- Haskell-Cafe - Haskell.org

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 24 Oct '12

24 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 24 Oct '12

24 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 24 Oct '12

24 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 23 Oct '12

23 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.

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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

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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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