January 2007 - Haskell-Cafe

mtl tweaks
by Nicolas Frisby 21 Jan '07

21 Jan '07

There have been some discussions of augmentations of the monad transformer library. I at least know there was a discussion regarding strictness of state/value components in the state monad transformer (I must admit I didn't track the conclusion of that one). I have another small mtl complaint: ReaderT, for example, requires the base type to be a Monad in order to make it a Functor. So it's Monad m => Functor (ReaderT r m) instead of Functor f => Functor (ReaderT r f), which is what's actually necessary for the instance. Remember, I did say _small_ complaint. I realize this has a bit to do with the class heirarchy and that's a sensitive issue. Regarding backwards compatibility and interface changes, note that the Monad m => Monad (ReaderT r m) instance would be unaffected by this... it's only the Functor => Functor instance. If someone wants to treat a transformed monad as a functor, then they're probably savvy enough to specify the base monad as a functor (especially with the @fmap f = (>>= return . f)@ recipe). They won't even see a difference unless they are rolling their own monad, since all mtl monads are also functors. 1) Is there a collection of these suggestions specifically for mtl? On the wiki? On the bug tracker? 2) Any chance we could make this change to the library? Thanks, Nick

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Re: [Haskell-cafe] Article review: Category Theory
by Yitzchak Gale 20 Jan '07

20 Jan '07

I wrote: >> Will (id :: A -> A $!) do the trick? Ulf Norell wrote: > The problem is not with id, it's with composition. For any f and g we > have > > f . g = \x -> f (g x) > > So _|_ . g = \x -> _|_ for any g. OK, so then how about f .! g = ((.) $! f) $! g -Yitz

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Article review: Category Theory
by David House 20 Jan '07

20 Jan '07

Hey all, I've written a chapter for the Wikibook that attempts to teach some basic Category Theory in a Haskell hacker-friendly fashion. http://en.wikibooks.org/wiki/Haskell/Category_theory >From the article's introduction: "This article attempts to give an overview of category theory, insofar as it applies to Haskell. To this end, Haskell code will be given alongside the mathematical definitions. Absolute rigour is not followed; in its place, we seek to give the reader an intuitive feel for what the concepts of category theory are and how they relate to Haskell." I'd love comments from newcomers and experts alike regarding my approach, the content, improvements and so on. Of course, it's on the wikibook, so if you have anything to add (that's not _too_ substantial otherwise I'd recommend discussion first) then go ahead. Thanks in advance. -- -David House, dmhouse(a)gmail.com

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File locked unnecessarily (the file)
by Arie Peterson 19 Jan '07

19 Jan '07

Except that the example was not attached. Sorry. Greetings, Arie

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File locked unnecessarily (the continuation)
by Arie Peterson 19 Jan '07

19 Jan '07

Hi all, Some time ago I reported a strange file locking problem. I have reduced my offending program to a relatively small size. This reduction process was quite entertaining. At one point, I could reliably turn the locking off and on by removing and reintroducing a certain unused import :s. I was happy to see this go away on cutting out all concurrency. The example is attached. Open the tarball and run "ghc --make Main -O && ./a.out" in the resulting directory. On my system (amd64 Gentoo linux, ghc 6.4.2), this gives the following output. ###### Chasing modules from: Main Compiling Interface ( ./Interface.hs, ./Interface.o ) Compiling Main ( Main.hs, Main.o ) Linking ... Compiling plugin P b a.out: a.txt: openFile: resource busy (file is locked) ###### There is no reason for the file "a.txt" to be locked. The problem is somewhat brittle: any one of the following changes makes it disappear: - in "Main.hs:main", replacing the 'c' argument of 'cat' (the one dynamically loaded from the plugin 'P') by 'Interface.c'; - in "Main.hs:main", removing one of the two 'cat c' calls; - in "Interface.hs:c", removing one of the two 'a' calls. Because of the first of these, it seems that the problem is partly caused by the dynamic linking, but I'm not sure about that (the "unused import" experience made me a bit paranoid ;-) ). Can someone comment on the cause of this locking issue, or maybe further simplify the example? Thanks and greetings, Arie

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Final Call for Papers: TFP 2007, New York, USA
by TFP 2007 19 Jan '07

19 Jan '07

CALL FOR PAPERS Trends in Functional Programming 2007 New York, USA April 2-4, 2007 http://cs.shu.edu/tfp2007/ NEW: Abstract submission is now open! Link: http://cs.shu.edu/tfp2007/submissions.html NEW: Invited Talk: John McCarthy, Standford University The symposium on Trends in Functional Programming (TFP) is an international forum for researchers with interests in all aspects of functional programming languages, focusing on providing a broad view of current and future trends in Functional Programming. It aspires to be a lively environment for presenting the latest research results through acceptance by extended abstracts. A formal post-symposium refereeing process then selects the best articles presented at the symposium for publication in a high-profile volume. TFP 2007 is co-hosted by Seton Hall University and The City College of New York (CCNY) and will be held in New York, USA, April 2-4, 2007 at the CCNY campus. SCOPE OF THE SYMPOSIUM The symposium recognizes that new trends may arise through various routes. As part of the Symposium's focus on trends we therefore identify the following five article categories. High-quality articles are solicited in any of these categories: Research Articles leading-edge, previously unpublished research work Position Articles on what new trends should or should not be Project Articles descriptions of recently started new projects Evaluation Articles what lessons can be drawn from a finished project Overview Articles summarizing work with respect to a trendy subject Articles must be original and not submitted for simultaneous publication to any other forum. They may consider any aspect of functional programming: theoretical, implementation-oriented, or more experience-oriented. Applications of functional programming techniques to other languages are also within the scope of the symposium. Articles on the following subject areas are particularly welcomed: o Dependently Typed Functional Programming o Validation and Verification of Functional Programs o Debugging for Functional Languages o Functional Programming and Security o Functional Programming and Mobility o Functional Programming to Animate/Prototype/Implement Systems from Formal or Semi-Formal Specifications o Functional Languages for Telecommunications Applications o Functional Languages for Embedded Systems o Functional Programming Applied to Global Computing o Functional GRIDs o Functional Programming Ideas in Imperative or Object-Oriented Settings (and the converse) o Interoperability with Imperative Programming Languages o Novel Memory Management Techniques o Parallel/Concurrent Functional Languages o Program Transformation Techniques o Empirical Performance Studies o Abstract/Virtual Machines and Compilers for Functional Languages o New Implementation Strategies o any new emerging trend in the functional programming area If you are in doubt on whether your article is within the scope of TFP, please contact the TFP 2007 program chair, Marco T. Morazan, at tfp2007(a)shu.edu. SUBMISSION AND DRAFT PROCEEDINGS Acceptance of articles for presentation at the symposium is based on the review of extended abstracts (6 to 10 pages in length) by the program committee. Accepted abstracts are to be completed to full papers before the symposium for publication in the draft proceedings and on-line. Further details can be found at the TFP 2007 website. POST-SYMPOSIUM REFEREEING AND PUBLICATION In addition to the draft symposium proceedings, we intend to continue the TFP tradition of publishing a high-quality subset of contributions in the Intellect series on Trends in Functional Programming. IMPORTANT DATES Abstract Submission: February 1, 2007 Notification of Acceptance: February 20, 2007 Registration Deadline: March 2, 2007 Camera Ready Full Paper Due: March 9, 2007 TFP Symposium: April 2-4, 2007 PROGRAMME COMMITTEE John Clements California Polytechnic State University, USA Marko van Eekelen Radboud Universiteit Nijmegen, The Netherlands Benjamin Goldberg New York University, USA Kevin Hammond University of St. Andrews, UK Patricia Johann Rutgers University, USA Hans-Wolfgang Loidl Ludwig-Maximilians Universität München, Germany Rita Loogen Philipps-Universität Marburg, Germany Greg Michaelson Heriot-Watt University, UK Marco T. Morazán (Chair) Seton Hall University, USA Henrik Nilsson University of Nottingham, UK Chris Okasaki United States Military Academy at West Point, USA Rex Page University of Oklahoma, USA Ricardo Pena Universidad Complutense de Madrid, Spain Benjamin C. Pierce University of Pennsylvania, USA John Reppy University of Chicago, USA Ulrik P. Schultz University of Southern Denmark, Denmark Clara Segura Universidad Complutense de Madrid, Spain Jocelyn Sérot Université Blaise Pascal, France Zhong Shao Yale University, USA Olin Shivers Georgia Institute of Technology, USA Phil Trinder Heriot-Watt University, UK David Walker Princeton University, USA ORGANIZATION Symposium Chair: Henrik Nilsson, University of Nottingham, UK Programme Chair: Marco T. Morazan, Seton Hall University, USA Treasurer: Greg Michaelson, Heriot-Watt University, UK Local Arrangements: Marco T. Morazan, Seton Hall University, USA ************************************************************************************ Dr. Marco T. Morazan TFP 2007 Program Committee Chair http://cs.shu.edu/tfp2007/

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Re: [Haskell-cafe] Fractional sqrt
by Novák Zoltán 19 Jan '07

19 Jan '07

Hi, Thanks for the answers. The best solution would be a general purpose arbitrary precision math library for Haskell. I found two: http://medialab.freaknet.org/bignum/ http://r6.ca/FewDigits/ I think both uses power series and have trigonometric functions too. (I only need sqrt, so probably I will not use them, just the simple Newton alg.) It would be great, if a fast implementation of arbitrary precision real arithmetic Class would be part of the Standard libraries. Because Haskell is so great for numerical computations already (functions, composing functions, higher order f. with typechecking), a good math library would make Matlab obsolete. :) Zoltan Novak ps. Call me Zoltan (that is my first name), just we write it in different order in Hungary. (Names are in hungarian notation here: the type description precedes the actual name)

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for loops and 2d arrays in haskell
by Fernan Bolando 19 Jan '07

19 Jan '07

hi all Since I am very new to haskell and still learning, I hope I will not annoy poeple by asking the following question. what is the simplest way to implement the following code in haskell? it's just printing the contents of 2D array. for(i = 0; i < imax; i++){ for(n = 0; n < nmax; n++){ printf("%i:%i = %f\n", array[i][n]); } } ,Fernan -- Public PGP/GnuPG key (http://www.fernski.com) pub 1024D/3576CA71 2006-02-02 Fernan Bolando Key fingerprint = FDFE C9A8 FFED C1A5 2F5C EFEB D595 AF1C 3576 CA71

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hs-plugins on ghc6.6?
by Adam Megacz 18 Jan '07

18 Jan '07

Has anybody been able to get hs-plugins to work with ghc6.6? http://www.nabble.com/Re%3A-hs-plugins-runtime-error-with-GHC-6.6%3A-index-… - a -- PGP/GPG: 5C9F F366 C9CF 2145 E770 B1B8 EFB1 462D A146 C380

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X11
by Philippe de Rochambeau 18 Jan '07

18 Jan '07

Hello, does anyone know where I can find examples that use the Graphics.X11.Xlib.Window module. Thanks. phiroc

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