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Haskell Implementors' Workshop 2012: Call for Participation
by Johan Tibell 01 Aug '12

01 Aug '12

===================================================================== Call for Participation ACM SIGPLAN Haskell Implementors' Workshop http://haskell.org/haskellwiki/HaskellImplementorsWorkshop/2012 Copenhagen, Denmark, September 14th, 2012 The workshop will be held in conjunction with ICFP 2012 http://www.icfpconference.org/icfp2012/ ===================================================================== There are less than nine days left until the early registration expires! The Haskell Implementors' Workshop is to be held alongside ICFP 2012 this year in Copenhagen, Denmark. There will be no proceedings; it is an informal gathering of people involved in the design and development of Haskell implementations, tools, libraries, and supporting infrastructure. This relatively new workshop reflects the growth of the user community: there is a clear need for a well-supported tool chain for the development, distribution, deployment, and configuration of Haskell software. The aim is for this workshop to give the people involved with building the infrastructure behind this ecosystem an opportunity to bat around ideas, share experiences, and ask for feedback from fellow experts. We intend the workshop to have an informal and interactive feel, with a flexible timetable and plenty of room for ad-hoc discussion, demos, and impromptu short talks. * Program: http://www.haskell.org/haskellwiki/HaskellImplementorsWorkshop/2012 * Local arrangements (including travel and accommodation): http://icfpconference.org/icfp2012/local.html * Registration is available via (workshop abbreviation: HIM): http://icfpconference.org/icfp2012/registration.html Workshop organizers: * Workshop Co-Chairs: Gregory Collins, Google Johan Tibell, Google * Program Committee: Lennart Augustsson, Standard Chartered Bank Manuel M T Chakravarty, University of New South Wales Simon Marlow, Microsoft Research David Terei, Stanford University =====================================================================

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Haskell Weekly News: Issue 238
by Daniel Santa Cruz 01 Aug '12

01 Aug '12

Welcome to issue 238 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of July 22 to 28, 2012. Quotes of the Week * clsmith: i find programming in haskell is mostly about reinventing things and then discovering them in Prelude * EvanR: ruby, cobol of the 2000s * monochrom: "customers who used == also liked: foldr, map, Ord, continuation passing style" * jmcarthur: we calmly talk trolls to death Top Reddit Stories * Haskell logo QR code Domain: self.haskell, Score: 44, Comments: 5 On Reddit: [1] http://goo.gl/DuSSU Original: [2] http://goo.gl/DuSSU * In Praise of EitherT Domain: ocharles.org.uk, Score: 43, Comments: 9 On Reddit: [3] http://goo.gl/V2R9B Original: [4] http://goo.gl/M7rTb * The must-know list of packages on hackage? Domain: self.haskell, Score: 39, Comments: 29 On Reddit: [5] http://goo.gl/N7i6J Original: [6] http://goo.gl/N7i6J * SmartCheck: a better QuickCheck for testing algebraic data and generalizing counterexamples found Domain: leepike.wordpress.com, Score: 39, Comments: 6 On Reddit: [7] http://goo.gl/ENFxH Original: [8] http://goo.gl/SZMCG * Compile Haskell (Fay) to Javascript using Javascript compiled with GHCJS Domain: ghcjs.github.com, Score: 36, Comments: 8 On Reddit: [9] http://goo.gl/YOSuG Original: [10] http://goo.gl/d7Af4 * Programming in Fay - Simple example Domain: youtube.com, Score: 30, Comments: 17 On Reddit: [11] http://goo.gl/Ieswf Original: [12] http://goo.gl/4DcLd * Flexibly-Phased Constraints in Haskell Domain: jonmsterling.com, Score: 28, Comments: 19 On Reddit: [13] http://goo.gl/uvYY4 Original: [14] http://goo.gl/3cRD4 * Do you use Haskell in business? What do you use it for? Domain: self.haskell, Score: 24, Comments: 57 On Reddit: [15] http://goo.gl/AcGlL Original: [16] http://goo.gl/AcGlL * ANN: CmdTheLine 0.2 released -- with new tutorial. Domain: elifrey.com, Score: 23, Comments: 11 On Reddit: [17] http://goo.gl/yezpc Original: [18] http://goo.gl/cM9kZ * State of the Shelly (Haskell scripting lib) Domain: yesodweb.com, Score: 23, Comments: 1 On Reddit: [19] http://goo.gl/siupj Original: [20] http://goo.gl/FyrDC * Hackage b0rked for cabal 0.10.2 Domain: self.haskell, Score: 22, Comments: 4 On Reddit: [21] http://goo.gl/HzgX8 Original: [22] http://goo.gl/HzgX8 * Pipes to Conduits part 0: Combining Functors Domain: unknownparallel.wordpress.com, Score: 22, Comments: 18 On Reddit: [23] http://goo.gl/sF95u Original: [24] http://goo.gl/Jepr5 * Deconstraining DSLs [PDF] Domain: doc.ic.ac.uk, Score: 19, Comments: 2 On Reddit: [25] http://goo.gl/Ur7Lz Original: [26] http://goo.gl/E2DhL * What is the reason for Haskell's cabal package problems? Domain: self.haskell, Score: 17, Comments: 41 On Reddit: [27] http://goo.gl/O0x9z Original: [28] http://goo.gl/O0x9z * Pipes to Conduits part 4: Recovering from Abort Domain: unknownparallel.wordpress.com, Score: 16, Comments: On Reddit: [29] http://goo.gl/tr9Jy Original: [30] http://goo.gl/XbAUR * Pipes to Conduits part 1: Yield and Await Domain: unknownparallel.wordpress.com, Score: 15, Comments: 5 On Reddit: [31] http://goo.gl/KV0io Original: [32] http://goo.gl/GuFqX * Pipes to Conduits part 2: Upstream Results Domain: unknownparallel.wordpress.com, Score: 15, Comments: 9 On Reddit: [33] http://goo.gl/ONYzN Original: [34] http://goo.gl/Tf3Si * Instance Templates: a proposed feature to change type class hierarchies without breaking compatibility Domain: github.com, Score: 15, Comments: 6 On Reddit: [35] http://goo.gl/7Jx8c Original: [36] http://goo.gl/LG1YK * Categories over pairs of types Domain: twanvl.nl, Score: 15, Comments: 1 On Reddit: [37] http://goo.gl/8ee77 Original: [38] http://goo.gl/jxLPR * Pipes to Conduits part 3: Abort Domain: unknownparallel.wordpress.com, Score: 14, Comments: 12 On Reddit: [39] http://goo.gl/IZbij Original: [40] http://goo.gl/2kgLg Top StackOverflow Questions * What does “Pure” mean, in the context of programming languages and paradigms? votes: 21, answers: 2 Read on SO: [41] http://goo.gl/a0IUe * Can a thunk be duplicated to improve memory performance? votes: 17, answers: 3 Read on SO: [42] http://goo.gl/rznKJ * What's the current status of restricted monads? votes: 14, answers: 1 Read on SO: [43] http://goo.gl/UnMbE * Is it good style to use a case expression on an Unit value just to use guards? votes: 13, answers: 3 Read on SO: [44] http://goo.gl/E8xn3 * Temporary namespace/context in Haskell votes: 12, answers: 3 Read on SO: [45] http://goo.gl/AWg4O * Using the LogicT Monad Transformer in Haskell votes: 12, answers: 1 Read on SO: [46] http://goo.gl/DOw0H * Irrefutable pattern does not leak memory in recursion, but why? votes: 11, answers: 1 Read on SO: [47] http://goo.gl/qK7rx * How to determine type of Haskell functions? votes: 11, answers: 3 Read on SO: [48] http://goo.gl/Rn5F7 * Haskell: map runST votes: 9, answers: 3 Read on SO: [49] http://goo.gl/mWnvL * How to play with Control.Monad.Writer in haskell? votes: 9, answers: 1 Read on SO: [50] http://goo.gl/GLTJc Until next time, Daniel Santa Cruz References 1. http://www.reddit.com/r/haskell/comments/xb3ig/haskell_logo_qr_code/ 2. http://www.reddit.com/r/haskell/comments/xb3ig/haskell_logo_qr_code/ 3. http://ocharles.org.uk/blog/posts/2012-07-24-in-praise-of-EitherT.html 4. http://www.reddit.com/r/haskell/comments/x3ikr/in_praise_of_eithert/ 5. http://www.reddit.com/r/haskell/comments/x4g18/the_mustknow_list_of_package… 6. http://www.reddit.com/r/haskell/comments/x4g18/the_mustknow_list_of_package… 7. http://leepike.wordpress.com/2012/07/26/smartcheck/ 8. http://www.reddit.com/r/haskell/comments/x7fyt/smartcheck_a_better_quickche… 9. http://ghcjs.github.com/fay-hello-js.jsexe/ 10. http://www.reddit.com/r/haskell/comments/x43nd/compile_haskell_fay_to_javas… 11. https://www.youtube.com/watch?v=dhFZ8vx9byc 12. http://www.reddit.com/r/haskell/comments/xbihk/programming_in_fay_simple_ex… 13. http://www.jonmsterling.com/posts/2012-07-22-flexibly-phased-constraints-in… 14. http://www.reddit.com/r/haskell/comments/wzwda/flexiblyphased_constraints_i… 15. http://www.reddit.com/r/haskell/comments/x8qnh/do_you_use_haskell_in_busine… 16. http://www.reddit.com/r/haskell/comments/x8qnh/do_you_use_haskell_in_busine… 17. http://elifrey.com/2012/07/23/CmdTheLine-Tutorial/ 18. http://www.reddit.com/r/haskell/comments/x0xda/ann_cmdtheline_02_released_w… 19. http://www.yesodweb.com/blog/2012/07/shelly-update 20. http://www.reddit.com/r/haskell/comments/x9etz/state_of_the_shelly_haskell_… 21. http://www.reddit.com/r/haskell/comments/x16h7/hackage_b0rked_for_cabal_010… 22. http://www.reddit.com/r/haskell/comments/x16h7/hackage_b0rked_for_cabal_010… 23. http://unknownparallel.wordpress.com/2012/07/24/pipes-to-conduits-part-0-co… 24. http://www.reddit.com/r/haskell/comments/x2lao/pipes_to_conduits_part_0_com… 25. http://www.doc.ic.ac.uk/~wlj05/files/Deconstraining.pdf 26. http://www.reddit.com/r/haskell/comments/x3bta/deconstraining_dsls_pdf/ 27. http://www.reddit.com/r/haskell/comments/x4knd/what_is_the_reason_for_haske… 28. http://www.reddit.com/r/haskell/comments/x4knd/what_is_the_reason_for_haske… 29. http://unknownparallel.wordpress.com/2012/07/28/pipes-to-conduits-part-4-re… 30. http://www.reddit.com/r/haskell/comments/xar9r/pipes_to_conduits_part_4_rec… 31. http://unknownparallel.wordpress.com/2012/07/25/pipes-to-conduits-part-1-yi… 32. http://www.reddit.com/r/haskell/comments/x4si1/pipes_to_conduits_part_1_yie… 33. http://unknownparallel.wordpress.com/2012/07/26/pipes-and-conduits-part-2-u… 34. http://www.reddit.com/r/haskell/comments/x6w9k/pipes_to_conduits_part_2_ups… 35. https://github.com/mgsloan/instance-templates 36. http://www.reddit.com/r/haskell/comments/x71ll/instance_templates_a_propose… 37. http://twanvl.nl/blog/haskell/categories-over-pairs-of-types 38. http://www.reddit.com/r/haskell/comments/xab88/categories_over_pairs_of_typ… 39. http://unknownparallel.wordpress.com/2012/07/27/pipes-to-conduits-part-3-ab… 40. http://www.reddit.com/r/haskell/comments/x8yx2/pipes_to_conduits_part_3_abo… 41. http://stackoverflow.com/questions/11607927/what-does-pure-mean-in-the-cont… 42. http://stackoverflow.com/questions/11675807/can-a-thunk-be-duplicated-to-im… 43. http://stackoverflow.com/questions/11599685/whats-the-current-status-of-res… 44. http://stackoverflow.com/questions/11620649/is-it-good-style-to-use-a-case-… 45. http://stackoverflow.com/questions/11661963/temporary-namespace-context-in-… 46. http://stackoverflow.com/questions/11695961/using-the-logict-monad-transfor… 47. http://stackoverflow.com/questions/11625840/irrefutable-pattern-does-not-le… 48. http://stackoverflow.com/questions/11638232/how-to-determine-type-of-haskel… 49. http://stackoverflow.com/questions/11662696/haskell-map-runst 50. http://stackoverflow.com/questions/11684321/how-to-play-with-control-monad-…

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GHC rendering of non-ASCII characters configurable?
by Richard Cobbe 01 Aug '12

01 Aug '12

I'm working on an application that involves processing a lot of Unicode data, and I'm finding the built-in Show implementation for Char to be really inconvenient. Specifically, it renders all characters at U+0080 and above with decimal escapes: Prelude> '\x80' '\128' This is annoying because all of the Unicode charts give the code points in hex, and indeed the charts are split into different PDFs at numbers that are nice and round in hex but not in decimal. So in order to figure out which character I'm looking at, I have to convert back to hex and then look it up in the charts. Is there any way to ask GHC to render super-ASCII characters with their hexadecimal escapes, instead? I'm perfectly happy to write my own custom Show instance, but I don't know how to hook that into ghci's REPL (or, for that matter, the routines that HUnit uses to generate the messages on failed tests, etc.). I'm using GHC 7.4.1 on MacOS 10.7.4. Thanks, Richard

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Re: [Haskell-cafe] specifying using type class
by Patrick Browne 01 Aug '12

01 Aug '12

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Re: [Haskell-cafe] The following is supposed to be initializing a 2D array but it doesn't seem to work.
by KC 31 Jul '12

31 Jul '12

That was it! On Tue, Jul 31, 2012 at 3:34 PM, Jan-Willem Maessen <jmaessen(a)alum.mit.edu>wrote: > > > On Tue, Jul 31, 2012 at 5:52 PM, KC <kc1956(a)gmail.com> wrote: > >> All I am getting is this: >> >> array ((1,5),(1,5)) [((1,5),1.0)] >> >> Maybe the behaviour of ghc was changed since the article was written. >> > > I think you've made a common mistake here. Array bounds are (lower in all > dimensions, upper in all dimensions). So perhaps you intended to use the > bounds ((1,1), (5,5)) in your code? > > I make this mistake frequently. > > -Jan-Willem Maessen > -- -- Regards, KC

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CRIP: the Curiously Reoccuring Instance Pattern
by oleg＠okmij.org 31 Jul '12

31 Jul '12

Ryan Ingram wrote: > I've been seeing this pattern in a surprising number of instance > definitions lately: > > instance (a ~ ar, b ~ br) => Mcomp a ar b br [1] > instance (b ~ c, CanFilterFunc b a) => CanFilter (b -> c) a [2] And here are a few more earlier instances of the same occurrence: http://okmij.org/ftp/Haskell/typecast.html > What I'm wondering is--are there many cases where you really want the > non-constraint-generating behavior? It seems like (aside from contrived, > ahem, instances) whenever you have instance CLASS A B where A and B share > some type variables, that there aren't any valid instances of the same > class where they don't share the types in that way. Instances of the form class C a b class C a a class C a b are very characteristic of (emulation) of disequality constraints. Such instances occur, in a hidden form, all the time in HList -- when checking for membership in a type-level list, for example. There are naturally two cases: when the sought type is at the head of the list, or it is (probably) at the tail of the list. class Member (h :: *) (t :: List *) instance Member h (Cons h t) instance Member h t => Member h (Cons h' t) Of course instances above are overlapping. And when we add functional dependencies (since we really want type-functions rather type relations), they stop working at all. We had to employ work-arounds, which are described in detail in the HList paper (which is 8 years old already).

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The following is supposed to be initializing a 2D array but it doesn't seem to work.
by KC 31 Jul '12

31 Jul '12

newArr :: (Ix i) => i -> i -> e -> Array i e newArr n m x = listArray (n,m) (repeat <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:repeat> x) ----- Prelude <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html> Data.Array> newArr 0 10 0 array (0,10) [(0,0),(1,0),(2,0),(3,0),(4,0),(5,0),(6,0),(7,0),(8,0),(9,0),(10,0)] Prelude <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html> Data.Array> newArr (0,0) (5,5) 0 array ((0,0),(5,5)) [((0,0),0),((0,1),0),((0,2),0),((0,3),0),((0,4),0),((0,5),0),((1,0),0),((1,1),0),((1,2),0),((1,3),0),((1,4),0),((1,5),0),((2,0),0),((2,1),0),((2,2),0),((2,3),0),((2,4),0),((2,5),0),((3,0),0),((3,1),0),((3,2),0),((3,3),0),((3,4),0),((3,5),0),((4,0),0),((4,1),0),((4,2),0),((4,3),0),((4,4),0),((4,5),0),((5,0),0),((5,1),0),((5,2),0),((5,3),0),((5,4),0),((5,5),0)] All I am getting is this: array ((1,5),(1,5)) [((1,5),1.0)] Maybe the behaviour of ghc was changed since the article was written. -- -- Regards, KC

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Reddy on Referential Transparency
by Chris Dornan 31 Jul '12

31 Jul '12

Café, For those who havent seen it Uday Reddy has a comprehensive answer to a request to explain referential transparency on Stack Overflow. http://stackoverflow.com/questions/210835/what-is-referential-transparency/9 859966#9859966 For good measure he finishes with a rather scathing assessment of functional programmers claim to ownership of RT: Functional programmers don't know much of this research. Their ideas on referential transparency are to be taken with a large grain of salt. Marvellous! 'Tis rare to find such a robust assessment combined with understanding (and it certainly makes me think a bit). Have we become a bit complacent about RT? Chris

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Explicit forall - Strange Error
by Shayan Najd Javadipour 31 Jul '12

31 Jul '12

Hi, I wonder why the following code doesn't typecheck in GHC 7.4.1: {-# LANGUAGE GADTs,RankNTypes #-}data T a where T1 :: (forall b. b -> b) -> (forall a. Int -> T a) {- Error: Data constructor `T1' returns type `forall a. Int -> T a' instead of an instance of its parent type `T a' In the definition of data constructor `T1' In the data type declaration for `T' Failed, modules loaded: none. -} While: {-# LANGUAGE GADTs,RankNTypes #-} f :: (forall b. b -> b) -> (forall a. Int -> Maybe a)f = undefined {- ghci> :t f f :: (forall b. b -> b) -> Int -> Maybe a -} Thanks, Shayan

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Detecting numeric overflows
by Евгений Пермяков 31 Jul '12

31 Jul '12

Can someone tell me if there are any primitives, that used to detect machine type overflows, in ghc haskell ? I perfectly understand, that I can build something based on preconditioning of variables, but this will kill any performance, if needed.

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