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ANNOUNCE: Haskell Communities and Activities Report (21st ed., November 2011)
by Janis Voigtländer 04 Dec '11

04 Dec '11

On behalf of all the contributors, I am pleased to announce that the Haskell Communities and Activities Report (21st edition, November 2011) is now available in PDF and HTML formats: http://haskell.org/communities/11-2011/report.pdf http://haskell.org/communities/11-2011/html/report.html Many thanks go to all the people that contributed to this report, both directly, by sending in descriptions, and indirectly, by doing all the interesting things that are reported. I hope you will find it as interesting a read as I did. If you have not encountered the Haskell Communities and Activities Reports before, you may like to know that the first of these reports was published in November 2001. Their goal is to improve the communication between the increasingly diverse groups, projects, and individuals working on, with, or inspired by Haskell. The idea behind these reports is simple: Every six months, a call goes out to all of you enjoying Haskell to contribute brief summaries of your own area of work. Many of you respond (eagerly, unprompted, and sometimes in time for the actual deadline to the call. The editor collects all the contributions into a single report and feeds that back to the community. When I try for the next update, six months from now, you might want to report on your own work, project, research area or group as well. So, please put the following into your diaries now: ========================================= End of April 2012: target deadline for contributions to the May 2012 edition of the HC&A Report ========================================= Unfortunately, many Haskellers working on interesting projects are so busy with their work that they seem to have lost the time to follow the Haskell related mailing lists and newsgroups, and have trouble even finding time to report on their work. If you are a member, user or friend of a project so burdened, please find someone willing to make time to report and ask them to "register" with the editor for a simple e-mail reminder in April (you could point me to them as well, and I can then politely ask if they want to contribute, but it might work better if you do the initial asking). Of course, they will still have to find the ten to fifteen minutes to draw up their report, but maybe we can increase our coverage of all that is going on in the community. Feel free to circulate this announcement further in order to reach people who might otherwise not see it. Enjoy! Janis Voigtlaender <hcar at haskell.org>

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Question about Interpreter
by Song Zhang 03 Dec '11

03 Dec '11

Hi I am a beginner of Haskell. I want to know why the interpreter was designed in a way that one function or a part fails the whole program fails. why we don't keep the parts that is correct functioning and also give error message for the wrong parts?

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emacs setup for haskell
by Rustom Mody 02 Dec '11

02 Dec '11

I am using haskell inside emacs with the usual setup ie .hs file in one window, ghci in another. Now the most usual workflow one would do is to load the file into ghci (C-c C-l) and try out something in ghci. A small niggle is that after the C-c C-l Ive to manually switch to the other window (current window does not switch to ghci automatically) This seems to be a strange default More importantly the state of the workspace in ghci seems to get messed up unless I do: 0. Prepare some code in .hs 2. Switch to ghci 3. Clear the ghci workspace with :m <nothing> 4. switch back to .hs 5. C-c C-l 6. Switch to ghci to try out new code Is there something I am missing?

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Parallelism?
by Michael Craig 02 Dec '11

02 Dec '11

I was writing some parallel code (asynchronous database writes for an event logger, but that's besides the point), and it seemed like the parallelized version (i.e. compiled with -threaded -with-rtsopts=-N2) wasn't running fast enough. I boiled it down to a dead-simple test: import Control.Concurrent import Data.Time.Clock.POSIX import System.Environment main :: IO () main = do n <- getArgs >>= return . read . head t1 <- getPOSIXTime work n t2 <- getPOSIXTime putStrLn $ show $ t2 - t1 putStrLn $ show $ (fromIntegral n :: Double) / (fromRational . toRational $ t2 - t1) work :: Integer -> IO () work n = do forkIO $ putStrLn $ seq (fact n) "Done" putStrLn $ seq (fact n) "Done" fact :: Integer -> Integer fact 1 = 1 fact n = n * fact (n - 1) (I know this is not the best way to time things but I think it suffices for this test.) Compiled with ghc --make -O3 test.hs, ./test 500000 runs for 74 seconds. Compiling with ghc --make -O3 -threaded -with-rtsopts=-N, ./test 500000 runs for 82 seconds (and seems to be using 2 cpu cores instead of just 1, on a 4-core machine). What gives? Mike S Craig

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discover data constructor without pattern matching?
by Philippe Sismondi 02 Dec '11

02 Dec '11

Is it possible to discover the data constructor used to make a value without doing pattern matching? In my current small project I have a data type with a zillion constructors. These use record syntax so that I can access the payload without pattern matching. But I just discovered that it I want to print out the constructor name in my error handling function. Can I do this without, say, hacking around with a string from 'show'? - P -

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Cabal on mac
by Tim Pizey 29 Nov '11

29 Nov '11

Hi, I have installed cabal and HaxMl cabal install HaXml but my .hs file cannot find import Text.XML.HaXml.Xml2Haskell (readXml) giving *** Chasing dependencies: Chasing modules from: *ch.hs ch.hs:3:8: Could not find module `Text.XML.HaXml.Xml2Haskell': locations searched: Text/XML/HaXml/Xml2Haskell.hs Text/XML/HaXml/Xml2Haskell.lhs It would seem only to be looking in the current directory. Hoping I have just missed something obvious. Tim -- Tim Pizey http://pizey.net/~timp

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Multi-parameter type classes
by David Beacham 28 Nov '11

28 Nov '11

Hi all, I've been reading through Okasaki's purely functional data structures and noticed that the source he provides for the Set implementations uses multi-parameter type classes, whereas those for the Heap structures does not. class Set s a where empty :: s a insert :: a -> s a -> s a ... class Heap h where empty :: Ord a => h a insert :: Ord a => a -> h a -> h a ... The only difference I can see is that the instance declarations for Set contain the Ord constraint, whereas they are defined on the class functions for Heap. instance (Ord a) => Set RedBlackSet a where ... I realise that the type of `a` needs to be constrained to Ord instances at some point in the code, but is there any particular reason for doing it differently in each case? If not, should one method be preferred over the other? Thanks for the help, David.

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help with a class definition
by Rustom Mody 28 Nov '11

28 Nov '11

The Data.Vector.Unboxed page on hackage has the following line class (Vector Vector a, MVector MVector a) => Unbox a Can someone explain whats the repeating Vector (and MVector) there?

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Problems linking code from the book Haskell School of Expression
by Philippe Sismondi 26 Nov '11

26 Nov '11

Greetings. Yesterday I decided to try some of the code in Paul Hudak's book The Haskell School of Expression. I am having problems when I do ghc --make of the code on page 40 of the book, which is the first graphics program therein. I am running OS X Snow Leopard, with the Haskell Platform (32 bit), ghc version 7.0.3. I am very green when it comes to looking under the covers at what gcc, ld, etc do at the behest of ghc. This is where things seem to go wrong, as I outline below. In setting up to try this code I discovered that I needed to install the package soegtk to get Graphics.SOE.Gtk. The cabal install of soegtk was not smooth: it appears to me that cabal does not always handle dependencies as one would hope. However, I eventually got soegtk and the packages upon which it depends installed. Here is my list of local user packages as reported by ghc-pkg: cairo-0.12.2 gio-0.12.2 glib-0.12.2 gtk-0.12.2 pango-0.12.2 soegtk-0.12.1 Oddly, in the course of running cabal install to get the above packages, at some point I got a flurry of messages similar to the one below. (This is not one of the actual messages from cabal install; I did not keep those. This warning message is from the ghc --make step, as described below.) ld: warning: ignoring file /opt/local/lib/libgtk-x11-2.0.dylib, file was built for unsupported file format which is not the architecture being linked (i386) This puzzled me, and the same thing came up at a later stage, as I shall describe shortly. But initially I ignored the messages and proceeded to try to make my sample program. Here is the code from Hudak's book, which I put in Main.hs: module Main where import Graphics.SOE.Gtk main = runGraphics ( do w <- openWindow "SOE First Try" (300,300) drawInWindow w (text (100,100) "Hello Graphics World") k <- getKey w closeWindow w ) When I do ghc --make Main, it grinds forever at the link stage. The link fails. It also gives ld warnings about ignoring libraries in /opt/local/lib, as in the example above. Then, the link eventually fails because of a huge number of unresolved symbols. Here is a sample link error from the verbose output: Undefined symbols for architecture i386: "_cairo_arc", referenced from: _sbnE_info in libHSsoegtk-0.12.1.a(Gtk.o) _se2n_info in libHSsoegtk-0.12.1.a(Gtk.o) _sf5B_info in libHSsoegtk-0.12.1.a(Gtk.o) _svSr_info in libHScairo-0.12.2.a(Paths.o) _swiH_info in libHScairo-0.12.2.a(Paths.o) "_cairo_arc_negative", referenced from: _seqb_info in libHSsoegtk-0.12.1.a(Gtk.o) _svPR_info in libHScairo-0.12.2.a(Paths.o) _swg7_info in libHScairo-0.12.2.a(Paths.o) and on and on. I have no idea what is happening here. First, I don't know whether the linker messages about ignoring libs in /opt/local/lib is connected with the unresolved symbols, but one would suspect that it is. Second, it is not clear to me why ghc would link against anything in /opt/local/lib, which it clearly does. Here are the first couple of lines of verbose output from ghc --make, showing the inclusion of the flag -L/opt/local/lib: /Developer/usr/bin/gcc -v -o Main -march=i686 -m32 -fno-stack-protector -DDONT_WANT_WIN32_DLL_SUPPORT Main.o -L/Users/Philippe/Library/Haskell/ghc-7.0.3/lib/soegtk-0.12.1/lib -L/Library/Haskell/ghc-7.0.3/lib/stm-2.2.0.1/lib -L/Users/Philippe/Library/Haskell/ghc-7.0.3/lib/gtk-0.12.2/lib -L/opt/local/lib -L/Users/Philippe/Library/Haskell/ghc-7.0.3/lib/pango-0.12.2/lib I have plenty of macports stuff installed in /opt/local. But I have not intentionally included /opt/local/lib in any path. That is, I do NOT have, for example, LD_LIBRARY_PATH or DYLD_LIBRARY_PATH set. So is it normal for ghc to link against /opt/local/lib libraries, or is it something in my environment that is triggering this? And, in any case, is that why the link fails? What about the references to "architecture i386"? Do they provide the necessary clue for the clueless (me)? Any help is appreciated. - Phil -

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Re: [Haskell-beginners] data design for a questionnaire (retitled + update)
by Alia 26 Nov '11

26 Nov '11

Hi folks, As the code (from my prior post) is not so relevant to the issue at hand, it was quite reasonably suggested that I should simplify my question somewhat. So here goes: In my model, I have a Question record type, which has a type parameter to take into account that answers can be evaluated to different types. For example, I can ask a question which requires a Double as an answer and another which requires an Int as an answer: data Question a = Question {text:: String, answer:: Maybe a} This question type is wrapped/boxed in another type to allow the different questions to referenced in a list. Hence, data Question' = QuestionS (Question String) | QuestionI (Question Int) | QuestionD (Question Double) and I can now store the questions as follows: questions = [ QuestionI {text="What's 1+1?", answer=Maybe 2} , QuestionD {text="What's 1.0+1.5?", answer=Maybe 2.5} ] Now to extract Question a from Question' I would have liked to write the following: extract :: Question' -> Question a extract q = case q of QuestionS x -> extractQString q QuestionI x -> extractQInt q QuestionD x -> extractQDouble q but instead, I had to produce the following instead: extractQString :: Question' -> Question String extractQString (QuestionS q) = q extractQInt :: Question' -> Question Int extractQInt (QuestionI q) = q extractQDouble :: Question' -> Question Double extractQDouble (QuestionD q) = q From what I understand so far, that specialized extraction or unboxing functions are the way to go unless one uses language extensions like GADTs which can be conceptually more complicated. Is this indeed the case, or are there other ways to achieve generalized type unboxing while remaining within the Haskell98 specification? Best, Alia

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