August 2002 - Haskell

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24 Aug '02

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Workshop on Rule-Based Programming (RULE'02): Call for Participation
by Eelco Visser 23 Aug '02

23 Aug '02

[three Haskell papers!] CALL FOR PARTICIPATION: RULE'02: The 2002 ACM SIGPLAN WORKSHOP ON RULE-BASED PROGRAMMING October 5, 2002 Pittsburgh, Pensylvania Co-located with PLI REGISTRATION SITE: http://www.regmaster.com/pli2002.html WORKSHOP SITE: http://www.program-transformation.org/rule02 WORKSHOP OVERVIEW: The rule-based programming paradigm is characterized by the repeated, localized transformation of a shared data object such as a term, graph, proof, or constraint store. The transformations are described by rules which separate the description of the sub-object to be replaced (the pattern ) from the calculation of the replacement. Optionally, rules can have further conditions that restrict their applicability. The transformations are controlled by explicit or implicit strategies. The basic concepts of rule-based programming appear throughout computer science, from theoretical foundations to practical implementations. Term rewriting is used in semantics in order to describe the meaning of programming languages, as well as in the implementation of program transformation systems. It is used implicitly or explicitly to perform computations, e.g., in Mathematica, OBJ, or ELAN , or to perform deductions, e.g., by using inference rules to describe or implement a logic, theorem prover or constraint solver. Extreme examples of rule-based programming include the mail system in Unix which uses rules in order to rewrite mail addresses to canonical forms, or the transition rules used in model checkers. Rule-based programming is currently experiencing a renewed period of growth with the emergence of new concepts and systems that allow a better understanding and better usability. On the theoretical side, after the in-depth study of rewriting concepts during the eighties, the nineties saw the emergence of the general concepts of rewriting logic and of the rewriting calculus. On the practical side, new languages such as ASM, ASF+SDF, BURG, Claire, ELAN, Maude, and Stratego, new systems such as LRR and commercial products such as Ilog Rules and Eclipse have shown that rules are a useful programming tool. The practical application of rule-based programming prompts research into the algorithmic complexity and optimization of rule-based programs as well as into the expressivity, semantics and implementation of rules-based languages. Here, a particular focus is the use and specific ation of strategies as a high-level control flow concept for the application of the rules. The purpose of this workshop is to bring together researchers from the various communities working on rule-based programming to foster fertilisation between theory and practice, as well as to favour the growth of this programming paradigm. INVITED TALK: * BURG, IBURG, WBURG, GBURG: So Many Trees to Rewrite, So Little Time by Todd Proebstring (Microsoft Research) CONTRIBUTED TALKS: * Design Patterns for Functional Strategic Programming by Ralf Laemmel (Vrije Universiteit) and Joost Visser (CWI) * Towards Generic Refactoring by Ralf Laemmel (CWI &Vrije Universiteit) * Simple Termination of Context-Sensitive Rewriting by Bernhard Gramlich (Technische Universitaet Wien) and Salvador Lucas (Universidad Politecnica de Valencia) * On Implementing Behavioral Rewriting Grigore Rosu by (University of Illinois at Urbana-Champaign) * Pattern-matching and Rewriting Rules for Group Indexed Data-Structure by Jean-Louis Giavitto and Olivier Michel (Universited Evry) * A Rule-Based Language for Programming Software Updates by Martin Erwig and Deling Ren (Oregon State University) * Some Prolog Macros for Rule-Based Programming: Why? How? by Tim Menzies (University of West Virgina) and Lindsay Mason (University of British Columbia) * An Asynchronous Rule-Based Approach for Business Process Automation Using Obligations by Alan Abrahams, David Eyers and Jean Bacon (University of Cambridge) PROGRAM COMMITTEE: Bernd Fischer (RIACS/NASA Ames Research Center, USA) [co-chair] Eelco Visser (Universiteit Utrecht, The Netherlands) [co-chair] Mark van den Brand (CWI & Vrije Universiteit, The Netherlands) James Cordy (Queens University at Kingston, Canada) Francois Fages (INRIA Rocquencourt, France) Thom Fruehwirth (Universitaet Ulm, Germany) Claude Kirchner (LORIA & INRIA Nancy, France) Herbert Kuchen (Universitaet Muenster, Germany) Mark Minas (Universitaet Erlangen-Nuernberg, Germany) Oege de Moor (Oxford University, UK)

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More suitable data structure needed
by Dr Mark H Phillips 23 Aug '02

23 Aug '02

Hi, Consider the following data structure, effectively of type [[(Int,Int)]]: (2,5) (1,3) (2,0) (2,5) (1,2) (1,1) (1,0) (2,5) (3,1) (1,5) (2,4) (2,0) (1,5) (1,4) (1,3) (1,1) (1,0) (1,5) (1,4) (2,2) (1,0) (1,5) (1,4) (1,2) (2,1) (1,5) (2,3) (1,2) (1,0) (1,5) (2,3) (2,1) (1,5) (1,3) (2,2) (1,1) (1,5) (4,2) Notice that the some of the "inner lists" start off the same. If we delete the repetitions, we can more clearly see an emerging structure. (2,5) (1,3) (2,0) (1,2) (1,1) (1,0) (3,1) (1,5) (2,4) (2,0) (1,4) (1,3) (1,1) (1,0) (2,2) (1,0) (1,2) (2,1) (2,3) (1,2) (1,0) (2,1) (1,3) (2,2) (1,1) (4,2) I would like to represent this structure in Haskell, but am not sure quite the best way of doing it. (I am relatively new to Haskell.) I think I want to do something like: [ [(2,5),[(1,3),[(2,0)]], [(1,2),[(1,1),[(1,0)]]], [(3,1)]], [(1,5),[(2,4),[(2,0)]], [(1,4),[(1,3),[(1,1),[(1,0)]]], [(2,2),[(1,0)]], [(1,2),[(2,1)]]], [(2,3),[(1,2),[(1,0)]], [(2,1)]], [(1,3),[(2,2),[(1,1)]]], [(4,2)]] ] But what is the best way to represent this in Haskell? (Clearly I can't do exactly this, because Haskell requires all list elements to be of the same type.) Thanks, Mark.

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Java byte code library anyone?
by Sven Panne 23 Aug '02

23 Aug '02

Before I start re-inventing the wheel, it might be better to ask the people on this list first: Has anybody already written a Haskell library for Java byte code manipulation (reading/writing class files, editing byte codes, etc.)? Something in the spirit of the Byte Code Engineering Library (http://jakarta.apache.org/bcel/) or the Jikes Bytecode Toolkit (http://www.alphaworks.ibm.com/tech/jikesbt/) would be extremely nice. It's not rocket science, but quite a bit of work. Given the ubiquity of Java I'd be surprised if no one has done this before... Cheers, S.

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OT: haskell, .net, microsoft and vs
by Hal Daume III 23 Aug '02

23 Aug '02

I apologize for the OT post, but I know there are a lot of @microsoft.com people here and was hoping to see if anyone there or elsewhere had answers to a few questions... I know MS has been touting .net as cross language and cross platform and I know that at least in many of their talks they mention languages like SML, Haskell, OCaml, Mercury, etc. and being developed for .net. I have a vague idea of how the progress is going on these. What I'm more intersted in right now is this: they have an *excellent* development environment (Visual Studio) and have had it for years. But, as far as I know, it is not configurable in the sense that I really couldn't use it to develop applications in any of the languages listed above -- it supports, C/++/#, ASP, and visual basic, as far as I know. Does anyone know if either it already does, or if MS plans to support other more esoteric languages in the development environment... - Hal -- Hal Daume III "Computer science is no more about computers | hdaume(a)isi.edu than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume

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Re: Word8-Based IO
by Warrick Gray 23 Aug '02

23 Aug '02

Hi, > > sendTo :: Socket -> [Word8] -> SockAddr -> IO Int > > recvFrom :: Socket -> Int -> IO ([Word8], Int, SockAddr) > > send :: Socket -> [Word8] -> IO Int > > recv :: Socket -> Int -> IO [Word8] > >Isn't "recv" an ugly abbreviation? I like these functions. I also want to mention buffer operations, following in the tradition of hPutBuf and hGetBuf (which improved performance in the haskell-web-server...), perhaps: sendBuf :: Socket -> UArray Int Word8 -> Int -> SockAddr -> IO Int recvBuf :: Socket -> UArray Int Word8 -> Int -> IO (Int,SockAddr) ...which would allow explicit memory reuse. Incidentally I was always disappointed at not being able to use explicitly buffered Socket IO in Windows (buffered ops already availible in most Unix-like systems because of the socket<->handle mapping). Yrs, WG. _________________________________________________________________ MSN Photos is the easiest way to share and print your photos: http://photos.msn.com/support/worldwide.aspx

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23 Aug '02

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ambiguous type variables in existential classes
by Hal Daume III 22 Aug '02

22 Aug '02

Sorry for the incomprehensible subject, I couldn't think of anything better. I've tried to come up with a short example of the error I'm getting, but haven't been able to get it too short, so here's what I have: class Foo p where foo :: Eq e => p e -> e -> (forall p' . Foo p' e -> a) -> a -- note, foo is basically supposed to be: -- foo :: (Eq a , exists p' . Foo p' e) => p e -> e -> p' e -- but we can't do this directly, so we use CPS data Const e = Const instance Foo Const where foo p e q = q p newtype Wrap p e = Wrap (p e) instance (Eq e, Foo p) => Foo (Wrap p) where foo (Bar p) e q = foo p e q data DList e = DNil | forall q . Foo q => (e, q e) :> (DList e) -- a list of members of this class instance Foo DList where foo DNil e q = q Const foo ((e',p) :> ps) e q | e == e' = q (Bar p) | otherwise = foo ps e q ---- Okay, that's the code. This produces the following error in GHC: /home/hdaume/projects/NLP/Foo.hs:23: Ambiguous type variable(s) `e' in the constraint `Eq e' arising from use of `q' at /home/hdaume/projects/NLP/Foo.hs:23 In the definition of `foo': q (Bar p) line 23 is the line which reads " | e == e' = q (Bar p)". The interesting thing is that if I change the instance declaration of Foo (Bar p) to: instance Foo p => Foo (Bar p) where ...same as before... this error goes away. Certainly having the constraint on the instance declaration doesn't really *do* anything since you need to have Eq to use any of the methods, but I have no idea why it would cause this program to be ill typed. Can someone explain this? -- Hal Daume III "Computer science is no more about computers | hdaume(a)isi.edu than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume

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