January 2012 - Haskell-Cafe

Haddock 2.8.1 and alex 3.0.1
by cwr＠netcom.co.uk 20 Jan '12

20 Jan '12

Building haddock 2.8.1 with alex 3.0.1 I get the error message: [ 7 of 33] Compiling Haddock.Lex ( dist/build/Haddock/Lex.hs, dist/build/Haddock/Lex.o ) src/Haddock/Lex.x:150:17: Couldn't match expected type `AlexInput' against inferred type `(t, t1, t2)' In the first argument of `alexScan', namely `inp' In the expression: alexScan inp sc In the expression: case alexScan inp sc of { AlexEOF -> [] AlexError _ -> error "lexical error" AlexSkip inp' _ -> go inp' sc AlexToken (inp'@(pos', _, _)) len act -> Haddock 2.8.1 builds correctly (or at least completely) with alex 2.3.5 I'm not sure if this is a bug or a feature - if it's a feature, then the dependencies for haddock 2.8.1 need updating. Will

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Not an isomorphism, but what to call it?
by Sean Leather 20 Jan '12

20 Jan '12

I have two types A and B, and I want to express that the composition of two functions f :: B -> A and g :: A -> B gives me the identity idA = f . g :: A -> A. I don't need g . f :: B -> B to be the identity on B, so I want a weaker statement than isomorphism. I understand that: (1) If I look at it from the perspective of f, then g is the right inverse or section (or split monomorphism). (2) If I look at from g, then f is the left inverse or retraction (or split epimorphism). But I just want two functions that give me an identity on one of the two types and I don't care which function's perspective I'm looking at it from. Is there a word for that? Regards, Sean

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Vector (or List) to CArray?
by Dominic Espinosa 19 Jan '12

19 Jan '12

Hello, I'm trying to use the fftw binding, and its functions operate on CArrays of Complex. My data is coming from hsndfile, so it starts out as a Vector of Double. How do I convert this data to CArray? The API functions in the CArray module don't seem to indicate how. Thanks.

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cabal-install bootstrap.sh
by cwr＠netcom.co.uk 19 Jan '12

19 Jan '12

I've just spent most of a morning trying to get bootstrap.sh from the cabal-install package to work. The trick is to use "ghc-pkg init pathname" to initialise the package file - simply adding an empty package file or directory doesn't work. Whoever is responsible for cabal-install, could you please update the README to reflect this requirement. Thanks - Will

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Is there a way to do *allocation* profiling with a breakdown by type?
by Eugene Kirpichov 19 Jan '12

19 Jan '12

Hi, I've got a program that seems to spend much of its time allocating short-lived objects, which therefore don't show up in +RTS -hy or alike. Is there a way to get a breakdown by type of the objects that are being *allocated* but not necessarily retained (disappear after gen0)? -- Eugene Kirpichov Principal Engineer, Mirantis Inc. http://www.mirantis.com/ Editor, http://fprog.ru/

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Conduit versions of wai and warp?
by Erik de Castro Lopo 19 Jan '12

19 Jan '12

Hi Michael, The current versions of wai and warp: http://hackage.haskell.org/package/wai http://hackage.haskell.org/package/warp still seem to be the versions that use enumerator. Any idea when the Conduit versions might show up on Hackage? Until then, should I be grabbing these packages from Github? Cheers, Erik -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/

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PhD program at Portland State accepting applications for Fall 2012
by Nathan Collins 19 Jan '12

19 Jan '12

Hi, Portland State University has a lot going on in functional programming. The Fall 2012 PhD program application deadline is March 1 for US students and February 1 for international students: http://cs.pdx.edu/programs/admissions FP related work at Portland: - Tim Sheard is working on the Trellys project: design and implementation (in Haskell) of a "practical" dependently typed programming language. Joint project with Aaron Stump at UIowa and Stephanie Weirich at UPenn. Here "practical" means intended for programming more than for theorem proving. Supports theorem proving, but also logically dubious features like general recursion and Type in Type. The key design issue is the interplay between safe and unsafe features. Project repo: https://code.google.com/p/trellys/ - Andrew Tolmach, Mark Jones, and James Hook are working on HASP: design and implementation (in Haskell) of a functional programming language for high-assurance systems programming. Project page: http://hasp.cs.pdx.edu/ - Sergio Antoy is working on narrowing in functional logic programming and the Curry FLP language. Intro to functional logic programming: http://web.cecs.pdx.edu/~antoy/homepage/publications/cacm/paper.pdf PAKCS Curry implementation: http://www.informatik.uni-kiel.de/~pakcs/ - Arthur Peters, a student of Sergio’s, is working on a new implementation of Curry based on a simplified graph rewriting model of functional logic computation. A paper about it and the prototype implementation are available at: http://web.cecs.pdx.edu/~amp4/vialois - Andrew Black is a co-author on a recent paper on Haskell for the Cloud: https://research.microsoft.com/en-us/um/people/simonpj/papers/parallel/ Other FP resources in Portland: - Galois, located a half mile from the computer science department, hosts many "Tech Talks", open to the public: http://corp.galois.com/blog/ - Functional programming study group: http://pdxfunc.org/ Living in Portland: - Portland is a very popular US city, known for beer, bikes, music, and street food: http://en.wikipedia.org/wiki/Portland_Oregon (wikipedia is blacked out today) Cheers, -nathan (PhD student in programming languages)

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Haskell Weekly News: Issue 211
by Daniel Santa Cruz 19 Jan '12

19 Jan '12

Welcome to issue 211 of the HWN, a newsletter covering developments in the Haskell community. This release covers top stories found around the net between January 01 to 14, 2012. This is going to be a shorted version, as I have been out of time. I did not want to let more time go by without a heartbeat from your friendly HWN. Happy coding! You can find an HTML version at: http://contemplatecode.blogspot.com/2012/01/haskell-weekly-news-issue-211.h… Quotes of the Week * <ddarius> xtalmath: #math or #not-math would probably be a more appropriate place to ask. <shachaf> ddarius: Those two channels pretty much cover it all. <ddarius> shachaf: You're clearly not an intuitionist. * monochrom: in fact, freedom is the freedom to write 2.2 = 4 * Jafet: caleskell is the standard golf course. * byorgey: Escaped skolem! Authorities mount massive search. News at 11. * BMeph: loves swapping function arguments, he said flippantly * kmc: "Haskell is great, because Curry-Howard! Proving things in the type system. We can prove that, uh, Ints exist, unless they're [bottom]." * Jafet: In Soviet Russia, State executes you * byorgey: that would be known as "low-dimensional linear algebra", where "low-dimensional" is said in as condescending a tone as possible * monochrom: at a crossroad you meet an angel (always tells the truth) or demon (always lies). the crossroad gives you two choices of roads, one to heaven, one to hell, and only the angel or demon knows which is which. if you ask "how would you answer if I asked 'what is a monad?'", what will happen? Top Reddit Stories * Dropbox tech blog » Using the Dropbox API from Haskell Domain: tech.dropbox.com, Score: 68, Comments: 4 On Reddit: [1] http://goo.gl/rsndV Original: [2] http://goo.gl/g3RyF * A new approach to iteratees Domain: self.haskell, Score: 48, Comments: 99 On Reddit: [3] http://goo.gl/ZRbGW Original: [4] http://goo.gl/ZRbGW * Haskell / Repa Real-Time Ray Tracing Demo Domain: youtube.com, Score: 45, Comments: 7 On Reddit: [5] http://goo.gl/rzyZs Original: [6] http://goo.gl/15P2V * Order theory for computer scientists Domain: matt.might.net, Score: 39, Comments: 1 On Reddit: [7] http://goo.gl/nTOfr Original: [8] http://goo.gl/HAVjQ * Generalizing Streams into Coroutines (with an introduction to functors, applicative functors and arrows) Domain: github.com, Score: 38, Comments: 15 On Reddit: [9] http://goo.gl/hUopY Original: [10] http://goo.gl/avTUf * Well-Typed are hiring: Haskell consultant Domain: well-typed.com, Score: 38, Comments: 71 On Reddit: [11] http://goo.gl/Chu7f Original: [12] http://goo.gl/VHPfJ * Why iteratees are hard to understand Domain: blog.ezyang.com, Score: 34, Comments: 11 On Reddit: [13] http://goo.gl/LrEoV Original: [14] http://goo.gl/d1scG * Modern reall-world Haskell? Domain: self.haskell, Score: 33, Comments: 21 On Reddit: [15] http://goo.gl/l0Aud Original: [16] http://goo.gl/l0Aud * FRP: reactive-banana is getting dynamic event switching Domain: apfelmus.nfshost.com, Score: 30, Comments: 3 On Reddit: [17] http://goo.gl/dGblT Original: [18] http://goo.gl/ovRE5 * wxHaskell and wxWidgets 2.9 Domain: wewantarock.wordpress.com, Score: 29, Comments: On Reddit: [19] http://goo.gl/sxiBx Original: [20] http://goo.gl/878Aj * Updated Yesod blog example: i18n, authentication, authorization, and database Domain: yesodweb.com, Score: 28, Comments: 22 On Reddit: [21] http://goo.gl/4eDAT Original: [22] http://goo.gl/duzSG * Experiments in Haskell Packaging Domain: justhaskell.org, Score: 27, Comments: 5 On Reddit: [23] http://goo.gl/y2vHk Original: [24] http://goo.gl/0nADv * http-conduit Domain: yesodweb.com, Score: 26, Comments: 26 On Reddit: [25] http://goo.gl/gisUJ Original: [26] http://goo.gl/mMhaf * Reasoning about Stream Processing with Effects Domain: personal.cis.strath.ac.uk, Score: 26, Comments: 6 On Reddit: [27] http://goo.gl/3Ckzr Original: [28] http://goo.gl/Uw8ZO * Simon Marlow's Parallel and Concurrent Programming Haskell (slides) Domain: self.haskell, Score: 26, Comments: 3 On Reddit: [29] http://goo.gl/Rg0Lz Original: [30] http://goo.gl/Rg0Lz * Who is my Community? -- wxHaskell maintainer's thoughts on C bindings Domain: wewantarock.wordpress.com, Score: 26, Comments: On Reddit: [31] http://goo.gl/tcqZN Original: [32] http://goo.gl/mRwQa Top StackOverflow Questions * Fun with repeated fmap votes: 26, answers: 1 Read on SO: [33] http://goo.gl/T2FPm * Can a compiler automatically detect pure functions without the type information about purity? votes: 23, answers: 2 Read on SO: [34] http://goo.gl/wgYim * What does “floated out” mean? votes: 21, answers: 2 Read on SO: [35] http://goo.gl/8J3no * Haskell projects to study and learn from? votes: 19, answers: 2 Read on SO: [36] http://goo.gl/SHwsZ * How do you identify monadic design patterns? votes: 19, answers: 4 Read on SO: [37] http://goo.gl/dE3EJ * how do I get good (small) shrinks out of QuickCheck? votes: 16, answers: 0 Read on SO: [38] http://goo.gl/X3ZOs * Which applicative functor is used for passing shared parameters? votes: 14, answers: 1 Read on SO: [39] http://goo.gl/UxmZK * Double map in haskell? votes: 12, answers: 2 Read on SO: [40] http://goo.gl/uWpuz * Higher ranked and impredicative types votes: 12, answers: 1 Read on SO: [41] http://goo.gl/zpkXn * Handling events in Haskell votes: 12, answers: 1 Read on SO: [42] http://goo.gl/e7sV9 * Does the term “monadic” in J have anything to do with its Haskell use? votes: 12, answers: 4 Read on SO: [43] http://goo.gl/hTa2j * Any good articles on optimizing Haskell code? votes: 12, answers: 2 Read on SO: [44] http://goo.gl/FzRrf * What is a “free variable”? votes: 11, answers: 2 Read on SO: [45] http://goo.gl/Og5SC * Haskell Thread Communication Pattern Scenario votes: 10, answers: 1 Read on SO: [46] http://goo.gl/3Rzq0 * Defining a function by equations with different number of arguments votes: 10, answers: 1 Read on SO: [47] http://goo.gl/hejqO * What functionality do you get for free with Functors or other type-classes? votes: 10, answers: 5 Read on SO: [48] http://goo.gl/9hSlv * Why are types with no data constructors valid? votes: 10, answers: 1 Read on SO: [49] http://goo.gl/Pp6Ze * Folding flatMap/bind over a list of functions (a.k.a. Name That Combinator!) votes: 9, answers: 2 Read on SO: [50] http://goo.gl/bBtBI Until next time, Daniel Santa Cruz References 1. http://tech.dropbox.com/?p=129 2. http://www.reddit.com/r/haskell/comments/o0j2a/dropbox_tech_blog_using_the_… 3. http://www.reddit.com/r/haskell/comments/ohjg7/a_new_approach_to_iteratees/ 4. http://www.reddit.com/r/haskell/comments/ohjg7/a_new_approach_to_iteratees/ 5. https://www.youtube.com/watch?v=jBd9c1gAqWs 6. http://www.reddit.com/r/haskell/comments/o0im2/haskell_repa_realtime_ray_tr… 7. http://matt.might.net/articles/partial-orders/ 8. http://www.reddit.com/r/haskell/comments/o6nnp/order_theory_for_computer_sc… 9. https://github.com/leonidas/codeblog/blob/master/2012/2012-01-08-streams-co… 10. http://www.reddit.com/r/haskell/comments/o87z5/generalizing_streams_into_co… 11. http://www.well-typed.com/blog/63 12. http://www.reddit.com/r/haskell/comments/odv3t/welltyped_are_hiring_haskell… 13. http://blog.ezyang.com/2012/01/why-iteratees-are-hard-to-understand/ 14. http://www.reddit.com/r/haskell/comments/o2p3p/why_iteratees_are_hard_to_un… 15. http://www.reddit.com/r/haskell/comments/o5iov/modern_reallworld_haskell/ 16. http://www.reddit.com/r/haskell/comments/o5iov/modern_reallworld_haskell/ 17. http://apfelmus.nfshost.com/blog/2012/01/01-frp-api-0-5.html 18. http://www.reddit.com/r/haskell/comments/nz0af/frp_reactivebanana_is_gettin… 19. http://wewantarock.wordpress.com/2012/01/11/wxhaskell-and-wxwidgets-2-9/ 20. http://www.reddit.com/r/haskell/comments/oc8qi/wxhaskell_and_wxwidgets_29/ 21. http://www.yesodweb.com/blog/2012/01/blog-example 22. http://www.reddit.com/r/haskell/comments/of4g9/updated_yesod_blog_example_i… 23. http://justhaskell.org/2012/01/02/experiments-in-haskell-packaging/ 24. http://www.reddit.com/r/haskell/comments/nzv56/experiments_in_haskell_packa… 25. http://www.yesodweb.com/blog/2012/01/http-conduit 26. http://www.reddit.com/r/haskell/comments/o5b4w/httpconduit/ 27. http://personal.cis.strath.ac.uk/~raa/posts/2012-01-06-streams.html 28. http://www.reddit.com/r/haskell/comments/o5ioy/reasoning_about_stream_proce… 29. http://www.reddit.com/r/haskell/comments/oaqsa/simon_marlows_parallel_and_c… 30. http://www.reddit.com/r/haskell/comments/oaqsa/simon_marlows_parallel_and_c… 31. http://wewantarock.wordpress.com/2012/01/12/who-is-my-community/ 32. http://www.reddit.com/r/haskell/comments/ogtos/who_is_my_community_wxhaskel… 33. http://stackoverflow.com/questions/8736995/fun-with-repeated-fmap 34. http://stackoverflow.com/questions/8760956/can-a-compiler-automatically-det… 35. http://stackoverflow.com/questions/8779390/what-does-floated-out-mean 36. http://stackoverflow.com/questions/8703319/haskell-projects-to-study-and-le… 37. http://stackoverflow.com/questions/8777216/how-do-you-identify-monadic-desi… 38. http://stackoverflow.com/questions/8788542/how-do-i-get-good-small-shrinks-… 39. http://stackoverflow.com/questions/8762943/which-applicative-functor-is-use… 40. http://stackoverflow.com/questions/8735072/double-map-in-haskell 41. http://stackoverflow.com/questions/8739741/higher-ranked-and-impredicative-… 42. http://stackoverflow.com/questions/8780183/handling-events-in-haskell 43. http://stackoverflow.com/questions/8807539/does-the-term-monadic-in-j-have-… 44. http://stackoverflow.com/questions/8832624/any-good-articles-on-optimizing-… 45. http://stackoverflow.com/questions/8857038/what-is-a-free-variable 46. http://stackoverflow.com/questions/8705332/haskell-thread-communication-pat… 47. http://stackoverflow.com/questions/8745597/defining-a-function-by-equations… 48. http://stackoverflow.com/questions/8785311/what-functionality-do-you-get-fo… 49. http://stackoverflow.com/questions/8829194/why-are-types-with-no-data-const… 50. http://stackoverflow.com/questions/8716668/folding-flatmap-bind-over-a-list…

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Error in converting List of Lists into PArray ( Parray a )
by mukesh tiwari 18 Jan '12

18 Jan '12

Hello all I am trying to convert List of Lists ( [[(Int , Double )]] ) into PArray ( PArray ( Int , Double )) but getting run time error. This code works fine and print list of PArray ( Int , Double ) but when i put print $ P.fromList ( map P.fromList c ) then i am getting runtime error. It says "Main: Data/Array/Parallel/PArray/PDataInstances.hs:337:10-30: No instance nor default method for class operation Data.Array.Parallel.PArray.PData.fromListPR". Could some one please tell me how to resolve this issue. Thank you --import ParallelMat import Data.List import System.Environment import Data.Array.Parallel import qualified Data.Array.Parallel.PArray as P processMatrix :: [ [ Double ] ] -> [ [ Double ] ] -> [ ( [ ( Int , Double ) ] , [ ( Int , Double ) ]) ] processMatrix [] [] = [] processMatrix ( x : xs ) ( y : ys ) | ( all ( == 0 ) x ) Prelude.|| ( all ( == 0 ) y ) = processMatrix xs ys | otherwise = ( filter ( \( x , y ) -> y /= 0 ) . zip [ 1..] $ x ,filter ( \( x , y ) -> y /= 0 ) . zip [1..] $ y ) : processMatrix xs ys main = do [ first , second ] <- getArgs a <- readFile first b <- readFile second let a' = transpose . tail . map ( map ( read :: String -> Double ) . words ) . lines $ a b' = tail . map ( map ( read :: String -> Double ) . words ) . lines $ b ( c , d ) = unzip $ processMatrix a' b' print $ ( map P.fromList c ) --print d Macintosh-0026bb610428:Haskell mukesh$ ghc --make -Odph -fdph-par Main.hs [1 of 1] Compiling Main ( Main.hs, Main.o ) Linking Main ... Macintosh-0026bb610428:Haskell mukesh$ ./Main A.in A.in [fromList<PArray> [(1,1.0),(6,1.0)],fromList<PArray> [(4,11.0),(9,11.0)],fromList<PArray> [(1,4.0),(4,2.0),(6,4.0),(9,2.0)]] Putting print $ P.fromList ( map P.fromList c ) Macintosh-0026bb610428:Haskell mukesh$ ghc --make -Odph -fdph-par Main.hs [1 of 1] Compiling Main ( Main.hs, Main.o ) Linking Main ... Macintosh-0026bb610428:Haskell mukesh$ ./Main A.in A.in Main: Data/Array/Parallel/PArray/PDataInstances.hs:337:10-30: No instance nor default method for class operation Data.Array.Parallel.PArray.PData.fromListPR Input file A.in 10 10 1 2 3 0 0 0 0 0 0 4 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 2 11 2 0 1 2 0 0 0 0 0 0 0 1 2 3 0 0 0 0 0 0 4 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 2 11 2 0 1 2 0 0 0 0 0 0 0

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Pattern-matching & substitution for haskell-src-exts?
by Conal Elliott 18 Jan '12

18 Jan '12

Has anyone implemented pattern-matching & substitution for haskell-src-exts? - Conal

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