January 2015 - Haskell-Cafe

cabal keeps relinking
by Semen Trygubenko / Семен Тригубен ко 23 Jan '15

23 Jan '15

Dear Haskell-Cafe, cabal seems to be re-linking every time I run cabal build Wait was not an issue for small projects but for big ones it is starting to get in the way… How could I persuade cabal to re-link only if there are changes in the source tree and only these exes that require it? Many thanks, Semen PS I googled around and found this https://www.haskell.org/pipermail/libraries/2005-August/004306.html which seems to imply relink is (was?) conditional on multiple exes being present in the project. However, for me it was always relinking regardless. -- Семен Тригубенко http://trygub.com

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Munich Haskell Meeting, 2015-01-26 @ 19:30
by Christian Neukirchen 23 Jan '15

23 Jan '15

Dear all, Next week, our monthly Munich Haskell Meeting will take place again on Monday, January 26 at Cafe Puck at 19h30. For details see here: http://www.haskell-munich.de/dates If you plan to join, please add yourself to this dudle so we can reserve enough seats! It is OK to add yourself to the dudle anonymously or pseudonymously. https://dudle.inf.tu-dresden.de/haskell-munich-jan-2015/ Everybody is welcome! cu, -- Christian Neukirchen <chneukirchen(a)gmail.com> http://chneukirchen.org

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hs-fltk build fails
by Jeremy Henty 23 Jan '15

23 Jan '15

I am building with " cabal sandbox init ; cabal install hs-fltk ". Logs attached. Any advice? Regards, Jeremy Henty

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HDBC installation error
by Arunram A 23 Jan '15

23 Jan '15

Hi, I am trying to install HDBC, using the following command. cabal install hdbc and getting the following error. Preprocessing library HDBC-2.4.0.0... [1 of 8] Compiling Database.HDBC.Locale ( Database\HDBC\Locale.hs, dist\build\Database\HDBC\Locale.o ) [2 of 8] Compiling Database.HDBC.SqlValue ( Database\HDBC\SqlValue.hs, dist\build\Database\HDBC\SqlValue.o ) Database\HDBC\SqlValue.hs:265:29: Ambiguous occurrence `defaultTimeLocale' It could refer to either `Data.Time.defaultTimeLocale', imported from `Data.Time' at Database\HDBC\SqlValue.hs:20:1-16 (and originally defined in `time-1.5:Data.Time.Form at.Locale') or `Database.HDBC.Locale.defaultTimeLocale', imported from `Database.HDBC.Locale' at Database\HDBC\SqlValue.hs:22:30-46 (and originally defined in `System.Locale') Seems to be some kind of import conflict. But, I am not able to identify the problem and resolve the issue. May I get a clarity on what the problem could be and is there a known solution for this? Thanks! Regards, Arunram A.

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Type synonyms considered harmful?
by Julian Ospald 23 Jan '15

23 Jan '15

Hi, I've recently had a discussion about type synonyms and when to use them in haskell. On the one hand, they make reading type signatures easier and tell you what is meant, not just what is inside. On the other hand they also sort of hide what is inside, although you will probably need to know exactly that when using them. This might make reading code for new collaborators more difficult if they have to memorize all type synonyms first. So there are basically a few questions: * What do you think is a good policy for when and how to use them? Or would you say just not use them at all and put equivalent information in parameter documentation? * What are the upsides, downsides, pitfalls and also alternatives? (for completeness, also since https://www.haskell.org/haskellwiki/Type_synonym is a bit sparse) * Can we do something to improve the downsides? Or is there already something? (e.g. editor/IDE that can tell me the underlying type, error messages etc.) -- Regards, Julian Ospald

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[ANNOUNCE] New release of SBV
by Levent Erkok 23 Jan '15

23 Jan '15

I'm happy to announce a new release of SBV (v4.0); a library for seamlessly integrating SMT solvers with Haskell. https://hackage.haskell.org/package/sbv Most of the changes in this release are due to Brian Huffman of Galois; essentially adding capabilities so end-users can define symbolic bit-vector types that are not natively supported by SBV. For instance, a user can define `SWord17` for representing 17-bit words and use them just like other symbolic types natively supported by SBV. This feature allows SBV to take advantage of arbitrary bit-size decision procedures for bit-vector logics as found in modern SMT solvers, which are becoming increasingly more valuable in (semi-)automated verification of software artifacts. (Note that SBV already supports other basic types such as unbounded Integers, IEEE Floats and Doubles, Rationals, Algebraic reals, and traditional machine word sizes such as Word8/Int8; 16; 32; and 64 symbolically.) We plan to also add automatic support for SWordN/SIntN for arbitrary N, once GHC gets proper support for type-level naturals. Thanks to Brian Huffman for his contributions to this release. Bug reports/comments are always welcome. -Levent.

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Overlapping instances with "instance F a => G a"
by Bryan Gardiner 22 Jan '15

22 Jan '15

Hi cafe! Happy New Year. I'm writing a version of the Binary typeclass that encodes values with host endianness, and I have the code at the bottom of the message. HostBinary provides the encoding/decoding interface, and with HostBinaryNum I want to be able to write only e.g. "instance HostBinaryNum Int32" for numeric types. I had the manual HostBinary Word8 and HostBinary Int8 instances before I wrote HostBinaryNum. What I can't see is why I get this error at all: Binary.hs:21:29: Overlapping instances for HostBinary Word8 arising from a use of ‘hget’ Matching instances: instance HostBinary Word8 -- Defined at Binary.hs:16:10 instance HostBinaryNum a => HostBinary a -- Defined at Binary.hs:26:10 In the second argument of ‘fmap’, namely ‘(hget :: Get Word8)’ In the expression: fmap fromIntegral (hget :: Get Word8) In an equation for ‘hget’: hget = fmap fromIntegral (hget :: Get Word8) and also why commenting out either the HostBinary Int8 or the HostBinaryNum a => HostBinary a instances fixes the problem; and yet, the HostBinaryNum Word8 is accepted! Doesn't "HostBinaryNum a => HostBinary a" create a HostBinary instance for all instances of HostBinaryNum only? So why would it cause problems with an Int8 instance, and why isn't the HostBinaryNum Word8 instance needed to trigger a collision with the explicit HostBinary Word8? GHC 7.8.3, if it matters. Thanks for any clarifiation you can provide, Bryan Binary.hs: {-# LANGUAGE FlexibleInstances, UndecidableInstances #-} module Binary where import Data.Binary.Get import Data.Binary.Put import Data.Bits import Data.Int import Data.Word import Foreign.Storable class HostBinary a where hget :: Get a hput :: a -> Put instance HostBinary Word8 where hget = getWord8 hput = putWord8 instance HostBinary Int8 where hget = fmap fromIntegral (hget :: Get Word8) hput = hput . (fromIntegral :: Int8 -> Word8) class (Bits a, Integral a, Storable a) => HostBinaryNum a instance HostBinaryNum a => HostBinary a where hget = getNum hput = putNum --instance HostBinaryNum Word8 getNum :: (Bits a, Integral a, Storable a) => Get a getNum = undefined putNum :: (Bits a, Integral a, Storable a) => a -> Put putNum = undefined

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Help with identity functor print instance and monad transformers
by Cody Goodman 22 Jan '15

22 Jan '15

I understand the error below, but I'm not really sure what an instance of liftIO would look like for the Identity functor. I guess I'm not all to clear on what an identity functor is. I'm a little fuzzy on what an identity monad is. I understand that id gives the identity of something back pretty well though. Can anyone help? If anyone could help me fix the code below as well as explain the questions I have above it would be a great help. import Control.Monad.Trans.State import Control.Monad.IO.Class type MyState = State Int modAndPrintState :: Int -> MyState () modAndPrintState x = do get >>= \i -> do liftIO . print $ i if even x then put (x + i) else put (i * (x + 1) - x) main = undefined -- Prelude Control.Monad.Trans.State> :r -- [1 of 1] Compiling Main ( blah2.hs, interpreted ) -- blah2.hs:9:5: -- No instance for (MonadIO Data.Functor.Identity.Identity) -- arising from a use of `liftIO' -- Possible fix: -- add an instance declaration for -- (MonadIO Data.Functor.Identity.Identity) -- In the first argument of `(.)', namely `liftIO' -- In the expression: liftIO . print -- In a stmt of a 'do' block: liftIO . print $ i -- Failed, modules loaded: none. Thanks!

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Haskell Weekly News: Issue 315
by Daniel Santa Cruz 22 Jan '15

22 Jan '15

Welcome to issue 315 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers from January 4 to 17, 2015 Quotes of the Week * mpickering: When is the right time to use Data.ByteString.Char8? edwardk: mpickering: some time around 1980 * tpolecat: any lecture by conal elliott has that flavor. he is the bob ross of functional programming. Top Reddit Stories * Write You a Haskell Domain: dev.stephendiehl.com, Score: 280, Comments: 24 Original: [1] http://goo.gl/isSXWc On Reddit: [2] http://goo.gl/kOevyS * Announcing LTS Haskell 1.0 Domain: fpcomplete.com, Score: 87, Comments: 14 Original: [3] http://goo.gl/QW25bK On Reddit: [4] http://goo.gl/PlWgfm * MSFT open sources production serialization system written partially in Haskell Domain: blog.nullspace.io, Score: 83, Comments: 6 Original: [5] http://goo.gl/SDm7i1 On Reddit: [6] http://goo.gl/wljTKD * haskell-emacs - Write Emacs extensions in Haskell Domain: github.com, Score: 64, Comments: 3 Original: [7] http://goo.gl/NkK9FE On Reddit: [8] http://goo.gl/75zURt * The Type Theory Podcast, Episode 3: Dan Licata on Homotopy Type Theory Domain: typetheorypodcast.com, Score: 58, Comments: 17 Original: [9] http://goo.gl/z30wH5 On Reddit: [10] http://goo.gl/5KDPNu * Cabal 1.22 and cabal-install 1.22 Domain: groups.google.com, Score: 54, Comments: 3 Original: [11] http://goo.gl/WdxPu0 On Reddit: [12] http://goo.gl/ih07XV * [Haskell-cafe] The amount of CPP we have to use is getting out of hand Domain: haskell.org, Score: 51, Comments: 94 Original: [13] http://goo.gl/0OYZ7C On Reddit: [14] http://goo.gl/4QCAIc * Boston Haskell lightning talks: Rust, Phantoms, ATS, Freezing, Music, and Code reuse Domain: self.haskell, Score: 50, Comments: 8 Original: [15] http://goo.gl/TAjkMi On Reddit: [16] http://goo.gl/TAjkMi * Haskell for all: total-1.0.0: Exhaustive pattern matching using traversals, prisms, and lenses Domain: haskellforall.com, Score: 47, Comments: 9 Original: [17] http://goo.gl/TJDioE On Reddit: [18] http://goo.gl/sx7aMu * Mutation analysis for Haskell (alpha) Domain: self.haskell, Score: 46, Comments: 9 Original: [19] http://goo.gl/MByMzL On Reddit: [20] http://goo.gl/MByMzL * Simple Algebraic Data Types by Bartosz Milewski Domain: bartoszmilewski.com, Score: 45, Comments: 1 Original: [21] http://goo.gl/Ns3UjL On Reddit: [22] http://goo.gl/uuqCt2 * The Frank programming language Domain: homepages.inf.ed.ac.uk, Score: 42, Comments: 29 Original: [23] http://goo.gl/msxrp5 On Reddit: [24] http://goo.gl/0NPQYY * GHC Weekly News - 2015/01/07 Domain: ghc.haskell.org, Score: 42, Comments: 3 Original: [25] http://goo.gl/HVsDLU On Reddit: [26] http://goo.gl/JiMf4b * The Rust community grapples with the menace of ORPHAN INSTANCES Domain: discuss.rust-lang.org, Score: 42, Comments: 19 Original: [27] http://goo.gl/HIl68S On Reddit: [28] http://goo.gl/dUWa48 * The state and future of supercompilation in GHC. Domain: self.haskell, Score: 39, Comments: 10 Original: [29] http://goo.gl/e9FDLV On Reddit: [30] http://goo.gl/e9FDLV Top StackOverflow Questions * Implementation of null function votes: 18, answers: 2 Read on SO: [31] http://goo.gl/cVqJa3 * What is the type of this self-applying factorial function? votes: 17, answers: 3 Read on SO: [32] http://goo.gl/vQaeQ0 * Haskell fast concurrent queue votes: 17, answers: 3 Read on SO: [33] http://goo.gl/HdAlul * Ghc: partially compile Haskell code? votes: 13, answers: 1 Read on SO: [34] http://goo.gl/hS2QNQ * What is the type of return 5 in Haskell when no context is given? votes: 12, answers: 1 Read on SO: [35] http://goo.gl/IC08Sk * Is it possible to write fmap for this data type involving a type family? votes: 11, answers: 1 Read on SO: [36] http://goo.gl/6V5Mps * How to get cabal and nix work together votes: 11, answers: 1 Read on SO: [37] http://goo.gl/TOXNDv * What is the most general way to compute the depth of a tree with something like a fold? votes: 8, answers: 2 Read on SO: [38] http://goo.gl/vlcioF * What is exactly an indexed functor in Haskell and what are its usages? votes: 8, answers: 1 Read on SO: [39] http://goo.gl/10PK8U * Functor instance for generic polymorphic ADTs in Haskell? votes: 8, answers: 2 Read on SO: [40] http://goo.gl/Oh0uFp * Finite State Transducers in Haskell? votes: 7, answers: 1 Read on SO: [41] http://goo.gl/gu80ZL * Is it possible to encode a generic “lift” function in Haskell? votes: 7, answers: 2 Read on SO: [42] http://goo.gl/R9B8N5 * Haskell Expression Equivalents votes: 6, answers: 1 Read on SO: [43] http://goo.gl/t54YiS Until next time, [44]+Daniel Santa Cruz References 1. http://dev.stephendiehl.com/fun/ 2. http://www.reddit.com/r/haskell/comments/2regln/write_you_a_haskell/ 3. https://www.fpcomplete.com/blog/2015/01/announcing-lts-haskell-1-0 4. http://www.reddit.com/r/haskell/comments/2rapgt/announcing_lts_haskell_10/ 5. http://blog.nullspace.io/bond-oss.html 6. http://www.reddit.com/r/haskell/comments/2rxkmk/msft_open_sources_productio… 7. https://github.com/knupfer/haskell-emacs 8. http://www.reddit.com/r/haskell/comments/2sdvyu/haskellemacs_write_emacs_ex… 9. http://typetheorypodcast.com/2015/01/episode-3-dan-licata-on-homotopy-type-… 10. http://www.reddit.com/r/haskell/comments/2rnumy/the_type_theory_podcast_epi… 11. https://groups.google.com/forum/#!msg/haskell-cafe/rRbP9oJjOBc/J8lf_sGVl_oJ 12. http://www.reddit.com/r/haskell/comments/2rcuak/cabal_122_and_cabalinstall_… 13. https://www.haskell.org/pipermail/haskell-cafe/2015-January/117683.html 14. http://www.reddit.com/r/haskell/comments/2shv2d/haskellcafe_the_amount_of_c… 15. http://www.reddit.com/r/haskell/comments/2sn8la/boston_haskell_lightning_ta… 16. http://www.reddit.com/r/haskell/comments/2sn8la/boston_haskell_lightning_ta… 17. http://www.haskellforall.com/2015/01/total-100-exhaustive-pattern-matching.… 18. http://www.reddit.com/r/haskell/comments/2s1tfx/haskell_for_all_total100_ex… 19. http://www.reddit.com/r/haskell/comments/2rj5sw/mutation_analysis_for_haske… 20. http://www.reddit.com/r/haskell/comments/2rj5sw/mutation_analysis_for_haske… 21. http://bartoszmilewski.com/2015/01/13/simple-algebraic-data-types/ 22. http://www.reddit.com/r/haskell/comments/2sbl93/simple_algebraic_data_types… 23. http://homepages.inf.ed.ac.uk/slindley/papers/frankly-draft-march2014.pdf 24. http://www.reddit.com/r/haskell/comments/2rao0t/the_frank_programming_langu… 25. https://ghc.haskell.org/trac/ghc/blog/weekly20150107 26. http://www.reddit.com/r/haskell/comments/2rq6zc/ghc_weekly_news_20150107/ 27. http://discuss.rust-lang.org/t/orphan-rules/1322/4 28. http://www.reddit.com/r/haskell/comments/2shyfj/the_rust_community_grapples… 29. http://www.reddit.com/r/haskell/comments/2s97d0/the_state_and_future_of_sup… 30. http://www.reddit.com/r/haskell/comments/2s97d0/the_state_and_future_of_sup… 31. http://stackoverflow.com/questions/27941690/implementation-of-null-function 32. http://stackoverflow.com/questions/27814714/what-is-the-type-of-this-self-a… 33. http://stackoverflow.com/questions/27933941/haskell-fast-concurrent-queue 34. http://stackoverflow.com/questions/27780282/ghc-partially-compile-haskell-c… 35. http://stackoverflow.com/questions/27932794/what-is-the-type-of-return-5-in… 36. http://stackoverflow.com/questions/27936725/is-it-possible-to-write-fmap-fo… 37. http://stackoverflow.com/questions/27968909/how-to-get-cabal-and-nix-work-t… 38. http://stackoverflow.com/questions/27769688/what-is-the-most-general-way-to… 39. http://stackoverflow.com/questions/27771474/what-is-exactly-an-indexed-func… 40. http://stackoverflow.com/questions/27856974/functor-instance-for-generic-po… 41. http://stackoverflow.com/questions/27997155/finite-state-transducers-in-has… 42. http://stackoverflow.com/questions/28003135/is-it-possible-to-encode-a-gene… 43. http://stackoverflow.com/questions/27761821/haskell-expression-equivalents 44. https://plus.google.com/105107667630152149014/about

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cabal-install crashes
by Henk-Jan van Tuyl 21 Jan '15

21 Jan '15

L.S., On my Windows PC, cabal-install crashes each time a file must be downloaded, i.e. when I try to install a package that is not in the cache and when I give command "cabal update". I deleted the file %appdata%\cabal\config to make sure it is not the cause of this crash. I tried several versions of cabal-install, the Haskell Platform I use is 2014.2.0.0. How can I solve this? Regards, Henk-Jan van Tuyl -- Folding@home What if you could share your unused computer power to help find a cure? In just 5 minutes you can join the world's biggest networked computer and get us closer sooner. Watch the video. http://folding.stanford.edu/ http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --

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