March 2003 - Haskell - Haskell.org

Scrap your Boilerplate
by Markus.Schnell＠infineon.com 21 Mar '03

21 Mar '03

Does the "zipper" fall into this category? http://haskell.org/wiki/wiki?TheZipper Markus > You might also find the 'cast' function in Section 3 of "Scrap your > boilerplate" useful. > http://research.microsoft.com/~simonpj/papers/hmap/ > I'm not certain, but it has the right smell. > Simon

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WFLP'03: Deadline extension --> April 2
by German Vidal 21 Mar '03

21 Mar '03

********************************************************************** NEW: The deadline for submissions to WFLP 2003 has been extended to April 2, 2003 ********************************************************************** ====================================================================== SECOND CALL FOR PAPERS WFLP 2003 12th Int'l Workshop on Functional and (Constraint) Logic Programming June 12-13, 2003, Valencia (Spain) http://www.dsic.upv.es/~rdp03/wflp/ ====================================================================== WFLP is the International Workshop on Functional and (Constraint) Logic Programming. WFLP'03, the twelfth in the series, will be held in Valencia, Spain, June 12-13, 2003, at the Federated Conference on Rewriting, Deduction and Programming (RDP'03) http://www.dsic.upv.es/~rdp03/ The WFLP workshops aim at bringing together researchers interested in functional programming, (constraint) logic programming, as well as their integration. It promotes the cross-fertilizing exchange of ideas and experiences among researches and students from the different communities interested in the foundations, applications, and combina- tions of high-level, declarative programming languages and related areas.The technical program of the workshop will include invited talks, presentations of refereed papers and demo presentations. Previous editions of the workshop have been held in Grado (Italy), Kiel (Germany), Benicassim (Spain), Grenoble (France), Bad Honnef (Germany), Schwarzenberg (Germany), Marburg (Germany), Rattenberg (Germany), and Karlsruhe (Germany). ---------------------------------------------------------------------- TOPICS OF INTEREST WFLP'03 solicits papers in all areas of functional and (constraint) logic programming, including (but not limited to): * Language Design: modules and type systems, multi-paradigm languages, concurrency and distribution, objects * Foundations: formal semantics, rewriting and narrowing, nonmonotonic reasoning, dynamics, type theory * Implementation: abstract machines, parallelism, compile-time and run-time optimizations, interfacing with external languages * Transformation and Analysis: abstract interpretation, specialization, partial evaluation, program transformation, meta-programming * Software Engineering: design patterns, specification, verification and validation, debugging, test generation * Applications: declarative programming in education and industry, domain-specific languages, visual/graphical user interfaces,embedded systems, WWW applications, knowledge representation and machine learning, deductive databases, advanced programming environments and tools The main focus is on new and original research results but submissions describing innovative products, prototypes under development or interesting experiments (e.g., benchmarks) are also encouraged. ---------------------------------------------------------------------- IMPORTANT DATES Submission of papers: Wednesday, April 2, 2003 Notification of acceptance: Friday, April 25, 2003 Camera-ready papers: Sunday, May 11, 2003 Workshop: June 12 - 13, 2003 ---------------------------------------------------------------------- SUBMISSION Authors are invited to submit an extended abstract (no longer than 12 pages including figures and references) or a system description (no longer than 4 pages) in Postscript or PDF format (Springer LNCS style) via email to gvidal(a)dsic.upv.es before April 2, 2003. Submissions should include the title, authors' names, affiliations, addresses, and e-mail. ---------------------------------------------------------------------- PUBLICATION Accepted papers will be included in the preliminary proceedings that will be available at the workshop. After the workshop, authors of the best abstracts will be invited to submit full papers.These full papers will be reviewed and accepted papers will appear in the Electronic Notes in Theoretical Computer Science (Elsevier). ---------------------------------------------------------------------- BEST NEWCOMER AWARD An award will be given to the best paper exclusively written by one or several young researchers who have not yet obtained their PhD degrees. Papers written in this category should be clearly marked "Student papers" in the submission. ---------------------------------------------------------------------- PROGRAM COMMITTEE Maria Alpuente (Technical University of Valencia, Spain) Sergio Antoy (Portland State University, USA) Annalisa Bossi (Universita Ca' Foscari di Venezia, Italy) Olaf Chitil (University of York, UK) Rachid Echahed (Institut IMAG, France) Sandro Etalle (University of Twente, The Netherlands) Moreno Falaschi (Universita di Udine, Italy) Michael Hanus (CAU Kiel, Germany) Yukiyoshi Kameyama (University of Tsukuba, Japan) Herbert Kuchen (University of Muenster, Germany) Michael Leuschel (University of Southampton, UK) Juan Jose Moreno-Navarro (Universidad Politecnica de Madrid, Spain) Ernesto Pimentel (University of Malaga, Spain) Mario Rodriguez-Artalejo (Universidad Complutense de Madrid, Spain) German Vidal (Technical University of Valencia, Spain) ---------------------------------------------------------------------- LOCAL ORGANIZING COMMITTEE Elvira Albert Santiago Escobar Cesar Ferri Jose Hernandez Carlos Herrero Pascual Julian Marisa Llorens Gines Moreno Javier Oliver German Vidal Alicia Villanueva ---------------------------------------------------------------------- FURTHER INFORMATION WFLP 2003: http://www.dsic.upv.es/~rdp03/wflp Workshop contact: gvidal(a)dsic.upv.es RDP 2003: http://www.dsic.upv.es/~rdp03

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simulating dynamic dispatch
by Markus.Schnell＠infineon.com 21 Mar '03

21 Mar '03

I think you are looking like something done in the HTML-Combinator-libraries. I tried something like that some time ago, but didn't finish. But perhaps you can get an idea from that. See the files with this mail. (The code is in unknown condition.) Markus > ah, yes. i was aware that would work. i forgot to mention > the constraint > that i don't want the user to have to use the MkFoo/MkBar > constructors. if i could use them internally to 'test', that would be > great, but that's what i couldn't get to work :).

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simulating dynamic dispatch
by Hal Daume III 21 Mar '03

21 Mar '03

hi all, i'm hoping to be able to simulate a sort of dynamic dispatch based on class instances. basically a function which takes a value and depending on what classes it is an instance of, does something. a simple example might be a 'maybeShow' function which sematically looks like: > maybeShow x > | typeOf x `instanceOf` Show = show x > | otherwise = "?" I've been trying for a while to simulate something along the lines of: > class Foo a where { foo :: a -> Bool } > class Bar a where { bar :: a -> Bool } > foo x > | typeOf x `instanceOf` Foo = Just (foo x) > | typeOf x `instanceOf` Bar = Just (bar x) > | otherwise = Nothing and I tried using dynamics but those require instances of typeable on universally quantified types, which doesn't work out too well in practice. also, we can do class sums using existential types: > data FB = forall a . Foo a => MkFoo a | forall a . Bar a => MkBar a and i tried something like: class FooBar a where wasFoo :: a -> Maybe FB -- will be MkFoo wasBar :: a -> Maybe FB -- will be MkBar wasFoo _ = Nothing wasBar _ = Nothing instance Foo a => FooBar a where wasFoo a = Just (MkFoo a) instance Bar a => FooBar a where wasBar a = Just (MkBar a) but this complains about duplicate instance declarations (for obvious reasons). i'm hoping one of the type class gurus out there can help me. Thanks in advance! -- Hal Daume III | hdaume(a)isi.edu "Arrest this man, he talks in maths." | www.isi.edu/~hdaume

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Re: simulating dynamic dispatch
by oleg＠pobox.com 21 Mar '03

21 Mar '03

> i'm hoping to be able to simulate a sort of dynamic dispatch based on > class instances. It seems you want to dispatch based not on a type but on the constraint of a type. You code almost worked. Here's the a bit updated and working version. class Foo a where { foo :: a -> Bool } class Bar a where { bar :: a -> Bool } data FB = forall a . Foo a => MkFoo a | forall a . Bar a => MkBar a instance Foo FB where foo (MkFoo x) = foo x instance Bar FB where bar (MkBar x) = bar x -- some instances for the test instance Foo Int where foo x = x == 0 instance Bar Char where bar x = x == 'a' test x = case x of (MkFoo a) -> Just $ foo a (MkBar a) -> Just $ bar a -- _ -> Nothing -- *Main> test $ MkFoo (0::Int) -- Just True -- *Main> test $ MkFoo (10::Int) -- Just False -- *Main> test $ MkBar 'a' -- Just True -- *Main> test $ MkBar 'b' -- Just False

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RE: flock and sockets
by oleg＠pobox.com 21 Mar '03

21 Mar '03

John Hughes wrote: > I didn't find this when I needed to lock files, so my solution (under > Unix) was to write a little C code and call it via the FFI. I used a > single lock file, since my application was a CGI script which runs fairly > rarely -- there's no need for a finer grain of locking. Many Linux/Unix systems come with 'procmail' installed. One part of that installation is a nifty program called 'lockfile': /tmp> lockfile -h Usage: lockfile -v | -nnn | -r nnn | -l nnn | -s nnn | -! | -ml | -mu | file... -v display the version number and exit -nnn wait nnn seconds between locking attempts -r nnn make at most nnn retries before giving up on a lock -l nnn set locktimeout to nnn seconds -s nnn suspend nnn seconds after a locktimeout occurred -! invert the exitcode of lockfile -ml lock your system mail-spool file -mu unlock your system mail-spool file The man page is quite informative. The program is specifically intended to be used in shell scripts. Therefore, we can use 'lockfile' in any language system that is capable of executing a shell command. In Hugs, we can use > import System (system, ExitCode(ExitSuccess)) > do { system ("lockfile -ml") >>= \ExitSuccess -> print "OK" } Therefore, we do not have to use FFI, we do not have to compile our scripts. We can lock files right in Hugs or GHCi. Daniel Luna wrote: > The second option that I have is to use a daemon and let the programs that > get started by mail connect via some sort of socket. Is there any socket > support in Haskell? One of the easiest ways of accomplishing this is by using inetd or xinetd. In this case, your application _only_ needs to read from the standard input and write into the standard output. An interactive (that is, not a single request-single reply) daemon should also be able to flush the standard output or set it to an unbuffered/line buffered discipline. The application needs to know nothing at all about sockets. Your network daemon could be just a (uncompiled) Haskell code, interpreted by Hugs or GHCi. For example, if you use inetd, you need to get your sysadm to add the following line mysrv 13720/tcp # my service, to listen on port 13720 into /etc/services and the following line mysrv stream tcp nowait yourname /home/you/daemon /home/you/daemon into /etc/inetd.conf and reHUP the inetd. The file /home/you/daemon (with executable permissions set) is a start-up shell script #!/bin/sh cd /home/you/directory PATH=right-path exec hugs -flags your-script.lhs

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Class or parameterized type?
by oleg＠pobox.com 21 Mar '03

21 Mar '03

Hello! > Example: I wish to define a structured container type, let's call it a > "RatsNest", that is type-variable in two ways: > (a) it is parameterized by a type of some class, let's call it "RatsTail", > such that a RatsNest is a structure of things that have common > properties. Certain operations between RatsNest values (e.g. a merge) are > valid only if they are containers for the same kind of RatsTail. Such > operations would not be different operations due to variation of thye > contained type. > (b) it can be implemented in various ways; e.g. a simple version is an > in-memory data structure, while another may be implemented in database > storage. This corresponds to the Haskell idea of overloading, or classes. Do you consider the standard design lacking? For example, -- sort of things to put into a RatNest class (Eq n) => RatTail n where is_the_same_rat:: n -> n -> Bool class (RatTail t) => RatNest c t where in_nest:: c t -> t -> Bool put:: c t -> t -> c t rat_fold:: (t -> z -> z) -> z -> (c t) -> z merge:: (RatNest c1 t) => c t -> c1 t -> c t merge c c1 = rat_fold put_perhaps c c1 where put_perhaps rat nest = if in_nest nest rat then nest else put nest rat instance RatTail Int where is_the_same_rat = (==) -- Pretend this is an in-memory implementation instance (RatTail t) => RatNest [] t where in_nest lst el = not $ null $ filter (is_the_same_rat el) lst put = (flip (:)) rat_fold = foldr -- Pretend this is a "disk" implementation -- At least it's unmarshalled. newtype Disk t = D String deriving Show instance (Read t, Show t, RatTail t) => RatNest Disk t where in_nest disk el = rat_fold (\item z -> z || is_the_same_rat item el) False disk put (D block) item = D $ '|' : ((show item) ++ block) rat_fold f z (D []) = z rat_fold f z (D ('|':block)) = f item $ rat_fold f z (D rest) where [(item,rest)] = readsPrec 1 block nest1 = [1,12,123] :: [Int] nest2 = D "" test1 = merge nest2 nest1 -- D "|1|12|123" test1' = merge test1 nest1 -- D "|1|12|123" test2 = merge [4,5,123] test1 -- [1,12,4,5,123] Haskell extensions are required, of course, because we use multi-parameter type classes.

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Looking for Libraries
by Matthew Donadio 20 Mar '03

20 Mar '03

Hi all, This is probably a long shot, but I am looking for a few libraries and don't want to put effort into something that has already been done. Is there a high level Haskell graphics library that would give functionality similar to gnuplot? I know I could build one myself, but I hate graphics programming. Does anyone have an FFI interface to LAPACK? Does anyone have an FFI interface to a free linear program solver? Or, a Haskell implmentation of the two-step simplex method? (I sold my copy of Chvatal's book in college for beer money). Thanks. -- Matthew Donadio (m.p.donadio(a)ieee.org)

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RE: Web server code in Haskell by Simon Marlow?
by Bayley, Alistair 20 Mar '03

20 Mar '03

This was mentioned earlier this month, see: http://haskell.org/pipermail/haskell/2003-March/011372.html http://haskell.org/pipermail/haskell/2003-March/011371.html -----Original Message----- From: Ahn Ki-yung [mailto:kyagrd@hitel.net] Sent: 20 March 2003 07:29 To: haskell(a)haskell.org Subject: Web server code in Haskell by Simon Marlow? *** WARNING - this email originates from the Internet *** I am very interested in writing server programs in haskell. There is a link to a paper and code of haskell web server in the following page. http://www.haskell.org/practice.html I can see the case study paper, but there is no code at the link location. I think inspecting through code might be very helpful, even if it does not compiling in the recent version of GHC. Please notice me where I can see the code if it is available. Also I hope the link at the haskell hompage be updated as well :-) Thanks. -- Ahn Ki-yung _______________________________________________ Haskell mailing list Haskell(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell *********************************************************************** INVESCO UK can accept no responsibility for the nature of this e-mail. If you have grounds to suspect that it is offensive or non-business related it should be deleted immediately. If you require an incoming Internet e-mail to be checked for content on your behalf please forward to the designated business mail parking area. If you wish to report the receipt of an offensive e-mail contact the IT Security section within London or Henley. It is an INVESCO UK policy that all Information Systems must be used for business related purposes. In order to protect the company's image and all personnel INVESCO UK Information Systems will be subject to monitoring. Disciplinary action will be taken against anyone who stores, distributes or displays inappropriate material. *********************************************************************** ***************************************************************** The information in this email and in any attachments is confidential and intended solely for the attention and use of the named addressee(s). This information may be subject to legal professional or other privilege or may otherwise be protected by work product immunity or other legal rules. It must not be disclosed to any person without our authority. If you are not the intended recipient, or a person responsible for delivering it to the intended recipient, you are not authorised to and must not disclose, copy, distribute, or retain this message or any part of it. *****************************************************************

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RE: Web server code in Haskell by Simon Marlow?
by Simon Marlow 20 Mar '03

20 Mar '03

> I am very interested in writing server programs in haskell. > > There is a link to a paper and code of haskell web server > in the following page. > > http://www.haskell.org/practice.html > > I can see the case study paper, but there is no code at > the link location. I think inspecting through code might be > very helpful, even if it does not compiling in the recent > version of GHC. The link is out of date. The code is now in the CVS repository, and it has a proper license. You can browse it on the web, here: http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/hws/ Cheers, Simon

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