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Fwd: Do you recognize this type?
by Corentin Dupont 20 Jun '14

20 Jun '14

Yes it looks like, but I don't think it is: the argument function in ListEvent is taking a list itself, so that's not a fold. It's a sort of interruptible computation (since as soon as the argument function returns Just, we remove all remaining events). But I'm sure there is a better way to write it. ListEvent :: [Event a] -> ([a] -> Maybe b) -> Event b On Fri, Jun 20, 2014 at 4:31 PM, Kyle Marek-Spartz < kyle.marek.spartz(a)gmail.com> wrote: > Isn’t this a fold? > > – > Kyle Marek-Spartz > > > > On Jun 20, 2014, 7:59:41 AM, Corentin Dupont <corentin.dupont(a)gmail.com> > wrote: > ------------------------------ > Nobody on that? > What I really want is: a computation that can be interrupted if enough > result is already there. > I would be surprise if that concept doesn't exist already somewhere :) > > > On Thu, Jun 19, 2014 at 5:17 PM, Corentin Dupont < > corentin.dupont(a)gmail.com> wrote: > >> Hi guys, >> I am making a DSL for event composition (inspired from >> digestive-functor & reactive-banana) and I find myself wanting a primitive >> like that: >> >> -- given a list of events, evaluate the function on those that have >> already fired. >> -- returns an event yelding the result when ready. >> ListEvent :: [Event a] -> ([a] -> Maybe b) -> Event b >> >> Does this type tell you something? Is there a better abstraction that >> can encompass it? >> My DSL is like that: >> >> -- | Composable events >> data Event a where >> SumEvent :: Event a -> Event a -> Event a -- The first event >> to fire will be returned >> AppEvent :: Event (a -> b) -> Event a -> Event b -- Both events >> should fire, and then the result is returned >> PureEvent :: a -> Event a -- Create a fake >> event. The result is useable with no delay. >> EmptyEvent :: Event a -- An event that is >> never fired. >> BaseEvent :: (Typeable a) => Field a -> Event a -- Embed a base >> event >> >> It's easy to define instances for Applicative and Alternative. >> But then I have a use case that I cannot solve: >> In the case of votations (which use this DSL), sometime you can end >> prematuraly the voting, even if not everybody voted yet: for example if the >> majority is already reached. >> In this case no need to wait for everybody, we can end the vote. >> >> This does not seem to be possible with Applicative or Monad interfaces: >> things have to be sequenced in a monad, they cannot be evaluated in >> parralel. >> For example: >> >> do >> v1 <- getVote1 >> v2 <- getVote2 >> v3 <- getVote3 >> evalMajority(v1, v2, v3) >> >> In this example I have to wait for all 3 votes to be completed, but in >> fact only two would suffice to achieve majority. >> That's why I added "ListEvent" to the DSL above which allow to achieve >> that but I'm wondering if there is a better/bigger abstraction to put it >> in. >> >> Thanks!! >> Corentin >> >> > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > > >

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Relations between Functor typeclass and kind
by Gautier DI FOLCO 19 Jun '14

19 Jun '14

Hi all, Some days ago, I was talking with someone about Kinds and GADTs. At some point he mention that types with this Kind: (* -> *) are called Functors. So, is there any relations between Functor typeclass and Functor kind? Is it considered as a "pattern"? if so, are there some other ones? If you have any link on this, I'll take them. Thanks in advance for your help.

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dead links on mailing list page
by Yomna Nasser 19 Jun '14

19 Jun '14

I was trying to look at some mailing list archives, and encountered several dead links in section 1.3 on this page: http://www.haskell.org/haskellwiki/Mailing_lists - 3rd paragraph, link to http://www.cse.unsw.edu.au/~dons/haskell-1990-2006/threads.html (a 403 is returned), where the link is under the text "hosted here" - 4th paragraph, link to the Google Coop Haskell Search Engine (a 404 is returned) I wasn't sure where else to report this.

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Haskell Weekly News: Issue 297
by Daniel Santa Cruz 19 Jun '14

19 Jun '14

Welcome to issue 297 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers from June 8 to 14, 2014 Quotes of the Week * dpwright: this afternoon I've literally taken an entire (portrait) screen's worth of code and reduced it to about three lines and a few lens operators * tommd: Haskell is a great language. I needed to test a program under high CPU load, so to burn some cycles I cabal-installed lens. Is there anything Haskell can't do? * bernalex: I imagine hell as a place where spj & edwardk are holding talks at the same time and I have to choose. Top Reddit Stories * Haxl on Github Domain: github.com, Score: 124, Comments: 25 Original: [1] http://goo.gl/zw8Y8q On Reddit: [2] http://goo.gl/FoV1zw * www.dohaskell.com Domain: dohaskell.com, Score: 92, Comments: 23 Original: [3] http://goo.gl/uxmtUy On Reddit: [4] http://goo.gl/xkGJUb * cloud-haskell is on hackage now Domain: hackage.haskell.org, Score: 87, Comments: 7 Original: [5] http://goo.gl/eCHMgO On Reddit: [6] http://goo.gl/h9j7H1 * Facebook releases Haxl, a library for async data access Domain: code.facebook.com, Score: 65, Comments: 0 Original: [7] http://goo.gl/D6xEnM On Reddit: [8] http://goo.gl/GCqeEt * The problem with mtl Domain: ro-che.info, Score: 61, Comments: 58 Original: [9] http://goo.gl/hDncB8 On Reddit: [10] http://goo.gl/2uRfm2 * Why programmers can’t make any money: dimensionality and the Eternal Haskell Tax Domain: michaelochurch.wordpress.com, Score: 56, Comments: 37 Original: [11] http://goo.gl/Ums4cZ On Reddit: [12] http://goo.gl/ytLSN2 * A Year of Functional Programming Domain: japgolly.blogspot.com.au, Score: 52, Comments: 17 Original: [13] http://goo.gl/torrXD On Reddit: [14] http://goo.gl/XTSVed * Haskell for all: Spreadsheet-like programming in Haskell Domain: haskellforall.com, Score: 52, Comments: 26 Original: [15] http://goo.gl/eaaeHa On Reddit: [16] http://goo.gl/biUwQf * GHC bug squashing at ZuriHac 2014 Domain: joachim-breitner.de, Score: 50, Comments: 0 Original: [17] http://goo.gl/zgd6qV On Reddit: [18] http://goo.gl/HDp0Wt * There is no Fork—our ICFP 2014 paper about Haxl Domain: community.haskell.org, Score: 47, Comments: 4 Original: [19] http://goo.gl/yvbvAH On Reddit: [20] http://goo.gl/pwcBxO * EclipseFP 2.6.0 released Domain: jpmoresmau.blogspot.fr, Score: 46, Comments: 18 Original: [21] http://goo.gl/fohXYx On Reddit: [22] http://goo.gl/duJMir * Haskell Development Workflow Demo, first video I've made, wanted to see if any found it useful Domain: youtube.com, Score: 44, Comments: 11 Original: [23] http://goo.gl/O1REsS On Reddit: [24] http://goo.gl/KTK1kk * Announcement: Bang, a drum machine DSL for Haskell Domain: self.haskell, Score: 42, Comments: 17 Original: [25] http://goo.gl/QVlj3E On Reddit: [26] http://goo.gl/QVlj3E * How do you learn to write systems in Haskell? (and a story) Domain: self.haskell, Score: 42, Comments: 66 Original: [27] http://goo.gl/PXiZUD On Reddit: [28] http://goo.gl/PXiZUD * The Haskell Cast #7 - Chris Done on Compiling to JavaScript and SQL Domain: haskellcast.com, Score: 41, Comments: 15 Original: [29] http://goo.gl/747Kpj On Reddit: [30] http://goo.gl/n0nlLJ * ICFP 2014 accepted papers Domain: github.com, Score: 40, Comments: 3 Original: [31] http://goo.gl/XZ962N On Reddit: [32] http://goo.gl/VnuyDW * cabal - the simple guide Domain: katychuang.com, Score: 40, Comments: 16 Original: [33] http://goo.gl/svwYE1 On Reddit: [34] http://goo.gl/oGidxo * Two failed attempts at extensible effects Domain: ro-che.info, Score: 37, Comments: 12 Original: [35] http://goo.gl/DD7WX9 On Reddit: [36] http://goo.gl/wh8fWW * Follow up blog post: "Understanding the RealWorld": the Haskell heap, function calls, and primitive types. Domain: well-typed.com, Score: 34, Comments: 12 Original: [37] http://goo.gl/9OMjWu On Reddit: [38] http://goo.gl/N0B4cJ * "Why Haskell?" Elevator Pitch Domain: self.haskell, Score: 33, Comments: 70 Original: [39] http://goo.gl/xsIOqw On Reddit: [40] http://goo.gl/xsIOqw Top StackOverflow Questions * Why should Applicative be a superclass of Monad? votes: 21, answers: 3 Read on SO: [41] http://goo.gl/Z1Q6uo * What do you call the data wrapped inside a monad? votes: 15, answers: 2 Read on SO: [42] http://goo.gl/iNtQAX * What algorithm is used in Haskell (GHC) for deriving types of recursive expressions? votes: 14, answers: 1 Read on SO: [43] http://goo.gl/7QvfqM * What container really mimics std::vector in Haskell? votes: 14, answers: 4 Read on SO: [44] http://goo.gl/k0vk2n * Why the cycle function cannot work with empty list? votes: 14, answers: 2 Read on SO: [45] http://goo.gl/B5Na0a Until next time, [46]+Daniel Santa Cruz References 1. https://github.com/facebook/Haxl 2. http://www.reddit.com/r/haskell/comments/27sv88/haxl_on_github/ 3. http://www.dohaskell.com/ 4. http://www.reddit.com/r/haskell/comments/27wjwk/wwwdohaskellcom/ 5. http://hackage.haskell.org/package/cloud-haskell-0.0.1.0 6. http://www.reddit.com/r/haskell/comments/281sbp/cloudhaskell_is_on_hackage_… 7. https://code.facebook.com/projects/854888367872565/haxl/ 8. http://www.reddit.com/r/haskell/comments/27syj2/facebook_releases_haxl_a_li… 9. http://ro-che.info/articles/2014-06-11-problem-with-mtl.html 10. http://www.reddit.com/r/haskell/comments/27vhd1/the_problem_with_mtl/ 11. http://michaelochurch.wordpress.com/2014/06/06/why-programmers-cant-make-an… 12. http://www.reddit.com/r/haskell/comments/27onwq/why_programmers_cant_make_a… 13. http://japgolly.blogspot.com.au/2014/06/a-year-of-functional-programming.ht… 14. http://www.reddit.com/r/haskell/comments/27oxr4/a_year_of_functional_progra… 15. http://www.haskellforall.com/2014/06/spreadsheet-like-programming-in-haskel… 16. http://www.reddit.com/r/haskell/comments/284enc/haskell_for_all_spreadsheet… 17. http://www.joachim-breitner.de/blog/archives/651-ZuriHac-2014.html 18. http://www.reddit.com/r/haskell/comments/27p2nc/ghc_bug_squashing_at_zuriha… 19. http://community.haskell.org/~simonmar/papers/haxl-icfp14.pdf 20. http://www.reddit.com/r/haskell/comments/27tjpj/there_is_no_forkour_icfp_20… 21. http://jpmoresmau.blogspot.fr/2014/06/eclipsefp-260-released.html 22. http://www.reddit.com/r/haskell/comments/27utrw/eclipsefp_260_released/ 23. https://www.youtube.com/watch?v=Li6oaO8x2VY 24. http://www.reddit.com/r/haskell/comments/27lcz3/haskell_development_workflo… 25. http://www.reddit.com/r/haskell/comments/27s6py/announcement_bang_a_drum_ma… 26. http://www.reddit.com/r/haskell/comments/27s6py/announcement_bang_a_drum_ma… 27. http://www.reddit.com/r/haskell/comments/28575l/how_do_you_learn_to_write_s… 28. http://www.reddit.com/r/haskell/comments/28575l/how_do_you_learn_to_write_s… 29. http://www.haskellcast.com/episode/007-chris-done-on-compiling-to-javascrip… 30. http://www.reddit.com/r/haskell/comments/27sh2d/the_haskell_cast_7_chris_do… 31. https://github.com/yallop/icfp2014-papers/blob/master/README.md 32. http://www.reddit.com/r/haskell/comments/27p8xo/icfp_2014_accepted_papers/ 33. http://katychuang.com/cabal-guide/ 34. http://www.reddit.com/r/haskell/comments/280h9h/cabal_the_simple_guide/ 35. http://ro-che.info/articles/2014-06-14-extensible-effects-failed.html 36. http://www.reddit.com/r/haskell/comments/285prk/two_failed_attempts_at_exte… 37. http://www.well-typed.com/blog/95/ 38. http://www.reddit.com/r/haskell/comments/27yg67/follow_up_blog_post_underst… 39. http://www.reddit.com/r/haskell/comments/282yky/why_haskell_elevator_pitch/ 40. http://www.reddit.com/r/haskell/comments/282yky/why_haskell_elevator_pitch/ 41. http://stackoverflow.com/questions/24112786/why-should-applicative-be-a-sup… 42. http://stackoverflow.com/questions/24127190/what-do-you-call-the-data-wrapp… 43. http://stackoverflow.com/questions/24105812/what-algorithm-is-used-in-haske… 44. http://stackoverflow.com/questions/24123967/what-container-really-mimics-st… 45. http://stackoverflow.com/questions/24223201/why-the-cycle-function-cannot-w… 46. https://plus.google.com/105107667630152149014/about

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Hackage "500 Internal Server Error"
by Conal Elliott 18 Jun '14

18 Jun '14

Is hackage uploading broken? I'm trying to upload a new version of vector-space and am getting "500 Internal Server Error". -- Conal

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ANNOUNCE: haste-perch
by Alberto G. Corona 18 Jun '14

18 Jun '14

Hi, haste-perch defines builder elements (perchs) for Haste.DOM elements that are appendable, so that dynamic HTML can be created in the client in a natural way, like textual HTML, but programmatically and with the advantage of static type checking. It can be ported to other haskell-js compilers. http://hackage.haskell.org/package/haste-perch This program, when compiled with haste: main= do withElem "idelem" $ build $ do div $ do div $ do p "hello" p ! atr "style" "color:red" $ "world" return () Creates these element: <div id= "idelem"> <-- was already in the HTML <div> <div> <p> hello </p> <p style= "color:red"> world </p> </div> </div> </div> Since the creation is in the browser, that permit quite dynamic pages for data presentation, and interctive textual (a.k.a "serious") applications and, in general the development of client-side web frameworks using haskell with the haste compiler. See the README in the git repository: https://github.com/agocorona/haste-perch -- Alberto.

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What are the problems with instances for polymorphic types?
by Gábor Lehel 18 Jun '14

18 Jun '14

In other words instances for forall-types, such as: instance Foo (forall a. [a]) where ... It feels obvious to me that there *would* be problems with this, but I'm curious about what, exactly, they are. Could someone familiar with the matter either elaborate on them, or refer me to an existing explanation, a previous discussion, or something of the sort? I *don't* have any kind of use case in mind, I'm merely seeking a better understanding of the type-system issues involved. (I attempted Google, but didn't have much success.) Thanks in advance.

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Building static libraries with -fPIC
by Nikolay Amiantov 18 Jun '14

18 Jun '14

Hello all, I'm trying to build a shared (.so) library written in Haskell with "foreign export"s, which is statically linked with all needed dependencies. To do this, as far as I understood, I need to build all my dependencies (I use Cabal) with -fPIC flag, and also somehow get RTS static library built with -fPIC. Two questions regarding to this: 1) How can I add a build flag which will be added while building all my dependencies in Cabal. 2) How can I get needed RTS library. 3) How can I deal with "base"? Can I just build it with Cabal, so this question reduces to (1)? Thanks, Nikolay.

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Warnings with OverloadedLists
by Nikolay Amiantov 18 Jun '14

18 Jun '14

Hello, I've bumped into strange GHC behaviour with OverloadedLists and pattern matching. Consider this example: {-# LANGUAGE OverloadedLists #-} len :: [a] -> Int len [] = 0 len (_:t) = 1 + len t If I load this with "ghci -Wall", I get a warning: Test.hs:4:1: Warning: Pattern match(es) are non-exhaustive In an equation for ‘len’: Patterns not matched: [] If I disable OverloadedLists extension, warning goes away. Can someone clear this behaviour out for me? It looks like a bug, but I'm not sure. Thanks for help, Nikolay.

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Support of multi-constructor types by UNPACK pragma
by Nikita Volkov 18 Jun '14

18 Jun '14

Currently the pragma only supports single-constructor types. So in the following example it will simply be ignored: data A = A1 Char | A2 {-# UNPACK #-} !B data B = B1 Int | B2 Bool However it seems to be easily solvable by changing the type A to something like the following during unpacking: data A = A1 Char | A2_1 Int | -- from B1 A2_2 Bool -- from B2 Am I missing something? Why is it not implemented?

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