April 2011 - Haskell-Cafe

parallel-haskell mailing list
by Eric Y. Kow 14 Apr '11

14 Apr '11

Hi everybody, I thought I should point out that there is a mailing list dedicated to parallelism and concurrency in Haskell. The group exists to bring together the various groups in the Haskell community that are working on parallelism. It is intended to provide some visibility into the various efforts and to encourage collaboration. So if you want to know what's going on in the parallel Haskell world, this is the place to hang out. You can join the list by sending an email to parallel-haskell+subscribe(a)googlegroups.com You can also visit the group on the web at http://groups.google.com/group/parallel-haskell Or for those of you who prefer GMane http://dir.gmane.org/gmane.comp.lang.haskell.parallel Enjoy! Eric -- Got news for the Parallel Haskell Digest? Send a mail to eric(a)well-typed.com!

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Munich functional programmers' UG
by "Dr. Heinrich Hördegen" 14 Apr '11

14 Apr '11

Dear all, last week, there have been thoughts to create a user group for functional programmers in Munich, Germany. People who are interessted can join the new mailing list: http://www.fs.lmu.de/cgi-bin/mailman/listinfo/high-order-munich The idea is to have meetings every now and then with talks on aspects of functional programming. However, it's not restricted to Haskell. Please spread the word to anyone who could be interested. Thank you, Heinrich -- hoerdegen(a)funktional.info www.funktional.info --

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Haskell on-line judge for Programming Challenges / Contests ?
by Dmitri O.Kondratiev 14 Apr '11

14 Apr '11

Hello, I am looking for a site providing on-line automatic judge for Programming Challenges that can be coded in Haskell. For example this site: http://www.programming-challenges.com provides lots of interesting programming provlems: http://www.programming-challenges.com/pg.php?page=index Yet, unfortunately only C, C++ and Java code can be submitted, judge on that site can't compile and run Haskell code. Is there any similar online judge for Haskell? Thanks, Dmitri

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IO and Cont as monads
by Burak Ekici 14 Apr '11

14 Apr '11

Dear List, I am quite new in Haskell's categorical manner of programming. However I have enough knowledge in Category Theory. I want to ask a question, maybe very well-known one by some of you, about monads of Haskell. For the type constructors like Maybe and [], I managed to prove that together with 2 natural transformations (bind + return), both of these triples construct a monad. But when I try to prove that IO and Cont type constructors with the same natural transformations (bind + return) are monads as well, it was failed. Here my question is: Is there anyone who knows how to prove that IO and Cont are monads with satisfing following properties: join . fmap join = join . joinjoin . fmap return = join . return = idreturn . f = fmap f . returnjoin . fmap (fmap f) = fmap f . join Thanks already now, Burak Ekici.

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Template Haskell tutorials?
by Kenneth Hoste 14 Apr '11

14 Apr '11

Hi, The links to the supposedly brilliant Template Haskell tutorials by Bulat are broken. http://www.haskell.org/bz/thdoc.htm http://www.haskell.org/bz/th3.htm Does anyone know if these tutorials moved to somewhere else? greetings, Kenneth

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Haskell Weekly News: Issue 177
by Daniel Santa Cruz 14 Apr '11

14 Apr '11

Welcome to issue 177 of the HWN, a newsletter covering developments in the [1]Haskell community. This release covers the week of April 3 to 9, 2011. You can find a HTML rendition of this issue at: http://bit.ly/hR2sMB Announcements Brent Yorgey [2]sent out a call for contributions for the second edition of Typeclassopedia. Roel van Dijk started what became a popular[3]thread on the mailing list about encodings of Haskell source files. What would you like to see here? I have gotten mixed feedback on what constitutes interesting items for the "announcements" section of the mailing list. Some have pointed out that package announcements tend to be noisy and not all that interesting to the community at large, or that have a bias towards only pointing out packages where the maintainer decides to send an email to the mailing list. What do *you* think? How about the "calls for papers"? Do those catch your attention? Let me hear what you think. Quotes of the Week * ion: Itâs a well-known historical fact cave men writing Haskell hated the ASCII pseudoglyphs. * shapr: I persuaded academics to show up and hang out. [...] I told the academics that there were academics worth chatting with on #haskell :-) * pjscott: If you really want to see something ridiculous, try looking at the auto-generated library documentation for anything involving regular expressions. The type declarations in Text.Regex.TDFA are like something H. P. Lovecraft might write about. * monochrom: the road to haskell is paved with good abstractions Top Reddit Stories * Typeclassopedia second edition: call for contributions From (byorgey.wordpress.com) scored 53 with 1 comments. Read on [4]reddit. Read the [5]original post. * High-Performance Web Applications In Haskell : Gregory Collins : QCon London :: PDF From (qconlondon.com) scored 45 with 8 comments. Read on [6]reddit. Read the [7]original post. * Haskell For the Cloud (new paper by Jeff Epstein, Andrew Black, and and Simon Peyton Jones) From (research.microsoft.com) scored 41 with 23 comments. Read on [8]reddit. Read the [9]original post. * Yhc is dead !? From (yhc06.blogspot.com) scored 32 with 27 comments. Read on [10]reddit. Read the [11]original post. * What is a functional language? :: Robert Harper From (existentialtype.wordpress.com) scored 29 with 35 comments. Read on [12]reddit. Read the [13]original post. * Functions are values From (existentialtype.wordpress.com) scored 27 with 16 comments. Read on [14]reddit. Read the [15]original post. * Understanding practical API design, static typing and functional programming. From (blog.tmorris.net) scored 27 with 2 comments. Read on [16]reddit. Read the [17]original post. * Platform Robots in Nikki and the Robots: a Haskell platform game From (joyridelabs.de), scored 22 with 3 comments. Read on [18]reddit. Read the [19]original post. * A Model for Denotative Continuous-Time Programming (AKA: FRP) From (creativelad.wordpress.com) scored 21 with 1 comments. Read on [20]reddit. Read the [21]original post. * Type Kata: Local data type From (blog.ezyang.com) scored 19 with 3 comments. Read on [22]reddit. Read the [23]original post. Top StackOverflow Questions * [24]Difference between State, ST, IORef, and MVar votes: 16, answers: 3 * [25]Duality approaches in functional programming votes: 11, answers: 1 * [26]What are the main differences of language features between Haskell and the functional part of F#? [closed] votes: 9, answers: 5 * [27]Using the Maybe Monad in "reverse" in Haskell votes: 9, answers: 3 * [28]How to reason about space complexity in Haskell votes: 8, answers: 1 About the Haskell Weekly News To help create new editions of this newsletter, please send stories to dstcruz(a)gmail.com. I'm in dire need of finding good "quotes of the week". If you happen to come across any, please don't hesitate to send it along. Until next time, Daniel Santa Cruz References 1. http://haskell.org/ 2. http://article.gmane.org/gmane.comp.lang.haskell.general/18636 3. http://article.gmane.org/gmane.comp.lang.haskell.cafe/87815 4. http://www.reddit.com/r/haskell/comments/ghs1x/typeclassopedia_second_editi… 5. http://byorgey.wordpress.com/2011/04/03/call-for-contributions-second-editi… 6. http://www.reddit.com/r/haskell/comments/gj7ud/highperformance_web_applicat… 7. http://qconlondon.com/dl/qcon-london-2011/slides/GregoryCollins_HighPerform… 8. http://www.reddit.com/r/haskell/comments/gjb52/haskell_for_the_cloud_new_pa… 9. http://research.microsoft.com/en-us/um/people/simonpj/papers/parallel/ 10. http://www.reddit.com/r/haskell/comments/ghtlf/yhc_is_dead/ 11. http://yhc06.blogspot.com/2011/04/yhc-is-dead.html 12. http://www.reddit.com/r/haskell/comments/gk1tp/what_is_a_functional_languag… 13. http://existentialtype.wordpress.com/2011/03/16/what-is-a-functional-langua… 14. http://www.reddit.com/r/haskell/comments/ghdvn/functions_are_values/ 15. http://existentialtype.wordpress.com/2011/04/02/functions-are-values/ 16. http://www.reddit.com/r/haskell/comments/gir70/understanding_practical_api_… 17. http://blog.tmorris.net/understanding-practical-api-design-static-typing-an… 18. http://www.reddit.com/r/haskell/comments/ghs81/platform_robots_in_nikki_and… 19. http://joyridelabs.de/blog/?p=612 20. http://www.reddit.com/r/haskell/comments/ghrmv/a_model_for_denotative_conti… 21. http://creativelad.wordpress.com/2011/04/03/dctp-model/ 22. http://www.reddit.com/r/haskell/comments/gk1w4/type_kata_local_data_type/ 23. http://blog.ezyang.com/2011/04/type-kata-local-data-type/ 24. http://stackoverflow.com/questions/5545517/difference-between-state-st-iore… 25. http://stackoverflow.com/questions/5557810/duality-approaches-in-functional… 26. http://stackoverflow.com/questions/5541487/what-are-the-main-differences-of… 27. http://stackoverflow.com/questions/5606228/using-the-maybe-monad-in-reverse… 28. http://stackoverflow.com/questions/5552433/how-to-reason-about-space-comple…

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Fucntion composing
by Adam Krauze 13 Apr '11

13 Apr '11

Hello, as I am newbie to Haskell and my introductory question is: given functions say f and g with type signatures f :: (Num a) => [a] -> [a] -> [(a,a)] // f takes two lists and zips them into one in some special way g :: (Num a) => a -> [(a,a)] -> [a] // g using some Num value calculates list of singletons from list of pairs of course g 0 :: (Num a) => [(a,a)] ->[a] now I want to create function h :: (Num a) => [a] -> [a] -> [a] in such way that (g 0) consumes output of f. But when I try Prelude> :t (g 0).f I get an error: <interactive>:1:9: Couldn't match expected type `[(a0, a0)]' with actual type `[a1] -> [(a1, a1)]' Expected type: [a1] -> [(a0, a0)] Actual type: [a1] -> [a1] -> [(a1, a1)] In the second argument of `(.)', namely `f' In the expression: (g 0) . f In pointfull representation it works well Prelude> let h x y = (g 0 (f x y)) How to do pointfree definition of h? Ajschylos.

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Service Component Architecture and Distributed Haskell
by James Keats 13 Apr '11

13 Apr '11

Hi all. There's recently been some talk about distributed haskell, but what I wish to see is interest from an experienced member of the Haskell community in implementing Haskell components with SCA/Tuscany. Please see this and consider it, it'd be great to be able to use Haskell in the same project with Java/Scala. Thanks. Link: http://www.haskell.org/pipermail/haskell/2010-April/022004.html

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Higher-kinded Quantification
by Leon Smith 13 Apr '11

13 Apr '11

I have a type constructor (Iterator i o m a) of kind (* -> * -> (* -> *) -> *), which is a monad transformer, and I'd like to use the type system to express the fact that some computations must be "pure", by writing the impredicative type (Iterator i o (forall m. m) a). However I've run into a bit of difficulty expressing this, due to the kind of m. I've attached a minimal-ish example. Is there a way to express this in GHC?

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Generating random graph
by Mitar 13 Apr '11

13 Apr '11

Hi! I have made this function to generate a random graph for Data.Graph.Inductive library: generateGraph :: Int -> IO (Gr String Double) generateGraph graphSize = do when (graphSize < 1) $ throwIO $ AssertionFailed $ "Graph size out of bounds " ++ show graphSize let ns = map (\n -> (n, show n)) [1..graphSize] es <- fmap concat $ forM [1..graphSize] $ \node -> do nedges <- randomRIO (0, graphSize) others <- fmap (filter (node /=) . nub) $ forM [1..nedges] $ \_ -> randomRIO (1, graphSize) gen <- getStdGen let weights = randomRs (1, 10) gen return $ zip3 (repeat node) others weights return $ mkGraph ns es But I noticed that graph has sometimes same weights on different edges. This is very unlikely to happen so probably I have some error using random generators. Could somebody tell me where? Mitar

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