April 2004 - Haskell - Haskell.org

[CFP] 2004 Scheme Workshop
by oscar waddell 15 Apr '04

15 Apr '04

The 2004 Scheme Workshop will be held with ICFP this September in Snowbird, Utah. Plans for this one-day event include presentations of technical and experience papers as well as panel discussions. It should be a useful and fun day for anyone with an interest in Scheme or functional programming languages. Please take a moment to read the "call for papers" appended below and consider submitting a paper. The submission deadline is June 9, 2004. See you in Utah, -Oscar Waddell workshop chair ======================================================================= CALL FOR PAPERS 2004 Scheme Workshop http://www.cs.indiana.edu/scheme2004 Snowbird, Utah, USA 22 September 2004 The workshop will be held in conjunction with ICFP 2004. http://www.cs.indiana.edu/icfp04/ ---------------------------------------------------------------------- Important dates Submission deadline June 9, 2004 Author notification July 23, 2004 Final paper due August 23, 2004 Workshop September 22, 2004 ---------------------------------------------------------------------- Purpose The 2004 Scheme Workshop is a forum for discussing experience with and future development of the Scheme programming language. The scope of the workshop includes all aspects of the design, implementation, theory, and application of Scheme. We encourage everyone interested in Scheme to participate. The workshop has been accepted by the ICFP'04 workshop committee; formal approval by the SIGPLAN executive committee is pending. ---------------------------------------------------------------------- Scope We invite submissions for both technical and experience papers. Topics of interest for Technical Papers include (but are not limited to): * Design Language critiques and extensions, concurrency and distribution, components and composition, language embedding, object systems, exception handling, syntactic abstraction, module systems and libraries, multiparadigm programming, scripting. * Implementation Compilers, interpreters, runtime systems, virtual machines, resource management, program analysis and transformation, partial evaluation, compile-time and run-time optimization, foreign function and operating system interfaces, embedded systems. * Development tools Profilers, tracers, debuggers, program development environments, program understanding tools, performance and conformance test suites. * Theory Formal semantics, correctness of analyses and transformations, lambda calculus, continuations. Topics of interest for Experience Papers include (but are not limited to): * Applications Domain-specific languages, graphical user interfaces, web programming, network applications, multimedia programming, systems programming, symbolic computing, large systems, use of Scheme as a scripting language. * Practice and experience Experience with Scheme in education and industry. * Scheme pearls Elegant, instructive uses of Scheme. Following the model of ICFP 2004, experience papers need not necessarily report original research results; they may instead report practical experience that will be useful to others, re-usable programming idioms, or elegant new ways of approaching a problem. The key criterion for such a paper is that it makes a contribution from which other practitioners can benefit. It is not enough simply to describe a program! System Demonstrations. Authors of both technical and experience papers are invited to describe, in an appendix, proposals for system demonstrations to be given during the workshop. Panel Discussions. We also invite proposals for panel discussions on topics of interest to the Scheme community. Authors of accepted proposals will give a brief presentation on the topic and then moderate the discussion that follows. ---------------------------------------------------------------------- Submission guidelines Authors should submit a 100-200 word abstract and a full paper to the program chair via e-mail by the end of Wednesday, June 9, Universal Coordinated Time. (The end of the day UTC corresponds to 8:00 PM EDT, 6:00 PM MDT, and 5:00 PM PDT.) Papers must be submitted in either PDF format or as PostScript documents that are interpretable by Ghostscript. Papers must be printable on US Letter sized paper. Submissions should be typeset in 10 point font on 12 point baseline in two columns 20pc (3.33in) wide and 54pc (9in) tall with a column gutter of 2pc (0.33in). Submissions should be be no more than 15 pages including text, figures, and bibliography. Authors wishing to supply additional material to the reviewers beyond the 15 page limit may do so in clearly marked appendices, on the understanding that reviewers are not required to read the appendices. Suitable LaTeX class files may be downloaded from the workshop web site. (This is not the standard ACM conference style; it is a larger format adopted by POPL'05 to be easier on reviewer's eyes.) We anticipate that experience papers will be shorter, generally around 10 pages in length. Submitted papers must have content that has not previously been published in other conferences or refereed venues. Simultaneous submission to other conferences or refereed venues is unacceptable. Each paper should explain its contributions in both general and technical terms, clearly identifying what has been accomplished, saying why it is significant, and comparing it with previous work. Authors should strive to make the technical content of their papers understandable to a broad audience. Submissions that do not meet these guidelines will not be considered. ------------------------------------------------------------------------------- Organizers Workshop chair: Oscar Waddell Computer Science Department Indiana University Bloomington, IN 47405-7104 owaddell(a)cs.indiana.edu Program committee: J. Michael Ashley (Beckman Coulter, Inc.) Danny Dubé (Université Laval) Robert Bruce Findler (University of Chicago) Richard Kelsey (Ember Corporation) Julia Lawall (University of Copenhagen) Michael Sperber (DeinProgramm) Steering committee: William D. Clinger (Northeastern University) Marc Feeley (University of Montreal) Matthias Felleisen (Northeastern University) Matthew Flatt (University of Utah) Dan Friedman (Indiana University) Christian Queinnec (University Paris 6) Manuel Serrano (INRIA) Olin Shivers (Georgia Institute of Technology) Mitchell Wand (Northeastern University)

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simulating dependent types; ghc/ghci discrepancy
by John D. Barnett 14 Apr '04

14 Apr '04

Hello all- I'm still playing with simulating dependent types using rank-2 polymorphism, and I've run into a small stumbling block on unwrapping existential datatypes: they can't be let-bound (monomorphic restriction?), but can be unwrapped in a case or lambda. Actually, I find that assignment in do-notation works nicely (since I'm already writing monadic code). As an example: module DepTest where data Show' = forall a . Show a => Show' a print' :: Show' -> IO () print' x = do (Show' x') <- return x print x' -- here, the type of x' is known test :: IO () test = print' (Show' True) This works fine. My problem is that I'd like to do this interactively, in ghci. *DepTest> :t Show' True Show' True :: Show' *DepTest> Show' x <- return $ Show' True *DepTest> :t x x :: forall a. a *DepTest> x getTcTyVar a {- tv aXJ -} getTcTyVar a {- tv aXJ -} getTcTyVar a {- tv aXJ -} getTcTyVar a {- tv aXJ -} getTcTyVar a {- tv aXJ -} getTcTyVar a {- tv aXJ -} Interrupted. First, the type of x seems like it should be either Bool or (forall a. Show a => a), but not (forall a.a). Second, egads, what did I do? Is it possible to unwrap an existential type in ghci? Thanks- John

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ANNOUNCE: HSQL 1.4 released
by Krasimir Angelov 14 Apr '04

14 Apr '04

I am pleased to announce the HSQL library version 1.4. The HSQL is a simple library, which provides interface to multiple databases. MySQL, PostgreSQL, ODBC and SQLite are currently supported. What is new? ~~~~~~~~~~~~ 1. The ODBC interface provides the "driverConnect" function which is an alternative to "connect" but allows to use a connection string instead of DSN, UserID and Pasword 2. The HSQL for Windows now supports MySQL and PostgreSQL databases. 3. A binary distributions are provided. There is an installation program for Windows-binary distribution and there are binary RPMs for Linux. All binaries are compiled for ghc-6.2.1 and Hugs Nov2003. 4. Some bugfixes Known bugs ~~~~~~~~~~ There is a bug in ghc-pkg from ghc-6.2.1. If you want to build libraries for GHCi you need to have ld.exe somewhere in the search path. The easiest way to workaround this is to copy ld.exe from ${GHC_INSTALL_DIR}/gcc-lib subdirectory to ${GHC_INSTALL_DIR}/bin before to install HSQL from binary under Windows. Credits ~~~~~~~ Great thanks to Victor Blomqvist and Conny Andersson for their work on binary distributions. Cheers, Krasimir __________________________________ Do you Yahoo!? Yahoo! Tax Center - File online by April 15th http://taxes.yahoo.com/filing.html

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ANNOUNCE: Happy version 1.14
by Simon Marlow 14 Apr '04

14 Apr '04

A new version of Happy, the parser generator for Haskell, has been released. Version 1.14 is a small increment relative to 1.13, whose purpose is mainly to introduce one new feature which will be reuqired to compile future versions of GHC. >From the release notes: * New meta-veriable $> represents the rightmost token. * Happy's OPTIONS pragma is merged with an existing one in the grammar file, if any. Sources, binaries and documentation are on Happy's web page: http://www.haskell.org/happy/ Cheers, Simon

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Call for Contributions - HC&A Report (May 2004 edition)
by Arthur van Leeuwen 14 Apr '04

14 Apr '04

Dear Haskellers, two changes of season and one of editor later, we strongly suspect the Haskell community has also moved along admirably. Thus comes the need to refresh our list of all things Haskell, and give *you* the opportunity to tell all the world about your passion. We invite your contributions to the imminent sixth edition of the ================================================================ Haskell Communities & Activities Report http://www.haskell.org/communities/ Submission deadline: 7 May 2004 (please send your contributions to hcar(a)haskell.org, in plain ASCII or LaTeX format) ================================================================ The Haskell Communities & Activities Reports biannually report on the state of Haskell over the last 6 months, and sometimes on plans for the next 6 months. If you have only recently joined the Haskell world, browse the November 2003 edition -- you will find an overview of what's going on as well as starting points and links that may provide many answers. We would much like you to send your contributions in by the first week of May, and will then endeavor to publish the collective report roughly a month before the start of summer. Note that this is a great opportunity to gauge the state of your research, update your web pages, release new versions, announce entirely new projects, summarize developments in sub-communities for every Haskeller to see, etc. :) The way we like to do things is have you update all existing summaries, as the purpose of these reports is to show *recent/current* activities. We will drop any topics that haven't had any activity since May 2003, so if you haven't updated your topic since then, now is a good time! Ofcourse, if you have new things to share, so much the better. We would love to hear of it, and include it in the report. Please keep your contributions short though, a couple hundred words maximum, as we would like to keep the size of the report reasonable. It is intended as an overview after all. Looking forward to your contributions, Arthur (interim editor, May 2004 edition) ----------------------------------- topics New suggestions for current hot topics, activities, projects, etc. are welcome - especially with names and addresses of potential contacts, but here is a non-exclusive list of likely topics (see also http://www.haskell.org/communities/topics.html ): General Haskell developments; Haskell implementations; Haskell extensions; Standardization and documentation; Haskell tutorials, howtos and wikis; Organisation of Haskell tool and library development; Haskell-related projects and publications; new research, fancy tools, long-awaited libraries, cool applications; Feedback from specialist mailing lists to the Haskell community as a whole; Haskell announcements; all (recent) things Haskell Announcements: if you've announced anything new on the Haskell list over the last six months, you'll want to make sure that is reflected in this edition! Project pings: if you're maintaining a Haskell tool or library or somesuch, you'll want to let everyone know that it is still alive and actively maintained, even if there have been no new additions, but all the more if there have been new developments. Tutorials: if you've fought with some previously undocumented corner of Haskell, and have been kind enough to write down how you did manage to build that graphical user interface, or if you' ve written a tutorial about some useful programming techniques, this is your opportunity to spread the word (short, topic-specific, and hands-on tutorials that only show how to achieve a certain practical task would do a lot to make things easier for new Haskellers - please write some!) Applications: if you've been working quietly, using Haskell for some interesting project or application (commercial or otherwise), you might want to let others know about what you're using Haskell for, and about your experiences using the existing tools and libraries; are you using Haskell on your job? An interesting thread about using Haskell and more generally functional programming for non-Haskell things seems to recur with reasonable frequency - why not write a sentence or two about your use of Haskell for our report? Feedback: if you're on one of the many specialist Haskell mailing lists, you'll want to report on whatever progress has been made there (GUI API discussions, library organisation, etc.) If you're unsure whether a contact for your area of work has come forward yet, have a look at the report's potential topics page, or get in touch with me. I've contacted last time's contributors, hoping they will volunteer to provide updates of their reports, and will update the contacts on the topics page fairly regularly. But where you don't yet see contacts listed for your own subject of interest, you are very welcome to volunteer, or to remind your local community/project team/mailing list/research group/etc. that they really ought to get their act together and let the Haskell community as a whole know about what they've been doing! :-) -------------------------- what should I write? That depends on your topic, but as a general rule, it shouldn't take you long. A simple sentence or two about your use of Haskell could go into the "Individual Haskellers and their projects" section. If you're a company, or if you're working on a project using Haskell as the implementation language, a paragraph on that could go into the "Commercial Applications" and "Non-Commercial Applications" sections. A typical summary report about a tool/library/project/application/.. would be between 1 and 3 paragraphs of ASCII text (what's it about? major topics and results since the last report? current hot topics? major goals for the next six months?) plus pointers to material for further reading (typically to a home page, or to mailing list archives, specifications and drafts, implementations, meetings, minutes,..). For those who prefer templates to fill in, the report is edited in LaTeX, and an entry template might look something like this: \subsubsection{<MYSTUFF>} \report{<MY NAME}\\ <WHAT IS IT?> <WHAT IS ITS STATUS? / WHAT HAS HAPPENED SINCE LAST TIME?> <CAN OTHERS GET IT?> <WHAT ARE THE IMMEDIATE PLANS?> <FURTHER READING> \url{<PROJECT URL>} Browsing through previous editions should give you a good idea of the variety of possibilities, ranging from very brief to extensive. -- Haskell Communities and Activities Report (May 2004 edition) All contributions are due in by the 7th of May 2004! http://www.haskell.org/communities/

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Re: concurrent haskell, higher-order types and parameterizing by typeclass
by oleg＠pobox.com 14 Apr '04

14 Apr '04

> data Showable = forall a. Show a => Showable a > > writer :: Chan Showable -> IO () > writer ch = mapM_ (writeChan ch) [Showable 42, Showable pi, Showable > "hello", Showable 'c'] > > printer :: Chan Showable -> IO () > printer ch = getChanContents ch >>= mapM_ (\(Showable a) -> print a) > > However, this solution requires a new wrapper datatype (or at least a > new constructor) to be defined for every typeclass to be used in > Chan-based communication; furthermore, all of the datatypes will be > identical except for the name of the typeclass. It seems like I > should be able to create a type that's parameterized by typeclass, > i.e. something like: > data Wrapper c = forall a. c a => Wrapper a > writer :: Chan (Wrapper Show) -> IO () We can do the following: > {-# OPTIONS -fglasgow-exts #-} > > import Data.IORef > > data W r = forall a. W a (a->r) > > -- emulate the channel > type Chan a = IORef [W a] > > writeChan ch x = do > c <- readIORef ch > writeIORef ch (x:c) > > getChanContents ch = readIORef ch > > -- Channel is parameterized by the answer type! > writer :: Chan String -> IO () > writer ch = mapM_ (writeChan ch) > [W 42 show, W pi show, W "hello" show, W 'c' show] > > printer :: Chan String -> IO () > printer ch = getChanContents ch >>= mapM_ (\(W a h) -> putStrLn $ h a) > > main = do > ch <- newIORef [] > writer ch > printer ch That is, pack the handler along with the data. The channel is parameterized by the answer type of the handler. Indeed, class Show means that we can apply the function 'show' to any value of the type in that class -- and get a string. If we use the Showable wrapper as in the original code, applying show is _the only thing_ we can do with that value. As we're interested in the answer type anyway (because there is nothing else we can do with the value), we can parameterize the class by the answer type. If we can perform several operations on the value, we can parameterize the channel by a tuple of all answer types. A cynic might remark that all this exercise with existentials is for nothing: Indeed, "W x show" is precisely equivalent to just "show x". Because of the non-strictedness, we can apply (or partially apply) all the handlers to the value in question and pass these applications through the channel. Only the needed ones would be evaluated.

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Announce: Haskell ACID Relational DBMS v0.1
by S. Alexander Jacobson 13 Apr '04

13 Apr '04

In thinking about a data storage model for a web app I wanted to develop, and in finding Haskell so concise and expressive, I wondered if one could write a relational DBMS in Haskell in under 1000 lines of code. The answer appears to be yes! Features: * non-destructive-update Haskell DBMS (can use a relational database without escaping to the IO monad!) * supports user defined types * supports user defined relations and functions * command pattern structure for write-ahead logging * Inner,Outer,Left,Right joins on arbitrary (user-defined) relations (not just "=") * in-memory/in-process means no disk/marshalling overhead Risks include: * functions/aggregates not yet implemented e.g. (a<b+c) or (a<max(b)) * no performance testing -- joins expensive! * no proof of correctness * written by non-academic new haskell developer * Not SQL. No Sockets. -- should be part of the app wrapper used to maintain consistency! * License: GPL Note: I am an expert neither in Haskell, nor in relational databases, nor in relational algebra/set theory/category theory. So comments/suggestions/recommendations on any aspect of this code are welcome. Comment on Haskell: WOW!!!!!! I basically wrote this without testing just thinking about my program in terms of transformations between types. I wrote the test/example code at the end and had almost no implementation errors in the code! The compiler/type-system is really really good at preventing you from making coding mistakes! I've never in my life had a block of code this big work on the first try!!! I am WAYYY impressed. Note: Code working means doing what I think it will do. I might be wrong about relational theory, but that is a separate class of problem. -Alex- _________________________________________________________________ S. Alexander Jacobson mailto:me@alexjacobson.com tel:917-770-6565 http://alexjacobson.com

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[Haskell] concurrent haskell, higher-order types and parameterizing by typeclass
by Abraham Egnor 13 Apr '04

13 Apr '04

I have a system composed of threads communicating via channels; I'd like the communicated types to be instances of a particular typeclass (say Show for purposes of discussion), but not all the same type. Since Chan is paramterized by the communicated datatype, the best way I've found to do this is via a wrapper type: data Showable = forall a. Show a => Showable a writer :: Chan Showable -> IO () writer ch = mapM_ (writeChan ch) [Showable 42, Showable pi, Showable "hello", Showable 'c'] printer :: Chan Showable -> IO () printer ch = getChanContents ch >>= mapM_ (\(Showable a) -> print a) However, this solution requires a new wrapper datatype (or at least a new constructor) to be defined for every typeclass to be used in Chan-based communication; furthermore, all of the datatypes will be identical except for the name of the typeclass. It seems like I should be able to create a type that's parameterized by typeclass, i.e. something like: data Wrapper c = forall a. c a => Wrapper a writer :: Chan (Wrapper Show) -> IO () ... Alternatively, are there any ideas as to how I could communicate heterogeneous instances of a given typeclass in a typesafe manner (i.e. without escaping to Data.Dynamic)? Abe

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RE: [Haskell] overuse of maybe and [] in prelude and libs
by Simon Marlow 13 Apr '04

13 Apr '04

> But, either way, I'm assuming I can't do this > (making up syntax): > > import Data.FiniteMap using MyPrelude as Prelude > > So that every place in FiniteMap that Prelude > functions are called, MyPrelude functions are > called instead? Then I could simply redefine just > and nothing as return and mzero. Not possible with GHC, I'm afraid. At least, not at the moment. We would have to retain a strong separation between Data.FiniteMap and Prelude, such as you would do if the Prelude were dynamically linked, and GHC doesn't do that. Indeed, doing so would probably have a significant impact on performance, because the compiler wouldn't be able to take advantage of any knowledge about the Prelude except for the types of functions when compiling Data.FiniteMap. Cheers, Simon

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haddock/ghc feature request
by Johannes Waldmann 13 Apr '04

13 Apr '04

I have a bunch of (polymorphic) function declarations like foo :: C a => A a -- ^ comment -> B a -- ^ comment foo = ... and haddock does a very fine job on them. but then I wanted (needed) to specialize them, and to avoid duplicating information, I wrote type Foo a = A a -- ^ comment -> B a -- ^ comment foo :: C a => Foo a {-# SPECIALIZE foo :: Foo Int #-} problem is, haddock does not want to parse the comments in the type Foo definition. (I think in case of such errors (malformed annotation), haddock should try to ignore and continue, instead of just give up.) On the other hand, it would be still better if ghc would have something like -fauto-specialize-all (that is, for all instances that are needed for the current `main' definition) so that I wouldn't have to change the source code at all. I vaguely remember there was a preprocessor (independent of ghc, written by someone else) that did "full specialization"? -- -- Johannes Waldmann, Tel/Fax: (0341) 3076 6479 / 6480 -- ------ http://www.imn.htwk-leipzig.de/~waldmann/ ---------

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