November 2004 - Haskell

RE: [Haskell] Better Exception Handling
by Bayley, Alistair 23 Nov '04

23 Nov '04

> From: John Goerzen [mailto:jgoerzen@complete.org] > > My other choice is to use Dynamic for my > exceptions, but that makes it even more > difficult to catch and handle Here's how I create "custom" exceptions; it doesn't seem onerous to me, but then I have a high tolerance for some kinds of coding pain: > data SqliteException = SqliteException Int String > deriving (Typeable) > catchSqlite :: IO a -> (SqliteException -> IO a) -> IO a > catchSqlite = catchDyn > throwSqlite :: SqliteException -> a > throwSqlite = throwDyn > Python can work that way, but also adds another feature: > > try: > blah > moreblah > finally: > foo And in Haskell we have catch(Dyn), bracket, and finally. Are these not enough? > Haskell's exception catching doesn't really have less > functionality than OCaml (some might argue it has more), > but it takes a lot more effort to compose, needing > to provide a function that returns a function in many cases. Does it? I'm not convinced... I think it's no more verbose than any other exception-handling mechanism, but maybe there's some cognitive overhead in translating an OO exception-handling idiom into Haskell. All I've ever used is catch and bracket, and I find them fairly straightforward. Could you post some code which you think would be clearer with an Ocaml or Python exception handling style? > The other annoying thing is forcing it to run in the IO monad. Note that you can throw exceptions from anywhere. They just have to be *caught* in the IO monad. 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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Exceptions
by Johannes Waldmann 23 Nov '04

23 Nov '04

in the following example, the handler won't catch the exception because of lazy evaluation. therefore, it's a different story than with exceptions in ML, Python, whatever strict language. main = do xs <- return [ 1, 2, error "throw" ] `catch` \ any -> do putStrLn "caught" return [ 4, 5, 6 ] print xs -- -- Johannes Waldmann, Tel/Fax: (0341) 3076 6479 / 6480 -- ------ http://www.imn.htwk-leipzig.de/~waldmann/ ---------

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RE: [Haskell] Re: Better Exception Handling
by Bayley, Alistair 23 Nov '04

23 Nov '04

> And in Haskell we have catch(Dyn), bracket, and finally. > > Are these not enough? > > We also have Control.Exception.try. :-) > > Peter Yes. Control.Exception.try is defined in terms of Control.Exception.catch: http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control.Exception .html#v%3Atry ... and I think catch is more similar to the Python-style try. ----------------------------------------- ***************************************************************** 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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RE: [Haskell] Re: Top Level TWI's again was Re: Re: Parameterized Show
by Simon Peyton-Jones 23 Nov '04

23 Nov '04

Can I suggest that this thread, interesting as it is, might usefully migrate to haskell-café. (Apart from anything else, there's another similar thread running there, and it makes sense to keep together.) Posting a summary back to the Haskell list, in due course, would be great. Simon | -----Original Message----- | From: haskell-bounces(a)haskell.org [mailto:haskell-bounces@haskell.org] On Behalf Of Benjamin | Franksen | Sent: 23 November 2004 13:21 | To: haskell(a)haskell.org | Subject: Re: [Haskell] Re: Top Level TWI's again was Re: Re: Parameterized Show | | On Tuesday 23 November 2004 00:10, Aaron Denney wrote: | > On 2004-11-22, Benjamin Franksen <benjamin.franksen(a)bessy.de> wrote: | > > On Monday 22 November 2004 09:38, Adrian Hey wrote: | > >> You have yet to | > >> explain how you propose to deal with stdout etc.. | > > | > > I see absolutely no reason why stdxxx must or should be top-level mutable | > > objects. They can and should be treated in the same way as environment | > > and command line arguments, i.e. | > > | > > getArgs :: IO [String] | > > getEnv :: String -> IO String | > > getStdin, getStdout, getStderr :: IO Handle | > > | > > Note that (just like environment and command line arguments) these | > > handles may refer to completely different things on different program | > > runs. | > | > Er, no. The handles can be considered as the same but _pointing_ to | > different things on different runs. | | I wrote "may refer to", not "are", so yes. | | > Keeping them outside the IO monad, | > and only accessing them inside -- i.e. the current situation -- would be | > fine. | | I beg to differ. Note, I do not claim they are unsafe. | | > They're not mutable in any sense. | | Well, a variable in C is not mutable in exactly the same sense: It always | refers (="points") to the same piece of memory, whatever value was written to | it. Where does that lead us? | | Ben | -- | Top level things with identity are evil. -- Lennart Augustsson | _______________________________________________ | Haskell mailing list | Haskell(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell

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PADL 2005: Call for participation
by dcabeza＠fi.upm.es 23 Nov '04

23 Nov '04

---------------------------------------------------------------------- Apologies for multiple copies ---------------------------------------------------------------------- CALL FOR PARTICIPATION Seventh International Symposium on Practical Aspects of Declarative Languages 2005 (PADL 05) http://www.unm.edu/~herme/padl05/ Long Beach, California, USA January 10-11, 2005 Co-located with ACM POPL, January 12-14, 2005 Early registration deadline: December 15th Declarative languages build on sound theoretical foundations to provide attractive frameworks for application development. These languages have been successfully applied to a wide array of different real-world situations, including database management, active networks, software engineering, decision support systems, or music composition. New developments in theory and implementation have opened up new application areas. At the same time, the application of declarative languages to novel problems raises numerous interesting research issues. Well-known questions include designing for scalability, language extensions for application deployment, and programming environments. Thus, applications often drive the progress in the theory and implementation of declarative systems, and benefit from this progress as well. PADL is a forum for researchers and practioners to present original work emphasizing novel applications and implementation techniques for all forms of declarative concepts, including, functional, logic, constraints, etc. Topics of interest include: * Innovative applications of declarative languages. * Declarative domain-specific languages and applications. * Practical applications of theoretical results. * New language developments and their impact on applications. * Evaluation of implementation techniques on practical applications. * Novel implementation techniques relevant to applications. * Novel uses of declarative languages in the classroom. * Practical experiences. PADL 05 welcomes new ideas and approaches pertaining to applications and implementation of declarative languages, and is not limited to the scope of the first six PADL symposia (past proceedings can be found in Springer Verlag LNCS, numbers 1551, 1753, 1990, 2257, 2562, and 3057). In this occasion PADL is co-located, as traditionally, with ACM POPL, which will be held immediately following PADL, January 12-14. INVITED SPEAKERS Norman Ramsey Harvard University Saumya Debray The University of Arizona ACCEPTED PAPERS Role-based Declarative Synchronization for Reconfigurable Systems Vlad Tanasescu, Pawel Wojciechowski Functional Framework for Sound Synthesis Jerzy Karczmarczuk Improved Fusion for Optimizing Generics Sjaak Smetsers, Artem Alimarine Solving Constraints on Sets of Spatial Objects Jesus M. Almendros-Jimenez, Antonio Corral A Full Pattern-based Paradigm for XML Query Processing Benzaken Véronique, Castagna Giuseppe, Miachon Cédric The Program Inverter LRinv and its Structure Masahiko Kawabe, Robert Glueck Discovery of Minimal Unsatisfiable Subsets of Constraints Using Hitting Set Dualization James Bailey, Peter Stuckey A Provably Correct Compiler for Efficient Model Checking of Mobile Processes Ping Yang, Yifei Dong, C.R. Ramakrishnan, Scott A. Smolka An Ordered Logic Program Solver Davy Van Nieuwenborgh, Stijn Heymans, Dirk Vermeir Type Class Directives Bastiaan Heeren, Jurriaan Hage Specializing Narrowing for Timetable Generation: A Case Study Nadia Brauner, Rachid Echahed, Gerd Finke, Hanns Gregor, Frederic Prost Solving Collaborative Fuzzy Agents Problems with CLP(FD) Susana Munoz-Hernandez, Jose Manuel Gomez-Perez Character-Based Cladistics and Answer Set Programming Daniel Brooks, Esra Erdem, James Minett, Donald Ringe Towards a More Practical Hybrid Probabilistic Logic Programming Framework Emad Saad, Enrico Pontelli Safe Programming with Pointers through Stateful Views Dengping Zhu, Hongwei Xi Improving Memory usage in the BEAM Ricardo Lopes, Vitor Santos-Costa Towards Provably Correct Code Generation via Horn Logical Continuation Semantics Qian Wang, Gopal Gupta, Michael Leuschel CONTACTS Please visit the conference web site: http://www.unm.edu/~herme/padl05/ for further information. For additional information about the conference please contact the General Chair: Gopal Gupta Department of Computer Science University at Texas at Dallas, Dallas, TX, U.S.A. gupta(a)utdallas.edu SPONSORED BY: ACM SIGPLAN (applied for), COMPULOG Americas (http://www.cs.nmsu.edu/~complog) The Association for Logic Programming (http://www.cs.kuleuven.ac.be/~dtai/projects/ALP/)

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Call for Papers - ICTAC05
by Bernhard K. Aichernig 23 Nov '04

23 Nov '04

Call for Papers - ICTAC05 INTERNATIONAL COLLOQUIUM ON THEORETICAL ASPECTS OF COMPUTING Hanoi, Vietnam - 17--21 October, 2005 http://www.iist.unu.edu/ictac05 ===================== BACKGROUND AND OBJECTIVES ICTAC is an International Colloquium on Theoretical Aspects of Computing founded by the International Institute for Software Technology of the United Nations University (UNU-IIST). The aim of the colloquium is to bring together practitioners and researchers from academia, industry and government to present research results, and exchange experience, ideas, and solutions for their problems in theoretical aspects of computing. We believe that this will help developing countries to strengthen their research, teaching and development in computer science and engineering, improve the links between developing countries and developed countries, and establish collaboration in research and education. The first ICTAC (ICTAC'04) was held in Guiyang, China. ICTAC'05 will have a technical program for five days including two days for tutorials and three days for a conference, and a training school for 5 days. The topics of the conference include, but are not limited to: - automata theory and formal languages - principles and semantics of programming languages - logics and their applications - software architectures and their description languages - software specification, refinement, and verification - model checking and theorem proving - formal techniques in software testing - models of object and component systems - coordination and feature interaction - integration of formal and engineering methods - service-oriented development - document-driven development - models of concurrency, security, and mobility - theory of parallel, distributed, and internet-based (grid) computing - real-time and embedded systems - type and category theory in computer science SPONSORS AND ORGANISATION ICTAC'05 will be organised jointly between the Institute of Information Technology of the Vietnamese Academy of Sciences and Technology (IoIT), the University of Technology of the Vietnam National University in Hanoi (UoT-VNU) and UNU-IIST. UNU-IIST, IoIT and UoT-VNU are also sponsors of ICTAC'05. There will be an one-week training school during 10--14 October 2005 before the conference. SUBMISSION AND PUBLICATION Submissions to the conference must not have been published or be concurrently considered for publication elsewhere. All submissions will be judged on the basis of originality, contribution to the field, technical and presentation quality, and relevance to the workshop. Papers should be written in English and not exceed 15 pages in LNCS format (see http://www.springer.de/comp/lncs/authors.html for details). The proceedings will be published by Springer in the Lecture Notes in Computer Science series by Springer-Verlag. A web site for uploading papers linked to the conference web page http://www.iist.unu.edu/ictac05 will be open at the appropriate time. All queries should be sent to: dvh(a)iist.unu.edu, or wirsing(a)informatik.uni-muenchen.de. IMPORTANT DATES - Submission deadline: 25 May 2005 - Acceptance notification: 1 July 2005 - Final version due: 25 July 2005 - Tutorial proposal submission deadline: 11 July 2005 - Tutorial proposal acceptance notification: 25 July 2005 - Tutorials: 17-18 October, 2005 - Conference dates: 19 -- 21 October, 2005 ADVISORY COMMITTEE Dines Bjorner, Singapore Manfred Broy, Germany Jifeng He, UNU-IIST Mathai Joseph, India Shaoying Liu, Japan Zhiming Liu, UNU-IIST Jim Woodcock, UK Jose Luiz Fiadeiro, UK Tobias Nipkow, Germany PUBLICITY CHAIR Bernhard K. Aichernig, UNU-IIST ORGANISING COMMITTEE Le Hai Khoi, IoIT (co-chair) Ho Si Dam, UoT-VNU (co-chair) Nguyen Tue, UoT-VNU Bui The Duy, UoT-VNU Le Quoc Hung, IoIT Do Nang Toan, IoIT Ngo Quoc Tao, IoIT PROGRAM COMMITTEE Marc Aiguier, France Keijiro Araki, Japan J.O.A. Ayeni, Nigeria Jay Bagga, USA Hubert Baumeister, Germany Michel Bidoit, France Jonathan Bowen, UK Victor A. Braberman, Argentina Cristian S. Calude, New Zealand Ana Cavalcanti, UK Yifeng Chen, UK Dang Van Hung, UNU-IIST (co-chair) Jim Davies, UK Janos Demetrovics, Hungary Jin Song Dong, Singapore Henning Dierks, Germany Do Long Van, Vietnam Marcelo F. Frias, Argentina Wan Fokkink, Netherlands Susanna Graf, France Valentin Goranko, South Africa Dimitar Guelev, Bulgaria Michael R. Hansen, Denmark Jozef Hooman, Netherlands Purush Iyer, USA Ryszard Janicki, Canada Takuya Katayama, Japan Maciej Koutny, UK Xuandong Li, China Antonia Lopes, Portugal Antoni Mazurkiewicz, Poland Hrushikesha Mohanty, India Ngo Quang Hung, USA Nguyen Cat Ho, Vietnam Paritosh Pandya, India Jean-Eric Pin, France Narjes Ben Rajeb, Tunisia R. Ramanujam, India. Anders P. Ravn, Denmark Gianna Reggio, Italy Wolfgang Reif, Germany Riadh Robbana, Tunisia Mark Ryan, UK Zaidi Sahnoun, Algeria Augusto Sampaio, Brazil Don Sannella, UK Bernhard Schaetz, Germany Andrea Maggiolo-Schettini, Italy Carolyn Talcott, USA P. S. Thiagarajan, Singapore Ji Wang, China Martin Wirsing, Germany (co-chair) Mingsheng Ying, China Jian Zhang, China Hongjun Zheng, USA -- Bernhard Aichernig, Research Fellow of UNU-IIST www.iist.unu.edu/~bka ---------------------------------------------------------------------- SEFM 2005: 3rd IEEE International Conference on Software Engineering and Formal Methods, Koblenz, Germany, 7.- 9. Sept. 2005

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HTK
by George Russell 23 Nov '04

23 Nov '04

> I don't know if HTk is still maintained. Yes it is! It's time I put some more binary bundles up though.

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Closed Projections on HLists?
by oleg＠pobox.com 21 Nov '04

21 Nov '04

Jared Warren wrote: > I'm doing some work with heterogeneous sets as provided by the HList > library <http://homepages.cwi.nl/~ralf/HList/>. My code uses > projections of sets internally and I keep running into the open-world > assumption when I ask the type checker to infer the result of > projections. For example (in ghci): > > *> hProject hNil > No instance for (HProject HNil l') > > ...When obviously the only valid projection from the empty set is the > empty set. > > So what I'm wondering: is it possible to rewrite HProject in such a > way as to allow the type checker to infer that the only possible > projections from a list are its (non-proper) sublists? If the inference is unique, then of course, we _can_ infer that the empty list can only be projected onto the empty list. However any non-empty list can be projected in several ways. How do we infer this ambiguous result? Either we have to specify which projection result we want, and the typechecker will tell us if that is possible. Or we obtain the powerset of a list -- and then enumerate it. The current HProject is written with the first approach in mind: > class HProject l l' > where > hProject :: l -> l' As we can see, there are no functional dependencies on the class -- as there can't be, in general. A list can, in general, be projected in many ways (and given a projection, there can be many lists it can be obtained from). So, we have to be explicit: which list is to project into what. In general, that is the best one can do, right? There is however, a special case: the empty list. The empty list can only be projected one way -- and, so we would like the typechecker to infer that (rather then wait for us to supply the result and then check it). If that behavior is desirable, it can be obtained. The key is the ability to specify functional dependencies on a per-instance basis. Here's the complete code > {-# OPTIONS -fglasgow-exts #-} > {-# OPTIONS -fallow-undecidable-instances #-} > -- Look, Ma! No overlapping instances! > > module Foo where > > import CommonMain hiding (HDeleteMany, hDeleteMany, TypeCast, > typeCast, HProject, hProject) > import TypeCastGeneric2 > import TypeEqTTypeable > import TypeEqBoolTTypeable > import TTypeable > class HProject l l' > where > hProject :: l -> l' > > instance TypeCast HNil l' => HProject HNil l' where > hProject _ = typeCast HNil > instance HProject (HCons a r) HNil > where > hProject l = HNil > instance ( HList l', HOccurs' e (HCons a r), HProject (HCons a r) l') > => HProject (HCons a r) (HCons e l') > where > hProject l = HCons (hOccurs' l) (hProject l) > > list1 = (HCons 'a' (HCons True HNil)) > > > test1 = hProject list1 :: HNil > test2 = hProject list1 :: (HCons Char HNil) > test3 = hProject list1 :: (HCons Bool HNil) > > -- NB: No explicit type of the result given. The typechecker _does_ > -- infer it > test4 = hProject HNil

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implicit parameters and the paper "prepose.pdf"
by John Velman 21 Nov '04

21 Nov '04

In a recent message to this list (msg15410) Oleg referenced a paper comparing implicit parameters and implicit configurations with url http://www.eecs.harvard.edu/~ccshan/prepose/prepose.pdf . I'd like to read this, (and examine the companion literate haskell file prepose.lhs), but www.eecs.harvard.edu rejects my connection. Nor can I find it anywhere else. Is this paper still available somewhere, or is it possible for someone to send me a copy? Best John Velman velman_at_cox.net

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WRS05: First Call For Papers
by Roberto Di Cosmo 20 Nov '04

20 Nov '04

******************************************************************************** 5th Int'l Workshop on Reduction Strategies in Rewriting and Programming WRS'05 Nara, Japan, April 22nd, 2005 First Call For Submissions ******************************************************************************** Reduction strategies in rewriting and programming have attracted an increasing attention within the last years. New types of reduction strategies have been invented and investigated, and new results on rewriting / computation under particular strategies have been obtained. Research in this field ranges from primarily theoretical questions about reduction strategies to very practical application and implementation issues. The need for a deeper understanding of reduction strategies in rewriting and programming, both in theory and practice, is obvious, since they bridge the gap between unrestricted general rewriting (computation) and (more deterministic) rewriting with particular strategies (programming). Moreover, reduction strategies provide a natural way to go from operational principles (e.g., graph and term rewriting, narrowing, lambda-calculus) and semantics (e.g., normalization, computation of values, infinitary normalization, head-normalization) to implementations of programming languages. Therefore any progress in this area is likely to be of interest not only to the rewriting community, but also to neighbouring fields like functional programming, functional-logic programming, and termination proofs of algorithms. The workshop wants to provide a forum for the presentation and discussion of new ideas and results, recent developments, new research directions, as well as of surveys on existing knowledge in this area. Furthermore we aim at fostering interaction and exchange between researchers and students actively working on such topics. Information on the previous, and forthcoming, WRS editions is available from the WRS permanent homepage. TOPICS Topics of interest include, but are not restricted to: * theoretical foundations for the definition and semantic description of reduction strategies * strategies in different frameworks (term rewriting, graph rewriting, infinitary rewriting, lambda calculi, higher order rewriting and explicit substitutions, conditional rewriting, rewriting with built-ins, narrowing, constraint solving, etc.) and their application in (equational, functional, functional-logic) programming (languages) * properties of reduction strategies / computations under strategies (e.g., completeness, computability, decidability, complexity, optimality, (hyper-)normalization, cofinality, fairness, perpetuality, context-freeness, neededness, laziness, eagerness, strictness) * interrelations, combinations and applications of reduction under different strategies (e.g., equivalence conditions for fundamental properties like termination and confluence, applications in modularity analysis, connections between strategies of different frameworks, etc.) * program analysis and other semantics-based optimization techniques dealing with reduction strategies * rewrite systems / tools / implementations with flexible / programmable strategies as essential concept / ingredient * specification of reduction strategies in (real) languages * data structures and implementation techniques for reduction strategies SUBMISSIONS We solicit papers on all aspects of reduction strategies in rewriting and programming. Submissions should describe unpublished work, except for survey papers which are explicitly welcome, too. Submissions should not exceed 10 pages (however, survey papers may be longer). Please, send a message containing the title, authors, and abstract of your submission before January 21, 2005 to the PC co-chairs at wrs05(a)pps.jussieu.fr The full version of your submission should be sent in postscript or PDF format before January 31, 2005. Submissions should include the title, authors' names, affiliations, addresses, and e-mail. Selection of papers by the PC will be based on originality, significance, and correctness. Final versions will be due by March 18, 2005. PUBLICATION Accepted papers will be included in the preliminary workshop proceedings that will be available at the workshop. Selected papers will be published in the Electronic Notes in Theoretical Computer Science (ENTCS) series of Elsevier. IMPORTANT DATES January 21, 2005 Deadline for electronic submission of abstracts January 31, 2005 Deadline for electronic submission of papers February 25, 2005 Notification of acceptance of papers March 18, 2005 Deadline for final versions of accepted papers April 22, 2005 Workshop INVITED SPEAKERS * Kokichi Futatsugi (JAIST, Japan) Additional invited speakers will be announced on the website. PROGRAM COMMITTEE * Sergio Antoy (Portland) * Eduardo Bonelli (Stevens) * Roberto Di Cosmo (Paris VII, Co-Chair) * Bernhard Gramlich (Wien) * Stefano Guerrini (Roma) * Salvador Lucas (Valencia) * Aart Middeldorp (Innsbruck) * Yoshihito Toyama (Tohoku, Co-Chair) PROGRAM CO-CHAIRS Roberto Di Cosmo Laboratoire PPS - Universite Paris 7, France Email: roberto(a)dicosmo.org Yoshihito Toyama Tohoku University, Japan Email: toyama(a)nue.riec.tohoku.ac.jp FURTHER INFORMATION * Call for papers in Text, PostScript and PDF format * WRS permanent website: http://www.dsic.upv.es/~wrs * WRS 2005 e-mail address: wrs05(a)pps.jussieu.fr * RDP 2005 website: http://www.kurims.kyoto-u.ac.jp/rdp05/

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