- Haskell-Cafe - Haskell.org

Auto-termination and leftovers in Conduits
by Myles C. Maxfield 28 Oct '12

28 Oct '12

Hey, Say I have a stream of Data.Text.Text objects flowing through a conduit, where the divisions between successive Data.Text.Text items occur at arbitrary boundaries (maybe the source is sourceFile $= decode utf8). I'd like to create a Sink that returns a tuple of (the first line, the rest of the input). My first attempt at this looks like this: sink = do out1 <- CT.lines C.=$ CL.head out2 <- CL.consume return (out1, T.concat out2) However, the following input provides: runIdentity $ CL.sourceList ["abc\nde", "f\nghi"] C.$$ sink (Just "abc","f\nghi") But what I really want is (Just "abc", "\ndef\nghi") I think this is due to the auto-termination you mention in [1]. My guess is that when CT.lines yields the first value, (CL.head then also yields it,) and execution is auto-terminated before CT.lines gets a chance to specify any leftovers. How can I write this sink? (I know I can just use CL.consume and T.break (== '\n'), but I'm not interested in that. I'm trying to figure out how to get the behavior I'm looking for with conduits.) Thanks, Myles [1] http://hackage.haskell.org/packages/archive/conduit/0.5.2.7/doc/html/Data-C…

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Overload function application? (was: foldr (.) id)
by Greg Fitzgerald 28 Oct '12

28 Oct '12

Should library functions target Control.Category? We write for Num, IsString, and ListLike. Haskell overloads number literals and string literals, and awkwardly not List literals. Should Haskell overload function application too? How cool would it be to be able to leverage all of Haskell's libraries, and effectively none of the runtime, by implementing instances for literals and function application? Say, for example, to generate a call graph, or make the language strict, or target LLVM more directly. Anyone tried this? Thanks, Greg On Sat, Oct 27, 2012 at 11:39 AM, Greg Fitzgerald <garious(a)gmail.com> wrote: > > Alternatively: flip (foldr id) > > Very cool, but... > > Prelude> import qualified Data.Foldable as F > Prelude F> :t F.foldr id > F.foldr id :: F.Foldable t => b -> t (b -> b) -> b > > > {- Generalizing -} > > Prelude F> import qualified Control.Category as C > Prelude F C> :t F.foldr (C..) C.id > F.foldr (C..) C.id :: (F.Foldable t, C.Category cat) => t (cat b b) -> cat > b b > > > {- Sneaky type-specialization -} > > Prelude F C> :t F.foldr C.id > F.foldr C.id :: F.Foldable t => b -> t (b -> b) -> b > > > On Sat, Oct 27, 2012 at 3:09 AM, Ross Paterson <ross(a)soi.city.ac.uk>wrote: > >> On Fri, Oct 26, 2012 at 07:41:18PM +0100, Greg Fitzgerald wrote: >> > I've recently found myself using the expression: "foldr (.) id" to >> > compose a list (or Foldable) of functions. It's especially useful >> > when I need to map a function over the list before composing. Does >> > this function, or the more general "foldr fmap id", defined in a >> > library anywhere? I googled and hoogled, but no luck so far. >> >> Alternatively: flip (foldr id) >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> > >

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Sparse records/ADTs
by Jon Fairbairn 27 Oct '12

27 Oct '12

Is there a convenient way of handling a data structure with lots of fields of different types that may or may not be filled in? Something equivalent to data D = D {a::Maybe A, b::Maybe B, c::Maybe C, …} but with better space efficiency and a more convenient empty object. An easy alternative is data E = Ea A | Eb B | Ec C | … type R = [E] which has a straightforward empty object, but one then must define getA e = listToMaybe [a | Ea a <- e] for each field, which is tedious (and O(n)). Obviously Templates would help, but is there an alternative I’ve missed? -- Jón Fairbairn Jon.Fairbairn(a)cl.cam.ac.uk

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Type-directed functions with data kinds
by Paul Visschers 27 Oct '12

27 Oct '12

Hello everyone, I've been playing around with the data kinds extension to implement vectors that have a known length at compile time. Some simple code to illustrate: > {-# LANGUAGE DataKinds, GADTs, KindSignatures #-} > > import Prelude hiding (repeat) > > data Nat = Zero | Succ Nat > data Vector (n :: Nat) a where > Nil :: Vector Zero a > Cons :: a -> Vector n a -> Vector (Succ n) a > > class VectorRepeat (n :: Nat) where > repeat :: a -> Vector n a > > instance VectorRepeat Zero where > repeat _ = Nil > > instance VectorRepeat n => VectorRepeat (Succ n) where > repeat x = Cons x (repeat x) In this code I have defined a repeat function that works in a similar way to the one in the prelude, except that the length of the resulting vector is determined by the type of the result. I would have hoped that its type would become 'repeat :: a -> Vector n a', yet it is 'repeat :: VectorRepeat n => a -> Vector n a'. As far as I can tell, this class constraint should no longer be necessary, as all possible values for 'n' are an instance of this class. I actually really just want to define a closed type-directed function and would rather not (ab)use the type class system at all. Is there a way to write the repeat function so that it has the type 'repeat :: a -> Vector n a' that I've missed? If not, is this just because it isn't implemented or are there conceptual caveats? Paul Visschers

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Enemy of the Mutable State
by Mark Lentczner 27 Oct '12

27 Oct '12

A Haskell poster I made that you all might enjoy: http://www.ozonehouse.com/mark/enemy/ - mzero

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ANNOUNCE: pretty-tree
by Ivan Lazar Miljenovic 27 Oct '12

27 Oct '12

For a project I'm working on, I got sick of writing out trees by hand. Data.Tree has a drawTree function, but whilst it's better than nothing, I prefer a more top-down "traditional" approach to drawing trees. So I wrote a package to do just that: http://hackage.haskell.org/package/pretty-tree (I didn't think to check whether diagrams had capabilities to do so until after I finished, though in this case I prefer a textual approach anyway.) I'm more than happy to consider adding other forms of drawing trees (though off-hand I can't think of any), or of course you can send me patches since I'm taking advantage of the nice new shiny hub.darcs.net (thanks Darcs team!). I was going to put an example here but it only really comes out nicely if you're using a monospaced font, so have a look at the Haddock docs for the module once Hackage builds it (in hindsight, I should probably have come up with better labels before releasing the package... oh well :) -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com http://IvanMiljenovic.wordpress.com

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Munich Haskell Meeting
by Heinrich Hördegen 26 Oct '12

26 Oct '12

Dear all, next Monday, 29th of October, our monthly Haskell Meeting will take place at 19h30 at Cafe Puck. Everybody interested in functional programming is wellcome. Please go to http://www.haskell-munich.de/dates for more information and push the button, if you intend to join. With hope to see you on Monday, Heinrich

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Problems translating Conor McBride's talk into Haskell + DataKind + KindPoly
by Ahn, Ki Yung 26 Oct '12

26 Oct '12

Most part of Conor's talk at ICFP, until just before the last stage where he heavily uses true value dependency for compiler correctness all the code seemed to be able to translate into Haskell with the new hot DataKinds and PolyKinds extension. I tried it in GHC 7.4.1 and it was possible to do it, but I got stuck and I had to make the generic list structure mono-kinded with kind signatures rather not use the PolyKinds extension. I wonder if this is just a but of GHC 7.4.1's implementation of PolyKinds, or a limitation of the DataKind design. I attached a literate Haskell script with this message that illustrates this problem. Thanks in advance for any comments including whether it runs in later versions or still has problems, and discussions about the DataKinds and PolyKinds extension. Ki Yung

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pragma to request tail recursion optimization (and perhaps [co]inductive recursion)
by Petr P 25 Oct '12

25 Oct '12

Hi, Haskell compilers optimize tail recursive functions as cycles, which improves both memory and CPU complexity. However, it's easy to make a mistake and break the conditions under which a function can be tail recursive and thus optimized. Is there a way to tell a Haskell compiler that a function must be tail recursive and if it cannot be done, fail the compilation? Like Scala's @tailrec? I'd say no, and believe it would be quite handy. I suppose this could be realized as a pragma. There could be also additional pragmas to ensure that a recursive function is inductive (the recursive call is on a strict sub-structure of one of the arguments) or co-inductive (the recursive call is guarded by a constructor). Best regards, Petr Pudlak

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Online haskell course
by graham＠fatlazycat.com 25 Oct '12

25 Oct '12

Anyone know of a similar course to the coursera scala one for haskell ? https://www.coursera.org/course/progfun Quite interesting online course that has graded assignments. Anything similar or anyone considering doing something similar for haskell ? Thanks Graham

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createProcess fails to find executable in Windows
by José Pedro Magalhães 25 Oct '12

25 Oct '12

Hi all, Consider the following program: module Test where > > import System.Process (readProcess) > > main :: IO () > main = readProcess "git" ["describe", "--tags"] "" >>= putStr > In Windows I get the following behaviour: > git --version > git version 1.7.10.msysgit.1 > > > ghc --version > The Glorious Glasgow Haskell Compilation System, version 7.6.1 > > > runghc Test > Test: git: createProcess: does not exist (No such file or directory) > The same happens with GHC 7.4.2. In Linux, however, it works as expected: $ git --version > git version 1.7.9.5 > $ ghc --version > The Glorious Glasgow Haskell Compilation System, version 7.4.1 > $ runghc Test.hs > Package-2.0-68-gacaf77a > Can anyone reproduce this result in Windows? Is this a bug or am I doing something wrong? Thanks, Pedro

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Re: [Haskell-cafe] serialize an unknown type
by Corentin Dupont 25 Oct '12

25 Oct '12

I'm trying to get around my problem (still stuck...). I have really to call a function like this: triggerEvent :: (Typeable e, Show e, Eq e) => Event e -> EventData e -> State Game () This function simply execute an event given the corresponding data and change the state of the game. Now as explained my web framework won't accept random types (like "e") to be passed around. So I said to myself "OK, so let's store the events in a list and ask the gui to just give me the number of the event". So I have an heterogeneous list of events [EventWrap]: data EventWrap where EW :: (Typeable e, Show e, Eq e) => {eventNumber :: Int, event :: Event e} -> EventWrap Do you think this solution can lead me somewhere?? I have actually doubts I can find back my "Event e" from the list because of the implicit forall in the GADT..... Thanks, Corentin On Wed, Oct 24, 2012 at 3:37 PM, Chris Smith <cdsmith(a)gmail.com> wrote: > "Corentin Dupont" <corentin.dupont(a)gmail.com> wrote: > > I could ask my user to make his new type an instance of a class as > suggested by Alberto... > > If you are working with unknown types, then your options are: (a) > constrain to some type class, or (b) have your clients pass in functions to > operate on the type alongside the values. Actually the first is just a > special case of the second with syntactic sugar... >

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Haskell Weekly News: Issue 248
by Daniel Santa Cruz 25 Oct '12

25 Oct '12

Welcome to issue 248 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of October 14 to October 20, 2012. Quotes of the Week * shachaf: Don't fixity fit ain't broken. * shachaf: SIKP, the cheap knockoff book that uses the less-known programming language "skeem" * shachaf: category-extras was so great until the antitrust lawsuit. * Cale: If there was a second syntax and we named it cata' would that be a catastrophe? * acowley: I just don't want edwardk to name them, because he'll choose a name like "x" * cmccann: so edwardk has abstracted things to the point where you can use his libraries without even installing them * Jafet: Agda has an evaluator? * johnw: but probably the other package is more "idiomatic". if by that you mean "uses mathematics to such a degree that most people walk away in consternation" * copumpkin: if I throw enough CT terms out there, one of them is bound to be right eventually * spj: "Our imperative colleagues do a lot of reasoning about their systems. Well, they have to." * spj: "You do the category theory, and you talk to Gordon. And he talks to Phil. And Phil talks to me." * ciaranm: a confusion is just the categorical dual of a fusion, which is just a fusoid in the category of brainofunctors Top Reddit Stories * The University of Heidelberg now uses Haskell for the intro cs course! Domain: self.haskell, Score: 85, Comments: 60 On Reddit: [1] http://goo.gl/abq2b Original: [2] http://goo.gl/abq2b * Cartesian Closed Comic #15: Iteratees Domain: ro-che.info, Score: 74, Comments: 2 On Reddit: [3] http://goo.gl/9NpD2 Original: [4] http://goo.gl/SB7HZ * Fast Code Nation: The Bright Side of High-Level Languages Domain: bos.github.com, Score: 63, Comments: 11 On Reddit: [5] http://goo.gl/KmBRt Original: [6] http://goo.gl/jm9s7 * VIDEO: Simon Marlow on High Performance Concurrency at the Haskell eXchange at Skills Matter Oct. 10th Domain: skillsmatter.com, Score: 44, Comments: On Reddit: [7] http://goo.gl/IuJhH Original: [8] http://goo.gl/BeHRg * Cellular automata as comonads Domain: blog.sigfpe.com, Score: 36, Comments: 10 On Reddit: [9] http://goo.gl/73nr2 Original: [10] http://goo.gl/vH4TK * VIDEO: Simon Peyton Jones at the Haskell eXchange about the origins and the future of Haskell Domain: skillsmatter.com, Score: 31, Comments: 6 On Reddit: [11] http://goo.gl/Iklcf Original: [12] http://goo.gl/WxJhc * Duality for Haskellers : Inside 206-105 Domain: blog.ezyang.com, Score: 31, Comments: 1 On Reddit: [13] http://goo.gl/qhohx Original: [14] http://goo.gl/VuHMS * Communication Patterns in Cloud Haskell (Part 4): K-Means Domain: well-typed.com, Score: 27, Comments: On Reddit: [15] http://goo.gl/9bT02 Original: [16] http://goo.gl/7S5XQ * VIDEO: Duncan Coutts on Cloud Haskell at the Haskell eXchange at Skills Matter Oct 10 Domain: skillsmatter.com, Score: 26, Comments: 29 On Reddit: [17] http://goo.gl/lLyGD Original: [18] http://goo.gl/eUd0W * How to use Yesod without Template Haskell Domain: yesodweb.com, Score: 26, Comments: On Reddit: [19] http://goo.gl/HT3aD Original: [20] http://goo.gl/s81mw * Variance Shadow Mapping with LambdaCube 3D Domain: lambdacube3d.wordpress.com, Score: 23, Comments: 5 On Reddit: [21] http://goo.gl/g3Hs0 Original: [22] http://goo.gl/jDv0S * Thinking Functionally with Haskell | PragPub 2012/10 Domain: pragprog.com, Score: 21, Comments: 1 On Reddit: [23] http://goo.gl/N9HRL Original: [24] http://goo.gl/6ptKX * Haskell Automatic Information Labeling System - new release and video Domain: self.haskell, Score: 18, Comments: 4 On Reddit: [25] http://goo.gl/ujURP Original: [26] http://goo.gl/ujURP * Livecoding with the Happstack web framework. (2 hour video) Domain: youtube.com, Score: 18, Comments: 22 On Reddit: [27] http://goo.gl/POkCB Original: [28] http://goo.gl/n5uNA * VIDEO: Blake Rain on Scalable web applications with YESOD at the Haskell eXchange at Skills Matter Oct 10 Domain: skillsmatter.com, Score: 17, Comments: 2 On Reddit: [29] http://goo.gl/kFPsJ Original: [30] http://goo.gl/zUNBb * Measuring the performance of Warp Domain: yesodweb.com, Score: 17, Comments: 13 On Reddit: [31] http://goo.gl/MBPnT Original: [32] http://goo.gl/p3HUq * Tail Recursive Sorting (QuickSort) Domain: self.haskell, Score: 15, Comments: 31 On Reddit: [33] http://goo.gl/qkECv Original: [34] http://goo.gl/qkECv * a class that generalizes over Functor, Functor2, Functor3, and so on — Redshift Phase 2: types classes for types with arbitrary numbers of * parameters Domain: csks.wordpress.com, Score: 15, Comments: 17 On Reddit: [35] http://goo.gl/gjQL2 Original: [36] http://goo.gl/0XsPN * Wiggling Sums: a somewhat interesting Traversable problem Domain: jaspervdj.be, Score: 13, Comments: 12 On Reddit: [37] http://goo.gl/FKPw3 Original: [38] http://goo.gl/xu2oX * VIDEO: Lennart Augustsson on 'Making EDSL's Fly at the Haskell eXchange at Skills Matter Oct 10 Domain: skillsmatter.com, Score: 12, Comments: 5 On Reddit: [39] http://goo.gl/hFlF6 Original: [40] http://goo.gl/TXMg4 Top StackOverflow Questions * Why not be dependently typed? votes: 25, answers: 1 Read on SO: [41] http://goo.gl/1TSVh * Writing cojoin or cobind for n-dimensional grid type votes: 23, answers: 0 Read on SO: [42] http://goo.gl/qbz7z * Monad Transformers vs Passing parameters to functions votes: 21, answers: 1 Read on SO: [43] http://goo.gl/UpU57 * SOAP web services in haskell? votes: 16, answers: 1 Read on SO: [44] http://goo.gl/1fiiM * How does one statisfy a class constraint in an instance of a class that requires a type constructor rather than a concrete type? votes: 13, answers: 1 Read on SO: [45] http://goo.gl/DD00Q * If “List” is a monoid, what is its “set”? votes: 13, answers: 2 Read on SO: [46] http://goo.gl/bYYar * Lambda calculus in Haskell: Is there some way to make Church numerals type check? votes: 13, answers: 2 Read on SO: [47] http://goo.gl/66qKc * Relation between Haskell Threads and OS Threads in the GHC votes: 13, answers: 1 Read on SO: [48] http://goo.gl/2u8pS * Can Scala's Cake Pattern be implemented in Haskell? votes: 12, answers: 2 Read on SO: [49] http://goo.gl/QiMfg * Agda Type-Checking and Commutativity / Associativity of + votes: 9, answers: 1 Read on SO: [50] http://goo.gl/CqnMJ Until next time, Daniel Santa Cruz References 1. http://www.reddit.com/r/haskell/comments/11klmw/the_university_of_heidelber… 2. http://www.reddit.com/r/haskell/comments/11klmw/the_university_of_heidelber… 3. http://ro-che.info/ccc/15.html 4. http://www.reddit.com/r/haskell/comments/11t7ru/cartesian_closed_comic_15_i… 5. http://bos.github.com/reaktor-dev-day-2012/reaktor-talk-slides.html 6. http://www.reddit.com/r/haskell/comments/11sl47/fast_code_nation_the_bright… 7. http://skillsmatter.com/podcast/home/high-performance-concurrency/ac-5258 8. http://www.reddit.com/r/haskell/comments/11geat/video_simon_marlow_on_high_… 9. http://blog.sigfpe.com/2006/12/evaluating-cellular-automata-is.html 10. http://www.reddit.com/r/haskell/comments/11o6nu/cellular_automata_as_comona… 11. http://skillsmatter.com/podcast/home/front-end-language-features/ac-5258 12. http://www.reddit.com/r/haskell/comments/11ge03/video_simon_peyton_jones_at… 13. http://blog.ezyang.com/2012/10/duality-for-haskellers/ 14. http://www.reddit.com/r/haskell/comments/11s0j0/duality_for_haskellers_insi… 15. http://www.well-typed.com/blog/74 16. http://www.reddit.com/r/haskell/comments/11iijo/communication_patterns_in_c… 17. http://skillsmatter.com/podcast/home/cloud-haskell/ac-5258 18. http://www.reddit.com/r/haskell/comments/11gia8/video_duncan_coutts_on_clou… 19. http://www.yesodweb.com/blog/2012/10/yesod-pure 20. http://www.reddit.com/r/haskell/comments/11jap2/how_to_use_yesod_without_te… 21. http://lambdacube3d.wordpress.com/2012/10/14/variance-shadow-mapping/ 22. http://www.reddit.com/r/haskell/comments/11ijjg/variance_shadow_mapping_wit… 23. http://pragprog.com/magazines/2012-10/thinking-functionally-with-haskell 24. http://www.reddit.com/r/haskell/comments/11o5oa/thinking_functionally_with_… 25. http://www.reddit.com/r/haskell/comments/11kpin/haskell_automatic_informati… 26. http://www.reddit.com/r/haskell/comments/11kpin/haskell_automatic_informati… 27. http://www.youtube.com/watch?v=7Wmszk4wZxQ 28. http://www.reddit.com/r/haskell/comments/11obty/livecoding_with_the_happsta… 29. http://skillsmatter.com/podcast/home/scalable-web-applications-with-yesod/a… 30. http://www.reddit.com/r/haskell/comments/11ggaf/video_blake_rain_on_scalabl… 31. http://www.yesodweb.com/blog/2012/10/measuring-warp 32. http://www.reddit.com/r/haskell/comments/11kfqu/measuring_the_performance_o… 33. http://www.reddit.com/r/haskell/comments/11lybk/tail_recursive_sorting_quic… 34. http://www.reddit.com/r/haskell/comments/11lybk/tail_recursive_sorting_quic… 35. http://csks.wordpress.com/2012/10/17/redshift-phase-2-types-classes-for-typ… 36. http://www.reddit.com/r/haskell/comments/11nj47/a_class_that_generalizes_ov… 37. http://jaspervdj.be/posts/2012-10-17-wiggling-sums.html 38. http://www.reddit.com/r/haskell/comments/11mnw5/wiggling_sums_a_somewhat_in… 39. http://skillsmatter.com/podcast/home/making-edsls-fly/ac-5258 40. http://www.reddit.com/r/haskell/comments/11gg80/video_lennart_augustsson_on… 41. http://stackoverflow.com/questions/12961651/why-not-be-dependently-typed 42. http://stackoverflow.com/questions/12963733/writing-cojoin-or-cobind-for-n-… 43. http://stackoverflow.com/questions/12968351/monad-transformers-vs-passing-p… 44. http://stackoverflow.com/questions/12904377/soap-web-services-in-haskell 45. http://stackoverflow.com/questions/12922187/how-does-one-statisfy-a-class-c… 46. http://stackoverflow.com/questions/12927152/if-list-is-a-monoid-what-is-its… 47. http://stackoverflow.com/questions/12942397/lambda-calculus-in-haskell-is-t… 48. http://stackoverflow.com/questions/12944151/relation-between-haskell-thread… 49. http://stackoverflow.com/questions/12947176/can-scalas-cake-pattern-be-impl… 50. http://stackoverflow.com/questions/12949323/agda-type-checking-and-commutat…

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Cabalizing LVM
by Jurriaan Hage 25 Oct '12

25 Oct '12

Dear all, in an effort to Cabalize the Helium compiler and make it available on Hackage, it would be helpful to have an example of a Cabal file that shows how to deal with compilation of platform dependent C-code. Anybody out there who has a sample for me? best, Jurriaan Hage

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testing if values can be applied to polymorphic functions
by alex 25 Oct '12

25 Oct '12

Hi all, A while ago I made an unusual visual front-end for Haskell: http://www.youtube.com/watch?v=5KtFlGEVFGE https://github.com/yaxu/texture It applies functions that are both proximal in Euclidean space, and type-compatible. It 'compiles' to Haskell, piped into ghci, but I (probably unwisely) wrote it in C, and ended up having to implement some crazed version of the Haskell type system there to get it working. Now I have idea for making this more interesting and practical (for live music-making, if nothing else), and would like to re-write it all in Haskell. Testing type compatibility of expressions is foxing me though. For example, say I have (++) and [1,2,3]. I then want to see if a value of type (Num a => [a]) can be applied to a function of type ([a] -> [a] -> [a]). I've been looking at Data.Typeable.funResultTy to do this, e.g.: funResultTy (typeOf ((++) :: [a] -> [a] -> [a])) (typeOf ([1,2,3] :: Num a => [a])) But I get this: Ambiguous type variable `a0' in the constraint: (Typeable a0) arising from a use of `typeOf' Being a bit more specific doesn't help: funResultTy (typeOf ((++) :: Typeable a => [a] -> [a] -> [a])) (typeOf ([1,2,3] :: (Typeable a, Num a) => [a])) I guess funResultTy doesn't work when polymorphism is involved.. Perhaps I could just use the ghc-as-a-library stuff to parse and typecheck code - would that be the way forward? Any pointers much appreciated! Best wishes alex -- http://yaxu.org/

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ANN: Cabal-1.16.0.2 and cabal-install-1.16.0.1
by Johan Tibell 24 Oct '12

24 Oct '12

Hi, On behalf of the cabal contributors, I'm proud to announce bugfix releases of Cabal and cabal-install. Here's a complete list of changes since the last release: Since Cabal-1.16.0.1: * Bump Cabal version number to 1.16.0.2 * Fixed warnings on the generated Paths module. The warnings are generated by the flag '-fwarn-missing-import-lists'. Since cabal-install-1.16.0: * Have bootstrap.sh use Cabal-1.16.0.2 * Fix installing from custom folder on Linux (#1058) * Change bootstrap.sh to require Cabal >= 1.16 && < 1.18 * Bump cabal-install version number to 1.16.0.1 * Bump network dependency in bootstrap.sh to 2.3.1.1 * Fix compilation error * Disable setting the jobs: $nprocs line in default ~/.cabal config * Fix building cabal-install with ghc-6.12 and older I expect this to be the last release from the 1.16 branch. I intend to make another in about 3 months, or a bit sooner if the sandboxing work is ready earlier. Cheers, Johan

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Re: [Haskell-cafe] generalizing the writer monad
by Strake 24 Oct '12

24 Oct '12

On 17/10/2012, Petr P <petr.mvd(a)gmail.com> wrote: > Hi, > > (this is a literate Haskell post.) > > lately I was playing with the Writer monad and it seems to me that it > is too tightly coupled with monoids. Currently, MonadWriter makes the > following assumptions: > > (1) The written value can be read again later. > (2) For that to be possible it has to be monoid so that multiple (or > zero) values can be combined. > > I fell say that this is a bit restricting. Sometimes, the written > value can be lost - either used to compute something else or for > example sent out using some IO action to a file, network etc. For > example, I'd like to create an IO-based writer monad whose `tell` logs > its argument somewhere - prints it, stores to a file etc. No need: newtype SequenceM m a = SequenceM (m a); instance (Monad m, Monoid a) => Monoid (SequenceM m a) where { mempty = SequenceM (return mempty); SequenceM mx `mappend` SequenceM my = SequenceM (liftM2 mappend mx my); } whatever :: (MonadWriter (SequenceM IO ()) m) => m (); whatever = tell (SequenceM (someIO :: IO ())); Cheers, Strake

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Re: [Haskell-cafe] A clarification about what happens under the hood with foldMap
by John Lato 24 Oct '12

24 Oct '12

> From: Alfredo Di Napoli <alfredo.dinapoli(a)gmail.com> > Subject: [Haskell-cafe] A clarification about what happens under the > hood with foldMap > > I'm sure I'm missing a point, but the "minimum" definition for a Foldable > instance is given in terms of foldMap, so I get the cake for free, foldr > included, right? > In the example I have defined my treeSum as: > > treeSum = Data.Foldable.foldr (+) 0 > > So the only thing Haskell knows it that I want to fold over a Foldable for > which foldMap (and therefore foldr) is defined, and specifically I want to > fold using (+) as function. > But foldMap is defined in terms of f, which in this case is Sum, because I > want to sum things. It it were (*) f would have been Product, right? > > So what I'm missing is the relation between foldMap and foldr, aka "How > Haskell infer from (+) that I want f = Sum and not something different"? What you're missing is that this isn't how foldr is defined. As you probably suspect, it isn't possible for Haskell to deduce "f = Sum" from just the function. And in general the function parameter to foldr isn't even equivalent to mappend for any monoid, because it operates on two values with different types. Rather, foldr is defined using the Endo monoid, which is > newtype Endo a = Endo (a -> a) instance Monoid (Endo a) where mempty = id mappend = (.) Here's the default instance of Data.Foldable.foldr > foldr :: (a -> b -> b) -> b -> t a -> b > foldr f z t = appEndo (foldMap (Endo . f) t) z What happens is that, as the tree is traversed, Leaf constructors are replaced with 'id' (mempty :: Endo b), and branch values are replaced with 'Endo . f', where f = (+) in this example. Look at Endo . f: -- Endo :: (b -> b) -> Endo b -- f :: a -> (b -> b) -- Endo . f :: a -> Endo b so at each branch (Endo . f) is applied to the value, resulting in the type 'Endo b' foldMap just composes everything together with mappend, which, after the Endo constructor is removed, is a giant function pipeline :: b -> b, which is then applied to the provided base value (0 here). John L.

2 1

Hackage Package Discoverability
by Myles C. Maxfield 23 Oct '12

23 Oct '12

Hello, I am the author/maintainer of the 'punycode' hackage package. After 4 months, I just found that punycode conversion already exists in the Data.Encoding.BootString package inside the 'encoding' package. I'd like to deprecate my package in favor of the 'encoding' package. However, I would also like to solve the discoverability problem of people not knowing to look in the 'encoding' package when they're looking for the punycode algorithm. (I certainly didn't look there, and as a result, I re-implemented the algorithm). My initial thought is to keep my package in the hackage database, but put a big label on it saying "DEPRECATED: Use Data.Encoding.BootString instead." Is there a better way to make this algorithm discoverable? Thanks, Myles

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Haskell job opening at Functor AB
by Alp Mestanogullari 23 Oct '12

23 Oct '12

Hello -cafe, There's a very cool Haskell job opening at Functor AB, involving some cool type theory, for use in nuclear fusion research. You can read about it here: http://alpmestan.com/posts/2012-10-23-haskell-job-opening-at-functor.html -- Alp Mestanogullari

1 0

A clarification about what happens under the hood with foldMap
by Alfredo Di Napoli 23 Oct '12

23 Oct '12

---------- Forwarded message ---------- From: Alfredo Di Napoli <alfredo.dinapoli(a)gmail.com> Date: 23 October 2012 10:35 Subject: Re: [Haskell-cafe] A clarification about what happens under the hood with foldMap To: Chaddaï Fouché <chaddai.fouche(a)gmail.com> I'm sure I'm missing a point, but the "minimum" definition for a Foldable instance is given in terms of foldMap, so I get the cake for free, foldr included, right? In the example I have defined my treeSum as: treeSum = Data.Foldable.foldr (+) 0 So the only thing Haskell knows it that I want to fold over a Foldable for which foldMap (and therefore foldr) is defined, and specifically I want to fold using (+) as function. But foldMap is defined in terms of f, which in this case is Sum, because I want to sum things. It it were (*) f would have been Product, right? So what I'm missing is the relation between foldMap and foldr, aka "How Haskell infer from (+) that I want f = Sum and not something different"? I hope to have been clearer, don't know if I'm missing something crucial, though :) Thanks, A.

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A clarification about what happens under the hood with foldMap
by oleg＠okmij.org 23 Oct '12

23 Oct '12

> I was playing with the classic example of a Foldable structure: Trees. > So the code you can find both on Haskell Wiki and LYAH is the following: > > data Tree a = Empty | Node (Tree a) a (Tree a) deriving (Show, Eq) > > instance Foldable Tree where > foldMap f Empty = mempty > foldMap f (Node l p r) = foldMap f l `mappend` f p `mappend` foldMap f r > > treeSum :: Tree Int -> Int > treeSum = Data.Foldable.foldr (+) 0 > > > What this code does is straighforward. I was struck from the following > sentences in LYAH: > > Notice that we didn't have to provide the function that takes a value and > > returns a monoid value. > > We receive that function as a parameter to foldMap and all we have to > > decide is where to apply > > that function and how to join up the resulting monoids from it. > > This is obvious, so in case of (+) f = Sum, so f 3 = Sum 3 which is a > Monoid. > What I was wondering about is how Haskell knows that it has to pass, for > example, Sum in case of (+) and Product in case of (*)? Hopefully the following paradox helps: > treeDiff :: Tree Int -> Int > treeDiff = Data.Foldable.foldr (-) 0 > > t1 = treeDiff (Node (Node Empty 1 Empty) 2 (Node Empty 3 Empty)) This works just as well. You might be surprised: after all, there is no Diff monoid that corresponds to (-). In fact, there cannot be since (-) is not associative. And yet treeDiff type checks and even produces some integer when applied to a tree. What gives?

1 0

Why Kleisli composition is not in the Monad signature?
by oleg＠okmij.org 23 Oct '12

23 Oct '12

Andreas Abel wrote: > I tell them that monads are for sequencing effects; and the > sequencing is visible clearly in > > (>>) :: IO a -> IO b -> IO b > (>>=) :: IO a -> (a -> IO b) -> IO b > > but not in > > fmap :: (a -> b) -> IO a -> IO b > join :: IO (IO a) -> IO a Indeed! I'd like to point out an old academic paper that was written exactly on the subject at hand: how Category Theory monads relate to monads in Haskell. Here is the relevant quotation: Monads are typically equated with single-threadedness, and are therefore used as a technique for incorporating imperative features into a purely functional language. Category theory monads have little to do with single-threadedness; it is the sequencing imposed by composition that ensures single-threadedness. In a Wadler-ised monad this is a consequence of bundling the Kleisli star and flipped compose into the bind operator. There is nothing new in this connection. Peter Landin in his Algol 60 used functional composition to model semi-colon. Semi-colon can be thought of as a state transforming operator that threads the state of the machine throughout a program. The work of Peyton-Jones and Wadler has turned full circle back to Landin's earlier work as their use of Moggi's sequencing monad enables real side-effects to be incorporated into monad operations such as print. Quoted from: Sec 3: An historical aside Jonathan M. D. Hill and Keith Clarke: An Introduction to Category Theory, Category Theory Monads, and Their Relationship to Functional Programming. 1994. http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.53.6497 I'd like to stress: the single-threadedness, the trick that lets us embed imperative language into a pure one, has *little to do* with category-theoretic monads with their Klesli star. The web page http://okmij.org/ftp/Computation/IO-monad-history.html describes the work of Landin in detail, contrasting Landin's and Peyton-Jones' papers.

1 0

A clarification about what happens under the hood with foldMap
by Alfredo Di Napoli 23 Oct '12

23 Oct '12

Hi Cafe, I was playing with the classic example of a Foldable structure: Trees. So the code you can find both on Haskell Wiki and LYAH is the following: data Tree a = Empty | Node (Tree a) a (Tree a) deriving (Show, Eq) instance Foldable Tree where foldMap f Empty = mempty foldMap f (Node l p r) = foldMap f l `mappend` f p `mappend` foldMap f r treeSum :: Tree Int -> Int treeSum = Data.Foldable.foldr (+) 0 What this code does is straighforward. I was struck from the following sentences in LYAH: Notice that we didn't have to provide the function that takes a value and > returns a monoid value. > We receive that function as a parameter to foldMap and all we have to > decide is where to apply > that function and how to join up the resulting monoids from it. This is obvious, so in case of (+) f = Sum, so f 3 = Sum 3 which is a Monoid. What I was wondering about is how Haskell knows that it has to pass, for example, Sum in case of (+) and Product in case of (*)? In other term, I'm missing the logical piece of the puzzle which maps the associative binary function passed to fold up to our foldMap declaration. Thanks for any explanation :) Cheers, A.

2 1

A yet another question about subtyping and heterogeneous collections
by oleg＠okmij.org 23 Oct '12

23 Oct '12

First of all, MigMit has probably suggested the parameterization of Like by the constraint, something like the following: data Like ctx = forall a. (ctx a, Typeable a) => Like a instance ALike (Like ALike) where toA (Like x) = toA x instance CLike (Like CLike) where toC (Like x) = toC x get_mono :: Typeable b => [Like ALike] -> [b] get_mono = catMaybes . map ((\(Like x) -> cast x)) lst_a :: [Like ALike] lst_a = [Like a1, Like b1, Like c1, Like d1] lst_c :: [Like CLike] lst_c = [Like c1, Like d1] t1 = map print_a lst_a t2 = map print_a lst_c (The rest of the code is the same as in your first message). You need the flag ConstraintKinds. Second, all your examples so far used structural subtyping (objects with the same fields have the same type) rather than nominal subtyping of C++ (distinct classes have distinct types even if they have the same fields; the subtyping must be declared in the class declaration). For the structural subtyping, upcasts and downcasts can be done mostly automatically. See the OOHaskell paper or the code http://code.haskell.org/OOHaskell (see the files in the samples directory).

2 2

Fast parsing of unboxed values without boxing them in the parser?
by Eugene Kirpichov 23 Oct '12

23 Oct '12

Hi cafe, Are there any libraries that can be used for very fast parsing of binary data into structures with unboxed fields? The "naive" way, as implemented in Data.Binary etc, has the parsing monad datatype (say, "Get a") be parametric in "a" and include a field of type "a", which makes it impossible for a parser to return an unboxed value (e.g. parse a word32 from a byte array). I'm thinking that a CPS-style parser type could allow returning an unboxed value as a result of the compiler inlining and fusing together the parsing code and the code that consumes the parsed value. Are there any libraries that work like this? -- Eugene Kirpichov http://www.linkedin.com/in/eugenekirpichov We're hiring! http://tinyurl.com/mirantis-openstack-engineer

2 1

A yet another question about subtyping and heterogeneous collections
by Dmitry Vyal 22 Oct '12

22 Oct '12

Hello list! I've been experimenting with emulating subtyping and heterogeneous collections in Haskell. I need this to parse a binary representation of objects of a class hierarchy in C++ program. So far I implemented upcasting using a chain of type classes and now I'm playing with heterogeneous lists. For future purposes It would be ideal to be able to have something like these functions: upcast_list :: [LikeC] -> [LikeA] downcast_list :: [LikeA] -> [LikeC] First one only replaces the existential wrapper leaving the actual value intact, and the second one also filters the list, passing the elements with specific enough type. I can implement this particular functions, but what's about a more general one? Something like cast_list :: [a] -> [b], where a and b are existential types from one hierarchy. Something like LikeA and LikeC in my example. Is my approach feasible? Is there a better one? Am I missing something obvious? Any relevant advices are welcome. The example code follows: {-# LANGUAGE FlexibleInstances, UndecidableInstances, OverlappingInstances, ExistentialQuantification, DeriveDataTypeable #-} import Data.Typeable import Data.Maybe data A = A {a_x :: Int} deriving (Show, Typeable) data B = B {b_x :: Int, b_a :: A} deriving (Show, Typeable) data C = C {c_z :: Int, c_b :: B} deriving (Show, Typeable) data D = D {d_w :: Int, d_c :: C, d_a :: A} deriving (Show, Typeable) class ALike x where toA :: x -> A class BLike x where toB :: x -> B class CLike x where toC :: x -> C class DLike x where toD :: x -> D instance ALike A where toA = id instance BLike B where toB = id instance CLike C where toC = id instance DLike D where toD = id instance ALike B where toA = b_a instance BLike C where toB = c_b instance CLike D where toC = d_c instance (BLike x) => (ALike x) where toA = (toA :: B -> A) . toB instance CLike x => BLike x where toB = toB . toC a1 = A 1 b1 = B 2 (A 2) c1 = C 3 b1 d1 = D 4 c1 (A 10) print_a :: ALike x => x -> String print_a v = "A = " ++ show (a_x $ toA v) sum_a :: (ALike x, ALike y) => x -> y -> String sum_a v1 v2 = "A1 = " ++ show (a_x $ toA v1) ++ " A2 = " ++ show (a_x $ toA v2) data LikeA = forall a. (ALike a, Typeable a) => LikeA a instance ALike LikeA where toA (LikeA x) = toA x get_mono :: Typeable b => [LikeA] -> [b] get_mono = catMaybes . map ((\(LikeA x) -> cast x)) data LikeC = forall c. (CLike c, Typeable c) => LikeC c instance CLike LikeC where toC (LikeC x) = toC x lst_a = [LikeA a1, LikeA b1, LikeA c1, LikeA d1] lst_c = [LikeC c1, LikeC d1]

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Re: [Haskell-cafe] Blocking IO & FIFOs
by Jason Dusek 22 Oct '12

22 Oct '12

2012/10/20 Asten, W.G.G. van (Wilfried, Student M-CSC) <w.g.g.vanasten(a)student.utwente.nl>: > Would you be happy with a solution like this: > > - First create two handles to two files in the tmp directory > - Then use StdStream's UseHandle to redirect std_err and std_out > (using CreatePipe for std_in) to these files > - Then write your query to the Handle for std_in > - waitForProcess > - Collect std_out and std_err from the temporary files. > > If that is not satisfactory you may want to check out conduit-process > (http://hackage.haskell.org/packages/archive/process-conduit/0.5.0.2/doc/htm…) > that also does some interaction with a process and interleaves std_in > and std_out. It should not be to hard to combine std_err into that > concept. I also faced this same problem and in one case solved it by > using a temporary file to hold my content which was also deleted > afterwards (This immediately prevented the content building up in > memory). For my application, it's important to be able to run multiple queries against the same Bash session. Waiting for Bash to shut down is thus not a viable way to finalize the response. Perhaps I can spawn two cats with their outputs connected to the FIFOs and wait for them to terminate. -- Jason Dusek pgp // solidsnack // C1EBC57DC55144F35460C8DF1FD4C6C1FED18A2B

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Oxford Haskell Users Group
by Nicolas Wu 22 Oct '12

22 Oct '12

Dear all, After a long pause over the summer, the Oxford Haskell Users Group will be meeting this week, at 8pm on Friday 26th October, at the Mitre pub on High Street, Oxford [1]. The group meets in an informal setting with no real agenda: typically we turn up, have a few pints, and enjoy a good chat about what we've been doing with Haskell. If you'd like to have more details about our meetings, then join our mailing list [2], where our meetings are usually announced. The format of OxHUG will be changing slightly: we'll be meeting once a month rather than fortnightly, and rotating the day of the week. The full details of the meetings can be found on the OxHUG calendar [3], though at the moment only this week's meeting is planned. Nick [1]: http://goo.gl/maps/QEN3o [2]: https://groups.google.com/group/oxhug [3]: https://www.google.com/calendar/embed?src=pjc31qnpq48rvvncq72utqtang%40grou…

1 0

Solving integer equations in Haskell
by Justin Paston-Cooper 22 Oct '12

22 Oct '12

Hello, Can anyone suggest a library written in Haskell which can solve equations of the form xM(transpose(x)) = y, where x should be an integer vector, M is an integer matrix and y is an integer? I'm aware that Mathematica can do this, but I would like something written in Haskell. I haven't been sure of what exact keywords I should be searching for, hence my asking here. Thanks, Justin

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ANN: hledger 0.19
by Simon Michael 22 Oct '12

22 Oct '12

I'm pleased to announce the release of hledger 0.19! This release has just two visible changes: a much faster balance command, and support for the latest GHC and libs. (A hledger-web update will follow.) hledger is a command-line tool and haskell library for tracking financial transactions, which are stored in a human-readable plain text format. In addition to reporting, it can also help you record new transactions, or convert CSV data from your bank. Add-on packages include hledger-web, a web interface. hledger is inspired by and compatible with John Wiegley's Ledger. For more, see http://hledger.org . Install it: cabal update; cabal install hledger . For installation help, see http://hledger.org/MANUAL.html#installing . Or, sponsor a ready-to-run binary for your platform: http://hledger.org/DOWNLOAD.html . Release notes: hledger, hledger-lib: support GHC 7.6 and latest cmdargs, haskeline, split balance report no longer has an O(n^2) slowdown with large numbers of accounts, and is generally more speedy. Benchmark on a 2010 macbook: +-------------------------------------------++--------------+--------------+--------+ | || hledger-0.18 | hledger-0.19 | ledger | +===========================================++==============+==============+========+ | -f data/100x100x10.journal balance || 0.21 | 0.07 | 0.09 | | -f data/1000x1000x10.journal balance || 10.13 | 0.47 | 0.62 | | -f data/1000x10000x10.journal balance || 40.67 | 0.67 | 1.01 | | -f data/10000x1000x10.journal balance || 15.01 | 3.22 | 2.36 | | -f data/10000x1000x10.journal balance aa || 4.77 | 4.40 | 2.33 | +-------------------------------------------++--------------+--------------+--------+ build version is set with CPP instead of cabal-file-th Release contributors: Simon Michael, Sergei Trofimovich Release stats: 106 days, 21 commits, 1 end-user fix, 0 end-user features since last release Project stats: 222 unit & functional tests, 7740 lines of code including hledger-web, 21 committers

1 0

serialize an unknown type
by Corentin Dupont 21 Oct '12

21 Oct '12

Hi the list! I have a simple question, how can I serialize/deserialize a structure like this: data InputChoice c deriving Typeable data Event a where InputChoice :: (Eq c, Show c) => [c] -> c -> Event (InputChoice c) (...) I'd like that the values of type "c" get serialized to a String... That's the easy part, but for deserializing, oops! Cheers, Corentin

5 10

How do I specify language literals in hsparql?
by Andrew Pennebaker 21 Oct '12

21 Oct '12

If the raw SPARQL is rdfs:label "D (programming language)"@en, what would the hsparql <http://hackage.haskell.org/package/hsparql> syntax be? The docs<https://github.com/robstewart57/hsparql/blob/master/tests/DBPedia.hs> don't include any language literal examples. -- Cheers, Andrew Pennebaker www.yellosoft.us

2 1

Blocking IO & FIFOs
by Jason Dusek 20 Oct '12

20 Oct '12

Hi all, I am developing a coroutine-like interface to Bash. http://hpaste.org/76523 The idea is that one can send multiple "queries" to an interpreter and then collect the results of each query. How do we know when Bash is done with each query? Waiting for "no more output" seems ambiguous; so the way CoBash works is: * Each query gets a tmp dir with two named pipes in it. * The query is wrapped in redirections to the pipes. * The pipes are removed when the query completes. This does work, sort of: +Prelude> :load CoBash.hs [1 of 1] Compiling CoBash ( CoBash.hs, interpreted ) Ok, modules loaded: CoBash. *CoBash> tuple@(i,o,e,p) <- start e :: Handle i :: Handle o :: Handle p :: ProcessHandle tuple :: (Handle, Handle, Handle, ProcessHandle) *CoBash> query tuple "for n in {1..4}; do sleep 1; echo $n; done" ("1\n2\n3\n4\n","") it :: (ByteString, ByteString) I say sort of because it is quite brittle. Many commands do not return at all, for example: *CoBash> query tuple "uname -a" The way I retrieve the output from the FIFOs seems dangerous: (,) <$> Bytes.hGetContents oh <*> Bytes.hGetContents eh Surely, the FIFO for STDERR can not be read from until the FIFO for STDOUT is finished; but if there is a great deal of error output then the process will fill the FIFO's buffer and get stuck. If we switch the order of the reads, the "for n in ..." example above blocks: (,) <$> Bytes.hGetContents eh <*> Bytes.hGetContents oh I have tried a few different ways to read from the two handles concurrently; for example, by giving each thread an MVar to put the contents in, or by using hGetNonBlocking on a list of handles in a loop. Using the latter method, I never get EOF; it just collects empty strings forever. For comparison's sake, the expect behaviour with FIFOs is: In the first terminal: :; mkfifo fifo :; cat > fifo a b c d ^D :; In the second terminal: :; cat < fifo a b c d :; Here I "open" the FIFO for reading with < while opening it for writing with >. As long as the writer writes, the reader reads; when the writer closes the write end of the pipe, the reader receives EOF. Trying to duplicate the read behaviour in Haskell, using hGetContents from GHCi while using cat to write to the FIFO, doesn't work; which seems a little bogus. There have a been a few past threads about FIFOs and their troublesome interaction with Haskell's async-by-default IO style. To switch to System.Posix for IO -- and deal with Ptr Word8, in order to handle binary data -- seems like an awful step down. -- Jason Dusek pgp // solidsnack // C1EBC57DC55144F35460C8DF1FD4C6C1FED18A2B {-# LANGUAGE OverloadedStrings , ScopedTypeVariables , ParallelListComp , TupleSections #-} module CoBash where import Control.Applicative import Control.Concurrent import Control.Concurrent.MVar import Control.Exception import Control.Monad import Data.Bits import Data.ByteString.Char8 (ByteString) import qualified Data.ByteString.Char8 as Bytes import Data.Maybe import Data.Monoid import qualified GHC.IO.Handle.FD import System.IO import System.IO.Error import System.Process import System.Posix.ByteString import System.IO.Temp import qualified Text.ShellEscape as Esc start :: IO (Handle, Handle, Handle, ProcessHandle) start = runInteractiveProcess "bash" [] Nothing (Just []) query :: (Handle, Handle, Handle, ProcessHandle) -> ByteString -> IO (ByteString, ByteString) query (i, _, _, _) query = withFIFOs query' where query' ofo efo = do Bytes.hPut i cmd hFlush i [oh, eh] <- mapM openFIFO [ofo, efo] (,) <$> Bytes.hGetContents oh <*> Bytes.hGetContents eh -- Works. -- (,) <$> Bytes.hGetContents eh <*> Bytes.hGetContents oh -- Blocks. where cmd = Bytes.unlines ["{", query, "} 1>" <> ofo <> " 2>" <> efo] shutdown :: (Handle, Handle, Handle, ProcessHandle) -> IO () shutdown (i, _, _, p) = () <$ hClose i <* waitForProcess p openFIFO path = GHC.IO.Handle.FD.openFileBlocking (Bytes.unpack path) ReadMode -- | Run an IO action with two FIFOs in scope, which will removed after it -- completes. withFIFOs :: (RawFilePath -> RawFilePath -> IO a) -> IO a withFIFOs m = withSystemTempDirectory "cobash." m' where m' = (uncurry m =<<) . mk . Bytes.pack mk d = (o, e) <$ (createNamedPipe o mode >> createNamedPipe e mode) where (o, e) = (d <> "/o", d <> "/e") mode = ownerReadMode .|. ownerWriteMode .|. namedPipeMode

1 0

Monad.Reader #21 call for copy
by Edward Z. Yang 20 Oct '12

20 Oct '12

Call for Copy: The Monad.Reader - Issue 21 -------------------------------------------- Whether you're an established academic or have only just started learning Haskell, if you have something to say, please consider writing an article for The Monad.Reader! The submission deadline for Issue 21 will be: **Tuesday, January 1** The Monad.Reader ~~~~~~~~~~~~~~~~ The Monad.Reader is a electronic magazine about all things Haskell. It is less formal than journal, but somehow more enduring than a wiki- page. There have been a wide variety of articles: exciting code fragments, intriguing puzzles, book reviews, tutorials, and even half-baked research ideas. Submission Details ~~~~~~~~~~~~~~~~~~ Get in touch with me if you intend to submit something -- the sooner you let me know what you're up to, the better. Please submit articles for the next issue to me by e-mail (ezyang(a)mit.edu) Articles should be written according to the guidelines available from http://themonadreader.wordpress.com/contributing/ Please submit your article in PDF, together with any source files you used. The sources will be released together with the magazine under a BSD license. If you would like to submit an article, but have trouble with LaTeX please let me know and we'll work something out.

1 0

Getting PID of a child process
by Jason Dusek 19 Oct '12

19 Oct '12

Hi All, Using `System.Process.runInteractiveProcess', I can start a process and get a handle to it: runInteractiveProcess :: FilePath -> [String] -> Maybe FilePath -> Maybe [(String, String)] -> IO (Handle, Handle, Handle, ProcessHandle) For diagnostic purposes, I'd like to print the PID of the process attached to this handle -- how best to do that? -- Jason Dusek pgp // solidsnack // C1EBC57DC55144F35460C8DF1FD4C6C1FED18A2B

3 3

Release Candidate 2: HP 2014.4.0.0
by Mark Lentczner 19 Oct '12

19 Oct '12

Haskellistas - The second release candidate of HP 2014.4.0.0 is now available: - Source tarball: haskell-platform-2012.4.0.0-rc2.tar.gz<http://ozonehouse.com/mark/platform/haskell-platform-2012.4.0.0-rc2.tar.gz> - Mac 32-bit installer: Haskell Platform 2012.4.0.0 32bit rc2 signed.pkg<http://ozonehouse.com/mark/platform/Haskell%20Platform%202012.4.0.0%2032bit…> Mac notes: This version is now digitally signed by me, and so will install without fuss on OS X 10.8. I'll put up 64-bit version soon. - Mark Changes since RC1: TODO ---- [x] update vector and primitive versions (they had a point release to fix a bad bug, and the fix was very very simple) [x] remove Build.hs - it did all its work by shelling out anyway! [x] main .cabal file should not have versions commented out [x] this will require a way to extract the list of packages that need to be part of the source release [x] why not also extract the list of packages needed for core.packages [x] fix to build.hs (ticket #195) [x] bugs in build.sh (ticket #197) [x] build instructions in the tarball (ticket #207) [x] merge in shared library patch (ticket #198) MAC TODO -------- [x] mac cabal script needs to quote things with $HOME or ~ in them (ticket #192) [x] If the cabal command is "update" then the wrapper script shouldn't do an update the first time through! [x] sign Mac installers (ticket #203)

2 2

acid-state audit trail
by Richard Wallace 19 Oct '12

19 Oct '12

Hey all, I've been looking at acid-state as a possible storage backend for an application. It looks like it fits my needs pretty damn well, but one thing that I'm curious about is if it is possible to get a list of "update events". You can obviously query for the current state, but it's not immediately apparent if you can see the history of your values state. This is useful in some things, like providing audit trails and debugging. As well as being able to re-create state in a different form. I was also curious if the createCheckpoint function eliminates the state history or does it just create a snapshot, it's not apparent from the docs. Thanks, Rich

3 3

Adding custom events to eventlog
by Janek S. 19 Oct '12

19 Oct '12

Dear list, I'm using ThreadScope to improve performance of my parallel program. It would be very helpful for me if I could place custom things in eventlog (e.g. "now function x begins"). Is this possible? Janek

2 2

Erroneous interaction between DataKinds and ExistentialQuantification?
by Stefan Holdermans 19 Oct '12

19 Oct '12

I am almost sure this is a known issue, but I noticed some erroneous (?) interaction between datatype promotion and existential quantification. Consider the following program: {-# LANGUAGE DataKinds #-} {-# LANGUAGE ExistentialQuantification #-} {-# LANGUAGE GADTs #-} {-# LANGUAGE KindSignatures #-} module Test where data K = forall a. T a -- promotion gives 'T :: * -> K data G :: K -> * where D :: G (T []) -- kind error! I would expect the type checker to reject it, but GHC (version 7.6.1) compiles it happily. Is this indeed a (known) bug? On a related note: is there a way to promote a type that involves an existential type variable of a kind other than *? Thanks, Stefan

2 1

Build GHC to i386 platform represent an error after run command "make bootstrapping-files"
by Chatsiri Ratana 19 Oct '12

19 Oct '12

Hello All, I using GHC for x86_64 platform compiles the GHC source in order to port code to the i386 platform. Steps for building GHC to i386 platform at link[1]. Configure before initial other steps as completed follow by $./configure --enable-hc-boot --build=i386-unknown-linux --host=i386-unknown-linux --target=i386-unknown-linux --with-gcc=/usr/bin/gcc --with-ld=/usr/bin/ld --with-nm=/usr/bin/nm --with-ghc=/usr/lib/ghc/bin/ghc ---------------------------------------------------------------------- Configure completed successfully. Building GHC version : 7.7.20120806 Build platform : i386-unknown-linux Host platform : i386-unknown-linux Target platform : i386-unknown-linux Bootstrapping from HC files. Using GCC : /usr/bin/gcc which is version : 4.6.3 Building a cross compiler : NO Porting to foreign arch : NO Alien script : ld : /usr/bin/ld Happy : () Alex : () Python : /usr/bin/python Perl : /usr/bin/perl dblatex : xsltproc : HsColour was not found; documentation will not contain source links Building DocBook HTML documentation : NO Building DocBook PS documentation : NO Building DocBook PDF documentation : NO ---------------------------------------------------------------------- Next step for making executable file by command "make bootstrapping-files". It's show an error such as below. $ make bootstrapping-files make -r --no-print-directory -f ghc.mk bootstrapping-files /usr/bin/find: `libraries/ghc-prim/dist-install/build': No such file or directory /usr/bin/find: `libraries/ghc-prim/dist-install/build': No such file or directory /usr/bin/find: `libraries/integer-gmp/dist-install/build': No such file or directory /usr/bin/find: `libraries/integer-gmp/dist-install/build': No such file or directory /usr/bin/find: `libraries/base/dist-install/build': No such file or directory /usr/bin/find: `libraries/base/dist-install/build': No such file or directory /usr/bin/find: `libraries/filepath/dist-install/build': No such file or directory /usr/bin/find: `libraries/filepath/dist-install/build': No such file or directory /usr/bin/find: `libraries/array/dist-install/build': No such file or directory /usr/bin/find: `libraries/array/dist-install/build': No such file or directory /usr/bin/find: `libraries/deepseq/dist-install/build': No such file or directory /usr/bin/find: `libraries/deepseq/dist-install/build': No such file or directory /usr/bin/find: `libraries/bytestring/dist-install/build': No such file or directory /usr/bin/find: `libraries/bytestring/dist-install/build': No such file or directory /usr/bin/find: `libraries/containers/dist-install/build': No such file or directory /usr/bin/find: `libraries/containers/dist-install/build': No such file or directory /usr/bin/find: `libraries/old-locale/dist-install/build': No such file or directory /usr/bin/find: `libraries/old-locale/dist-install/build': No such file or directory /usr/bin/find: `libraries/old-time/dist-install/build': No such file or directory /usr/bin/find: `libraries/old-time/dist-install/build': No such file or directory /usr/bin/find: `libraries/time/dist-install/build': No such file or directory /usr/bin/find: `libraries/time/dist-install/build': No such file or directory /usr/bin/find: `libraries/unix/dist-install/build': No such file or directory /usr/bin/find: `libraries/unix/dist-install/build': No such file or directory /usr/bin/find: `libraries/directory/dist-install/build': No such file or directory /usr/bin/find: `libraries/directory/dist-install/build': No such file or directory /usr/bin/find: `libraries/process/dist-install/build': No such file or directory /usr/bin/find: `libraries/process/dist-install/build': No such file or directory /usr/bin/find: `libraries/hpc/dist-boot/build': No such file or directory /usr/bin/find: `libraries/hpc/dist-boot/build': No such file or directory /usr/bin/find: `libraries/hpc/dist-install/build': No such file or directory /usr/bin/find: `libraries/hpc/dist-install/build': No such file or directory /usr/bin/find: `libraries/pretty/dist-install/build': No such file or directory /usr/bin/find: `libraries/pretty/dist-install/build': No such file or directory /usr/bin/find: `libraries/template-haskell/dist-install/build': No such file or directory /usr/bin/find: `libraries/template-haskell/dist-install/build': No such file or directory /usr/bin/find: `libraries/Cabal/Cabal/dist-boot/build': No such file or directory /usr/bin/find: `libraries/Cabal/Cabal/dist-boot/build': No such file or directory /usr/bin/find: `libraries/Cabal/Cabal/dist-install/build': No such file or directory /usr/bin/find: `libraries/Cabal/Cabal/dist-install/build': No such file or directory /usr/bin/find: `libraries/binary/dist-boot/build': No such file or directory /usr/bin/find: `libraries/binary/dist-boot/build': No such file or directory /usr/bin/find: `libraries/binary/dist-install/build': No such file or directory /usr/bin/find: `libraries/binary/dist-install/build': No such file or directory /usr/bin/find: `libraries/bin-package-db/dist-boot/build': No such file or directory /usr/bin/find: `libraries/bin-package-db/dist-boot/build': No such file or directory /usr/bin/find: `libraries/bin-package-db/dist-install/build': No such file or directory /usr/bin/find: `libraries/bin-package-db/dist-install/build': No such file or directory /usr/bin/find: `libraries/hoopl/dist-boot/build': No such file or directory /usr/bin/find: `libraries/hoopl/dist-boot/build': No such file or directory /usr/bin/find: `libraries/hoopl/dist-install/build': No such file or directory /usr/bin/find: `libraries/hoopl/dist-install/build': No such file or directory /usr/bin/find: `libraries/transformers/dist-install/build': No such file or directory /usr/bin/find: `libraries/transformers/dist-install/build': No such file or directory /usr/bin/find: `libraries/xhtml/dist-install/build': No such file or directory /usr/bin/find: `libraries/xhtml/dist-install/build': No such file or directory /usr/bin/find: `libraries/terminfo/dist-install/build': No such file or directory /usr/bin/find: `libraries/terminfo/dist-install/build': No such file or directory /usr/bin/find: `libraries/haskeline/dist-install/build': No such file or directory /usr/bin/find: `libraries/haskeline/dist-install/build': No such file or directory /usr/bin/find: `libraries/integer-gmp/dist-install/build': No such file or directory /usr/bin/find: `compiler/stage1/build': No such file or directory /usr/bin/find: `compiler/stage1/build': No such file or directory /usr/bin/find: `compiler/stage2/build': No such file or directory /usr/bin/find: `compiler/stage2/build': No such file or directory /usr/bin/find: `libraries/haskell98/dist-install/build': No such file or directory /usr/bin/find: `libraries/haskell98/dist-install/build': No such file or directory /usr/bin/find: `libraries/haskell2010/dist-install/build': No such file or directory /usr/bin/find: `libraries/haskell2010/dist-install/build': No such file or directory /usr/bin/find: `libraries/random/dist-install/build': No such file or directory /usr/bin/find: `libraries/random/dist-install/build': No such file or directory /usr/bin/find: `libraries/primitive/dist-install/build': No such file or directory /usr/bin/find: `libraries/primitive/dist-install/build': No such file or directory /usr/bin/find: `libraries/vector/dist-install/build': No such file or directory /usr/bin/find: `libraries/vector/dist-install/build': No such file or directory /usr/bin/find: `libraries/dph/dph-base/dist-install/build': No such file or directory /usr/bin/find: `libraries/dph/dph-base/dist-install/build': No such file or directory /usr/bin/find: `libraries/dph/dph-prim-interface/dist-install/build': No such file or directory /usr/bin/find: `libraries/dph/dph-prim-interface/dist-install/build': No such file or directory /usr/bin/find: `libraries/dph/dph-prim-seq/dist-install/build': No such file or directory /usr/bin/find: `libraries/dph/dph-prim-seq/dist-install/build': No such file or directory /usr/bin/find: `libraries/dph/dph-prim-par/dist-install/build': No such file or directory /usr/bin/find: `libraries/dph/dph-prim-par/dist-install/build': No such file or directory /usr/bin/find: `libraries/dph/dph-lifted-base/dist-install/build': No such file or directory /usr/bin/find: `libraries/dph/dph-lifted-base/dist-install/build': No such file or directory /usr/bin/find: `libraries/dph/dph-lifted-boxed/dist-install/build': No such file or directory /usr/bin/find: `libraries/dph/dph-lifted-boxed/dist-install/build': No such file or directory /usr/bin/find: `libraries/dph/dph-lifted-copy/dist-install/build': No such file or directory /usr/bin/find: `libraries/dph/dph-lifted-copy/dist-install/build': No such file or directory /usr/bin/find: `libraries/dph/dph-lifted-vseg/dist-install/build': No such file or directory /usr/bin/find: `libraries/dph/dph-lifted-vseg/dist-install/build': No such file or directory /bin/sh: 1: Syntax error: Unterminated quoted string make[1]: *** [includes/dist-derivedconstants/build/tmp/mkDerivedConstants] Error 2 make: *** [bootstrapping-files] Error 2 Who have strategies or methods for solving build step at here? [1] http://hackage.haskell.org/trac/ghc/wiki/Building/Porting Thank in advance Chatsiri Rattana -- :--------------------------------------------------------

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How to correctly benchmark code with Criterion?
by Janek S. 19 Oct '12

19 Oct '12

Dear list, during past few days I spent a lot of time trying to figure out how to write Criterion benchmarks, so that results don't get skewed by lazy evaluation. I want to benchmark different versions of an algorithm doing numerical computations on a vector. For that I need to create an input vector containing a few thousand elements. I decided to create random data, but that really doesn't matter - I could have as well use infinite lists instead of random ones. My problem is that I am not certain if I am creating my benchmark correctly. I wrote a function that creates data like this: dataBuild :: RandomGen g => g -> ([Double], [Double]) dataBuild gen = (take 6 $ randoms gen, take 2048 $ randoms gen) And I create benchmark like this: bench "Lists" $ nf L.benchThisFunction (L.dataBuild gen) The question is how to generate data so that its evaluation won't be included in the benchmark. I already asked this question on StackOverflow ( http://stackoverflow.com/questions/12896235/how-to-create-data-for-criterio… ) and got answer to use evaluate + force. After spending one day on testing this approach I came to conclusion that doing this does not seem to influence results of a benchmark in any way (I did stuf like unsagePerformIO + delayThread). On the other hand I looked into sources of criterion and I see that the benchmark code is run like this: evaluate (rnf (f x)) I am a Haskell newbie and perhaps don't interpret this correctly, but to me it looks as though criterion did not evaluate the possibly non-evaluated parameter x before running the benchmark, but instead evaluates the final result. Can someone provide an explanation on how this exactly works and how should I write my benchmarks so that results are correct? Janek

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ANNOUNCE: apphointments - A simple functional calendar
by Steffen Schuldenzucker 19 Oct '12

19 Oct '12

Dear Café, I just wrote myself a small (~ 200 LOC) calendar application today. https://bitbucket.org/rainmaker/apphointments Comments/Patches welcome. From the readme: apphointments - A simple functional calendar ============================================ This is a "UI = Code" calendar application: You create an event by writing code that defines it. To see what's up next, you create a "report", i.e. a summary of the events in a certain time range such as 'thisWeek'. This is done by haskell functions again from GHCi or ghc -e. See example.hs. Using just haskell instead of our own language or GUI allows great flexibility in both defining events and generating reports. See e.g. the 'lecture' combinator from Apphointments.Util. Status ------ Works for me, but has no features yet. See TODO. Credits ------- Created by Steffen Schuldenzucker <sschuldenzucker(a)uni-bonn.de>

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Fwd: hdbc parametrized select
by Wilfried van Asten 18 Oct '12

18 Oct '12

This was not sent to the cafe since I used the wrong from address. ---------- Forwarded message ---------- From: Asten, W.G.G. van (Wilfried, Student M-CSC) <w.g.g.vanasten(a)student.utwente.nl> Date: Thu, Oct 18, 2012 at 12:27 PM Subject: Re: [Haskell-cafe] hdbc parametrized select To: s9gf4ult(a)gmail.com Cc: haskell-cafe(a)haskell.org To my knowledge you can combine execute and the fetch functions like this: find conn id = do stmt <- prepare conn "SELECT * FROM table WHERE id = ? LIMIT 1" excute stmt [id] row <- fetchRow stmt On Thu, Oct 18, 2012 at 10:51 AM, <s9gf4ult(a)gmail.com> wrote: > does hdbc have parametrized selects ? > There is execute > execute :: Statement -> [SqlValue] -> IO Integer > and some other functions which get list of parameters but return IO Integer or > IO () > and there is other couple of functions > fetchRow :: Statement -> IO (Maybe [SqlValue]) > which return values of select but do not get parameters > > How can i execute parametrized select with hdbc ? > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe

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hdbc parametrized select
by s9gf4ult＠gmail.com 18 Oct '12

18 Oct '12

does hdbc have parametrized selects ? There is execute execute :: Statement -> [SqlValue] -> IO Integer and some other functions which get list of parameters but return IO Integer or IO () and there is other couple of functions fetchRow :: Statement -> IO (Maybe [SqlValue]) which return values of select but do not get parameters How can i execute parametrized select with hdbc ?

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Re: [Haskell-cafe] Safe Haskell and instance coherence
by Mikhail Glushenkov 18 Oct '12

18 Oct '12

Hello David, On Thu, Oct 18, 2012 at 6:21 AM, David Mazieres expires 2013-01-15 PST <mazieres-f97tx4hr7aw7n8sqs9kpcvuwd2(a)temporary-address.scs.stanford.edu> wrote: > > What's interesting is that these examples doesn't violate the goals of > Safe Haskell--i.e., they don't violate type safety, encapsulation, or > semantic consistency as we defined it--but they are worrisome. I think that my example violates encapsulation in a similar way to how GND violates encapsulation in the MinList example from the Safe Haskell paper. IIUC, the problem is that all instances are assumed to be coherent (that's why it's an error to have two Monoid Int instances in the same module), but this assumption isn't actually enforced across module boundaries, even though it is reflected in the type system. GHC does print a warning after having encountered orphan instances, but that's all. > This is essentially the same thing as my Monoid example. You've got > two different dictionaries for the same type, and can pass either one > around depending on what module you imported. Are "dictionaries" something that is a part of Haskell semantics or an artefact of the implementation? > It's > not hard to cook up a contrived example where some sort of security > monitor hands over the keys to the kingdom if ever it encounters > duplicate items in a set. Auditing the code, that might seem fine > unless you understand the implementation of Set, which makes reasoning > about security a lot harder. Agreed. > What we really want is for the dictionary to be associated with the > data structure at the time of creation, not passed in as an argument > for each operation. But that's not even implementable without the > existential types extension, and also would require re-writing all the > containers, which is absolutely not what we want. This is what Scala does. Unfortunately, this can make some operations (e.g. set union) asymptotically less efficient (as it's now impossible to rely on the fact that both sets use the same associated dictionary). > Failing that, I guess we could disallow overlapping instances even > when they don't overlap in a single module. This is a whole-program > check similar to what type families requires, so could possibly be > implemented in a similar way. I'm really interested in the link-time check that is performed for type families. Is it described somewhere? > However, as with type families, making > it work with dynamic loading is going to be kind of hard. Essentially > there would have to be some run-time inspectable information about all > instances defined. I think that's why Rust chose to just disallow orphan instances :-) Even without dynamic loading, supporting all GHC extensions (e.g. FlexibleInstances) will probably be non-trivial. -- () ascii ribbon campaign - against html e-mail /\ www.asciiribbon.org - against proprietary attachments

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Re: [Haskell-cafe] Safe Haskell and instance coherence
by Mikhail Glushenkov 17 Oct '12

17 Oct '12

Hello David, On Wed, Oct 17, 2012 at 6:02 PM, David Mazieres expires 2013-01-15 PST <mazieres-sebg4mvthwuzk9r27pqmr8jndi(a)temporary-address.scs.stanford.edu> wrote: > Can you elaborate on how this can be used to break the data structure > invariant? If in safe Haskell you import two modules that have > overlapping instances, you will not be able to use the two instances. > Modules that import only one instance will be able to use that > instance. Please take a look at the code example I provided: https://gist.github.com/3854294 I don't use overlapping instances or any other extensions besides Safe Haskell. By defining two orphan Ord instances for U I'm able to construct a value of type Set U that contains two equal elements: > test fromList [X,Y,X] This is what I meant by "breaking the data structure invariant". This shouldn't normally be possible: the documentation for Data.Set.insert says: "If the set already contains an element equal to the given value, it is replaced with the new value." Regarding your Monoid example, it will still be possible to make it work even if instance coherence is enforced by using a newtype wrapper (in fact, Data.Monoid already includes Sum and Product newtype wrappers that provide this functionality). -- () ascii ribbon campaign - against html e-mail /\ www.asciiribbon.org - against proprietary attachments

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generalizing the writer monad
by Petr P 17 Oct '12

17 Oct '12

Hi, (this is a literate Haskell post.) lately I was playing with the Writer monad and it seems to me that it is too tightly coupled with monoids. Currently, MonadWriter makes the following assumptions: (1) The written value can be read again later. (2) For that to be possible it has to be monoid so that multiple (or zero) values can be combined. I fell say that this is a bit restricting. Sometimes, the written value can be lost - either used to compute something else or for example sent out using some IO action to a file, network etc. For example, I'd like to create an IO-based writer monad whose `tell` logs its argument somewhere - prints it, stores to a file etc. So what I'm suggesting is to have another type class between Monad and MonadWriter, let's say MonadTell, which only allows to write values, not to retrieve them later: > {-# LANGUAGE FlexibleInstances, MultiParamTypeClasses, FunctionalDependencies #-} > import Control.Monad > import Control.Monad.Trans > import qualified Control.Monad.Writer as W > import qualified Control.Monad.Reader as R > import Data.Monoid > > class Monad m => MonadTell w m where > tell :: w -> m () > tell w = writer ((), w) > writer :: (a, w) -> m a > writer ~(a, w) = tell w >> return a (We don't need fun.deps. here, they're needed in MonadWriter because of `listen`. IDK if it'd be still better to add fun.dep. just to eliminate typing problems?) And MonadWriter would be defined by inheriting from MonadTell: > class (MonadTell w m, Monoid w) => MonadWriter' w m | m -> w where > listen :: m a -> m (a, w) > pass :: m (a, w -> w) -> m a Now we could use MonadWriter as before, but we could also make more generic writers like: > newtype Log = Log String deriving Show > -- Prints logs to stdout. > instance MonadTell Log IO where > tell (Log s) = putStrLn s > > -- Collects the length of written logs. > instance Monad m => MonadTell Log (W.WriterT (Sum Int) m) where > tell (Log s) = W.tell (Sum $ length s) > > > main = do > let l = Log "Hello world" > tell l > print . getSum . W.execWriter $ (tell l :: W.Writer (Sum Int) ()) The same applies to MonadReader. We could make another type class between Monad and MonadReader just with `ask`: > class Monad m => MonadAsk r m | m -> r where > ask :: m r This would allow us to write instances like > instance MonadAsk Log IO where > ask = liftM Log getLine Does it make sense? Best regards, Petr Pudlak

2 1

poor performance when generating random text
by Dmitry Vyal 17 Oct '12

17 Oct '12

Hello anyone I've written a snippet which generates a file full of random strings. When compiled with -O2 on ghc-7.6, the generation speed is about 2Mb per second which is on par with interpreted php. That's the fact I find rather disappointing. Maybe I've missed something trivial? Any suggestions and explanations are welcome. :) % cat ext_sort.hs import qualified Data.Text as T import System.Random import Control.Exception import Control.Monad import System.IO import qualified Data.Text.IO as TI gen_string g = let (len, g') = randomR (50, 450) g in T.unfoldrN len rand_text (len, g') where rand_text (0,_) = Nothing rand_text (k,g) = let (c, g') = randomR ('a','z') g in Just (c, ((k-1), g')) write_corpus file = bracket (openFile file WriteMode) hClose $ \h -> do let size = 100000 sequence $ replicate size $ do g <- newStdGen let text = gen_string g TI.hPutStrLn h text main = do putStrLn "generating text corpus" write_corpus "test.txt" % cat ext_sort.prof Wed Oct 17 10:59 2012 Time and Allocation Profiling Report (Final) ext_sort +RTS -p -RTS total time = 32.56 secs (32558 ticks @ 1000 us, 1 processor) total alloc = 12,742,917,332 bytes (excludes profiling overheads) COST CENTRE MODULE %time %alloc gen_string.rand_text.(...) Main 70.7 69.8 gen_string Main 17.6 15.8 gen_string.rand_text Main 5.4 13.3 write_corpus.\ Main 4.3 0.8 individual inherited COST CENTRE MODULE no. entries %time %alloc %time %alloc MAIN MAIN 67 0 0.0 0.0 100.0 100.0 main Main 135 0 0.0 0.0 100.0 100.0 write_corpus Main 137 0 0.0 0.0 100.0 100.0 write_corpus.\ Main 138 1 4.3 0.8 100.0 100.0 write_corpus.\.text Main 140 100000 0.0 0.0 95.7 99.2 gen_string Main 141 100000 17.6 15.8 95.7 99.2 gen_string.g' Main 147 100000 0.0 0.0 0.0 0.0 gen_string.rand_text Main 144 25109743 5.4 13.3 77.5 83.2 gen_string.rand_text.g' Main 148 24909743 0.6 0.0 0.6 0.0 gen_string.rand_text.(...) Main 146 25009743 70.7 69.8 70.7 69.8 gen_string.rand_text.c Main 145 25009743 0.8 0.0 0.8 0.0 gen_string.len Main 143 100000 0.0 0.0 0.0 0.0 gen_string.(...) Main 142 100000 0.6 0.3 0.6 0.3

3 4

License of CloudHaskell code write by Haskell language.
by Chatsiri Ratana 17 Oct '12

17 Oct '12

Hello All, License of term of CloudHaskell[1]. It isn't GNU & GPL public license. I not sure to include it's to my project. Who know MPI write by haskell pure and not dependent on C/C++ wrapper such as Haskell-mpi[2]? I need to idea for implement OpenMPI standard by Haskell language. Who have an idea for implementing MPI by Haskell? [1]https://github.com/jepst/CloudHaskell/blob/master/LICENSE [2]http://hackage.haskell.org/package/haskell-mpi Thank in advance, Chatsiri Rattana. -- :--------------------------------------------------------

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