October 2012 - Haskell-Cafe

Using CatchIO on StateT
by Dmitry Vyal 12 Oct '12

12 Oct '12

Greetings, I found The MonadCatchIO-mtl package while looking for a way to catch exceptions in my custom StateT s (ReaderT r IO) monad. CatchIO worked flawlessly when handlers used the environment r from Reader, but things broken when I tried to use state. According to source, the handler is run with the same state which was passed to runStateT. instance MonadCatchIO m => MonadCatchIO (StateT s m) where m `catch` f = StateT $ \s -> (runStateT m s) `Control.Monad.CatchIO.catch` (\e -> runStateT (f e) s) And my finalizing action depends on the current state. Can you give an advice how to get the behavior I need? Best regards, Dmitry Vyal

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Hackage Documentation issue
by Jonas Almström Duregård 12 Oct '12

12 Oct '12

Hi, Short question: can I or someone else make the Hackage build bot rebuild a package with a new dependency (even though the initial build was successful)? Long question: Does anyone know what would cause the documentation of a successful Hackage build not to appear on the web? Specifically I'm trying to get docs working for: http://hackage.haskell.org/package/BNFC-meta-0.4 Doing cabal haddock works well for me, and I managed to find the log of the Hackage build: http://hackage.haskell.org/packages/archive/BNFC-meta/0.4/logs/success/ghc-… The problem seems to be related to one of the dependencies (Alex-meta) which had a similar problem: the documentation step failed (but not the build). I was able to fix it (or at least work around it) so Alex-meta has working documentation now. Can I or someone else prompt the BNFC-meta documentation to rebuild with the new dependency (even though the initial build was successful)? I wouldn't want to upload a new version just to get docs rebuilt, and i'm not even sure it solves the issue. Regards, Jonas

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Ordering of BigFloats in numbers-3000.0.0.0
by Michael Orlitzky 11 Oct '12

11 Oct '12

I'm trying to use, http://hackage.haskell.org/package/numbers-3000.0.0.0 to get better precision "for free" out of some numerical code. I ran into an issue pretty quickly, though. In Data.Number.BigFloat, we have, data BigFloat e = BF (Fixed e) Integer deriving (Eq, Ord) and the derived Ord is obviously incorrect: Prelude Data.Number.BigFloat> let x = 0.1 :: BigFloat Prec50 Prelude Data.Number.BigFloat> let y = 0.02 :: BigFloat Prec50 Prelude Data.Number.BigFloat> x < y True But, I'm not sure who I should contact as the maintainer. The LICENSE file has, Copyright (c) 2007-2012 Lennart Augustsson, Russell O'Connor, Richard Smith, Daniel Wagner, Dan Burton The cabal file says (no email address), Maintainer: Lennart Augustsson And the repo is at, https://github.com/DanBurton/numbers Does anyone know who I should bug for a fix?

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London Haskell User Group is back
by Derek Wright 11 Oct '12

11 Oct '12

[Sorry if you have already seen / heard this already, this really is the last announcement...] After 3 years away the London Haskell User Group is back. Our first meeting is in a couple of weeks. Wednesday 24-Oct-2012 6-30pm City University, College Building, St John Street, London. EC1V 4PB http://www.meetup.com/London-HUG/events/85763942/ Why Do Monads Matter? Speaker: Derek Wright A quick tour through the why rather than the how of monads. Trying to give an intuition about why these concepts matter in the first place. From The Four Horsemen of the Catapocalypse (Failure, Dependence, Uncertainty and Destruction) through Kleisli arrows and ending up with monads and categorical programming. See why these abstract concepts should be part of every Haskell programmers toolkit. Join us for the first talk of London Haskell 2.0, then participate in a discussion about the future meetings and shape the vision! We are using Meetup to organise everything and the main page for London Haskell is: http://www.meetup.com/London-HUG/ There has already been a lot interest and 35 (out of a possible 40) have signed up already! Hope to see you there. Yours, Derek.

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64-bit vs 32-bit haskell platform on Mac: misleading notice on Platform website?
by Eugene Kirpichov 11 Oct '12

11 Oct '12

Hi, I installed Haskell Platform 32-bit on a fresh 64-bit mac, because the page http://www.haskell.org/platform/mac.html says: "Pick the 32-bit vesion, unless you have a specific reason to use the 64-bit version. The 32-bit one is slightly faster for most programs." Then, during the installation of a package, the following happeed: Loading package cairo-0.12.3.1 ... <command line>: can't load .so/.DLL for: /opt/local/lib/libz.dylib (dlopen(/opt/local/lib/libz.dylib, 9): no suitable image found. Did find: /opt/local/lib/libz.dylib: mach-o, but wrong architecture) That libz.dylib is a 64-bit library and it can't be used by 32-bit Haskell platform. QUESTION: It seems to me that most people, at least most who care about "slightly faster" programs, are likely to run into something like this - using native 64-bit libraries. Compatibility exists only in the opposite direction. Wouldn't it be appropriate to remove this notice and ask people to use the 64-bit version unless they have a specific reason not to? -- Eugene Kirpichov http://www.linkedin.com/in/eugenekirpichov We're hiring! http://tinyurl.com/mirantis-openstack-engineer

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Safe Haskell and instance coherence
by Mikhail Glushenkov 11 Oct '12

11 Oct '12

Hello, It's a relatively well-known fact that GHC allows for multiple type class instances for the same type to coexist in a single program. This can be used, for example, to construct values of the type Data.Set.Set that violate the data structure invariant. I was mildly surprised to find out that this works even when Safe Haskell is turned on: https://gist.github.com/3854294 Note that the warnings tell us that both instances are "[safe]" which gives a false sense of security. I couldn't find anything on the interplay between orphan instances and Safe Haskell both in the Haskell'12 paper and online. Is this something that the authors of Safe Haskell are aware of/are intending to fix? -- () ascii ribbon campaign - against html e-mail /\ www.asciiribbon.org - against proprietary attachments

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ANN: data-dword: Long binary words from short ones
by Mikhail Vorozhtsov 11 Oct '12

11 Oct '12

Hi. I'm pleased to announce my new little library, data-dword[1]. It provides Template Haskell utilities for defining binary word data types from low and high halves, e.g. data Word96 = Word96 Word32 Word64 -- strictness is configurable data Int96 = Int96 Int32 Word64 -- All instances are fully implemented (including `quotRem`, etc) instance Bounded, Enum, Eq, Integral, Num, Ord, Read, Real, Show, Ix, Bits, Hashable -- Extra bit-manipulating functions, unwrapped addition and -- multiplication, etc. instance BinaryWord, DoubleWord -- Rewrite rules for converting to/from the standard integral types {-# RULES "fromIntegral/..." ... #-} The library comes with a pretty thorough test suite (that ATM has some failures on x86-32 due to bug #7233[2] in the base library). [1] http://hackage.haskell.org/package/data-dword [2] http://hackage.haskell.org/trac/ghc/ticket/7233

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Haskell Weekly News: Issue 247
by Daniel Santa Cruz 11 Oct '12

11 Oct '12

Welcome to issue 247 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of September 30 to October 6, 2012. Quotes of the Week * monochrom: 98% of people lack the property that they're members of the other 2% * ManuelChakravarty: We used to say, "Learn Haskell, it makes you a better Java programmer." Now we say, "Learn Agda, it makes you a better Haskell programmer." * Cale: data Nature a = Rock a | Stream (Nature a) | Bush (Nature a) (Nature a) * rwbarton: type inference is supposed to be the compiler's job, not the reader's job * ShelSilverstein: Lazy Lazy Lazy Lazy Lazy Lazy Jane. She wants a drink of water so she waits and waits and waits and waits and waits for it to rain. * Cale: We're up to 950 users which apparently means that any beginner question is now answered by 4 people. * YayMe: When I first started playing with haskell I could barely sleep for 2 days because of how confused it made me * EvanLaforge: Then we can tell them about how we had to walk in the snow both ways uphill fighting space leaks with bare hands while they just have to read about it. * nand: every time I try arguing semantics, I come to the conclusion that every word is the exact same thing and refers to anything * bfig: i have been reborn as a haskell programer after finding Debug.Trace Top Reddit Stories * Factorization diagrams Domain: mathlesstraveled.com, Score: 82, Comments: 11 On Reddit: [1] http://goo.gl/Jrk0M Original: [2] http://goo.gl/kXkiV * The New Cloud Haskell Domain: well-typed.com, Score: 73, Comments: 21 On Reddit: [3] http://goo.gl/hcRJJ Original: [4] http://goo.gl/0oWfj * GHC HEAD now features Agda-like Holes Domain: hpaste.org, Score: 66, Comments: 43 On Reddit: [5] http://goo.gl/J8bIu Original: [6] http://goo.gl/297jv * FizzBuzz Revisited using Monoids Domain: dave.fayr.am, Score: 55, Comments: 68 On Reddit: [7] http://goo.gl/nqIsE Original: [8] http://goo.gl/995Zq * Which advanced Haskell topics interest you? (Survey) Domain: docs.google.com, Score: 39, Comments: 12 On Reddit: [9] http://goo.gl/Djy63 Original: [10] http://goo.gl/K5pRm * A fast new SipHash implementation Domain: serpentine.com, Score: 34, Comments: 8 On Reddit: [11] http://goo.gl/ZGSxD Original: [12] http://goo.gl/GiD3F * Communication Patterns in Cloud Haskell (Part 1): master-slave, work-stealing and work-pushing Domain: well-typed.com, Score: 33, Comments: 10 On Reddit: [13] http://goo.gl/IRr8s Original: [14] http://goo.gl/6vUyY * Category and lenses Domain: web.jaguarpaw.co.uk, Score: 28, Comments: 2 On Reddit: [15] http://goo.gl/r6Ajl Original: [16] http://goo.gl/VwtQP * Yesod Tutorial 1 at FP Complete Domain: fpcomplete.com, Score: 28, Comments: 15 On Reddit: [17] http://goo.gl/wnUkh Original: [18] http://goo.gl/kvCMu * The usual type signatures for foldl and foldr are confusing Domain: self.haskell, Score: 27, Comments: 63 On Reddit: [19] http://goo.gl/XGHq8 Original: [20] http://goo.gl/XGHq8 * Haskell Symposium 2012 (with youtube links) Domain: haskell.org, Score: 27, Comments: 1 On Reddit: [21] http://goo.gl/7Dz89 Original: [22] http://goo.gl/I1XXa * Solving a Partial Differential Equation Comonadically Domain: idontgetoutmuch.wordpress.com, Score: 27, Comments: 2 On Reddit: [23] http://goo.gl/Qxwjq Original: [24] http://goo.gl/lBm12 * Need a math related Haskell project suggestion Domain: self.haskell, Score: 22, Comments: 20 On Reddit: [25] http://goo.gl/o3lYl Original: [26] http://goo.gl/o3lYl * Algebraically Interpreting Polymorphism Domain: stackoverflow.com, Score: 21, Comments: 12 On Reddit: [27] http://goo.gl/3SCA9 Original: [28] http://goo.gl/wC6rm Top StackOverflow Questions * Why is seq bad? votes: 18, answers: 1 Read on SO: [29] http://goo.gl/IwVYS * Can I speed up this Haskell algorithm? votes: 18, answers: 3 Read on SO: [30] http://goo.gl/PSVUB * Is there a library that uses ConstraintKinds to generalize all the base type classes to allow constraints? votes: 15, answers: 1 Read on SO: [31] http://goo.gl/MfTMC * What are skolems? votes: 15, answers: 2 Read on SO: [32] http://goo.gl/qEnEI * What does it usually mean when a Haskell record accessor leads with an underscore? votes: 14, answers: 3 Read on SO: [33] http://goo.gl/3YD1n * How does this piece of obfuscated Haskell code work? votes: 12, answers: 1 Read on SO: [34] http://goo.gl/rLtFG * How to implement a game loop in reactive-banana? votes: 12, answers: 1 Read on SO: [35] http://goo.gl/nx3zR * How do Haskell currying and pattern matching work together? votes: 9, answers: 1 Read on SO: [36] http://goo.gl/jBCz0 * How does the constraints package work? votes: 9, answers: 1 Read on SO: [37] http://goo.gl/mb2na * In haskell, What does :+: mean in data type definition? votes: 9, answers: 1 Read on SO: [38] http://goo.gl/kv0Yn * Monad m => a -> [a -> m a] -> m a votes: 9, answers: 1 Read on SO: [39] http://goo.gl/ns8FG * How to decorate a Tree in Haskell votes: 7, answers: 2 Read on SO: [40] http://goo.gl/kBn66 Until next time, Daniel Santa Cruz References 1. http://mathlesstraveled.com/2012/10/05/factorization-diagrams/ 2. http://www.reddit.com/r/haskell/comments/1117h2/factorization_diagrams/ 3. http://www.well-typed.com/blog/70 4. http://www.reddit.com/r/haskell/comments/10y089/the_new_cloud_haskell/ 5. http://hpaste.org/75668 6. http://www.reddit.com/r/haskell/comments/10u7xr/ghc_head_now_features_agdal… 7. http://dave.fayr.am/posts/2012-10-4-finding-fizzbuzz.html 8. http://www.reddit.com/r/haskell/comments/10zlyy/fizzbuzz_revisited_using_mo… 9. https://docs.google.com/spreadsheet/viewform?formkey=dE1QZFNRLTFMdkllYWIyR2… 10. http://www.reddit.com/r/haskell/comments/10r2yk/which_advanced_haskell_topi… 11. http://www.serpentine.com/blog/2012/10/02/a-fast-new-siphash-implementation… 12. http://www.reddit.com/r/haskell/comments/10u381/a_fast_new_siphash_implemen… 13. http://www.well-typed.com/blog/71 14. http://www.reddit.com/r/haskell/comments/10zo6t/communication_patterns_in_c… 15. http://web.jaguarpaw.co.uk/~tom/blog/2012/09/30/category-and-lenses.html 16. http://www.reddit.com/r/haskell/comments/10ph2c/category_and_lenses/ 17. http://fpcomplete.com/yesod-tutorial-1-my-first-web-site/ 18. http://www.reddit.com/r/haskell/comments/10seol/yesod_tutorial_1_at_fp_comp… 19. http://www.reddit.com/r/haskell/comments/10q2ls/the_usual_type_signatures_f… 20. http://www.reddit.com/r/haskell/comments/10q2ls/the_usual_type_signatures_f… 21. http://www.haskell.org/haskell-symposium/2012/index.html 22. http://www.reddit.com/r/haskell/comments/10xafx/haskell_symposium_2012_with… 23. http://idontgetoutmuch.wordpress.com/2012/04/01/solving-a-partial-different… 24. http://www.reddit.com/r/haskell/comments/10zy51/solving_a_partial_different… 25. http://www.reddit.com/r/haskell/comments/10zf9c/need_a_math_related_haskell… 26. http://www.reddit.com/r/haskell/comments/10zf9c/need_a_math_related_haskell… 27. http://stackoverflow.com/questions/10453558/algebraically-interpreting-poly… 28. http://www.reddit.com/r/haskell/comments/10rt4y/algebraically_interpreting_… 29. http://stackoverflow.com/questions/12687392/why-is-seq-bad 30. http://stackoverflow.com/questions/12730503/can-i-speed-up-this-haskell-alg… 31. http://stackoverflow.com/questions/12697348/is-there-a-library-that-uses-co… 32. http://stackoverflow.com/questions/12719435/what-are-skolems 33. http://stackoverflow.com/questions/12746229/what-does-it-usually-mean-when-… 34. http://stackoverflow.com/questions/12659951/how-does-this-piece-of-obfuscat… 35. http://stackoverflow.com/questions/12685430/how-to-implement-a-game-loop-in… 36. http://stackoverflow.com/questions/12658936/how-do-haskell-currying-and-pat… 37. http://stackoverflow.com/questions/12728159/how-does-the-constraints-packag… 38. http://stackoverflow.com/questions/12733171/in-haskell-what-does-mean-in-da… 39. http://stackoverflow.com/questions/12755655/monad-m-a-a-m-a-m-a 40. http://stackoverflow.com/questions/12658443/how-to-decorate-a-tree-in-haske…

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ANNOUNCE: luachunk-0.1
by Anupam Jain 10 Oct '12

10 Oct '12

Hi all, I just released luachunk-0.1 on Hackage (http://github.com/ajnsit/luachunk) Luachunk is a small library to read and write Lua 5.1 bytecode chunks. It is modeled after ChunkSpy.lua (http://luaforge.net/projects/chunkspy/) though the code is written from scratch. A pretty listing printer for lua bytecode is in the works. I used LuaChunk in a project to generate Lua bytecode via a DSL, and then interpret it with HsLua. I hope to release the DSL itself sometime in the future. -- Anupam

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Re: [Haskell-cafe] CPS Streams
by jeff p 10 Oct '12

10 Oct '12

Here is an haste of the original message since it seems like the formatting was lost. http://hpaste.org/76082 sorry about that, Jeff

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