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Usage of rewrite rule specialization in Hackage
by Scott Kilpatrick 06 May '11

06 May '11

I'm looking for "real" code that uses the kind of GHC rewrite rule that specializes a polymorphic function with another one, as described here <http://www.haskell.org/ghc/docs/latest/html/users_guide/rewrite- rules.html#rule-spec>. Can anyone point me to any popular packages in Hackage that make use of this? Thanks, Scott

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Haskell statistics for use of TypeFamilies and FunctionalDependencies
by Gwern Branwen 05 May '11

05 May '11

Continuing my investigation of my local copies of patch-tag+github+c.h.o+misc repositories (, I rewrote my haskell-src-exts script to examine LANGUAGE extensions in use (ignoring any .cabal flags): import Language.Haskell.Exts import qualified Data.Foldable as F (concat) import Control.Monad (when) main :: IO () main = do args <- fmap lines $ getContents mapM_ (\a -> checkAndPrint a `catch` (\_ -> return ())) args checkAndPrint :: FilePath -> IO () checkAndPrint fs = do x <- readFile fs let exts = F.concat $ readExtensions x let exts' = filter (\y -> y == FunctionalDependencies || y == TypeFamilies) exts when (not $ null exts') $ putStr (fs++":") >> (putStrLn $ unwords $ map show exts) return () The attachment is the gzipped* result of `find . -type f -name "*?hs" | sort | runhaskell ~/extensions.hs >> ~/exts.txt`. Notes: - 3597 files matched either LANUAGE - there were: 2685 users of TypeFamilies 1030 users of FunctionalDependencies 118 users of TypeFamilies+FunctionalDependencies 2.7k TypeFamilies may be surprisingly large; if so, it may be due to GHC and its duplicates; 947 lines match the string 'ghc' and 721 lines match both 'ghc' and 'TypeFamilies'. * 500k is a bit much for an attachment -- gwern http://www.gwern.net

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How often is the planet updated?
by Magnus Therning 05 May '11

05 May '11

I see that Planet Haskell hasn't been updated since April 26. Is something wrong with it, or does it really not update more often than that? /M -- Magnus Therning OpenPGP: 0xAB4DFBA4 email: magnus(a)therning.org jabber: magnus(a)therning.org twitter: magthe http://therning.org/magnus

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2nd CFP: ACM SIGPLAN 7th Workshop on Generic Programming (WGP 2011)
by Shin-Cheng Mu 05 May '11

05 May '11

====================================================================== CALL FOR PAPERS WGP 2011 7th ACM SIGPLAN Workshop on Generic Programming Tokyo, Japan Sunday, September 18th, 2011 http://flolac.iis.sinica.edu.tw/wgp11/ Collocated with the International Conference on Functional Programming (ICFP 2011) ====================================================================== Goals of the workshop --------------------- Generic programming is about making programs more adaptable by making them more general. Generic programs often embody non-traditional kinds of polymorphism; ordinary programs are obtained from them by suitably instantiating their parameters. In contrast with normal programs, the parameters of a generic program are often quite rich in structure; for example they may be other programs, types or type constructors, class hierarchies, or even programming paradigms. Generic programming techniques have always been of interest, both to practitioners and to theoreticians, and, for at least 20 years, generic programming techniques have been a specific focus of research in the functional and object-oriented programming communities. Generic programming has gradually spread to more and more mainstream languages, and today is widely used in industry. This workshop brings together leading researchers and practitioners in generic programming from around the world, and features papers capturing the state of the art in this important area. We welcome contributions on all aspects, theoretical as well as practical, of * generic programming, * programming with (C++) concepts, * meta-programming, * programming with type classes, * programming with modules, * programming with dependent types, * polytypic programming, * adaptive object-oriented programming, * component-based programming, * strategic programming, * aspect-oriented programming, * family polymorphism, * object-oriented generic programming, * and so on. Organizers ---------- Co-Chair Jaakko Järvi, Texas A&M University, USA Co-Chair Shin-Cheng Mu, Academia Sinica, Taiwan Programme Committee ------------------- Dave Abrahams, BoostPro Computing, USA Magne Haveraaen, Universitetet i Bergen, Norway Akimasa Morihata, Tohoku University, Japan Pablo Nogueira, Universidad Politécnica de Madrid, Spain Ulf Norell, Chalmers University of Technology and University of Gothenberg, Sweden Ross Paterson, City University London, UK Rinus Plasmeijer, Radboud University Nijmegen, The Netherlands Sibylle Schupp, Technische Universität Hamburg-Harburg, Germany Andrew Sutton, Kent State University, USA Tarmo Uustalu, Institute of Cybernetics, Estonia Important Information --------------------- We plan to have formal proceedings, published by the ACM. Submission details Deadline for submission: Monday 2011-06-06 Notification of acceptance: Tuesday 2011-07-01 Final submission due: Monday 2011-07-25 Workshop: Sunday 2011-09-18 Authors should submit papers, in postscript or PDF format, formatted for A4 paper, to the WGP11 EasyChair instance by the above deadline. The length should be restricted to 12 pages in standard (two-column, 9pt) ACM format. Accepted papers are published by the ACM and will additionally appear in the ACM digital library. History of the Workshop on Generic Programming ---------------------------------------------- This year: * Tokyo, Japan 2011 (affiliated with ICFP11) Earlier Workshops on Generic Programming have been held in * Baltimore, Maryland, US 2010 (affiliated with ICFP10) * Edinburgh, UK 2009 (affiliated with ICFP09) * Victoria, BC, Canada 2008 (affiliated with ICFP), * Portland 2006 (affiliated with ICFP), * Ponte de Lima 2000 (affiliated with MPC), * Marstrand 1998 (affiliated with MPC). Furthermore, there were a few informal workshops * Utrecht 2005 (informal workshop), * Dagstuhl 2002 (IFIP WG2.1 Working Conference), * Nottingham 2001 (informal workshop), There were also (closely related) DGP workshops in Oxford (June 3-4 2004), and a Spring School on DGP in Nottingham (April 24-27 2006, which had a half-day workshop attached). Additional information: The WGP steering committee consists of J. Gibbons, R. Hinze, P. Jansson, J. Järvi, J. Jeuring, B. Oliveira, S. Schupp, and M. Zalewski

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Haskell Weekly News - Issue 180
by Daniel Santa Cruz 05 May '11

05 May '11

Welcome to issue 180 of the HWN, a newsletter covering developments in the [1]Haskell community. This release covers the week of April 24 to 30, 2011. Announcements Eric Kow suggested (http://goo.gl/bc4JM) the creation of a Haskell User Group starter kit. If you have ideas in this department, please see what Eric has started! Christopher Done wrote (http://goo.gl/2m7AJ) a response to a blog from Cedric entitled "Why Scala's Option and Haskell's Maybe types won't save you from Null." A lively conversation followed. Henning Thielemann started an very interesting thread (http://goo.gl/FWyDE) with the title "Python is lazier than Haskell." (See the quotes of the week for the full quote) Quotes of the Week These quotes are taken from the #haskell channel, and from the haskell/haskell-cafe mailing lists. * Henning Thielemann: If Haskell is great because of its laziness, then Python must be even greater, since it is lazy at the type level. * Ben Lippmeier: Laziness at the value level causes space leaks, and laziness at the type level causes mind leaks. Neither are much fun. When people start wanting laziness at the kind level we'll have to quarantine them before the virus spreads... * Gregg Reynolds: If the designers could find a way to support laziness at the programmer level, I for one would be very grateful. * Felipe Almeida Lessa: But I am sure that john-millikin-is-great.txt must be increasing like a memory leak ;) * James Cook: I think Haskell questions on SO tend to the opposite extreme; no matter how poorly thought-out the question, the Haskell community will descend on it like a swarm of helpful piranhas. * Philippa: C++ seeing a renaissance as a functional language, is this a bit like rusty nails seeing a renaissance as sex toys for masochists? * #haskell: <aristid> preflex: seen FunctorSalad * preflex_ is now known as preflex. * copumpkin is now known as postflex. * xplat is now known as circumflex. <circumflex> FunctorSalad was never seen on #haskell-world-conquest -4 years, 3 days, 7i minutes and 0 seconds ago, saying: RELEASE THE KRAKEN! * tolkad: Without a standard you are lost, adrift in a sea of unspecified semantics. Hold fast to the rules of the specification lest you be consumed by ambiguity. * Saizan: it's much easier via type theory, 1) prove bottom 2) ???? 3) quodlibet * dons: Think of a monad as a spacesuit full of nuclear waste in the ocean next to a container of apples. Now, you can't put oranges in the space suite or the nuclear waste falls in the ocean, *but* the apples are carried around anyway, and you just take what you need. * tolkad: Perhaps by using it we could modernise haskell by applying the principles of the new kind of science Top Reddit Stories * The Irish Times: "Functional programming is more than just esoteric; it's becoming somewhat cool." Domain: irishtimes.com, Score: 44, Comments: 6 On Reddit: http://goo.gl/cfUbb Original: http://goo.gl/1nHEY * Why Silkapp uses Haskell Domain: blog.silkapp.com, Score: 43, Comments: 0 On Reddit: http://goo.gl/IFREi Original: http://goo.gl/1SwNa * Harper again: The real point of laziness Domain: existentialtype.wordpress.com, Score: 38, Comments: 89 On Reddit: http://goo.gl/vA8oD Original: http://goo.gl/hqcpy * Patent 5,893,120 reduced to mathematical formulae (via Haskell) Domain: paulspontifications.blogspot.com, Score: 34, Comments: 13 On Reddit: http://goo.gl/PmB4K Original: http://goo.gl/QknmV * IO evaluates the Haskell heap Domain: blog.ezyang.com, Score: 33, Comments: 3 On Reddit: http://goo.gl/FzaGV Original: http://goo.gl/KNHku * Why Haskell? Domain: blog.mired.org, Score: 33, Comments: 19 On Reddit: http://goo.gl/t1r8I Original: http://goo.gl/cV6KL * Filter and Visualize Data in Seconds with Silk (Haskell-based startup) Domain: thenextweb.com, Score: 33, Comments: 18 On Reddit: http://goo.gl/hA9HF Original: http://goo.gl/NIR3U * Haskell libraries you should use Domain: blog.johantibell.com, Score: 29, Comments: 3 On Reddit: http://goo.gl/PPTWG Original: http://goo.gl/LmbT6 * Tying the Knot - a really mind bending Haskell technique Domain: haskell.org, Score: 27, Comments: 8 On Reddit: http://goo.gl/iEdSM Original: http://goo.gl/5Zg6f * haskell.org GSoC accepted projects are up. Domain: self.haskell, Score: 26, Comments: 32 On Reddit: http://goo.gl/mnrl9 Original: /r/haskell/comments/gxj1h/haskellorg_gsoc_accepted_projects_are_up/ Top StackOverflow Questions * What are the best Haskell libraries to operationalize a program? votes: 57, answers: 3 Read on SO: http://goo.gl/rYQiL * How do you design programs in Haskell or other functional programming languages? [migrated] votes: 24, answers: 0 Read on SO: http://goo.gl/IW7Z5 * Haskell: why the convention to name a helper function "go"? votes: 22, answers: 3 Read on SO: http://goo.gl/CE1O7 * What haskell topics need to be addressed in a Real-World-Haskell style? votes: 21, answers: 5 Read on SO: http://goo.gl/3tgE4 * What are some interesting uses of high order functions? votes: 14, answers: 14 Read on SO: http://goo.gl/WdGeF * How are mutable arrays implemented in Haskell? votes: 10, answers: 3 Read on SO: http://goo.gl/LMK8u * Performance problem with Euler problem and recursion on Int64 types votes: 10, answers: 6 Read on SO: http://goo.gl/CLXD2 About the Haskell Weekly News To help create new editions of this newsletter, please send stories to dstcruz(a)gmail.com. Until next time, Daniel Santa Cruz

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Can't install haskellnet with ghc7
by Charles-Pierre Astolfi 04 May '11

04 May '11

Hi -cafe, Did anybody managed to install haskellnet from hackage with ghc7? I tried on windows and mac and I get the following type error: [ 4 of 11] Compiling Network.HaskellNet.SMTP ( Network/HaskellNet/SMTP.hs, dist/build/Network/HaskellNet/SMTP.o ) Network/HaskellNet/SMTP.hs:269:25: Couldn't match expected type `Data.Text.Internal.Text' with actual type `[Char]' Expected type: Data.Text.Internal.Text Actual type: String In the first argument of `simpleMail', namely `to' In a stmt of a 'do' expression: myMail <- simpleMail to from subject plainBody htmlBody attachments Am I missing something? -- Cp

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warning - Euler problem spoiler enclosed
by Barbara Shirtcliff 04 May '11

04 May '11

Hi, In the following solution to problem 24, why is nub ignored? I.e. if you do lexOrder of "0012," you get twice as many permutations as with "012," even though I have used nub. puzzled, Bar -- file Euler.hs module Euler where import Data.List {- problem 24 A permutation is an ordered arrangement of objects. For example, 3124 is one possible permutation of the digits 1, 2, 3 and 4. If all of the permutations are listed numerically or alphabetically, we call it lexicographic order. The lexicographic permutations of 0, 1 and 2 are: 012 021 102 120 201 210 What is the millionth lexicographic permutation of the digits 0, 1, 2, 3, 4, 5, 6, 7, 8 and 9? -} lexI :: Char -> String -> Int lexI c s = maybe 1 (id) $ elemIndex c s lexOrder :: [Char] -> [[Char]] lexOrder s | length s == 1 = [s] | length s == 2 = z : [reverse z] | otherwise = concat $ map (\n -> h n) [0..((length s) - 1)] where z = sort $ nub s -- why is the nub ignored here? h :: Int -> [String] h n = map (z!!n :) $ lexOrder $ filter (\c -> lexI c z /= n) z p24 = (lexOrder "1234567890")!!999999 main :: IO() main = do putStrLn $ show $ p24

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Advertisement: the Haskell Stack Overflow Q & A site
by Don Stewart 04 May '11

04 May '11

Hey all, I thought I'd just make a quick advertisement for the Haskell Stack Overflow community: http://stackoverflow.com/questions/tagged/haskell as a forum for questions and answers on beginner to advanced Haskell problems. The site is very active, with roughly as many questions being asked on SO as on haskell-cafe these days. One of the benefits of a site like SO as a forum is the ability to record and link to prior work, edit for technical errors, and easily search and categorize past answers. It is also less prone to noise, for those suffering from cafe overload. Cheers, Don

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Another good video tutorial on monads
by C K Kashyap 04 May '11

04 May '11

http://vimeo.com/20717301 I really liked how he starts off with the let statement. Regards, Kashyap

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Re: [Haskell-cafe] Please add a method for optimized concat to the Semigroup class
by John Lato 04 May '11

04 May '11

> > From: Edward Kmett <ekmett(a)gmail.com> > > On Tue, May 3, 2011 at 3:43 PM, Yitzchak Gale <gale(a)sefer.org> wrote: > > > I'm sure there are countless other natural examples of semigroups > > in the wild, and that the typical non-trivial ones will benefit > > from an optimized sconcat. > > > > Sold! (modulo the semantic considerations above) > Somewhat academic, but would there be a case for implementing sconcat in terms of Foldable (or similar)? John L

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