- Haskell-Cafe - Haskell.org

Converting non-primitive(?) recursive function to use scanl (or foldl maybe?)
by Cody Goodman 15 Dec '14

15 Dec '14

I've had some trouble wrapping my brain around how to use a scan to build this list: code below, but here's an lpaste link: http://lpaste.net/116494 {-# LANGUAGE OverloadedStrings #-} import Control.Monad import Data.Maybe import Text.Regex type RegexRes = Maybe (String, String, String, [String]) re = mkRegex "\$(\\w+)\$:([[:digit:]]+)" getParenNum :: String -> [String] getParenNum s = case matchRegexAll re s of Nothing -> [] Just (_,_,after,[word,num]) -> (word ++ num):getParenNum after getParenNumOnce' :: String -> String getParenNumOnce' s = case matchRegexAll re s of Nothing -> [] Just (_,_,after,[word,num]) -> (word ++ num) -- trying to accumulate the results of regexMatching repeatedly over a string f1 str = (\ (_, _, xs, acc) -> Just (xs, acc)) <=< (matchRegexAll re) $ str f2 str = fromMaybe "" $ (\ (_, _, after, [word,num]) -> Just after) <=< (matchRegexAll re) $ str -- f3 :: RegexRes -> RegexRes -- f3 :: RegexRes -> String f3 match = fromMaybe [] $ (\(_,_,after,[word,num]) -> Just (word ++ num)) =<< match f4 str = scanl (const f3) "" [(matchRegexAll re str)] f5 acc str = fromMaybe [] $ (\ (_, _, after, [word,num]) -> Just (acc ++ [word ++ num])) <=< (matchRegexAll re) $ str -- λ> scanl f5 [] ["(fish):1 sausage (cow):3 tree (boat):4"] -- [[],["fish1"]] str = "(fish):1 sausage (cow):3 tree (boat):4" -- λ> foldl (const . f3 . (matchRegexAll re)) "(fish):1 sausage (cow):3 tree (boat):4" ["(fish):1 sausage (cow):3 tree (boat):4"] -- "fish1" -- λ> -- want ["fish1","cow3","boat4"] -- λ> -- I feel like I should be using something in place of const that smartly chooses whether to pass String or... idk main = undefined

2 1

Typesafe API calls
by Fabian Bergmark 13 Dec '14

13 Dec '14

I just released a library on hackage (https://hackage.haskell.org/package/apis-0.0.0) The goal of the library is to provide typesafe API calls to a large amount of existing APIs. By typesafe I mean that, for each API the library provides two types, Input and Output, and a function :: Input -> IO Output. Internally the library knows how to serialize Input to make an HTTP request and how to deserialize the response into Output. Whats cool about this library is that it uses Template Haskell to generate the API interfaces. A combination of Open Data Tables, an internal YQL engine and JSON Schemas, all written in Haskell makes this possible. Right now only two APIs are included (Swedish weather data and Swedish public announcement), but more APIs are soon to follow. I have also implemented FFI exports of all types and functions so that, in combination with SWIG, this library can be used in almost any language. So far I have tried Java and Python with success. If anyone would like to try it and respond with feedback it would be greatly appreciated! If you want any specific API supported, I can try to import that right away Fabian Bergmark

4 4

Standard output of exec
by Ernesto Rodriguez 13 Dec '14

13 Dec '14

Dear All, I have a program which invokes an external program via exec. I wish to capture the standard output of the program without using external (temporary) files. Is it possible to create an in memory handle which I can tell exec to use as standard output of the invoked program. It would be great if it can be done without any extra dependencies. Best, -- Ernesto Rodriguez Masters Student Computer Science Utrecht University www.netowork.me github.com/netogallo

6 7

Having a very annoying problem related with (I think) lazy IO
by Ömer Sinan Ağacan 12 Dec '14

12 Dec '14

Hi all, I'm having a horrible bug in my program: a print statement that doesn't force any values(e.g. it just prints a static string) effecting the behavior of the program. I tried hard to produce a minimal example, but couldn't manage it. So instead I'm just going to point to the source code of my program: https://github.com/osa1/rho-torrent/blob/master/src/Rho/Listener.hs#L125 If I remove this print statement everything goes weird. I think when I remove this statement some effects are not happening, or happening in wrong times. Here's how I'm testing the different behavior: This is a network application and I'm creating an environment in my system and making this program talk to another program. I run my program 10 times without the print statement, and run 10 times with it. Both groups produce same output in every time, but outputs of first group and second group are different from each other. (apart from extra prints, of course) So this print statement effects some other things, and expected behavior is the one happening with print statement. I know the program is big for anyone to figure things in short amount of time, I'm just looking for tips and pitfalls that I may be falling. Does anyone have any ideas what I may be doing wrong? Note that the program doesn't use any unsafe stuff. Thanks in advance.

2 4

Typesafe multi-table PostgreSQL queries in Yesod with schema names?
by Manuel Gómez 12 Dec '14

12 Dec '14

Hi list, I’m looking for a rather specific point in the design space for database access libraries in Haskell and I’m having some trouble figuring out whether any existing library covers my use cases. Specifically, I need to 1. get a Yesod application to access a PostgreSQL database without having to reinvent the universe to get them decently integrated, 2. write queries in an EDSL with a good approach to composability and decent static verification against some declared form of the database objects, 3. be able to perform joins 4. be able to specify the schema name for each database object used in queries These are the closest solutions I’ve found: * HaskellDB: seems like the best approach to the EDSL issue and it’s certainly capable of performing joins, and I haven’t looked into schema names, but it seems possible — but it seems to be poorly maintained and somewhat abandoned, and comfortable Yesod integration doesn’t sound like a lot of fun. * Persistent: it’s perfectly integrated into Yesod, and together with Esqueleto, it provides a nice, typesafe and fully expressive EDSL — but, as far as I can tell, there is no support for specifying schema names, which is essential for my use case. * Groundhog: Yesod integration is reasonable since Persistent borrowed a good bit of its design, and after a few weeks of experimenting, I’ve got it pretty much worked out by now, but while it certainly takes an interesting and solid approach to the EDSL issue, it seems completely incapable of performing joins, and hence its usability is very heavily limited — I don’t fancy creating SQL views for each and every join I need. * postgresql-simple: minimal Yesod integration shouldn’t be too uncomfortable, since Persistent uses it internally anyway, and it can certainly do joins and use whatever fancy SQL magic I could ever want to write, certainly including schema names — but it doesn’t really do anything for statically checking queries against a schema specification or for composability and code reuse, and I might as well just use Persistent to do raw SQL. I’ve breifly looked at modifying Persistent to add support for schema names, but I got lost in the source code very quickly, and I worry about the fact that it requires serial primary keys — Groundhog is *very* nice in that it has a great approach to DBMS-specific syntax and other things I’ll probably need (non-numeric and composite keys), but it’s really a shame that it can’t do joins, and the workaround with views sort of defeats the purpose of SQL abstraction. I’m not sure what to do about this. Is there some grand unified relational database access library I just haven’t found, or is a pull request the only solution?

5 6

Using Docker to ease the pain of building GHC
by Sean Seefried 12 Dec '14

12 Dec '14

Hi all, I wrote a blog post recently on my experience of using Docker to relieve the pain of developing a GHC cross compiler build script. I thought it might be helpful. http://lambdalog.seanseefried.com/posts/2014-12-12-docker-build-scripts.html Sean

1 0

Linker troubles with cabal sandbox
by Zach Moazeni 12 Dec '14

12 Dec '14

Hello folks, I'm getting some linker trouble in the form of: "Undefined symbols for architecture x86_64: ... ld: symbol(s) not found for architecture x86_64" When trying to compile a simple hashtables example https://hackage.haskell.org/package/hashtables-1.2.0.0 I came across a previous thread with a solution https://www.haskell.org/pipermail/haskell-cafe/2014-October/116469.html but it's not working for me. Here are the steps I'm taking and the verbose errors: https://gist.github.com/zmoazeni/b68c66f5e15654948946 Can someone give me a hand? Thanks! Zach

1 1

Re: [Haskell-cafe] Standard output of exec
by Michael Snoyman 11 Dec '14

11 Dec '14

That's not going to be sufficient; you could end up in a deadlock by not flushing the output buffer of the child process. Have a look at the implementation of readProcess to see how to deal with some of these tricky issues in the lazy I/O world: http://hackage.haskell.org/package/process-1.2.0.0/docs/src/System-Process.… On Thu Dec 11 2014 at 6:40:54 PM Derek McLoughlin < derek.mcloughlin(a)gmail.com> wrote: > Something like this would work: > > import System.Process > import System.IO > import System.Exit > > readExecOutput :: String -> [String] -> IO () > readExecOutput cmdName cmdOptions = do > (_, Just hOut, _, jHandle) <- > createProcess (proc cmdName cmdOptions) > { cwd = Just "." > , std_out = CreatePipe > } > exitCode <- waitForProcess jHandle > exec_output <- hGetContents hOut > case exitCode of > ExitSuccess -> putStr $ "Success!\n" ++ exec_output > _ -> putStrLn "Command failed" > > > Derek. > > On Thursday, 11 December 2014 13:29:34 UTC, Ernesto Rodriguez wrote: > >> Hi Michael, >> >> Thank you for your answer. Conduits certainly are the best approach, but >> the output I am going to read is very short and the external program will >> only be called once or twice per execution so I think Lazy IO won't be >> problematic here. >> >> Best regards, >> >> Ernesto >> > On Thu, Dec 11, 2014 at 10:18 AM, Michael Snoyman <mic...@snoyman.com> >> wrote: >> > You can use readProcess[1], but it has some downsides: >>> >>> 1. Can't specify all options to CreateProcess, e.g., working directory >>> and environment variables. >>> 2. Relies on lazy I/O >>> >>> I use Data.Conduit.Process[2] for this kind of thing, which has a full >>> tutorial[3]. However, it *does* involve non-base dependencies. >>> >>> [1] http://www.stackage.org/haddock/2014-12-10-ghc78-exc/ >>> process-1.2.0.0/System-Process.html#v:readProcess >>> [2] http://www.stackage.org/haddock/2014-12-10-ghc78-exc/ >>> conduit-extra-1.1.4.2/Data-Conduit-Process.html >>> [3] https://www.fpcomplete.com/user/snoyberg/library- >>> documentation/data-conduit-process?show=tutorials >>> >>> On Thu Dec 11 2014 at 11:11:12 AM Ernesto Rodriguez <ne...@netowork.me> >>> wrote: >>> >> Dear All, >>>> >>>> I have a program which invokes an external program via exec. I wish to >>>> capture the standard output of the program without using external >>>> (temporary) files. Is it possible to create an in memory handle which I can >>>> tell exec to use as standard output of the invoked program. It would be >>>> great if it can be done without any extra dependencies. >>>> >>>> Best, >>>> >>>> -- >>>> Ernesto Rodriguez >>>> >>>> Masters Student >>>> Computer Science >>>> Utrecht University >>>> >>>> www.netowork.me >>>> github.com/netogallo >>>> >>>> >>>> _______________________________________________ >>>> Haskell-Cafe mailing list >>>> >>> Haskel...@haskell.org >>>> http://www.haskell.org/mailman/listinfo/haskell-cafe >>>> >>> >> >> >> -- >> Ernesto Rodriguez >> >> Masters Student >> Computer Science >> Utrecht University >> >> www.netowork.me >> github.com/netogallo >> >> >>

1 0

Haskell Weekly News: Issue 313
by Daniel Santa Cruz 11 Dec '14

11 Dec '14

Welcome to issue 313 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers from November 16 to December 6, 2014 Quotes of the Week * Dijkstra: How do we convince people that in programming simplicity and clarity (in short: what mathematicians call "elegance") are not a dispensable luxury, but a crucial matter that decides between success and failure? * Dijkstra: Simplicity is a great virtue but it requires hard work to achieve it and education to appreciate it. And to make matters worse: complexity sells better. * monochrom: those who have learned from history are bound to helplessly watch other people repeat it. Top Reddit Stories * 24 Days of GHC Extensions: Welcome! Domain: ocharles.org.uk, Score: 140, Comments: 24 Original: [1] http://goo.gl/HvjuvJ On Reddit: [2] http://goo.gl/4Skf5Z * How I Start: Haskell Domain: howistart.org, Score: 120, Comments: 11 Original: [3] http://goo.gl/T2OCwj On Reddit: [4] http://goo.gl/1bGl7I * Lucid: templating DSL for HTML Domain: chrisdone.com, Score: 105, Comments: 59 Original: [5] http://goo.gl/JJsJlT On Reddit: [6] http://goo.gl/jyvkHu * Erik Meijer planning to make Category theory for programmers MOOC Domain: imgur.com, Score: 105, Comments: 45 Original: [7] http://goo.gl/ZnCi6Z On Reddit: [8] http://goo.gl/8vOFP0 * An opportunity in Singapore Domain: self.haskell, Score: 98, Comments: 31 Original: [9] http://goo.gl/X0OBAE On Reddit: [10] http://goo.gl/X0OBAE * Dan Piponi working on an audio synthesis tool written in Haskell Domain: plus.google.com, Score: 90, Comments: 1 Original: [11] http://goo.gl/J83NRS On Reddit: [12] http://goo.gl/Cbhx8x * GHC and LLVM: The State of the Union Domain: smart-cactus.org, Score: 86, Comments: 13 Original: [13] http://goo.gl/CA7R4C On Reddit: [14] http://goo.gl/l3L8H7 * Learning Haskell as a Nonprogrammer Domain: superginbaby.wordpress.com, Score: 84, Comments: 77 Original: [15] http://goo.gl/9Q1ufS On Reddit: [16] http://goo.gl/Yqni7Y * Announcing Opaleye: SQL-generating embedded domain specific language for Postgres Domain: self.haskell, Score: 84, Comments: 85 Original: [17] http://goo.gl/HoMXjj On Reddit: [18] http://goo.gl/HoMXjj * Deploy any Haskell application. Instantly. Domain: halcyon.sh, Score: 79, Comments: 27 Original: [19] http://goo.gl/sxQWKk On Reddit: [20] http://goo.gl/5V2dMj * Haskell for all: How to build library-agnostic streaming sources Domain: haskellforall.com, Score: 75, Comments: 26 Original: [21] http://goo.gl/1QcyRf On Reddit: [22] http://goo.gl/eYwlBp * Image Processing with Comonads Domain: jaspervdj.be, Score: 74, Comments: 33 Original: [23] http://goo.gl/GZu2oc On Reddit: [24] http://goo.gl/Xsh0z9 * 24 Days of GHC Extensions: View Patterns Domain: ocharles.org.uk, Score: 69, Comments: 39 Original: [25] http://goo.gl/uN3PLf On Reddit: [26] http://goo.gl/2YZ1r0 * Well-Typed are hiring! Domain: well-typed.com, Score: 68, Comments: 36 Original: [27] http://goo.gl/DxYgrT On Reddit: [28] http://goo.gl/2imRKu * 24 Days of GHC Extensions: Pattern Synonyms Domain: ocharles.org.uk, Score: 65, Comments: 20 Original: [29] http://goo.gl/j75DP8 On Reddit: [30] http://goo.gl/VTvCof * How lazy evaluation works Domain: hackhands.com, Score: 63, Comments: 30 Original: [31] http://goo.gl/YPOFaM On Reddit: [32] http://goo.gl/w2uKGJ * Learn You an Agda Domain: williamdemeo.github.io, Score: 59, Comments: 61 Original: [33] http://goo.gl/hQ5DaT On Reddit: [34] http://goo.gl/2I00Zh * 24 Days of GHC Extensions: Record Wildcards Domain: ocharles.org.uk, Score: 59, Comments: 38 Original: [35] http://goo.gl/BnXqjA On Reddit: [36] http://goo.gl/yBzMoY * Functional Programming Patterns (BuildStuff '14) Domain: slideshare.net, Score: 55, Comments: 15 Original: [37] http://goo.gl/fzo56I On Reddit: [38] http://goo.gl/XCy482 * The GHCJS guys are working on TH cross compiling Domain: thread.gmane.org, Score: 54, Comments: 15 Original: [39] http://goo.gl/XyA4FS On Reddit: [40] http://goo.gl/eRULJH * Silk is looking for a Haskell engineer Domain: self.haskell, Score: 53, Comments: 21 Original: [41] http://goo.gl/eB10Wa On Reddit: [42] http://goo.gl/eB10Wa * Keera studios: Haskell games & adventure engine for Android, beta testers wanted! Domain: keera.co.uk, Score: 53, Comments: 8 Original: [43] http://goo.gl/rk7Wrx On Reddit: [44] http://goo.gl/ciCbYp * 24 Days of GHC Extensions: Bang Patterns Domain: ocharles.org.uk, Score: 51, Comments: 34 Original: [45] http://goo.gl/X4Huvh On Reddit: [46] http://goo.gl/eHmymQ * Let's Build a Browser Engine in Haskell: part 6 Domain: hrothen.github.io, Score: 49, Comments: 4 Original: [47] http://goo.gl/mDnmpX On Reddit: [48] http://goo.gl/96aLMl * A Docker build script for GHC 7.8.3 cross-compiler targeting Android Domain: github.com, Score: 49, Comments: 21 Original: [49] http://goo.gl/uCsMNK On Reddit: [50] http://goo.gl/KvY3JX * amazonka - Comprehensive Amazon Web Services SDK Domain: self.haskell, Score: 46, Comments: 17 Original: [51] http://goo.gl/Yi18LS On Reddit: [52] http://goo.gl/Yi18LS * 24 Days of GHC Extensions: Rebindable Syntax (guest post by Benjamin Kovach) Domain: ocharles.org.uk, Score: 46, Comments: 14 Original: [53] http://goo.gl/7e3J1I On Reddit: [54] http://goo.gl/jJJONL * Haskell Communities and Activities Report — November 2014 Domain: haskell.org, Score: 45, Comments: 6 Original: [55] http://goo.gl/7dfgQk On Reddit: [56] http://goo.gl/fME8UF * A taste of State: parsers are easy Domain: nikita-volkov.github.io, Score: 44, Comments: 4 Original: [57] http://goo.gl/HF4R3O On Reddit: [58] http://goo.gl/d7bLLV * Hybrid server/client Haskell web apps / Michael Snoyman Domain: youtube.com, Score: 42, Comments: 31 Original: [59] http://goo.gl/IbxCkI On Reddit: [60] http://goo.gl/Yjx8wj * Nix: The Cabal Purgatory HOWTO Domain: cse.chalmers.se, Score: 41, Comments: 45 Original: [61] http://goo.gl/pxdnYA On Reddit: [62] http://goo.gl/2uE8ya * Ranjit Jhala talks about Liquid Haskell Domain: youtu.be, Score: 41, Comments: 14 Original: [63] http://goo.gl/EAubrl On Reddit: [64] http://goo.gl/f1D8iz * A quick and easy Emacs configuration for Haskell Domain: github.com, Score: 39, Comments: 16 Original: [65] http://goo.gl/8akoJo On Reddit: [66] http://goo.gl/Ma8iDO Top StackOverflow Questions * Turning A => M[B] into M[A => B] votes: 22, answers: 2 Read on SO: [67] http://goo.gl/P4AXC2 * When are type signatures necessary in Haskell? votes: 19, answers: 3 Read on SO: [68] http://goo.gl/9YcgHo * Boilerplate-free annotation of ASTs in Haskell? votes: 18, answers: 4 Read on SO: [69] http://goo.gl/dNv5kU * Can fusion see through newtype wrappers? votes: 17, answers: 1 Read on SO: [70] http://goo.gl/00ONhp * Haskell: Why use Proxy? votes: 17, answers: 2 Read on SO: [71] http://goo.gl/bWRvVN * Is Haskell's laziness an elegant alternative to Python's generators? votes: 16, answers: 3 Read on SO: [72] http://goo.gl/oblTSW * Type inference interferes with referential transparency votes: 16, answers: 7 Read on SO: [73] http://goo.gl/2nIW1x * What exactly is the kind “*” in Haskell? votes: 15, answers: 2 Read on SO: [74] http://goo.gl/sOXwgi * What's the difference between partial evaluation and function inlining in a functional language? votes: 14, answers: 1 Read on SO: [75] http://goo.gl/AOZuCH Until next time, [76]+Daniel Santa Cruz References 1. https://ocharles.org.uk/blog/posts/2014-12-01-24-days-of-ghc-extensions.html 2. http://www.reddit.com/r/haskell/comments/2nyy5z/24_days_of_ghc_extensions_w… 3. http://howistart.org/posts/haskell/1 4. http://www.reddit.com/r/haskell/comments/2mkbay/how_i_start_haskell/ 5. http://chrisdone.com/posts/lucid 6. http://www.reddit.com/r/haskell/comments/2my5bc/lucid_templating_dsl_for_ht… 7. http://imgur.com/OQbeouW 8. http://www.reddit.com/r/haskell/comments/2o5m95/erik_meijer_planning_to_mak… 9. http://www.reddit.com/r/haskell/comments/2nkjdq/an_opportunity_in_singapore/ 10. http://www.reddit.com/r/haskell/comments/2nkjdq/an_opportunity_in_singapore/ 11. https://plus.google.com/107913314994758123748/posts/9EQP1xqsQZ6 12. http://www.reddit.com/r/haskell/comments/2nsdkq/dan_piponi_working_on_an_au… 13. http://smart-cactus.org/~ben/posts/2014-11-28-state-of-llvm-backend.html 14. http://www.reddit.com/r/haskell/comments/2nphz5/ghc_and_llvm_the_state_of_t… 15. http://superginbaby.wordpress.com/2014/11/18/learning-haskell-as-a-nonprogr… 16. http://www.reddit.com/r/haskell/comments/2mp3fb/learning_haskell_as_a_nonpr… 17. http://www.reddit.com/r/haskell/comments/2nxx7n/announcing_opaleye_sqlgener… 18. http://www.reddit.com/r/haskell/comments/2nxx7n/announcing_opaleye_sqlgener… 19. https://halcyon.sh/ 20. http://www.reddit.com/r/haskell/comments/2ogoot/deploy_any_haskell_applicat… 21. http://www.haskellforall.com/2014/11/how-to-build-library-agnostic-streamin… 22. http://www.reddit.com/r/haskell/comments/2n8xo4/haskell_for_all_how_to_buil… 23. http://jaspervdj.be/posts/2014-11-27-comonads-image-processing.html 24. http://www.reddit.com/r/haskell/comments/2nmf5l/image_processing_with_comon… 25. https://ocharles.org.uk/blog/posts/2014-12-02-view-patterns.html 26. http://www.reddit.com/r/haskell/comments/2o341e/24_days_of_ghc_extensions_v… 27. http://www.well-typed.com/blog/101/ 28. http://www.reddit.com/r/haskell/comments/2nlbei/welltyped_are_hiring/ 29. https://ocharles.org.uk/blog/posts/2014-12-03-pattern-synonyms.html 30. http://www.reddit.com/r/haskell/comments/2o6bq5/24_days_of_ghc_extensions_p… 31. https://hackhands.com/lazy-evaluation-works-haskell/ 32. http://www.reddit.com/r/haskell/comments/2n0991/how_lazy_evaluation_works/ 33. http://williamdemeo.github.io/2014/02/27/learn-you-an-agda/ 34. http://www.reddit.com/r/haskell/comments/2o49aw/learn_you_an_agda/ 35. https://ocharles.org.uk/blog/posts/2014-12-04-record-wildcards.html 36. http://www.reddit.com/r/haskell/comments/2oaj7e/24_days_of_ghc_extensions_r… 37. http://www.slideshare.net/ScottWlaschin/fp-patterns-buildstufflt 38. http://www.reddit.com/r/haskell/comments/2ntyqz/functional_programming_patt… 39. http://thread.gmane.org/gmane.comp.lang.haskell.ghc.devel/7299 40. http://www.reddit.com/r/haskell/comments/2o9fez/the_ghcjs_guys_are_working_… 41. http://www.reddit.com/r/haskell/comments/2mok2d/silk_is_looking_for_a_haske… 42. http://www.reddit.com/r/haskell/comments/2mok2d/silk_is_looking_for_a_haske… 43. http://keera.co.uk/blog/2014/11/24/haskell-android-games-adventure-engine-b… 44. http://www.reddit.com/r/haskell/comments/2nesiz/keera_studios_haskell_games… 45. https://ocharles.org.uk/blog/posts/2014-12-05-bang-patterns.html 46. http://www.reddit.com/r/haskell/comments/2oe1kk/24_days_of_ghc_extensions_b… 47. http://hrothen.github.io/2014/11/19/lets-build-a-browser-engine-in-haskell-… 48. http://www.reddit.com/r/haskell/comments/2mthmd/lets_build_a_browser_engine… 49. https://github.com/sseefried/docker-build-ghc-android 50. http://www.reddit.com/r/haskell/comments/2n5njy/a_docker_build_script_for_g… 51. http://www.reddit.com/r/haskell/comments/2n2sgc/amazonka_comprehensive_amaz… 52. http://www.reddit.com/r/haskell/comments/2n2sgc/amazonka_comprehensive_amaz… 53. https://ocharles.org.uk/blog/guest-posts/2014-12-06-rebindable-syntax.html 54. http://www.reddit.com/r/haskell/comments/2ohm6p/24_days_of_ghc_extensions_r… 55. https://www.haskell.org/communities/11-2014/report.pdf 56. http://www.reddit.com/r/haskell/comments/2mirc9/haskell_communities_and_act… 57. http://nikita-volkov.github.io/a-taste-of-state-parsers-are-easy/ 58. http://www.reddit.com/r/haskell/comments/2mk0ww/a_taste_of_state_parsers_ar… 59. https://www.youtube.com/watch?v=XfINRj5OzGw 60. http://www.reddit.com/r/haskell/comments/2mhzj3/hybrid_serverclient_haskell… 61. http://www.cse.chalmers.se/~bernardy/nix.html 62. http://www.reddit.com/r/haskell/comments/2mv3vo/nix_the_cabal_purgatory_how… 63. http://youtu.be/vYh27zz9530 64. http://www.reddit.com/r/haskell/comments/2nc9w6/ranjit_jhala_talks_about_li… 65. https://github.com/chrisdone/emacs-haskell-config 66. http://www.reddit.com/r/haskell/comments/2oib5x/a_quick_and_easy_emacs_conf… 67. http://stackoverflow.com/questions/27267848/turning-a-mb-into-ma-b 68. http://stackoverflow.com/questions/27067905/when-are-type-signatures-necess… 69. http://stackoverflow.com/questions/27157717/boilerplate-free-annotation-of-… 70. http://stackoverflow.com/questions/27014235/can-fusion-see-through-newtype-… 71. http://stackoverflow.com/questions/27044209/haskell-why-use-proxy 72. http://stackoverflow.com/questions/26952366/is-haskells-laziness-an-elegant… 73. http://stackoverflow.com/questions/27019906/type-inference-interferes-with-… 74. http://stackoverflow.com/questions/27095011/what-exactly-is-the-kind-in-has… 75. http://stackoverflow.com/questions/26996110/whats-the-difference-between-pa… 76. https://plus.google.com/105107667630152149014/about

1 0

Polykinded promoted types
by Gautier DI FOLCO 10 Dec '14

10 Dec '14

Hi all, Is there a way to get a Polykinded promoted type. For example with *Maybe* I get: data *Maybe* a = Just a | Nothing Prelude> :k *Maybe* *Maybe* :: ** -> ** Is it possible to get a *k -> **? If so, how can I do that? Thanks in advance for your help, Regards.

3 8