August 2014 - Haskell-Cafe

Re: [Haskell-cafe] Does GHC compare pointers when eval'ing (==)
by Johan Holmquist 21 Aug '14

21 Aug '14

To summarise, the points raised sofar against this optimisation are, roughly, as follows. * More expensive to compare two values which aren't equal. I think the potential gain of the optimisation might outweigh the cost in the general case. (Also, the optimisation would not have to be triggered for simple values like Char, Int and Double.) * Dodgy Eq instances may break the optimisation But may also break other stuff and should be avoided anyway. * _|_ will not necessarily yield _|_ on comparison This seems the biggest problem sofar to me, that (5,_|_) == (5,_|_) would be _|_ if both pairs where at different memory locations and True otherwise. But perhaps not a big deal in practice. * Disrupts other compiler optimisation opportunities My knowledge about compilers is lacking so I cannot comment this. Interesting that this was implemented by hbc. Would be nice to know if it led to problems. 2014-08-20 14:32 GMT+02:00 Jan-Willem Maessen <jmaessen(a)alum.mit.edu>: > On Wed, Aug 20, 2014 at 2:52 AM, Johan Holmquist <holmisen(a)gmail.com> > wrote: > >> Comparing two structures for equality (structurally) can be expensive. >> But if their references are the same they would for sure be equal (unless >> (==) was defined in some funny way). Does GHC perform any such optimization? >> > > The big problem in practice are the Float / Double instances in the > presence of NaN, as noted by other folks. > > Back in the day, hbc had a flag that would do a pointer equality check in > the derived Eq instances it generated before falling back to a traversal. > Interestingly, it does actually change the way you think about programming > when you're working with multiple values that are very likely to share > sub-structure. Suddenly it seems OK to do equality checks as you traverse > (because most of them will succeed), which would be O(n^2) if you weren't > doing the pointer test. > > -Jan-Willem Maessen > > >> (Likely a question for another list but I make a stab here first. ) >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >> >

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Taking over setlocale
by Sven Bartscher 21 Aug '14

21 Aug '14

Greetings, I would like to take over the setlocale package. Following the steps described on http://www.haskell.org/haskellwiki/Taking_over_a_package I need to state my intention on a public forum. The original author (Lukas Mai) published his package in the Public Domain. This would be fine, but the Author seems to be living in Germany. The problem with that is that in Germany an author is not allowed to give up his copyright in this way. This means the licensing of the package defaults to "All rights reserved", making the package undistributable. I already tried to contact the author about this problem, but he didn't respond in a week. This isn't a very long time, but I didn't get an answer on past attempts to contact him (this was at least a month, but probably more, ago). To solve this problem, I rewrote the setlocale binding with the same API under the BSD3-clause. PS: Please keep the CC to Lukas Mai and the debian-haskell list intact. Regards Sven

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Haskell Weekly News: Issue 302
by Daniel Santa Cruz 21 Aug '14

21 Aug '14

Welcome to issue 302 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers from August 3 to 16, 2014 Quotes of the Week * alpha123: I don't think I have the gonads for monads. :( * edwardk: There is nothing Natural about a Word. * bernalex: I very often have to deal with people who think I'm pretentious for caring about what programming language I use. "it's just programming". lol. sopvop: I used to think the same sopvop: Then haskell corrupted me sopvop: Haskell is a ghetto Top Reddit Stories * ಠ_ಠ :: String -> a Domain: hackage.haskell.org, Score: 122, Comments: 56 Original: [1] http://goo.gl/S87evt On Reddit: [2] http://goo.gl/vsfwgr * An inspiring Haskell story: the green screen of FP Domain: keera.co.uk, Score: 120, Comments: 13 Original: [3] http://goo.gl/z8Aecw On Reddit: [4] http://goo.gl/fevzzz * Haskell Platform 2014.2.0.0 is released Domain: haskell.org, Score: 117, Comments: 56 Original: [5] http://goo.gl/Wc6TAF On Reddit: [6] http://goo.gl/hMPM7F * criterion 1.0: a major new release Domain: serpentine.com, Score: 110, Comments: 13 Original: [7] http://goo.gl/7Qf3ox On Reddit: [8] http://goo.gl/Pa9fCW * What’s a module system good for anyway? Domain: blog.ezyang.com, Score: 102, Comments: 20 Original: [9] http://goo.gl/qtm97E On Reddit: [10] http://goo.gl/XlWrM2 * Easy Haskell Profiling Domain: danielvelkov.blogspot.com, Score: 83, Comments: 33 Original: [11] http://goo.gl/SWdxdC On Reddit: [12] http://goo.gl/z6clxp * Type Theory Podcast: Peter Dybjer on QuickCheck, verification, and semantics Domain: typetheorypodcast.com, Score: 57, Comments: 21 Original: [13] http://goo.gl/jN12Ak On Reddit: [14] http://goo.gl/tLVPpw * Fantasy World Haskell Domain: self.haskell, Score: 52, Comments: 165 Original: [15] http://goo.gl/UmJLhG On Reddit: [16] http://goo.gl/UmJLhG * Haskell for all: managed-1.0.0: A monad for managed resources Domain: haskellforall.com, Score: 51, Comments: 10 Original: [17] http://goo.gl/tbw8JN On Reddit: [18] http://goo.gl/cKB6xZ * Announcing auto-update Domain: yesodweb.com, Score: 47, Comments: 4 Original: [19] http://goo.gl/xhlw04 On Reddit: [20] http://goo.gl/GO6udg * Codeworld: An open-source educational programming environment using Haskell Domain: github.com, Score: 47, Comments: 8 Original: [21] http://goo.gl/7NTgnF On Reddit: [22] http://goo.gl/wJBgw5 * PureScript By Example Domain: leanpub.com, Score: 46, Comments: 25 Original: [23] http://goo.gl/vzcfP6 On Reddit: [24] http://goo.gl/p3gkEB * Lenses from the ground up Domain: taylor.fausak.me, Score: 45, Comments: 43 Original: [25] http://goo.gl/S31V8w On Reddit: [26] http://goo.gl/DAGBPz * Algebraic Terraforming: Trees from Magma Domain: fpcomplete.com, Score: 42, Comments: 22 Original: [27] http://goo.gl/1nGpnz On Reddit: [28] http://goo.gl/iX8ZST * Denotational Design: from meanings to programs By Conal Elliott at BayHac (Video) Domain: youtube.com, Score: 42, Comments: 5 Original: [29] http://goo.gl/rHeDrK On Reddit: [30] http://goo.gl/SLCZ5D * Just LOOK at the humongous type that Hindley-Milner infers for this tiny program! Domain: spacemanaki.com, Score: 38, Comments: 27 Original: [31] http://goo.gl/cAypnJ On Reddit: [32] http://goo.gl/ztDpK7 * Recommended Reading Material Domain: reinh.com, Score: 37, Comments: 12 Original: [33] http://goo.gl/LwGkEr On Reddit: [34] http://goo.gl/ED5pqI * (Monoidal) transducers are monoid homomorphisms Domain: oleksandrmanzyuk.wordpress.com, Score: 37, Comments: 43 Original: [35] http://goo.gl/VPDl6Y On Reddit: [36] http://goo.gl/5RY9Nf * λ Lessons: Pattern matching, first-class functions, and abstracting over recursion in Haskell Domain: stevekrouse.github.io, Score: 36, Comments: 3 Original: [37] http://goo.gl/NkxhS5 On Reddit: [38] http://goo.gl/4275GH Top StackOverflow Questions * What are the problems with an ADT encoding that associates types with data constructors? (Such as Scala.) votes: 41, answers: 1 Read on SO: [39] http://goo.gl/k4bw4R * Why is Haskell missing “obvious” Typeclasses votes: 26, answers: 7 Read on SO: [40] http://goo.gl/2YElO8 * Is there a way to elegantly represent this pattern in Haskell? votes: 22, answers: 6 Read on SO: [41] http://goo.gl/sQdRMt * Why does Scala not have a return/unit function defined for each monad (in contrast to Haskell)? votes: 16, answers: 4 Read on SO: [42] http://goo.gl/MQlZTi * How did Haskell add Turing-completeness to System F? votes: 15, answers: 1 Read on SO: [43] http://goo.gl/weJo4Z * How to avoid superfluous variables in do notation? votes: 14, answers: 2 Read on SO: [44] http://goo.gl/b3IgC5 Until next time, [45]+Daniel Santa Cruz References 1. https://hackage.haskell.org/package/acme-lookofdisapproval-0.1/docs/Acme-Lo… 2. http://www.reddit.com/r/haskell/comments/2dce3d/%E0%B2%A0_%E0%B2%A0_string_… 3. http://keera.co.uk/blog/2014/08/13/most-inspiring-green-screen-you-will-eve… 4. http://www.reddit.com/r/haskell/comments/2djh3b/an_inspiring_haskell_story_… 5. http://www.haskell.org/platform/#2014.2.0.0 6. http://www.reddit.com/r/haskell/comments/2d3p2c/haskell_platform_2014200_is… 7. http://www.serpentine.com/blog/2014/08/08/criterion-1-0/ 8. http://www.reddit.com/r/haskell/comments/2cyuv4/criterion_10_a_major_new_re… 9. http://blog.ezyang.com/2014/08/whats-a-module-system-good-for-anyway/ 10. http://www.reddit.com/r/haskell/comments/2d3xuh/whats_a_module_system_good_… 11. http://danielvelkov.blogspot.com/2014/08/easy-haskell-profiling.html 12. http://www.reddit.com/r/haskell/comments/2d8ob4/easy_haskell_profiling/ 13. http://typetheorypodcast.com/2014/08/episode-1-peter-dybjer-on-type-theory-… 14. http://www.reddit.com/r/haskell/comments/2df24l/type_theory_podcast_peter_d… 15. http://www.reddit.com/r/haskell/comments/2chb2h/fantasy_world_haskell/ 16. http://www.reddit.com/r/haskell/comments/2chb2h/fantasy_world_haskell/ 17. http://www.haskellforall.com/2014/08/managed-100-monad-for-managed-resource… 18. http://www.reddit.com/r/haskell/comments/2d7109/haskell_for_all_managed100_… 19. http://www.yesodweb.com/blog/2014/08/announcing-auto-update 20. http://www.reddit.com/r/haskell/comments/2crqyg/announcing_autoupdate/ 21. https://github.com/google/codeworld 22. http://www.reddit.com/r/haskell/comments/2drcwx/codeworld_an_opensource_edu… 23. https://leanpub.com/purescript/read 24. http://www.reddit.com/r/haskell/comments/2cswdd/purescript_by_example/ 25. http://taylor.fausak.me/2014/08/03/lenses-from-the-ground-up/ 26. http://www.reddit.com/r/haskell/comments/2cl28r/lenses_from_the_ground_up/ 27. https://www.fpcomplete.com/user/bss/magma-tree 28. http://www.reddit.com/r/haskell/comments/2corq6/algebraic_terraforming_tree… 29. https://www.youtube.com/watch?v=zzCrZEil9iI 30. http://www.reddit.com/r/haskell/comments/2dh0sd/denotational_design_from_me… 31. http://spacemanaki.com/blog/2014/08/04/Just-LOOK-at-the-humongous-type/ 32. http://www.reddit.com/r/haskell/comments/2cka6d/just_look_at_the_humongous_… 33. http://reinh.com/notes/posts/2014-07-25-recommended-reading-material.html 34. http://www.reddit.com/r/haskell/comments/2cpr4z/recommended_reading_materia… 35. http://oleksandrmanzyuk.wordpress.com/2014/08/09/transducers-are-monoid-hom… 36. http://www.reddit.com/r/haskell/comments/2d5ael/monoidal_transducers_are_mo… 37. https://stevekrouse.github.io/hs.js/ 38. http://www.reddit.com/r/haskell/comments/2d1dpg/%CE%BB_lessons_pattern_matc… 39. http://stackoverflow.com/questions/25330359/what-are-the-problems-with-an-a… 40. http://stackoverflow.com/questions/25191659/why-is-haskell-missing-obvious-… 41. http://stackoverflow.com/questions/25107101/is-there-a-way-to-elegantly-rep… 42. http://stackoverflow.com/questions/25238622/why-does-scala-not-have-a-retur… 43. http://stackoverflow.com/questions/25255413/how-did-haskell-add-turing-comp… 44. http://stackoverflow.com/questions/25314793/how-to-avoid-superfluous-variab… 45. https://plus.google.com/105107667630152149014/about

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Re: [Haskell-cafe] Does GHC compare pointers when eval'ing (==)
by Barak A. Pearlmutter 20 Aug '14

20 Aug '14

You don't need to define a new weird datatype in order to violate reflexivity, as an eminent IEEE committee has already designed one for you. Prelude> let x = 0/0 in x == x False

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Does GHC compare pointers when eval'ing (==)
by Johan Holmquist 20 Aug '14

20 Aug '14

Comparing two structures for equality (structurally) can be expensive. But if their references are the same they would for sure be equal (unless (==) was defined in some funny way). Does GHC perform any such optimization? (Likely a question for another list but I make a stab here first. )

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Re: [Haskell-cafe] Reducing CPU load when using Helm
by Kaspar Emanuel 19 Aug '14

19 Aug '14

On 18 August 2014 20:24, Kaspar Emanuel <kaspar.bumke(a)gmail.com> wrote: > Hi, > > are there some simple tricks to reduce the CPU load? This simple > square rendering sits at 20% CPU even when the window size remains > constant. > > import FRP.Helm > import qualified FRP.Helm.Window as Window > > render :: (Int, Int) -> Element > render (w, h) = collage w h [move (100, 100) $ filled red $ square 64] > > main :: IO () > main = do > engine <- startup defaultConfig > run engine $ render <~ Window.dimensions engine > > The binary fractal tree I submitted in another thread on helm-dev > sits at 100% CPU when the mouse isn't moving. It seems to me that > Helm keeps re-drawing and re-computing when it doesn't need to. > > I started digging through the source a bit but it will take me a while > to understand Elerea and how it samples the inputs and causes things > to update. > > Are there any quick fixes? > So I tried stepping the render function using a time-constant. But that just causes the resulting programs to freeze on the first frame. https://github.com/kasbah/helm/commit/9ef9607d8057100b5b7930fa2b5e85c6040d1… I modified the driveNetwork function from here: www.formicite.com/dopage.php?frp/frp.html Bit out of my element with this stuff. I'd just like to _not_ max out the CPU with simple graphical programs.

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[ANN] extended-categories
by Ian Milligan 19 Aug '14

19 Aug '14

extended-categories <github.com/ian-mi/extended-categories> is an implementation of categories designed to make use of GHC's recently enriched kind system. This project is a work in progress (only a few basic constructs are implemented), but your comments are welcomed. extended-categories requires a recent version of GHC (I have only tested it with 7.8.3). Categories are poly-kinded and have an Object constraint family: class Category m where type Object (m :: k -> k -> *) (a :: k) :: Constraint id :: Object m a => m a a (.) :: (Object m a, Object m b, Object m c) => m b c -> m a b -> m a c This means in addition to our usual categories of kind * -> * -> * we can define categories of other kinds, for example (:-) from the constraints package forms a category with kind Constraint -> Constraint -> * (whose instance is used in this package)! Functors are named (tagged) as done in the data-category package. This allows a much wider variety of functors to be defined. Additionally functors must provide proof that they are closed under the object constraint, i.e. forall a. Object (Domain f c1) a :- Object (Codomain f c2) (FMap f a). For any two categories c1: k1 -> k1 -> * and c2: k2 -> k2 -> *, the product category (c1 :><: c2) has kind (k1, k2) -> (k1, k2) -> *. The functor (Diag c) is defined to be the functor from c to c :><: c which sends the object X to '(X, X) and morphism f to the morphism f :><: f. Categories which contain their products (the class ProductCategory) are defined by the universal property described here < http://en.wikipedia.org/wiki/Universal_morphism#Products>, i.e. by a terminal morphism, for every object a and b of c, from (Diag c) to the object '(a, b) of c :><: c. From this terminal morphism, the functions fst, snd, (&&&), and (***), as well as the product functor from c :><: c to c mapping '(a, b) to a >< b and f :><: g to f *** g, are defined. Here are some examples of how this works with the standard Haskell functors: *Functor> fmap (CanonicalF :.: CanonicalF) P.succ [P.Just 2, P.Nothing] [Just 3,Nothing] *Product> fmap (CanonicalF :.: ProductF) ((P.+1) :><: (P.+2)) [(1,2), (3,4), (5,6)] [(2,4),(4,6),(6,8)] or alternatively fmap CanonicalF ((P.+1) *** (P.+2)) [(1,2), (3,4), (5,6)] Next I will likely be adding more instances as well as some documentation. Thanks for reading, Ian Milligan

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Reducing CPU load when using Helm
by Kaspar Emanuel 19 Aug '14

19 Aug '14

Forwarding this to haskell-cafe as it failed before. On 18 August 2014 20:24, Kaspar Emanuel <kaspar.bumke(a)gmail.com> wrote: > Hi, > > are there some simple tricks to reduce the CPU load? This simple > square rendering sits at 20% CPU even when the window size remains > constant. > > import FRP.Helm > import qualified FRP.Helm.Window as Window > > render :: (Int, Int) -> Element > render (w, h) = collage w h [move (100, 100) $ filled red $ square 64] > > main :: IO () > main = do > engine <- startup defaultConfig > run engine $ render <~ Window.dimensions engine > > The binary fractal tree I submitted in another thread on helm-dev > sits at 100% CPU when the mouse isn't moving. It seems to me that > Helm keeps re-drawing and re-computing when it doesn't need to. > > I started digging through the source a bit but it will take me a while > to understand Elerea and how it samples the inputs and causes things > to update. > > Are there any quick fixes? > > Ciao, > > Kaspar

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Is there something for emacs to view project module structure?
by Magicloud Magiclouds 19 Aug '14

19 Aug '14

Hi, I think a structure viewing in editor could be very help for long file or big project. Something like java/c# IDEs do. But I could not find a hint on google. Any suggestions? -- 竹密岂妨流水过山高哪阻野云飞 And for G+, please use magiclouds#gmail.com.

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Type-level numbers problem
by Konstantine Rybnikov 18 Aug '14

18 Aug '14

Hi! I wrote one thing with type-level numbers that were represented as simple data Zero data Succ a Then, in order to not invent my own peano numbers, I decided to go look for ones, and found GHC.TypeLits. But unfortunately, it shows a weird error for me. Here is a piece of code: https://gist.github.com/k-bx/a7b49165399658f87358 The one that's below "main" is working properly, while the one on top of it gives this error: typenats.hs:17:16: No instance for (Op D (0 + 1)) arising from a use of `f' Possible fix: add an instance declaration for (Op D (0 + 1)) In the expression: f (VJ i :: VJ D (0 + 1)) In an equation for `f': f (VJ i) = f (VJ i :: VJ D (0 + 1)) In the instance declaration for `Op D 0' Maybe I worked too much on type-level numbers today to not understand this simple error I'm getting, but could someone be so kind and explain me what am I doing wrong? Thank you very much!

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