- Haskell - Haskell.org

Proposal: Allow "\=" for field update in record update syntax
by S. Alexander Jacobson 05 Mar '05

05 Mar '05

I don't know what it takes to get this sort of change into circulation, but I assume it starts with a clear and specific description of the problem, exploration of alternatives, and a specific proposed solution. So here goes: ---------- Proposal: Allow "\=" for field update in record update syntax Specifically, intepret rec {field\=fn} as rec {field = fn (field rec)} Jusitification: It is extremely common when updating records to be updating a field value based on its prior value rather than simply assigning a new value from nothing. Unforunately the syntax supports the later clearly but makes the former awkward to read, understand, and debug: myFun db myType1 = db { tMyType1 = insert myType1 $ tMyType1 db nextId = 1 + (nextId db) } One solution to this problem that does not involve changing syntax is to generate field update functions for every field of a type e.g.: update_tMyType1 f s = s {tMyType1= f $ myType1 s} update_nextId f s = s {nextId = f $ nextId s} Once you have those functions, myFun looks much better: myFun db myType = update_nextId (+1) $ update_tMyType1 f db where f= insert myType1 But, generating the update_ functions is not programming; its just manual syntax generation. And, even if you have template haskell to do it, * it is annoying to have to find/write the TH code to implement * update_ is incredibly verbose * shortening it risks proliferation of psuedo-syntax: u_tMyType, u__tMyType, or utMyType? * it adds clutter to have to call the TH for each record type, and * it pollutes the name space/increases risk of name collision Rather than force the user to jump through these hoops, why not make the syntax flow naturally from the existing record update syntax? It makes sense to use \= for this task as it is the functional generalization of C or Java's += or *= . Then myFun would have the elegant and clear syntax: myFun db myType1 =db {tMyType1\=insert myType1, nextId\=(+1)} -Alex- ______________________________________________________________ S. Alexander Jacobson tel:917-770-6565 http://alexjacobson.com

13 39

ANNOUNCE: The GHC Survey 2005
by Simon Marlow 04 Mar '05

04 Mar '05

Just ahead of the 6.4 release, we've put together a GHC user survey. The idea is to help us get a better understanding of who uses GHC, what for, what aspects of GHC they consider most valuable, and what they would most like to see improved. The survey is anonymous (if you wish), and should take about 5 minutes to complete. We will use the information to help guide what we do in future, and to help justify what to do. Have your say! We'll also be publishing lots of cool stats once we have enough responses. Here's the link: http://www.haskell.org/ghc/survey/start.cgi Many thanks to Tomasz Zielonka for putting together the survey, which is implemented using WASH on GHC (of course). Simon M, on behalf of the GHC Team

1 0

RE: [Haskell] sugar for extensible types (was: class associated types, via GADTs.)
by Simon Marlow 04 Mar '05

04 Mar '05

On 16 February 2005 08:08, Ralf Laemmel wrote: > Assoc types and GADTs are great but > I am still too fond of encoding extensible datatypes with (open) > classes. > I contributed to some related discussion at comp.compilers a while > ago [1]. The Haskell code samples are worth sharing (because they are > sooooo simple): > > http://homepages.cwi.nl/~ralf/OOHaskell/src/interpreter/nonextensible.hs > http://homepages.cwi.nl/~ralf/OOHaskell/src/interpreter/extensible.hs > (Note: *no* OOHaskell idioms used.) I wish we'd done exceptions like this. An extensible exception type is entirely possible, and using it is not hard either - see the attached file implemented in terms of the existing Control.Exception. For example: test = do x `catch` (\(IOException e) -> print e) `catch` (\(ArithException e) -> print e) and I make a new type into an exception like this: data T = ... deriving (Typeable, Show) instance Exception T this can replace DynExceptions and the strange catchJust/tryJust stuff in Control.Exception. Perhaps we can figure out a reasonable migration path that wouldn't break too much code in one go? Cheers, Simon

3 2

integration of functional and imperative programming concepts
by Derek Elkins 04 Mar '05

04 Mar '05

> for my diploma thesis, I need to find information about integration of > functional and imperative programming concepts. Could somebody of you > point me to good websites, papers, etc. about this topic? You haven't been too specific about what kinds of things you are looking for. One thing that you might find useful is, http://www.mm.informatik.tu-darmstadt.de/~kuehne/fps/. Also there are plenty of functional-object-oriented languages. Checking the paper listings of FOOL conferences should provide many other papers.

1 0

RE: [Haskell] Re: help with some basic code that doesn't work
by Simon Peyton-Jones 03 Mar '05

03 Mar '05

Happily, with GHC 6.4, your code works fine, both with 'deriving' and with a hand-written instance. It works because 6.4 can build recursive dictionaries. Simon | -----Original Message----- | From: haskell-bounces(a)haskell.org [mailto:haskell-bounces@haskell.org] On Behalf Of Shin-Cheng | Mu | Sent: 14 February 2005 05:32 | To: haskell(a)haskell.org | Cc: Malcolm.Wallace(a)cs.york.ac.uk | Subject: [Haskell] Re: help with some basic code that doesn't work | | Malcolm Wallace <Malcolm.Wallace(a)cs.york.ac.uk> wrote: | > Just a comment, since a couple of people have made similar statements. | > Haskell will derive Eq for arbitrarily complex types - there is no | > restriction to "simple" types, whatever they might be. | | Now that this topic is brought up... | | Occasionally I would need to define recursive datatypes | using an explicit fixed-point operator, such as: | | > data Fix f = In (f (Fix f)) deriving (Show, Eq) | > data L a x = Nil | Cons a x deriving (Show, Eq) | | However, Haskell was not able to derive from Fix f any | instances. The following is what happens in GHCi: | | *Main> In Nil == In Nil | | Context reduction stack overflow; size = 21 | Use -fcontext-stack20 to increase stack size to (e.g.) 20 | `Eq (L e (Fix (L e)))' arising from use of `==' at <interactive>:1 | `Eq (Fix (L e))' arising from use of `==' at <interactive>:1 | <<deleted>> | | *Main> In Nil | | Context reduction stack overflow; size = 21 | Use -fcontext-stack20 to increase stack size to (e.g.) 20 | `Show (L e (Fix (L e)))' | arising from use of `print' at <interactive>:1 | <<deleted>> | | Probably Malcolm meant that Haskell will derive standard instances | for arbitrarily complex type **if they are definable manually**. | If so Malcolm's statement is true -- indeed I cannot even declare | the instances by hand. The following declaration is acceptable | by GHC after using the option -fallow-undecidable-instances, | but even though, I got the same error message as I try to print | In Nil. | | > Instance Show (f (Fix f)) => Show (Fix f) where | > showsPrec _ (In x) = ("In ("++) . showsPrec 1 x . (')':) | | This is rather unsatisfactory, because I would not be able | to inspect values of type Fix f in the interpreter. Is there | a way to get around this? | | sincerely, | Shin | | _______________________________________________ | Haskell mailing list | Haskell(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell

1 0

integration of functional and imperative programming concepts
by Wolfgang Jeltsch 03 Mar '05

03 Mar '05

Hello, for my diploma thesis, I need to find information about integration of functional and imperative programming concepts. Could somebody of you point me to good websites, papers, etc. about this topic? Thanks a lot. Best wishes, Wolfgang

2 1

How to import user's modules with hs-plugins
by Javier García-Vivó Albors 02 Mar '05

02 Mar '05

Hello everybody. I'm using the hs-plugins function "eval" in my source code and I need to import a Haskell module at runtime. Do you know if it's possible? I've seen many examples of hs-plugins and never had a user's module import in the "eval" call. Thanks for your time ---------------------------------------------------------------- This message was sent using IMP, the Internet Messaging Program.

2 1

Summer School on Reliable Computing: Call for Participation
by Steve Zdancewic 02 Mar '05

02 Mar '05

Call for Participation Summer School on Reliable Computing July 20-29, 2005 University of Oregon Eugene, Oregon USA Registration Deadline: March 31, 2005. http://www.cs.uoregon.edu/research/summerschool/summer05/index.html e-mail: summer-school-security(a)cs.uoregon.edu Program ------- This Summer School will cover current research in reliability of software systems ranging from foundational materials on type systems, program analyses, and model checking to advanced applications of the techniques in practice. Material will be presented at a tutorial level that will help graduate students and researchers from academia or industry understand the critical issues and open problems confronting the field. The course is open to anyone interested. Prerequisites are an elementary knowledge of logic and mathematics that is usually covered in undergraduate classes on discrete mathematics. Some knowledge of programming languages at the level provided by an undergraduate survey course will also be expected. Our primary target group is PhD students. We also expect attendance by faculty members who would like to conduct research on this topic or introduce new courses at their universities. The program consists of more than twenty-five 80-minute lectures presented by internationally recognized leaders in programming languages, model checking, and software reliability research. Speakers and their topics include: * Counterexample-driven Refinement Thomas Ball Microsoft Research * Specifying and Checking Stateful Software Interfaces Manuel Fahndrich Microsoft Research * Checking Software Properties with Contracts Robby Findler University of Chicago * Domain-Specific Languages: For Fun and Profit Kathleen Fisher AT&T Research * Domain-Specific Languages: For Fun and Profit Kathleen Fisher AT&T Research * Improving Software Quality with Type Qualifiers Jeff Foster University of Maryland * Lightweight Analyses for Reliable Concurrency Stephen Freund Williams College * Type Systems: A Foundation for Reliable Computing Robert Harper Carnegie Mellon University * Program Verification by Lazy Abstraction Ranjit Jhala University of California at San Diego * An Introduction to Model Checking Orna Kupferman Hebrew University, Jerusalem Venue ----- The summer school will be held at the University of Oregon, located in the southern Willamette Valley city of Eugene, close to some of the world's most spectacular beaches, mountains, lakes and forests. On the weekend, students will have the option of participating in a group activity in Oregon's countryside. Housing ------- The school will provide on-campus housing and meals. To share a room with another student attending the school, the cost is $450.00 (USD) per person. Housing rates are based on check-in Tuesday, July 19 and check-out before noon on Saturday, July 30. Some single rooms may be available for an additional fee of $130.00 (USD). If you'd like a single room, please indicate your choice and we will try to accommodate you on a first-come/first-served basis. Registration ------------ The cost for registration is $175.00 (USD) for graduate students, and $300.00 (USD) for other participants. There are a limited number of grants available to fund part of the cost of student participation. The registration deadline is March 31, 2005. Additional information about the program, registration, venue, and housing options is available on the web site. Or, you may request more information by e-mailing summer-school-security(a)cs.uoregon.edu. Organizers ---------- Zena Ariola, University of Oregon David Walker, Princeton University Steve Zdancewic, University of Pennsylvania

1 0

Deadlines Extended: CALL FOR PAPERS IN THREE SYMPOSIUMS
by Dr. M. Sarfraz 02 Mar '05

02 Mar '05

Deadlines Extended ==================================== CALL FOR PAPERS IN THREE SYMPOSIUMS ==================================== Symposium - 1 International Symposium of Graphical Models and Imaging (IV2005-GMAI) http://www.graphicslink.demon.co.uk/IV05/GMAI.htm IN The 9th IEEE International Conference on INFORMATION VISUALISATION 06- 07 - 08 July 2005 * LONDON * ENGLAND * IMPORTANT - PLEASE NOTE THAT: 1- The deadline for the paper submission is extended to: 21 March 2005. 2- The main conference IV05 details are available from: http://www.graphicslink.demon.co.uk/IV05/ ==================================== Symposium - 2 International Symposium of Computer Aided Geometric Design & Graphics (CGIV2005-CAGDAG) http://www.graphicslink.demon.co.uk/cgiv05/CAGDAG.htm IN The 2nd International Conference INFORMATION VISUALISATION 26 - 27 - 28 - 29 July 2005 * Xi jiao Hotel?* Beijing * China IMPORTANT - PLEASE NOTE THAT: 1- The deadline for the paper submission is extended to: 15 March 2005. 2- The main conference CGIV05 details are available from: http://www.graphicslink.demon.co.uk/cgiv05 ==================================== Symposium - 3 International Symposium of Computer Aided Geometric Design & Graphics (CGIV2005-CAGDAG) http://www.graphicslink.demon.co.uk/cgiv05/IRTAST.htm IN The 2nd International Conference INFORMATION VISUALISATION 26 - 27 - 28 - 29 July 2005 * Xi jiao Hotel?* Beijing * China IMPORTANT - PLEASE NOTE THAT: 1- The deadline for the paper submission is extended to: 15 March 2005. 2- The main conference CGIV05 details are available from: http://www.graphicslink.demon.co.uk/cgiv05 ==================================== CONTACT: All submissions should be addressed to: Dr Muhammad Sarfraz (Chair) Department of Information and Computer Science King Fahd University of Petroleum and Minerals KFUPM # 1510, Dhahran 31261 Saudi Arabia. Tel: +966-3-860-2763 Fax: +966-3-860-2763 Email: sarfraz(a)kfupm.edu.sa WWW: http://www.ccse.kfupm.edu.sa/~sarfraz/

1 0

linear implicit parameters
by Immanuel Litzroth 02 Mar '05

02 Mar '05

Has anybody got a good example of how to use linear implicit parameters? I seem to be unable to understand the description in the ghc manual and I couldn't find a paper on the web. Immanuel *************************************************************************** I can, I can't. Tubbs Tattsyrup -- Immanuel Litzroth Software Development Engineer Enfocus Software Antwerpsesteenweg 41-45 9000 Gent Belgium Voice: +32 9 269 23 90 Fax : +32 9 269 16 91 Email: Immanuell(a)enfocus.be web : www.enfocus.be ***************************************************************************

1 0