June 2014 - Haskell-Cafe

Re: [Haskell-cafe] ANN: set-cover solves Sudoku, Soma cube, 8 Queens etc.
by Johannes Waldmann 22 Jun '14

22 Jun '14

Henning Thielemann <lemming <at> henning-thielemann.de> writes: > .. package set-cover for solving exact set cover problems. > http://hackage.haskell.org/package/set-cover It's hard to evaluate whether one could use the library because there's essentially no visible documentation. E.g., what does Math.SetCover.Exact.search do? Its type refers to "State" which seems implementation-dependent, and hides the connection to the specification of the set cover problem. Putting some text and examples into the haddocks might help. But of course putting the specification there would be even better. Also, care to explain what algorithm your solver uses, and give some performance data (e.g., N-queens for N=10,20,40,..)? - J.W.

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Re: [Haskell-cafe] diagrams
by Dominic Steinitz 22 Jun '14

22 Jun '14

> Message: 3 > Date: Fri, 20 Jun 2014 13:30:29 -0700 > From: Joachim Breitner <mail(a)joachim-breitner.de> > To: haskell-cafe(a)haskell.org > Subject: Re: [Haskell-cafe] diagrams > Message-ID: <1403296229.16156.3.camel@kirk> > Content-Type: text/plain; charset="utf-8" > > Hi, > > Am Mittwoch, den 18.06.2014, 23:44 -0400 schrieb Anthony Cowley: > >>> I have no doubt that diagrams can make the diagrams themselves. What >>> I wonder about is the animation capabilities---ability to have >>> elements move or cross-fade (dissolve), in particular. > > I very recently created this animation with diagrams, which does move > and fade stuff: > http://joachim-breitner.de/publications/haskell_bytes_portland_2014-06-13.w… > > But the ride was not as smooth as it should have been. See > https://gist.github.com/nomeata/cde96a2e693a23cca8ee for the code. The > function positionSpec contains a ?script?, i.e. a sequence of animated > modifications to the image, using lenses to position the change. > > I guess with the right combinators, this can be greatly simplified, but > it is hard to come up with the right combinators. Especially as our code > is one-dimensional, and we want to compose in three dimensions (x, y and > time). It’s still a nice example though - thanks for posting it. Dominic Steinitz dominic(a)steinitz.org http://idontgetoutmuch.wordpress.com

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diagrams
by Dennis Raddle 20 Jun '14

20 Jun '14

I'm a math tutor and I want to create videos to explain certain math concepts, and I'm wondering if the diagrams library would help me. There are two questions--first, whether it has the capability to do what I want, and second how hard it would to integrate the animations into YouTube videos with voice-over. To describe the capability, I'm looking first to typeset equations. Then, second, to represent manipulation of equations by having elements of equations move locations or cross-fade. Also "elements" could mean pictures or non-mathematical notation. I have no doubt that diagrams can make the diagrams themselves. What I wonder about is the animation capabilities---ability to have elements move or cross-fade (dissolve), in particular. As far as the ability to integrate these into YouTube videos, that is probably beyond the scope of this list, but maybe someone happens to have done this and knows the answer. Mike

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

20 Jun '14

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

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How to represent a board?
by Rafael Almeida 20 Jun '14

20 Jun '14

Hello, There are many examples of games which use boards: chess, checkers, go, etc. Many people starting out at programming and/or game programming are very much tempted to code a board game. It looks like such a game would be easy to build at first sight. Even more sophisticated games sometimes use a grid system, which looks a lot like boards as well. In a grid system the movements of a given agent are bound do discrete positions. Although the user may not see the board underneath pretty graphics, it is a board game. Having in mind the sort of operations such games have to make on boards, I ask: what are the best representations for a board in Haskell? In many languages a NxN array seems like a good pick. In Haskell, most would translate that into lists of lists. I know I have. However, traversing those lists to get a position, calculate where a piece or agent could go, etc., feels awkward and unefficient. Beside the point already made, we have no type safe guarantee that our 64x64 won't become a 63x63 + 65x1 board due to some misbehaving function. It strikes me that list of lists is not a great board representation. I think STArrays are a better pick; from the perspective of translating an NxN array into Haskell, at least. However, maybe there is an even more elegant way to deal with a board in Haskell. I hope you can help me out figuring it out. []'s Rafael

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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 18 Jun '14

18 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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