it didn't do anything. (I suppose that is for DLLs anyway, but I was trying everything I could think of before asking.) I removed it though, and now clicking on Add in that dialog doesn't seem to do anything. I thought I would ask, because although I have been told on the "FP for Eclipse" website that it is very much a work-in-progress, I figure that importing packages would have been implemented long before we got syntax coloring. Sorry this turned out so long, Thank you, -- Geoff C

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RE: [Haskell-cafe] Exceptions
by Simon Peyton-Jones 05 Oct '04

05 Oct '04

Actually GHC does exactly that when you compile with -prof -auto-all. Then if you run with +RTS -xc, you get a backtrace of sorts. (I have not tested this recently!) The backtrace is not yet reified into a data structure that can be examined, but that'd be quite doable if someone wanted to try. The downside is that -prof makes many optimisations work less well, and all modules must be compiled this way. Simon | -----Original Message----- | From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of MR | K P SCHUPKE | Sent: 01 October 2004 18:29 | To: alastair(a)reid-consulting-uk.ltd.uk; haskell-cafe(a)haskell.org | Cc: k.schupke(a)imperial.ac.uk; jgoerzen(a)complete.org | Subject: Re: [Haskell-cafe] Exceptions | | >But, being able to see the context in which a thunk was constructed would be | >extremely useful. | | Yes of course... I was thinking along the lines of what is possible, rather | than what is desirable. If only ghc had a -g (debug) flag like gcc that would | force each funtion to push its entry onto some kind of call stack that would | be returned by exceptions... | | Keean. | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

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Re: [Haskell-cafe] Optmiization of recursion
by Jon Cast 04 Oct '04

04 Oct '04

Fergus Henderson <fjh-mailbox-18(a)galois.com> held forth: > On 28-Sep-2004, John Goerzen <jgoerzen(a)complete.org> wrote: > > As I'm investigating Haskell, it's occured to me that most of the > > Haskell tutorials out there have omitted something that was quite > > prominent in the OCaml material I had read: making functions properly > > tail-recursive. > > > > The OCaml compiler was able to optimize tail-recursive functions such > > that they could be compiled using a loop. This would achieve two main > > benefits: performance due to not needing to allocate/deallocate stack > > frames, and the ability to work on very large lists since each recursive > > call did not consume extra stack space. <snip> > If analyzing the performance and space usage of your programs is > important, then Haskell may not be the best choice of language. I disagree. If performance and space usage are sufficiently important, Haskell may not be best. But it's really not that much easier to analyze performance or space usage under strict languages. In either case, the golden rule is: profile. Reading the source code will tell you very little. Jon Cast p.s. Of course, I know that reading the source code tells you very little about strict languages and nothing about lazy ones. But in both cases it is overwhelmingly dominated by profiling.

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Predicate logic (FOL) reasoning in Haskell
by Graham Klyne 04 Oct '04

04 Oct '04

I was recently passed this reference, which I thought was worthy of sharing here... I've not yet read it myself in detail, but at a glance it looks very readable. http://staff.science.uva.nl/~bcate/core/rwt.pdf [[ Reasoning with Tableaux Jan van Eijck CWI and ILLC, Amsterdam, Uil-OTS, Utrecht ... The paper contains the full Haskell [5] code of a free variable tableau theorem prover for predicate logic, written in `literate programming' style. ]] #g ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

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