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Re: [Haskell-cafe] The State Monad
by J.Garrett.Morris＠Dartmouth.EDU 08 Oct '04

08 Oct '04

--- John Goerzen wrote: tick :: Int -> State Int Int tick newval = do put newval return newval Or this: tick :: State Int Int tick = do n <- get return n That is even more incomprehensible to me -- why would removing a line before the return cause a type error? --- end of quote --- I can run both of those without a problem - could you post more of your code? It's probably somewhere else. /gXm

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IPv6?
by John Goerzen 07 Oct '04

07 Oct '04

Hi, Network.Socket in GHC6 seems to be restricted to IPv4, despite the prsence of AF_INET6. How do I support IPv6 in Haskell?

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more shootout madness -- hashes in GHC
by Robert 07 Oct '04

07 Oct '04

I've been playing around with the "Hashes, Part II" code from the shootout. I wanted to try to implement this test using Data.HashTable instead of Data.FiniteMap to see if that would buy us anything. In fact, the HashTable implementation is consistantly faster than FiniteMap, but not by a lot (thus making the transition to the IO monad not worthwhile IMO). The interesting thing, however, is that at a certain number of iterations (106 in my case), the Hashtable code segfaults. GDB shows that it is blowing the top off the program stack and dying when it tries to write to kernel space. (can't write to 0xbfffffff). My question is, why does this happen? Is it well known that sequence_ ing longish lists has this effect? It seems to me that there is no reason to consume stack for sequences of IO like this (especially using sequence_ or >> where we ONLY care about the side effects of the operation). I don't have a stong grasp of how the RTS works, perhaps someone could explain in small words? The code in question is attached. As you can see, I've tried several approaches to reduce the program stack usage, but they seem to generate very similar code. -- Robert robdockins(a)fastmail.fm

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Re: [Haskell-cafe] Re: OCaml list sees abysmal Language Shootout results
by MR K P SCHUPKE 07 Oct '04

07 Oct '04

I thought that you could use chained buffers to implement the lazy lists, that way the top level interface would still be simple. Also there should be an easy graph-rewrite operation that turns your 3 sequential operations (in the wc example) into 3 parallel ones... put these two together in the compiler and the simple coding should perform near to the complex form. Keean.

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Re: [Haskell-cafe] Re: OCaml list sees abysmal Language Shootout
by MR K P SCHUPKE 07 Oct '04

07 Oct '04

results Okay... read the license, seems whoever wrote that article on Clean didn't read the license... You can use it under LGPL, for commercial purposes... Still cant help thinking its yet-another-lazy-functional language - I thought the whole point of Haskell was to stop prolifertion of many non-standard lazy functional languages... (have to say I like the idea of basing the formal semantics on graph rewriting though...) # Keean.

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Re: [Haskell-cafe] Re: OCaml list sees abysmal Language Shootout results
by MR K P SCHUPKE 07 Oct '04

07 Oct '04

what about, quote: N.B. CleanLanguage is only available under a restrictive license. It's LGPL so long as you're not selling anything you write. Otherwise you have to purchase a commercial license. This probably means CleanLanguage will not survive long. Keean.

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RE: [Haskell-cafe] What Functions are Standard?
by Simon Peyton-Jones 07 Oct '04

07 Oct '04

John | > 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. This is the GHC library documentation, but Hugs and NHC use the same source-code repository, but omitting some functionality. What's omitted is explicitly documented, at the exact link you give above, under "Implementation-specific notes". The language definition for Haskell 98 defines a small collection of libraries, standardised a long time ago. But all implementations come with a much richer set of libraries, many of which are written in Haskell 98, and almost all of which are usable by Haskell 98 programs. These are the ones documented above. Meanwhile lots of people are writing libraries too. In particular, there are several libraries around for reading and writing binary files, and a long recent email thread on the subject. What I'm hoping is that someone, or a small group, will decide on a good interface (in the light of experience thus far) and implement it, so that the rest of us don't have to choose. Simon

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Can I determin the function name passed in?
by Mark Carter 07 Oct '04

07 Oct '04

I'm an utter noob to Haskell, and FP in gnereal. I would like to create a function taking an argument which is a function (actually an equation taking no args, but dependent on other equations), which prints the name of the function and and it's result. Is there a way of doing this in Haskell? Sounds like a job for macros, or some kind of introspection-type stuff.

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Re: [Haskell-cafe] Re: OCaml list sees abysmal Language Shootout results
by MR K P SCHUPKE 07 Oct '04

07 Oct '04

Well, the sootout itself is using GHC, as it lists both language and implementation. There is no entry for NHC or Hugs... So I would say if the GHC specific code is neat and fast put it in... If people hant to see a NHC then there should be a separate implentation of haskell (for example I can see at least 3 schemes in the sootout) Keean.

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Re: [Haskell-cafe] Re: OCaml list sees abysmal Language Shootout results
by MR K P SCHUPKE 07 Oct '04

07 Oct '04

looking at ghc's scores... this would seem to be the next candidate for a rework: -- $Id: reversefile.ghc.html,v 1.6 2004/10/03 00:44:58 bfulgham Exp $ -- http://www.bagley.org/~doug/shootout/ -- from Julian Assange main = interact $ unlines . reverse . lines Compare this with the gcc implementation which uses buffers and is 68 lines long! Keean.

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Re: [Haskell-cafe] Re: OCaml list sees abysmal Language Shootout results
by MR K P SCHUPKE 07 Oct '04

07 Oct '04

Also, its nice to see GHC ahead of Java now in the standard results... Would be nice to beat OCaml & Clean though ... Keean.

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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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Re: [Haskell-cafe] Exceptions
by Dominic Steinitz 02 Oct '04

02 Oct '04

I'd be cautious about using exceptions too liberally. When I implemented a Haskell equivalent to traceroute, I wanted to stop sending packets when a certain condition occurred. It was very tempting to throw an exception and catch it at the top level. However, I think the code below is easier for the maintainer (me) to understand. Dominic. sequenceWhile_ :: Monad m => (a -> Bool) -> [m a] -> m () sequenceWhile_ p [] = return () sequenceWhile_ p (x:xs) = x >>= \c -> if (p c) then sequenceWhile_ p xs else return ()

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Compiling GHC for AIX5.1L
by John Goerzen 01 Oct '04

01 Oct '04

Hello, Following the directions at http://www.haskell.org/ghc/docs/6.2.1/html/building/sec-porting-ghc.html#UN…... I have a Debian unstable system on i386 as the host machine and an IBM PowerPC system as the target. I have configured the files as specified in the docs. I am to the "make boot && make" phase in the ghc directory on the (Linux) host. All goes well until: ==fptools== make all -r; in /home/jgoerzen/no-backup/programs/ghc-6.2.1/ghc/rts ------------------------------------------------------------------------ ../../ghc/compiler/ghc-inplace -optc-O -optc-Wall -optc-W -optc-Wstrict-prototypes -optc-Wmissing-prototypes -optc-Wmissing-declarations -optc-Winline -optc-Waggregate-return -optc-Wbad-function-cast -optc-I../includes -optc-I. -optc-Iparallel -optc-DCOMPILING_RTS -optc-fomit-frame-pointer -H16m -O -O2 -static -c Adjustor.c -o Adjustor.o /tmp/ghc10917.s: Assembler messages: /tmp/ghc10917.s:54: Error: no such instruction: `dcbf 0,%eax' /tmp/ghc10917.s:55: Error: no such instruction: `sync' /tmp/ghc10917.s:56: Error: no such instruction: `icbi 0,%eax' /tmp/ghc10917.s:63: Error: no such instruction: `sync' /tmp/ghc10917.s:64: Error: no such instruction: `isync' make[1]: *** [Adjustor.o] Error 1 make: *** [all] Error 1 Why it's trying to use PowerPC assembler on an x86 host I don't know. (I assume that's what's going on here; but I don't really know.) What is the problem?

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Re: [Haskell-cafe] Exceptions
by Niklas Broberg 01 Oct '04

01 Oct '04

Alastair Reid wrote: > > 3. Can I define my own exception types? > >Sadly, no. There is only one 'exception type'. > >You can use dynamics and encode new kinds of exceptions as Strings > but that isn't very satisfactory. But not at all, allowing you to declare your own exception types is *exactly* what using dynamic exceptions is all about. No need for String encodings, we can do far better than that. =) If I have my own type of exceptions, by making that type an instance of Typeable I can use the functions 'throwDyn' and 'catchDyn' instead of the normal 'throw' and 'catch'. That way I can declare exception handlers that only exceptions of a particular type, and just re-raises any other. I could for instance write main = do ... a bunch of computations ... `catchDyn` (\e :: MyExceptionType1 -> ... handle exceptions of MyExceptionType1 ...) `catchDyn` (\e :: MyExceptionType2 -> ... handle exceptions of MyExceptionType2 ...) `catch` (\e -> ... handle exceptions of the built-in Exceptions type ...) (Note that the type annotations on the patterns are not needed if the types can be inferred properly from within the handler. Note also that the Typeable constraint can be automatically derived by ghc with -fglasgow-exts, so no difficulty there either) Check the section about dynamic exceptions at http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control.Exceptio… for more info. > > 4. Can I write code that can catch and handle multiple different > > exception types from a single block of code? > >You use standard Haskell pattern matching so, yes, you can catch multiple >kinds of exceptions. They're not different types though. But yes they are, see above. Or rather, they can be, or they can be constructors of a common user-defined type, combining common pattern matching with dynamic matching on types. =) > > 5. Is there anything different about working with exceptions in monads? > >Use raiseIO instead of raise to raise exceptions. The functions for raising exceptions are called 'throw' and 'throwIO' (and 'throwDyn'), not raise. /Niklas _________________________________________________________________ The new MSN 8: advanced junk mail protection and 2 months FREE* http://join.msn.com/?page=features/junkmail

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Re: [Haskell-cafe] Exceptions
by MR K P SCHUPKE 01 Oct '04

01 Oct '04

>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.

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Re: [Haskell-cafe] Exceptions
by MR K P SCHUPKE 01 Oct '04

01 Oct '04

>1. Can exceptions be used in "pure" functions (outside of monads?) Exceptions can be thrown from pure functions, and can be used exactly like in the OCaml example (but in Haskell syntax) Exceptions can only be caught in the IO monad. >2. How are these exceptions caught and handled aside from using bracket? bracket, catch, catchIO >3. Can I define my own exception types? There is a User exception type, I don't think you can define your own completely new exception type. >4. Can I write code that can catch and handle multiple different >exception types from a single block of code? Yes but see (1) only in the IO monad >5. Is there anything different about working with exceptions in monads? No. >6. Can I get a stack trace from ghc or hugs if an exception is never >caught and thus causes the program to terminate? Stack traces in lazy laguages are not very useful. Best work-around is to produce an explicit call trace by inserting prints into functions... obviously this requires the functions to be in the IOMonad. Things are still odd because of lazyness though. Keean.

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Exceptions
by John Goerzen 01 Oct '04

01 Oct '04

One of the most important features of a modern language is, to me, exceptions. I have found very little coverage of exceptions in Haskell, what what coverage there was seemed to be centered on I/O. One reason I'm asking is this: what to do when pattern matching is incomplete due to an error. For instance, this is a common pattern in my OCaml code: let queuedir_of_type t = match t with "foo" -> "/data/queue5" | "bar" -> "/data/queue3" | _ -> raise (Exception ("Invalid type " ^ t));; This is often useful when "t" is something that was originally read from a user. Python has an expressive exception mechanism, where one can catch various different exceptions from a single block of code, etc. So, I have some questions about exceptions in Haskell: 1. Can exceptions be used in "pure" functions (outside of monads?) 2. How are these exceptions caught and handled aside from using bracket? 3. Can I define my own exception types? 4. Can I write code that can catch and handle multiple different exception types from a single block of code? 5. Is there anything different about working with exceptions in monads? 6. Can I get a stack trace from ghc or hugs if an exception is never caught and thus causes the program to terminate? Thanks! -- John

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Re: [Haskell-cafe] OCaml list sees abysmal Language Shootout results]
by Greg Buchholz 30 Sep '04

30 Sep '04

Sam Mason wrote: > > You probably want some strictness annotations in there. . . Now we're getting somewhere. When I replace the tuples with my own (strict) data structure, it gets about 7.5 times faster than the original shootout example (or about 24 times faster than the version with tuples). I get another 2x speedup when I pass '+RTS -G1' to the executable. So the version below is about 15 times faster than the original using the 3MB data file from the shootout. (Now we're only about 40x slower than ocaml). Don't forget to turn on '-funbox-strict-fields' for an additional improvement. -- Compile with... -- ghc -O2 -ddump-simpl -fvia-c -funbox-strict-fields -o wc_fold2 wc_fold2.hs -- execute as... ./wc_fold2 +RTS -G1 <input.txt import IO main = do file <- getContents putStrLn $ show (foldl wc (St 0 0 0) file) data Stuple = St !Int !Int !Int deriving Show wc (St l w c) '\n' = St (l+1) w (c+1) wc (St l w c) ' ' = St l (w+1) (c+1) wc (St l w c) x = St l w (c+1) ...It still seems like it's using a lot of memory (or at least doing a lot of garbage collecting). But it it still vastly better than before. Is there any way to reduce this more? (60,000,000 bytes divided by 3,000,000 chars = 20 bytes per char). Here's the memory usage report... 61,387,752 bytes allocated in the heap 11,837,148 bytes copied during GC 99,780 bytes maximum residency (234 sample(s)) 234 collections in generation 0 ( 0.11s) 1 Mb total memory in use INIT time 0.00s ( 0.00s elapsed) MUT time 0.15s ( 0.15s elapsed) GC time 0.11s ( 0.14s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 0.26s ( 0.29s elapsed) %GC time 42.3% (48.3% elapsed) Alloc rate 409,251,680 bytes per MUT second Productivity 57.7% of total user, 51.7% of total elapsed Greg Buchholz

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RE: Simple example using Parsec (was: [Haskell-cafe] Rethinking O O idioms)
by Bayley, Alistair 30 Sep '04

30 Sep '04

Sure. Learning Parsec took me a few hours, whereas John's parsing task might be done in much less time, if it's simple enough. OTOH, there's value in being able to use Parsec, so it's a good excuse to learn. > -----Original Message----- > From: Graham Klyne [mailto:GK@ninebynine.org] > Sent: 30 September 2004 10:34 > To: Bayley, Alistair; 'John Goerzen'; haskell-cafe(a)haskell.org > Subject: Simple example using Parsec (was: [Haskell-cafe] > Rethinking OO idioms) > > At 09:01 30/09/04 +0100, Bayley, Alistair wrote: > >Ben suggested using Parsec for reading the config file. > Parsec is a fine > >library, but there is a learning curve, and you might find > it quicker to do > >the parsing yourself, if it's very simple. Your call. > > Hmmm... Parsec *can* be simple too. Here's one I did earlier: > http://www.ninebynine.org/Software/HaskellUtils/RegexParser.hs > > #g ----------------------------------------- ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

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Simple example using Parsec (was: [Haskell-cafe] Rethinking OO idioms)
by Graham Klyne 30 Sep '04

30 Sep '04

At 09:01 30/09/04 +0100, Bayley, Alistair wrote: >Ben suggested using Parsec for reading the config file. Parsec is a fine >library, but there is a learning curve, and you might find it quicker to do >the parsing yourself, if it's very simple. Your call. Hmmm... Parsec *can* be simple too. Here's one I did earlier: http://www.ninebynine.org/Software/HaskellUtils/RegexParser.hs #g ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

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Seeking reference(s) relating to FP performance
by Graham Klyne 30 Sep '04

30 Sep '04

I've taken it as an article of faith that performance of FP language implementations has been improving quite steadily over the past few years. I'd like to assert this, but I can't find any clear evidence to support such an assertion. I note that the "about Haskell" page makes a similar assertion, but it doesn't offer any hint of supporting evidence: [[ Aren't functional programs very slow? They used to be, but the compilers have now caught up. Haskell programs run fast enough for all but the most performance-demanding applications. ]] -- http://www.haskell.org/aboutHaskell.html I'm looking for a reference -- informal will be enough -- that can give an perspective of progress in functional language implementation performance. I'm not looking for a single benchmark that shows a case of blindingly-fast functional code, but a pointer to trends of improving performance. It would also serve my purpose to have indications based on languages other than Haskell (e.g. ML and friends). Any ideas, please? #g ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

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RE: [Haskell-cafe] Rethinking OO idioms
by Bayley, Alistair 30 Sep '04

30 Sep '04

There are three pieces to this story: - read a config file into some structure - query/modify elements of that structure - write structure out to a file Create a data structure (analogous to a class with no methods) and functions to query/manipulate that structure. Nested FiniteMaps might be a good, simple, first attempt. Unlike the functions to read and write the file (which must be in the IO monad), the functions to query and manipulate the structure can (and probably should) be non-monadic. Ben suggested using Parsec for reading the config file. Parsec is a fine library, but there is a learning curve, and you might find it quicker to do the parsing yourself, if it's very simple. Your call. > ... Haskell has no mutable variables. A call like > config.setOption("main", "initpath", "/usr") in Python -- which alters > the state of the config object and returns nothing -- would be > impossible in Haskell (unless perhaps the FiniteMaps are mutable > somehow?) When you "modify" a structure, you return a new copy of it, where the bit that you wanted to change is replaced by the new value. For example, this is what happens when you call addToFM/delFromFM for FiniteMaps. This isn't necessarily as expensive as it sounds, as there's a lot of data sharing going on in Haskell programs, due to the fact that everything /is/ immutable. If it does turn out to be too expensive, then you could look at using a state monad. But assuming you need one immediately looks like premature optimisation to me. Alistair. ----------------------------------------- ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

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Rethinking OO idioms
by John Goerzen 30 Sep '04

30 Sep '04

I've worked with languages with object-oriented features for awhile now. Python and OCaml, the two with which I work the most, both have OO. One of my first projects in Haskell would be to write a Haskell version of Python's ConfigParser[1] class. I wrote[2],[3] a version of this for OCaml that works very well. In a nutshell, ConfigParser is a utility for working with sectioned configuration files in a style similar to the familiar .ini files from Windows. It has methods to read a configuration file, get/set the items that are being configured, and write a new file back out. This, then, is a fairly typical metaphor for OO programs: an instance of a class has some state that can be accessed or modified, and possibly stored and retrieved. So I am thinking about a ConfigParser for Haskell. The first thing that occured to me is that Haskell has no OO features, so I'm not sure what is the best way to handle the "class" and its various methods. The next thing that occured to me is that, unlike OCaml and Python classes, Haskell has no mutable variables. A call like config.setOption("main", "initpath", "/usr") in Python -- which alters the state of the config object and returns nothing -- would be impossible in Haskell (unless perhaps the FiniteMaps are mutable somehow?) I guess I'm having trouble translating this common OO language paradigm into the Haskell world. Thanks for any insight. -- John BTW, if I get a working ConfigParser for Haskell, I will publish it under the GPL like all the rest of my code. [1] http://www.python.org/doc/current/lib/RawConfigParser-objects.html [2] http://gopher.quux.org:70/devel/missinglib/html/ConfigParser.html [3] http://gopher.quux.org:70/devel/missinglib/html/ConfigParser.rawConfigParse…

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Re: OCaml list sees abysmal Language Shootout results
by Aaron Denney 29 Sep '04

29 Sep '04

On 2004-09-29, Tomasz Zielonka <t.zielonka(a)students.mimuw.edu.pl> wrote: > I remember SPJ's motto: ,,Avoid success at all cost''. Is this motto > still valid? Maybe. The existing user-base is being used as an argument for not "doing the right thing" with regards to binary-IO, strings, localization and text IO. -- Aaron Denney -><-

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Question about function on data type
by Peter Robinson 29 Sep '04

29 Sep '04

Hello! My question concerns a general term datatype: data Term = Not Term | Term :&&: Term | Term :||: Term | Literal Char Is it somehow possible to write a generic function that applies the associativity rules on a "Term" (by using pattern matching) and works with both data constructors or is it necessary to write one for :&&: and :||: ? Something like: assoc :: Term -> Term assoc ((t1 `op` t2) `op` t3) = .... -- this doesn't work Regards, Peter

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Strings - why [Char] is not nice
by Einar Karttunen 29 Sep '04

29 Sep '04

Hello Strings in haskell seem to be one major source of problems. I try to outline some of the problems I have faced and possible solutions. Size Handling large amounts of text as haskell strings is currently not possible as the representation (list of chars) is very inefficient. Serialization Most of the time when serializing a string, we want to handle it as an array of Word8. With lists one has to account for cycles and infinite length which make the encoding much more verbose and slow. The problem is not that some strings may not be trivially serializable, but rather that it is hard to find the easy cases. Typeclass instances It is currently hard to define typeclass instances for strings as String ( = [Char]) overlaps with [a]. Implementations provide solutions for this, but there should not be a need for workarounds in the first place. Show/Read The current Show/Read implementation makes it impossible to use with large strings. A read implementation needs to traverse the file looking for the terminating '"' and handling escape codes. A better solution would be to have an efficient (size prefixed) representation, maybe in a separate Serializable typeclass. But this would need the ablity to derive Serializable instances for algebraic datatypes automatically to avoid lots of useless code. Possible solutions The optimal solution should be compact and fast. A list of chunks is one solution - it would make it possible to e.g. mmap files (modulo encoding) and support fast concatenation. In addition one would need to support infinite and cyclic structures. Adding an alternative which corresponds to the current string abstraction would be sufficient. type CharT = Word8 data Str = S [(Ptr CharT, Int)] | I [CharT] A major problem is breaking old code. The main problem is that many functions are defined only on lists which forces strings to be lists. Making the functions polymorphic would solve a lot of problems and not only for Strings. There should be no performance penalty (at least in theory) when the compiler knows which instance is used at compile time. - Einar Karttunen

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k-th floorRoot
by fool＠sdf-eu.org 29 Sep '04

29 Sep '04

Hi. > floorRootDef, floorRoot :: Integer -> Integer -> Integer > > floorRootDef k n | k>=1 && n>=1 = last (takeWhile (\x->x^k<=n) [1..]) > > floorRoot k n | k>=1 && n>=1 = h n where > h x = let y=((k-1)*x+n`div`x^(k-1))`div`k in if y<x then h y else x (Oddly enough, you get an iteration formula for the _floor_ of the k-th root just by putting _floor_ brackets around every quotient in Newton's iteration formula for the k-th root.) Exercise: Transform floorRootDef into floorRoot by extensional-equality-preserving steps. I don't know how to do this. To conventionally prove the correctness of floorRoot, it suffices to show for all integers k>=1, n>=1, x>=1, y:=[((k-1)*x+[n/x^(k-1)])/k], r:=[n^(1/k)] ([] denoting floor): (1) r<=y (so r<=x is preserved in the iteration) and (2) x<=y --> x<=r (so r<=x<=r when the break condition is met). Using a<=[b] <--> a<=b for all integers a and reals b, (2) is straightforward and (1) is equivalent to k*r-(k-1)*x <= n/x^(k-1), which by r^k<=n can be sharpened to (3) k*(r-x)+x <= r^k/x^(k-1), which is true by induction over k if (3) implies k*(r-x)+x + r-x <= r^k/x^(k-1) * r/x, which indeed by (3) can be sharpened to k*(r-x)+r <= (k*(r-x)+x)*r/x, equivalently (multiply by x and collect terms) 0 <= k*(r-x)^2, done. -- fool(a)sdf-eu.org SDF-EU Public Access UNIX System - http://sdf-eu.org

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Silly I/O question
by John Goerzen 29 Sep '04

29 Sep '04

I'm trying to write a program that will copy an arbitrarily large text file to a destination, and duplicate it 100 times. Thus: ./myprog < Input > Output would be the same as running: cat Input > Output # once cat Input >> Output # 99 times My first attempt was this: import IO main = disp 100 disp 0 = return () disp n = do c <- getContents putStr c hSeek stdin AbsoluteSeek 0 disp (n-1) That failed, though, because getContents closes the file after it's been completely read (ugh -- why?). So then I tried to work on various options around hGetLine and hPutStrLn. But I couldn't figure out a way to make this either properly tail-recursive while handling the exception, or to avoid polling for EOF each time through the function. I also don't want to store the entire file in memory -- the idea is to seek back to the beginning for each iteration. I'm assuming that stdin is a seekable fd. I checked the wiki for a pattern here but didn't see any. Suggestions?

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GHC and OS X (10.4)
by Tom Davie 28 Sep '04

28 Sep '04

Hi, I've been attempting to use GHC on a beta copy of Mac OS X 10.4, I've been attmepting to use the panther version of the install package, but have hit a problem with tinkering with it - I get the following error when I attempt to run ghc: Verenia:~/Documents/Development/XBridgeAI tatd100$ ghc dyld: Library not found: HaskellSupport.framework/Versions/A/HaskellSupport Referenced from: /usr/local/lib/ghc-6.2.1/ghc-6.2.1 Reason: file not found Trace/BPT trap The framework is present in /Library/Frameworks, and /Library/Frameworks is in dyld's framework search path. Any Mac/Haskell gurus able to help I would much appreciate it. Thanks Tom Davie

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Optmiization of recursion
by John Goerzen 28 Sep '04

28 Sep '04

Hello, 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. So I have several questions about Haskell: 1. Do Haskell interpreters/compilers perform similar optimizations? 2. If the answer to 1 is No, then am I correct in saying that my ability to handle very large lists in recursive functions is limited by the maximum stack size on my system? 3. Does it make a difference for optimization purposes whether a particular recursive function is tail-recursive? 4. Is a reference available documenting which Prelude/standard functions are properly tail-recursive (and thus suitable for very large lists) and which are not? (OCaml system docs generally specify when a function is not tail-recursive... for instance, one of foldr/foldl is and the other isn't.) 5. Are there any examples/patterns anywhere that illustrate standard tail-recursion practice in Haskell? Thanks!

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Re: [Haskell-cafe] Re: Strings - why [Char] is not nice
by MR K P SCHUPKE 28 Sep '04

28 Sep '04

Can this not be handled in a nicer way by the compiler? What if the compiler tried to allocate the lists in chunks? For example when dealing with strings, why cant the compiler allocate the string as a fixed length unit, terminated with a link to the next unit. (IE allow the atoms in the list to be variable size). In this way functions which return blocks of data (for example reading a file in 8k buffers) can be treated just like reading a normal list. This would not only save memory, but would make IO more efficient, whilst retaining a Haskell-like style of coding. Keean.

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Re: [Haskell-cafe] mutually recursive modules
by Henning Thielemann 28 Sep '04

28 Sep '04

On Fri, 24 Sep 2004, Malcolm Wallace wrote: > Hugs doesn't support mutually-recursive modules at all. Ghc and nhc98 > support them only if you hand-write a .hi-boot file to bootstrap the > compilation. I would guess that better support from the mainstream > implementations is unlikely, because it would be a large amount of > effort for a situation which occurs only very rarely, and for which > there is a relatively easy workaround. Namely? The only workaround I know of is putting all type declarations and functions that depend on each other into one module, import them in the modules they logically belong to and document that the objects should be imported from the redirecting modules rather than from the modules they are defined in. I know of two projects where different tasks had to be merged into one module because of the lack of mutually recursive imports, namely functionalMetaPost and Haskore. It's interesting how other languages solve this problem. In Modula-3 it is solved by explicit module interfaces and partial revelation. One can define type T in module A and type T in module B very abstractly as pointers to something and the complete data structures (which may contain references to either interface) are usually revealed in the implementations of the modules. I'm curious how Oberon solves it, because it doesn't need explicit interfaces but derives them from the implementation files. Probably one can say that Oberon extracts something like a .hiboot file from every module file automatically. Why can't GHC and Hugs go this way?

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job ads I saw on DICE
by Christopher Milton 25 Sep '04

25 Sep '04

It is rare to see ads looking for Haskell programming experience... http://seeker.dice.com/jobsearch/servlet/JobSearch?op=1002&dockey=xml/6/2/6… http://seeker.dice.com/jobsearch/servlet/JobSearch?op=1002&dockey=xml/b/8/b…

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Re: [Haskell-cafe] mutually recursive modules
by Malcolm Wallace 24 Sep '04

24 Sep '04

Henning Thielemann <iakd0(a)clusterf.urz.uni-halle.de> writes: > As far as can see neither Hugs or GHC really support them. Is this still > on the to-do list or is it almost dropped due to implementation > difficulties? Hugs doesn't support mutually-recursive modules at all. Ghc and nhc98 support them only if you hand-write a .hi-boot file to bootstrap the compilation. I would guess that better support from the mainstream implementations is unlikely, because it would be a large amount of effort for a situation which occurs only very rarely, and for which there is a relatively easy workaround. Some lesser-known Haskell implementations do fully support mutually-recursive modules however. hhc (formerly Freja, from Henrik Nilsson) allows multiple modules to be stored in a single file, and hence mutual recursion can be achieved by ensuring all the recursive modules are compiled simultaneously from the same file. The PacSoft Haskell system (from the Programatica project at OGI) also fully supports mutually recursive modules, and I believe they can even be in separate files. Regards, Malcolm

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mutually recursive modules
by Henning Thielemann 24 Sep '04

24 Sep '04

As far as can see neither Hugs or GHC really support them. Is this still on the to-do list or is it almost dropped due to implementation difficulties?

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Job
by Mark Carroll 23 Sep '04

23 Sep '04

The company I'm involved with - Aetion, a tiny defense contractor in Columbus, Ohio - is now looking for an affordable Haskell programmer to hire. So, on the offchance that any of you guys are interested, or know of someone who might be, feel free to e-mail me for more information or to supply your resume. Unfortunately, the nature of the work is such that US citizenship or a security clearance would help, and telecommuting isn't really an option. On the upside, the work should be varied and interesting - more challenging than mundane. Given that fact, we are more interested in aptitude than specific experience. -- Mark

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The implementation of functional languages
by mattr 21 Sep '04

21 Sep '04

I am working thru "Implementing Functional Languages: a tutorial" and I want to look up one of the references to get more details. I am looking for the book "The implementation of Functional Programming languages" by S. L. Peyton Jones. Anyone know how I can get it? I would be willing to pay for a second hand copy if anyone has one. Matt

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no dynamic binding
by Andrew Harris 21 Sep '04

21 Sep '04

Hi - I have another question. I am still working on a soccer server and thought it would be neat to create command objects that had a "toString" method. Then, I was going to keep a list of these command objects and at the right time stringify them and send them to the server. So I created a class with a toString method: class ServerCommandClass a where toString :: a -> String And then a few instances: -- dash command data DashCommand = DashCommand { dashpower :: Double } instance ServerCommandClass DashCommand where toString c = "(dash " ++ show (dashpower c) ++ ")\n" -- move command data MoveCommand = MoveCommand { x :: Double, y :: Double } instance ServerCommandClass MoveCommand where toString c = "(move " ++ show (x c) ++ " " ++ show (y c) ++ ")\n" The problem is, I am not quite sure how to describe a *list* of command objects where the list could have both DashCommands and MoveCommands in it. Ideally the list could contain both, and then for each item in the list I could call the toString method. I was reading Simon Thompson's Haskell: The Craft of Functional Programming and I read that Haskell 98 does not support dynamic binding, which (it seems) is what I'm trying to do. Does anyone have a suggestion on an alternative approach? thanks, -andrew

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Re: [Haskell-cafe] Strings - why [Char] is not nice
by MR K P SCHUPKE 20 Sep '04

20 Sep '04

Would it not bet better is String were a typeclass instead of a data type. Then the string operations could be redefined by implementation, so you may have strings as lists or strings as arrays? Keean.

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Re: [Haskell-cafe] no dynamic binding
by Abraham Egnor 20 Sep '04

20 Sep '04

You can use exisential types to do what you'd like. From memory, so there are probably errors: newtype ServerCommand = forall a. ServerCommandClass a => ServerCommand a instance ServerCommandClass ServerCommand where toString (ServerCommand c) = toString c commands :: [ServerCommand] commands = [ServerCommand $ DashCommand ..., ServerCommand $ MoveCommand ...] Also, for a less type-safe approach, see Data.Dynamic. On Sun, 19 Sep 2004 13:48:53 -0400, Andrew Harris <andrew.unit(a)gmail.com> wrote: > Hi - > > I have another question. I am still working on a soccer server and > thought it would be neat to create command objects that had a > "toString" method. Then, I was going to keep a list of these command > objects and at the right time stringify them and send them to the > server. So I created a class with a toString method: > > class ServerCommandClass a where > toString :: a -> String > > And then a few instances: > > -- dash command > data DashCommand = > DashCommand { dashpower :: Double } > > instance ServerCommandClass DashCommand where > toString c = "(dash " ++ show (dashpower c) ++ ")\n" > > -- move command > data MoveCommand = > MoveCommand { x :: Double, > y :: Double } > > instance ServerCommandClass MoveCommand where > toString c = "(move " ++ show (x c) ++ " " ++ show (y c) ++ ")\n" > > The problem is, I am not quite sure how to describe a *list* of > command objects where the list could have both DashCommands and > MoveCommands in it. Ideally the list could contain both, and then for > each item in the list I could call the toString method. > > I was reading Simon Thompson's Haskell: The Craft of Functional > Programming and I read that Haskell 98 does not support dynamic > binding, which (it seems) is what I'm trying to do. Does anyone have > a suggestion on an alternative approach? > > thanks, > -andrew > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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