July 2014 - Haskell-Cafe

No `base-4.7.0.1` in hackage
by Alois Cochard 19 Jul '14

19 Jul '14

Hi, GHC 7.8.3 ships with base 4.7.0.1, but this module is not published in hackage. Is there any reason why? That is an issue in my tool `codex` because he used hackage to download the sources and tags them [1]. [1]: https://github.com/aloiscochard/codex/issues/15 -- *A\ois* http://twitter.com/aloiscochard http://github.com/aloiscochard

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Unable to build vector on OSX (ghc 7.4, gcc 4.8)
by Edsko de Vries 18 Jul '14

18 Jul '14

Hi guys, I am currently unable to build vector on OSX: Loading package base ... <command line>: can't load .so/.DLL for: /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.9.sdk/usr/lib/libiconv.dylib (dlopen(/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.9.sdk/usr/lib/libiconv.dylib, 9): no suitable image found. Did find: /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.9.sdk/usr/lib/libiconv.dylib: mach-o, but wrong filetype) cabal: Error: some packages failed to install: vector-0.10.11.0 failed during the building phase. The exception was: This has been reported before as some kind of clang issue, but I am using gcc 4.8 (ghc-7.4.2/settings has "gcc-4.8" for "C compiler command"). Any suggestions? Thanks! Edsko

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Functional Programing-Oriented Machine Learning Startup Seeks DevOps Guru
by charles＠charlesweitzer.com 18 Jul '14

18 Jul '14

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Re: [Haskell-cafe] Conduit & state
by Toby Goodwin 18 Jul '14

18 Jul '14

Hi Pavel, >runUnixServer (serverSettings socket) $ \appData -> > ... >I want to store some data in priority queue in Monad.State and be able to >access it inside the handleBS (now: handleBS :: ByteString -> ByteString). Funnily enough, I've just been looking at an analogous problem (with runTCPClient and Persistent). It seems that in the recent move from conduit-network to conduit-extra, the run*Client and run*Server functions were specialized to the IO monad. I don't know what the motivation for this change was (simpler error messages perhaps?) but it does seem unhelpful. The workaround I deployed last night was to roll my own runTCPClient' which merges the code from the (deprecated) conduit-network package and the (current) streaming-commons, but this is hardly satisfactory. I'm sure I must be missing something. (I don't know if there's a better list to discuss conduit.) Toby.

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Haskell Weekly News: Issue 299
by Daniel Santa Cruz 16 Jul '14

16 Jul '14

Welcome to issue 299 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers from June 29 to July 12, 2014 Quotes of the Week * donri: maybe good thing Applicative isn't called StrongLaxSemimonoidalEndofunctor * ninja_code: haskell is used to debug thinking * johnw: the first rule of category theory club is to state your identity Top Reddit Stories * The new haskell.org design Domain: new-www.haskell.org, Score: 130, Comments: 95 Original: [1] http://new-www.haskell.org/ On Reddit: [2] http://goo.gl/qG7683 * GHC-7.8.3 is out! Domain: haskell.org, Score: 115, Comments: 40 Original: [3] http://goo.gl/fyHQZd On Reddit: [4] http://goo.gl/7bsDXj * Announcing rest - A Haskell REST framework Domain: engineering.silk.co, Score: 97, Comments: 41 Original: [5] http://goo.gl/F3W628 On Reddit: [6] http://goo.gl/VqWdi9 * GHC plans for 7.10.1 Domain: ghc.haskell.org, Score: 77, Comments: 54 Original: [7] http://goo.gl/YH6516 On Reddit: [8] http://goo.gl/g16PE3 * unagi-chan: Fast and scalable concurrent queues for x86, with a Chan-like API Domain: hackage.haskell.org, Score: 61, Comments: 17 Original: [9] http://goo.gl/7dE0e4 On Reddit: [10] http://goo.gl/zaMC6o * Come practice Haskell *and* make a difference in the world by helping us build a new Free/Libre/Open patronage economy Domain: snowdrift.coop, Score: 59, Comments: 19 Original: [11] http://goo.gl/vU2cx On Reddit: [12] http://goo.gl/PRkG3B * What's wrong with String Domain: haskell.org, Score: 55, Comments: 123 Original: [13] http://goo.gl/8etnBS On Reddit: [14] http://goo.gl/k2FZ9u * Why is package management so awful? Domain: self.haskell, Score: 55, Comments: 100 Original: [15] http://goo.gl/7h7cWn On Reddit: [16] http://goo.gl/7h7cWn * Reﬂection without Remorse: Revealing a hidden sequence to speed up monadic reﬂection [pdf] Domain: homepages.cwi.nl, Score: 49, Comments: 36 Original: [17] http://goo.gl/QgiwtW On Reddit: [18] http://goo.gl/BpYmcS * Mutable Algorithms in Immutable Languages, Part 1 Domain: tel.github.io, Score: 49, Comments: 27 Original: [19] http://goo.gl/hn6218 On Reddit: [20] http://goo.gl/apKO9f * Where is Haskell going in industry? Domain: self.haskell, Score: 47, Comments: 114 Original: [21] http://goo.gl/Io2hvy On Reddit: [22] http://goo.gl/Io2hvy * Why Dependently Typed Programming Will (One Day) Rock Your World Domain: ejenk.com, Score: 45, Comments: 154 Original: [23] http://goo.gl/d3zkiG On Reddit: [24] http://goo.gl/z3ZhDf * Using Phantom Types for Extra Safety Domain: blog.jakubarnold.cz, Score: 41, Comments: 19 Original: [25] http://goo.gl/WnZH0Z On Reddit: [26] http://goo.gl/EJTpxl * Designing Dependently Typed Languages - OPLSS '14 Domain: self.haskell, Score: 37, Comments: 11 Original: [27] http://goo.gl/P2mV1A On Reddit: [28] http://goo.gl/P2mV1A * Hackage update, part 3 Domain: lambda.xyz, Score: 37, Comments: 13 Original: [29] http://goo.gl/sLHVfX On Reddit: [30] http://goo.gl/G7dHLc * Announce: The most complete prelude formed from only the "base" package Domain: hackage.haskell.org, Score: 37, Comments: 55 Original: [31] http://goo.gl/lSoKVh On Reddit: [32] http://goo.gl/UeNUYL * New generic programming library (with paper accepted in WGP 2014) Domain: andres-loeh.de, Score: 34, Comments: 11 Original: [33] http://goo.gl/PgV7yV On Reddit: [34] http://goo.gl/qFOlLB * Ideal programming language for a new modern OS built from scratch? Domain: self.haskell, Score: 32, Comments: 63 Original: [35] http://goo.gl/eTkOFd On Reddit: [36] http://goo.gl/eTkOFd * Neil Mitchell's Haskell Blog: Optimisation with Continuations Domain: neilmitchell.blogspot.com.es, Score: 30, Comments: 15 Original: [37] http://goo.gl/lvo8Oa On Reddit: [38] http://goo.gl/haJyX4 * can we do better than cabal sandbox? Domain: self.haskell, Score: 30, Comments: 38 Original: [39] http://goo.gl/RS3j0s On Reddit: [40] http://goo.gl/RS3j0s Top StackOverflow Questions * Is there a monad that doesn't have a corresponding monad transformer (except IO)? votes: 37, answers: 1 Read on SO: [41] http://goo.gl/wBH70C * Arrows are exactly equivalent to applicative functors? votes: 21, answers: 3 Read on SO: [42] http://goo.gl/WRjQx0 * Why does GHC typecheck before desugaring? votes: 19, answers: 1 Read on SO: [43] http://goo.gl/EzaZVr * List of Functors votes: 15, answers: 3 Read on SO: [44] http://goo.gl/RmJvDC * Or-patterns in Haskell votes: 13, answers: 2 Read on SO: [45] http://goo.gl/ukvJm1 * What does this list permutations implementation in Haskell exactly do? votes: 12, answers: 2 Read on SO: [46] http://goo.gl/CXOCch * Should Latitude, Longitude and Elevation have their own type in Haskell? votes: 12, answers: 2 Read on SO: [47] http://goo.gl/fkElae * Haskell List Comprehension Speed Inconsistencies votes: 12, answers: 1 Read on SO: [48] http://goo.gl/0o4rwI * What are some examples of type-level programming? [closed] votes: 11, answers: 3 Read on SO: [49] http://goo.gl/fowZ6b * QuickCheck Gen is not a monad votes: 11, answers: 1 Read on SO: [50] http://goo.gl/hiJ0q8 Until next time, [51]+Daniel Santa Cruz References 1. http://new-www.haskell.org/ 2. http://www.reddit.com/r/haskell/comments/2a77f2/the_new_haskellorg_design/ 3. http://www.haskell.org/ghc/docs/7.8.3/html/users_guide/release-7-8-3.html 4. http://www.reddit.com/r/haskell/comments/2af4jf/ghc783_is_out/ 5. http://engineering.silk.co/post/90354057868/announcing-rest-a-haskell-rest-… 6. http://www.reddit.com/r/haskell/comments/29h32i/announcing_rest_a_haskell_r… 7. https://ghc.haskell.org/trac/ghc/wiki/Status/GHC-7.10.1 8. http://www.reddit.com/r/haskell/comments/29v4lj/ghc_plans_for_7101/ 9. http://hackage.haskell.org/package/unagi-chan-0.1.0.1 10. http://www.reddit.com/r/haskell/comments/2adh2f/unagichan_fast_and_scalable… 11. https://snowdrift.coop/ 12. http://www.reddit.com/r/haskell/comments/29zw5j/come_practice_haskell_and_m… 13. http://www.haskell.org/pipermail/haskell-cafe/2014-June/114745.html 14. http://www.reddit.com/r/haskell/comments/29jw0s/whats_wrong_with_string/ 15. http://www.reddit.com/r/haskell/comments/2a9n0t/why_is_package_management_s… 16. http://www.reddit.com/r/haskell/comments/2a9n0t/why_is_package_management_s… 17. http://homepages.cwi.nl/~ploeg/papers/zseq.pdf 18. http://www.reddit.com/r/haskell/comments/29i94o/re%EF%ACection_without_remo… 19. http://tel.github.io/2014/07/12/mutable_algorithms_in_immutable_languges_pa… 20. http://www.reddit.com/r/haskell/comments/2ahorr/mutable_algorithms_in_immut… 21. http://www.reddit.com/r/haskell/comments/2a310v/where_is_haskell_going_in_i… 22. http://www.reddit.com/r/haskell/comments/2a310v/where_is_haskell_going_in_i… 23. http://ejenk.com/blog/why-dependently-typed-programming-will-one-day-rock-y… 24. http://www.reddit.com/r/haskell/comments/29kv2y/why_dependently_typed_progr… 25. http://blog.jakubarnold.cz/2014/07/08/using-phantom-types-for-extra-safety.… 26. http://www.reddit.com/r/haskell/comments/2a5duv/using_phantom_types_for_ext… 27. http://www.reddit.com/r/haskell/comments/2a115h/designing_dependently_typed… 28. http://www.reddit.com/r/haskell/comments/2a115h/designing_dependently_typed… 29. http://lambda.xyz/blog/hackage-part-3/ 30. http://www.reddit.com/r/haskell/comments/2abi06/hackage_update_part_3/ 31. http://hackage.haskell.org/package/base-prelude 32. http://www.reddit.com/r/haskell/comments/2ahq11/announce_the_most_complete_… 33. http://www.andres-loeh.de/TrueSumsOfProducts/ 34. http://www.reddit.com/r/haskell/comments/2a535x/new_generic_programming_lib… 35. http://www.reddit.com/r/haskell/comments/29tgjd/ideal_programming_language_… 36. http://www.reddit.com/r/haskell/comments/29tgjd/ideal_programming_language_… 37. http://neilmitchell.blogspot.com.es/2014/06/optimisation-with-continuations… 38. http://www.reddit.com/r/haskell/comments/29fjg4/neil_mitchells_haskell_blog… 39. http://www.reddit.com/r/haskell/comments/2a2ec7/can_we_do_better_than_cabal… 40. http://www.reddit.com/r/haskell/comments/2a2ec7/can_we_do_better_than_cabal… 41. http://stackoverflow.com/questions/24515876/is-there-a-monad-that-doesnt-ha… 42. http://stackoverflow.com/questions/24668313/arrows-are-exactly-equivalent-t… 43. http://stackoverflow.com/questions/24639279/why-does-ghc-typecheck-before-d… 44. http://stackoverflow.com/questions/24589995/list-of-functors 45. http://stackoverflow.com/questions/24700762/or-patterns-in-haskell 46. http://stackoverflow.com/questions/24484348/what-does-this-list-permutation… 47. http://stackoverflow.com/questions/24521976/should-latitude-longitude-and-e… 48. http://stackoverflow.com/questions/24690406/haskell-list-comprehension-spee… 49. http://stackoverflow.com/questions/24481113/what-are-some-examples-of-type-… 50. http://stackoverflow.com/questions/24481648/quickcheck-gen-is-not-a-monad 51. https://plus.google.com/105107667630152149014/about

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help with multiparam type class
by gbwey9 16 Jul '14

16 Jul '14

Hi, Given any Event constructor, the 'conv' method passes in the field name to 'val' for each field. There is a Z instance for each Event arity. The call "conv E1::E1" works but requires that I explicitly specify the type. Is there any way to avoid that having to do that? For some reason 'conv' does not have enough information to know the type. Is there a way to do this with functional dependencies or type classes or something else? Here is the paste http://lpaste.net/107632 and here is the error: *Main> conv E1 <interactive>:3:1: No instance for (Z e0 (String -> E1)) arising from a use of `conv' The type variable `e0' is ambiguous Possible fix: add a type signature that fixes these type variable(s) Note: there are several potential instances: instance (Event e, Field a) => Z e (a -> e) -- Defined at testfamily.hs:28:10 instance Event e => Z e e -- Defined at testfamily.hs:20:10 Possible fix: add an instance declaration for (Z e0 (String -> E1)) In the expression: conv E1 In an equation for `it': it = conv E1 Thanks for any help, Grant

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Feature Proposal: GHC Flag for implicit external Prelude
by Andrew Gibiansky 16 Jul '14

16 Jul '14

Hey all, I would like to propose a very minor flag to add to GHC. I would like GHC to have a --with-prelude flag, which would specify an alternate Prelude to use instead of the default Haskell prelude. This would have an effect similar to -XNoImplicitPrelude and an additional import MyNewPrelude in the source file. It might be a *little* different semantically, as a qualified import would disable the original implicit import, just like it does with the default Haskell prelude. The benefit this would have is that this would give alternate preludes a more first-class status. Instead of having to import an alternative prelude everywhere, you could just have a ghc-options: --with-prelude=... flag in your *.cabal file, and have a different prelude be used. This is important for my own work, as I highly prefer other preludes for my non-library development; I think this is a feature which will be very useful as Haskell develops and we try to figure out how to get rid of the warts in the current Prelude. In order to specify the actula module, I propose that you just pass the module name as the --with-prelude argument. You could thus invoke GHC like this: ghc --with-prelude=MyNewPrelude Main.hs Then, the same rules apply for figuring out which MyNewPrelude module you are referring to as if you had written a `import MyNewPrelude` statement in the file. I am willing to implement this for GHC if people agree that this is a useful flag (or at least agree that isn't not a *bad* idea). What are your thoughts on this proposal? -- Andrew

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Looking for architecture/infrastructure advice on coping with thousands of concurrent clients
by Isak Hansen 16 Jul '14

16 Jul '14

Let's just imagine there aren't enough poker sites out there already and that it makes sense to build another one. Basically, there will be players connecting to one or more tables, most likely talking some kind of JSON based protocol with the server(s). #1 - What's a good way to set up and maintain compressed and encrypted connections from the clients to my cluster? I've been wanting to try out websockets, think that'll do? #2 - I'm new to Haskell and would appreciate thoughts on how to store and process game state. Imagine a plain application managing 1000 tables of poker, responding to player input (that magically appears) and timer events (e.g. folding players that fail to take action). What data structures and libraries should I be looking at here? Thoughts on concurrency and how I organize program flow? #3 - I'm thinking of ZMQ to wire components together. Would I be better off basing my cluster on Cloud Haskell or some other library for distributed work? Would love some quick pointers and feedback here, thanks in advance, Isak

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[ANN](and feedback request) unagi-chan: Fast and scalable concurrent queues for x86, with a Chan-like API
by Brandon Simmons 15 Jul '14

15 Jul '14

I'm happy to finally release unagi-chan, an implementation of high-performance concurrent FIFO queues that have an API very similar to Control.Concurrent.Chan. You can see benchmarks and documentation here: http://hackage.haskell.org/package/unagi-chan If you have a moment (especially if you're on a non-x86 architecture) please take 10 minutes and run the tests with: $ cabal configure --enable-tests $ cabal build $ time ./dist/build/test/test Thanks to Ryan Newton for helping answer some low-level questions about his atomic-primops package which provides the CAS and fetch-and-add implementations that are the core of unagi-chan. REQUEST FOR FEEDBACK: would anyone be interested in any functionality like the following: - concurrent builder for Text, something like: new :: IO (InTextChan , Lazy.Text) write :: String -> InTextChan -> IO () - something similar for ByteString (is there a class for types convertable to ByteString?) - concurrent Text and/or ByteString readers, something like: reader :: Text -> IO TextReader takeText :: Int -> TextReader -> IO Text That's very sketchy, but I ask because I'm very close to that already with Unagi.Unboxed, and should be able to implement something like any of the above without too much trouble if it would be useful. Brandon http://brandon.si

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extensible-transformers
by Ben Foppa 15 Jul '14

15 Jul '14

Hi cafe, I whipped up extensible-transformers ( https://github.com/RobotGymnast/extensible-transformers) this afternoon. The idea is to make Monad transformer code more like extensible-effects code (http://hackage.haskell.org/package/extensible-effects) Here's a sample: {-# LANGUAGE FlexibleContexts #-} module Main(main) where import Control.Monad.Trans.Flexible import Control.Monad.Trans.List import Control.Monad.Trans.State.Strict -- A flexible transformer stack built from existing transformers using `liftT`. bar :: (In (StateT Int) t, In ListT t) => t () bar = do n <- liftT get liftT $ ListT $ return $ replicate n () -- A flexible transformer stack built from existing transformers using `liftT`. baz :: In (StateT Int) t => t () baz = do liftT $ state $ \i -> ((), i + (1 :: Int)) -- A flexible transformer monad stack composed of two other flexible -- transformer monad stacks. foo :: (In (StateT Int) t, In ListT t) => t () foo = do bar baz main :: IO () main = do evalStateT (runListT foo) (1 :: Int) >>= putStrLn . show runListT (evalStateT foo (2 :: Int)) >>= putStrLn . show Any feedback on this? Does such a package already exist? Thanks, Ben

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