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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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StateT and ContT
by Ivan Lazar Miljenovic 20 Jan '15

20 Jan '15

In the transformers library, there are two implementations for callCC for use with StateT (both lazy and strict) [1,2], the second of which is documented to not satisfy the laws of a monad transformer. [1]: http://hackage.haskell.org/package/transformers-0.4.2.0/docs/Control-Monad-… [2]: http://hackage.haskell.org/package/transformers-0.4.2.0/docs/Control-Monad-… However, in mtl the MonadCont class [3] uses the second definition for the StateT instance. Is there a good reason for this, or just to match the behaviour of pre-transformers mtl [4] ? [3]: http://hackage.haskell.org/package/mtl-2.2.1/docs/Control-Monad-Cont-Class.… [4]: http://hackage.haskell.org/package/mtl-1.1.1.1/docs/src/Control-Monad-State… (I did raise an issue for this on mtl's Github issue tracker, but it hasn't had any responses for two months.) -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com http://IvanMiljenovic.wordpress.com

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MonoidHashMap?
by Konstantine Rybnikov 20 Jan '15

20 Jan '15

Hi! I'm working with a lot of HashMap's and it's very frustrating how many times I've "lost" my data because of usage of either a Monoid instance of a HashMap (which is defined as H.union, which, upon collision, takes value from first hm and discards from second), or just using fromList in the wrong place. Whereas the data I'm working is is mostly defined as (Monoid v => HashMap k v), so what I need "by default" is actually something like `H.unionWith (<>)`. What I was wondering is this: is something like MonoidHashMap is desired to be in unordered-containers, or is this use-case only popular in my programs? I'm asking because I have a feeling that this thing might be useful quite a lot for others also. If not -- sorry for bothering :) Cheers!

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garbage collection for a data structure
by Elise Huard 20 Jan '15

20 Jan '15

Hi, I was wondering if there was a way to check whether a particular data structure gets garbage collected in a program. A friendly person pointed me to System.Mem.Weak on the Haskell-Beginner list - however I've been unable to verify how it works, so I'm bumping it to this list. See the following toy program: I was trying to see whether the output would contain "garbage collected". I wondered if performGC is a nudge rather than an immediate "garbage collect now" instruction, and performGC is not actually performed? Or I've misunderstood finalizers in this context and they would not actually be executed when z gets garbage collected? import System.Mem.Weak import System.Mem (performGC) import Control.Concurrent (threadDelay) main :: IO () main = do let x = 5 y = "done" z = 3 a <- mkWeak z x (Just (putStrLn "garbage collected")) performGC threadDelay 20000000 print y Thank you, Elise

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New Functional Programming Job Opportunities
by Functional Jobs 19 Jan '15

19 Jan '15

Here are some functional programming job opportunities that were posted recently: Functional Software Developer at Moixa Technology http://functionaljobs.com/jobs/8778-functional-software-developer-at-moixa-… Cheers, Sean Murphy FunctionalJobs.com

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http-client: proxy environment variable support
by Michael Snoyman 19 Jan '15

19 Jan '15

Neil Mitchell opened an issue[1] for http_proxy and https_proxy environment variable support in http-client. I've written that support, and it's ready to go, but there's an open question: what should the default behavior be? In particular, should environment variables, by default, be checked to determine the proxy, or not? Arguments each way: In favor of using environment variables: * Matches behavior of many other tools and libraries * Allows application users control without requiring a code change from application writers Against using environment variables: * It's a change in behavior vs what http-client does today (though that could certainly be seen as just a missing feature) * Environment variables will implicitly change the behavior of code, which generally speaking can be problematic I'm leaning towards having the default behavior be: * If the user explicitly chooses a proxy setting on the manager, use that * If the user explicitly sets a proxy value on the Request, use that * If the environment variable is set, use that * Otherwise, no proxy In addition to whether we should make this change in behavior, I'm also interested in whether people think this should require a major version bump. [1] https://github.com/snoyberg/http-client/issues/94

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enable-executable-profiling failing with cabal 1.22
by Michael Jones 19 Jan '15

19 Jan '15

Using cabal 1.22 for some reason does not like —enable-executable-profiling. I am working on a rebuilt machine that had 1.18 from the distribution, with 1.22 installed afterwards. I found a little info on people having this problem, but I failed to fix the problem. Note than the library profiling option is accepted. Only the executable fails. Is there any know way to start with platform 2014 with GHC 7.8.3 and install cabal 1.22 and can use this option?

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This code produces an infinite output... why? it's a bug in ghc?
by mariopal 19 Jan '15

19 Jan '15

This code produces an infinite output... why? it's a bug in ghc? > -- Glasgow Haskell Compiler, Version 7.6.3 on Debian Sid > import System.Locale (defaultTimeLocale) > import Data.Time > > > mkDated n = formatTime defaultTimeLocale "%FT%TZ" $ addUTCTime (3600*n) $ UTCTime (fromGregorian 2015 1 1) (timeOfDayToTime $ TimeOfDay 0 0 0) > > main = mapM_ putStrLn $ map mkDated [0..2]

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