March 2002 - Haskell - Haskell.org

first-class polymorphism beats rank-2 polymorphism
by Ralf Laemmel 06 Mar '02

06 Mar '02

Suppose you want to define a combinator like this: combinator :: (forall y. Class y => y -> y) -> (forall x. Class x => x -> x) This is a function combinator which turns a polymorphic function into another polymorphic function. As you can see, there is Class constraint. So in fact we would like to define the combinator by class overloading on the x in the result type. So how do we get our hands on the x while it is explicitly quantified by forall. Well, we try with implicit quantification: class Class x where combinator' :: (forall y. Class y => y -> y) -> x -> x So far so, good: We still need to define combinator in terms of combinator'. What is the trick to do this? Sounds like a homework question? Well, I know how to do it for first-class polymorphism. So let's restart the example. We define a function type to capture the kind of functions from above: data Bar = Foo (forall x. Class x => x -> x) Hence, the rank-2 type of combinator looks a bit different: combinator :: Bar -> Bar The class looks as follows: class Class x where combinator' :: Bar -> x -> x And here is how to define combinator in terms of combinator'. combinator f = Foo (combinator' f) This works both with GHC and hugs. In the pure rank-2 setting above, one would like to define combinator accordingly: combinator f = combinator' f But GHC 5.02.2 tells me: rank2.hs:11: Could not deduce (Class x) from the context () Probable fix: Add (Class x) to the type signature(s) for combinator arising from use of `combinator'' at rank2.hs:11 In the definition of `combinator': combinator' f This is not a bug, I guess although the probable fix proposal sends you in the wrong direction I claim. I tried a few other tricks but I can't get it done although my first-class polymorphic experience makes me think that it should be possible. Anyway, can this problem be solved? Any hint appreciated. Ralf -- Dr.-Ing. Ralf Laemmel CWI & VU, Amsterdam, The Netherlands http://www.cwi.nl/~ralf/ http://www.cs.vu.nl/~ralf/

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Icsm 2002 Call for Papers: deadline is approaching
by icsm2002.info＠unisannio.it 06 Mar '02

06 Mar '02

Dear Colleague I would like to remind you that the ICSM full paper submission deadline is approaching. ICSM (IEEE International Conference on Software Maintenance), 2002, and associated workshops: SCAM, WSE, and WESS will be held next October 2002 in Montreal, Canada. Outstanding Keynotes such as Jon Pincus, Microsoft, Industrial and experience papers, research papers, tutorials, tool expositions, dissertation forum, workshops, panels, and other exciting activities have been planned. Papers are due by 18 March 2002 You can sumbit papers following the electronic sumbission process at: http://serg.ing.unisannio.it/~icsmpapers Alternatively, you can also submit the papers via email to: icsm2002.full(a)unisannio.it If you wish to submit papers via email, you are kindly encouraged to fill the Submission Form: http://web.ing.unisannio.it/icsm2002/SubForm.txt we hope that this "Call For Papers" will be useful for your work. Please forward the following to anybody who you think may be interested. Our apologies if you received multiple copy of this. If you would like to be removed from our list please send an email to icsm2002.info(a)unisannio.it with REMOVE in the subject. ICSM2002 Giuliano Antoniol, Program CoChair Ira D. Baxter, Program CoChair ============================================================================ IEEE International Conference on Software Maintenance 2002 Sponsored by IEEE ICSM-2002, Montreal, Canada, 3-6 October 2002 http://www.icsm2002.org icsm2002.info(a)unisannio.it Call for Papers Theme: Maintaining distributed heterogeneous systems ICSM is the major international conference in the field of software and systems maintenance, evolution, and management. This year's theme elaborates on last year's theme of "systems and software evolution in the era of the internet". Standalone legacy applications of yesterday and novel applications using today's technologies are rapidly becoming integrated as part of enterprise-wide and industry-wide systems. The need for rapid integration has led to many distributed heterogeneous systems that are very challenging to maintain and evolve. ICSM 2002 will address these new scenarios and the major challenges of maintenance and evolution. The focus of the conference will be on the new challenges that heterogeneous systems pose for software maintenance, and the new opportunities for researchers and practitioners. A main goal of ICSM is to promote interaction between researchers and practitioners. ICSM 2002 will bring together researchers, practitioners, developers and users of tools, technology transfer experts, and project managers. Jon Pincus (Microsoft) will describe how Microsoft handles software enhancements. Other outstanding Keynote speakers are being arranged. Past conferences have had Parnas, Rombach, McCabe, and Jacobson. ICSM-2002 will offer technical presentations and demonstrations from academia and industry. We are particularly interested in exchanging concepts, prototypes, research ideas, and other results that could contribute to the academic arena and also benefit business and the industrial community. ICSM 2002 will be participatory, with working collaborative sessions and presentations of industry projects. The Conference will be held in conjunction with: WESS -- Workshop on Empirical Studies of Software Maintenance. SCAM -- Source Code Analysis and Manipulation WSE -- Workshop on WEBsite Evolution Topics of interest include but are not restricted to the following aspects of maintenance and evolution: - Methods and theories - Maintenance and/or productivity metrics - Organizational frameworks - Preventive maintenance - Design for maintenance - Tools and environments - Life cycle and process control - Models/methods for error prediction - User interface evolution - Commercial off-the-shelf (COTS) - Third party maintenance - Freeware and open source applications - Program comprehension - Software and system visualisation - Knowledge based systems - Measurement of software - Formal methods - Legal aspects and standards - Software reusability - Internet and distributed systems - Empirical studies - Testing and regression testing - Remote, tele-work - Version and configuration management - Processes and strategies - Management and organisation - Co-operative applications - Source code analysis and manipulation - Processes and strategies - Impact of new software practices - Programming languages - Reengineering and reverse engineering - Multimedia systems RESEARCH PAPERS: Research papers should describe original and significant work in the research and practice of software maintenance and evolution. Case studies, empirical research, and experiments are particularly welcome. We also welcome papers that present leading edge and novel ideas in maintenance. Papers must not exceed 5000 words (10 pages IEEE style) in length, in English. To encourage strong submissions a prize will be awarded for the "Best Paper as Submitted". This Prize is offered by John Wiley & Sons, the publishers of the Journal of Software Maintenance and Evolution. The Program Committee will select the winner and a formal presentation of the Prize will be made in Montreal at ICSM 2002. FAST TRACK PAPERS: Fast Track papers should describe on-going research activities, or summarize the experience in software maintenance and evolution. Papers must not exceed 4 pages, IEEE style, in length, in English. Accepted fast-track papers will be included in a dedicated section of the conference proceedings. DISSERTATION FORUM: We welcome submissions of young researchers that have delivered their dissertation (Masters or Ph.D.) in the last three years. An award will be issued for the best submission. Four page summaries of accepted dissertations will be included in the conference proceedings and a special forum section will be organised at the conference. INDUSTRIAL APPLICATIONS: Proposals for presentations of Industrial Applications are welcome. These can be state-of-the-art descriptions, experience reports and survey reports from real projects, industrial practices and models, or tool demonstrations. A dedicated sub-committee of the program committee will review Industrial Application proposals and a 1-page summary of accepted proposals will be included in the conference proceedings. TUTORIALS: Tutorials should present software maintenance and evolution topics of interest to practitioners. Tutorials may be full day or half-day in length. SUBMISSION STYLE: PDF and/or Postscript electronic submission only. Papers longer than 15 pages or 6000 words will be returned to the authors. Acceptance notification will occur July 1, 2002. IMPORTANT DATES You can sumbit papers following the electronic sumbission process at: http://serg.ing.unisannio.it/~icsmpapers Alternatively, you can also submit the papers via email to: icsm2002.full(a)unisannio.it If you wish to submit papers via email, you are kindly encouraged to fill the Submission Form: http://web.ing.unisannio.it/icsm2002/SubForm.txt Research Papers deadline: 18 March 2002 mailto:icsm2002.full@unisannio.it Fast Track Papers deadline: 1 May 2002 mailto:icsm2002.short@unisannio.it Dissertation Forum deadline: 1 May 2002 mailto:icsm2002.thesis@unisannio.it Industrial Applications deadline: 1 May 2002 mailto:icsm2002.industry@unisannio.it Tutorials deadline: 18 March 2002 mailto:icsm2002.tutorial@unisannio.it

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New Deadline: Special Journal Issue on Proof-Carrying Code
by Amy Felty 06 Mar '02

06 Mar '02

* * * * New Deadline: April 12, 2002 Call for Papers JOURNAL OF AUTOMATED REASONING Special issue on PROOF-CARRYING CODE Proof-carrying code is a technology that allows a host, or code consumer, to safely run code delivered by an untrusted code producer. The code producer delivers both a program and a formal proof that verifies that the code meets the desired safety policies. Safety policies generally express properties that prohibit the code from harmful behavior such as accessing private data, overwriting important data, accessing unauthorized resources, or consuming too many resources. Ideally, a proof is constructed automatically, and proof search may be directed using information that is passed on from the code producer's compiler. Proofs provided semi-automatically or interactively using a proof assistant are possible as well. Upon receiving the code and proof, the code consumer uses a mechanical proof checker to check the proof before executing the code. This special issue will be devoted to the topic of proof-carrying code and related approaches which use formal reasoning to enhance safety and reliability of software. Manuscripts emphasizing original results and study of tools and methods for proof generation, proof checking, and their integration with code generation/compilation are of particular interest. Manuscripts should be unpublished works and not submitted elsewhere. Revised and enhanced versions of papers published in conference proceedings that have not appeared in archival journals are eligible for submission. All submissions will be reviewed according to the usual standards of scholarship and originality. Information on JAR submissions can be found at URL http://www-unix.mcs.anl.gov/JAR/. Please format your submissions using JAR macros (available at http://www.wkap.nl/kaphtml.htm/STYLEFILES) Submissions should be sent to the guest editor in postscript format, preferably gzipped and uuencoded. In addition, please send as plain text, the title, authors, abstract, and contact information for the paper. The submission deadline is April 12, 2002. Guest Editor: Amy Felty, University of Ottawa, afelty(a)site.uottawa.ca This information is available on the web at: http://www.site.uottawa.ca/~afelty/jar-pcc/

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how to read a file's content to a string?
by Zhe Fu 06 Mar '02

06 Mar '02

I want to read a file's content into a string. content :: FilePath -> String content f = readFile f But when there is a type error: Type error in explicitly typed binding *** Term : content *** Type : FilePath -> IO String *** Does not match : FilePath -> String How can I solve it? Thanks. Zhe

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CFP: 2002 Haskell Workshop
by Manuel M. T. Chakravarty 05 Mar '02

05 Mar '02

[My apologies if you receive multiple copies of this message] ------------------------------- C A L L F O R P A P E R S ------------------------------- ACM SIGPLAN 2002 Haskell Workshop Pittsburgh, PA, USA 3rd October 2002 (as part of PLI'02) =================================== The purpose of the Haskell Workshop is to discuss experience with Haskell, and possible future development of the language. The scope of the workshop includes all aspects of the design, semantics, theory, application, implementation, and teaching of Haskell. Previous Haskell Workshops have been held in La Jolla (1995), Amsterdam (1997), Paris (1999), Montreal (2000), and Firenze (2001). For detailed information please consult http://www.cse.unsw.edu.au/~chak/hw2002/ Submission deadline: *** 24th May 2002 *** ** P L E A S E N O T E ** * If there is sufficient demand, we will try to organise facilities for system demonstrations. If you are generally interested in demonstrating an application or tool written in Haskell, please contact the PC chair <chak(a)cse.unsw.edu.au> at your earliest convenience, so that we can judge demand. * Submission and publication information changed with respect to the provisional CFP published earlier.

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Lazy Evaluation
by Nguyen Phan Dung 05 Mar '02

05 Mar '02

Hello, Sorry for asking such a silly question: Haskell is using lazy evaluation. So, it means we should be able to declare and use a list of billions elements without any trouble. So I declare my list as follow: mylist :: [Integer] mylist = [1..1000000000] In Hugs, I type mylist to print out all the elements inside. However, after printing about 22000 elements, the system crashs & outputs: "Garbage collection fails to reclaim sufficient memory" Would you please help me with this problem? Thank you very much, Phan Dung.

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Standard Library report: List union
by Jon Fairbairn 05 Mar '02

05 Mar '02

The current library report defines unionBy like this: unionBy eq xs ys = xs ++ deleteFirstsBy eq (nubBy eq ys) xs why does it take the nub of ys, but not xs? I'd have expected unionBy eq xs ys = (nubBy eq xs) ++ deleteFirstsBy eq (nubBy eq ys) xs Jón -- Jón Fairbairn Jon.Fairbairn(a)cl.cam.ac.uk 31 Chalmers Road jf(a)cl.cam.ac.uk Cambridge CB1 3SZ +44 1223 570179 (after 14:00 only, please!)

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ANNOUNCE: GCJNI, a Java Native Interface forHaskell
by Antony Courtney 04 Mar '02

04 Mar '02

I am pleased to announce release 1.1 of GCJNI. GCJNI is a library that allows Haskell to invoke Java code via the Java Native Interface (JNI). The implementation uses GreenCard to make the JNI (a C language interface) available to Haskell. GCJNI includes a few convenient features, such as: - Integration of the Haskell and Java garbage collectors, so that Java objects are garbage collected when they are no longer accessible from Haskell. - Type class based overloading, which makes it easy to pass common types (like Int, Float and String) to or from Java code. - A tool (GenBindings) which uses Java reflection on a set of compiled Java classes to generate a Haskell module with a simple, high-level, type-safe interface to the underlying Java code. GCJNI has been succesfully tested using both hugs98 and ghc under both Linux and Windows. The distribution includes a unified Makefile system and detailed release notes that makes it very easy to configure, compile and install for any supported combination of Haskell implementation and platform. More information (including pointers to the relevant distributions) is available from the GCJNI web page at: http://www.haskell.org/gcjni Bug reports and feedback should be sent to antony(a)apocalypse.org. Enjoy! -------- News: Release 1.1 (03-March-2002): * Substantial improvements to configuration and build process. Unified Makefile structure for Linux and Windows using Cygwin and GNU make. -- Antony Courtney Grad. Student, Dept. of Computer Science, Yale University antony(a)apocalypse.org http://www.apocalypse.org/pub/u/antony

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Why is this function type-correct
by Rijk J. C. van Haaften 04 Mar '02

04 Mar '02

Hello, Recently, I wrote a function similar to x :: a x = x 42 which is type-correct (Hugs, Ghc, THIH). Still, from the expression it is clear that the type shoud have a function type. The definition x :: a -> b x = x 42 is equally well accepted, though I can't see why this type would be correct. (I'd expect it to be too general.) For what reasons are these types considered correct? Thanks, Rijk-Jan

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RE: Why is this function type-correct
by Simon Marlow 04 Mar '02

04 Mar '02

> Recently, I wrote a function similar to > > x :: a > x = x 42 > > which is type-correct (Hugs, Ghc, THIH). > Still, from the expression it is clear > that the type shoud have a function type. > The definition > > x :: a -> b > x = x 42 > > is equally well accepted, though I can't > see why this type would be correct. (I'd > expect it to be too general.) In two words: polymorphic recursion. You'll find that the compiler won't be able to derive a type for 'x' in either of the two examples you gave, but x has several types the most general of which is 'forall a. a' (ie. your first example). The fact that x has type 'forall a. a' is also a useful hint as to its behaviour - the only value that has such a type is bottom. Cheers, Simon

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