- Haskell-Cafe - Haskell.org

Socket Programming
by Mark Goldman 03 Jan '07

03 Jan '07

I am trying to write a toy echo server that can handle multiple connections. I would like to be able to test and see if there are any connections waiting to be accepted on a socket. In C and related languages I would use something like select or poll to be nice to the OS, what would I use with the current haskell libs given that I can't seem to find a function to test if accept would block or not? -mdg -- Our problems are mostly behind us, now all we have to do is fight the solutions.

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Possible (GHC or HGL) bug or ??
by Calvin Smith 03 Jan '07

03 Jan '07

Dear haskell-cafe patrons, I've been working through an exercise in Hudak's _The Haskell School of Expression_ (ex. 3.2, creating a snowflake fractal image), and am seeing some strange drawing behavior that I'm hoping somebody can shed some light on. My initial solution is below (it requires HGL for Graphics.SOE): module Main where import Graphics.SOE main = runGraphics ( do w <- openWindow "Snowflake Fractal" (600, 600) fillStar w 300 125 256 (cycle $ enumFrom Blue) spaceClose w ) spaceClose w = do k <- getKey w if k == ' ' then closeWindow w else spaceClose w minSize = 2 :: Int fillStar :: Window -> Int -> Int -> Int -> [Color] -> IO () fillStar w x y h clrs | h <= minSize = return () fillStar w x y h clrs = do drawInWindow w (withColor (head clrs) (polygon [t1p1,t1p2,t1p3,t1p1])) drawInWindow w (withColor (head clrs) (polygon [t2p1,t2p2,t2p3,t2p1])) sequence_ $ map recur [t1p1,t1p2,t1p3,t2p1,t2p2,t2p3] where tanPiOverSix = tan(pi/6) :: Float halfSide = truncate $ tanPiOverSix * fromIntegral h hFrag = truncate $ tanPiOverSix * tanPiOverSix * fromIntegral h (t1p1,t1p2,t1p3) = ((x, y), (x-halfSide, y+h),(x+halfSide, y+h)) (t2p1,t2p2,t2p3) = ((x-halfSide, y+hFrag),(x, y+h+hFrag),(x+halfSide, y+hFrag)) reVert y = y - ((h - hFrag) `div` 3) recur pnt = fillStar w (fst pnt) (reVert (snd pnt)) (h`div`3) (tail clrs) This basically works, in that it does exactly what I want in Hugs, but GHC sometimes pauses partway through rendering, and does not continue rendering until I type any key (except space, which exits) or unfocus/refocus the window, or move the mouse pointer across the window. Sometimes, more often the first time in a GHCI session, it renders completely with no pauses, and it seems to pause more and more if I evaluate main, then close the window, evaluate again in the same GHCI session, repeatedly. The same pausing behavior is observed in a GHC-compiled executable. When the problem occurs, there is a message to the console that says: "thread blocked indefinitely". Versioning info: CPU: Pentium M OS: Gentoo GNU/Linux, kernel 2.6.18 GCC: 4.1.1 GHC: 6.6 HGL: 3.1 HUGS: March 2005 [all software compiled from source using gentoo ebuilds] Is anybody else familiar with this behavior? If not, any suggestions as to where I should file this as a potential bug? GHC? HGL? Both? Elsewhere? Thanks in advance for any information. Calvin p.s. Any stylistic or other comments about the code welcome too.

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Extensible static checking of dimensions?
by Mike Gunter 03 Jan '07

03 Jan '07

The very nice Buckwalter and Denney dimensional-numbers packages both work on a fixed set of base dimensions. This is a significant restriction for me--I want to avoid adding apples to oranges as well as avoiding adding meters to grams. Is it possible to have an extensible set of base dimensions? If so, how usable can such a system be made? Is it very much worse than a system with a fixed set of base dimensions? thanks, -m

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Generating Source Code with Haskell
by Martin Huschenbett 02 Jan '07

02 Jan '07

Hi, my aim is to transform an XML file into C++ source code with a Haskell program. The part that parses the XML is already finished (using HaXML) but the part that generates the C++ code is still remaining. Is there already any work on this topic? Maybe even on generating Java or any other object oriented or imperative language. My first approach was to simply generate the C++ code as text using a pretty printing library but this becomes ugly very fast. Next I thought about generating and rendering the AST of the resulting C++ code. But I don't want to reinvent the wheel. Regards, Martin.

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Glasgow Distributed Haskell
by Joel Reymont 02 Jan '07

02 Jan '07

Is anyone using GdH? Can someone tell me why it's not part of the GHC distribution? It seems that GdH is not being developed anymore and I think it's a real pity! Thanks, Joel -- http://wagerlabs.com/

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Re: [Haskell-cafe] Composing functions with runST
by Yitzchak Gale 02 Jan '07

02 Jan '07

Hi Thomas, You wrote: > How do I "import" Control.Monad.ST so I can experiment with it from > ghci and just do > runST > like you had. Instead of qualifying it in some way. In GHCi, use the :module command (:m) for built-in modules, and :load and :add for source files. In Hugs, use :load and :also for both. You need to run Hugs with the -98 flag to use Control.Monad.ST. > how do I do runST as fully qualified? Like this: Prelude> Control.Monad.ST.runST (do {r <- Data.STRef.newSTRef 2007; Data.STRef.readSTRef r}) 2007 Have fun with monads! Regards, Yitz

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porting ghc
by Brian McQueen 01 Jan '07

01 Jan '07

I was trying to get a ghc going in my shell account the other day and found that the data at http://haskell.org/ghc/docs/6.6/html/building/sec-porting-ghc.html didn't apply at all. The system is a netbsd alpha which turns up as alpha-unknown-netbsd through configure. I didn't find any configure.in to modify, but there is a config.guess. Is that what I'm supposed to modify? I was using the source distribution of ghc 6.6. So how do I get off to s start on the netbsd system at my ISP? Here is the uname data for my system with the hostname x'd out: NetBSD xxx 2.1.0_STABLE NetBSD 2.1.0_STABLE (sdf) #0: Mon Nov 13 08:11:09 UTC 2006 root@ol:/var/src/netbsd/src/sys/arch/alpha/compile/sdf alpha Brian McQueen

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Strange type behavior in GHCi 6.4.2
by Grady Lemoine 01 Jan '07

01 Jan '07

Hello, I've been playing around with Dan Piponi's work on automatic differentiation (from http://sigfpe.blogspot.com/2005/07/automatic-differentiation.html and http://sigfpe.blogspot.com/2006/09/practical-synthetic-differential.html) and I'm getting some odd behavior with the inferred types of functions in GHCi 6.4.2. My test case is as follows: > data Dual a = D a a deriving (Show,Eq) > instance Num a => Num (Dual a) where > fromInteger i = D (fromInteger i) 0 > (D a a')+(D b b') = D (a+b) (a'+b') > (D a a')-(D b b') = D (a-b) (a'-b') > (D a a')*(D b b') = D (a*b) (a*b'+a'*b) > evalDeriv f x = (v, v') > where D v v' = f (D x 1) > f x = x^3 > f' = snd . (evalDeriv f) When I load this in GHCi, I get: *Main> :t f f :: (Num a) => a -> a *Main> :t snd . (evalDeriv f) snd . (evalDeriv f) :: (Num a, Num (Dual a)) => a -> a *Main> :t f' f' :: Integer -> Integer Why is the type of f' Integer -> Integer, especially when the type of the expression it's defined as is more general? Is this something I'm not understanding about Haskell, or is it more to do with GHC 6.4.2 specifically? Any help appreciated, --Grady Lemoine

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Second Call for Papers: TFP 2007, New York, USA
by TFP 2007 31 Dec '06

31 Dec '06

CALL FOR PAPERS Trends in Functional Programming 2007 New York, USA April 2-4, 2007 http://cs.shu.edu/tfp2007/ 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] Literate Haskell source files. How do I turn them into something I can read?
by Kirsten Chevalier 31 Dec '06

31 Dec '06

[redirecting to the mailing list] On 12/31/06, Michael T. Richter <ttmrichter(a)gmail.com> wrote: > On Sat, 2006-30-12 at 23:48 -0800, Kirsten Chevalier wrote: > > I'm probably not the right person to explain why David Roundy chose to > > write his code the way he did, since I've never even met him. However, > > since as of 12/8/2006 there were 115 contributors to darcs, perhaps > > reading the source code isn't as difficult as you seem to think it is. > > What's the secret then? pdflatex vomits. unlit generates a few dozen > pages of whitespace per five-line block of code. I'm at a loss to take > this "literate" format and turn it into something which I can read. > The secret lies in adjusting your perspective. The literate format is readable by humans as is. If there are specific things about it that you find difficult, you can post about them here with *specific* examples you find perplexing. But one reason why you may feel like people aren't being helpful is that your question isn't specific enough. > > > I fail to see how making code which can't be read makes it more... > > > readable. > > > "Can't" is an awfully strong word, isn't it? > > Nope. The source files as they are are gibberish. Again, that's an awfully strong statement. Clearly, there exist any number of people who can read them without finding them gibberish. Perhaps it might be better to listen to what they are telling you than to make contradictory declarations. > There's some kind of > markup (of a type I can't identify) mixed up with Haskell source. Given > that I can't identify the markup and I'm a little hazy on Haskell at > this point (part of the point of this exercise was to learn to read > Haskell as it's used in real projects), the net effect is executable > line noise. You know, like a perl program written circa 3.0. > Right, if you're trying to learn to read Haskell as it's used in real projects, reading the .lhs files as they are is the way to go. Again, if you find specific things hard to comprehend, ask here, or on the IRC channel. A general "all Haskell looks like line noise!" complaint is hard for us to answer. > > I can only explain how I did it myself, which was through practice. > > OK. How do you read the literate source code? Which tools do you use > to separate the code from the commentary? Which tools actually work to > turn .lhs files into something human-readable? (pdflatex failed on the > GHC code as well.) > My eyes. And brain. I'm sorry that I can't be more helpful, but that's what most other people I've watched reading Haskell code use, too. Cheers, Kirsten -- Kirsten Chevalier* chevalier(a)alum.wellesley.edu *Often in error, never in doubt "Memo to myself: Do the dumb things I gotta do. Touch the puppet head." -- They Might Be Giants

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