- Haskell-Cafe - Haskell.org

Categorical description for vector-space instance of AdditiveGroup (Maybe a)
by trevor cook 11 Sep '14

11 Sep '14

Hi Haskell Cafe, In Vector-space-0.8.7, there is the cited inconsistency of the definition of AdditiveGroup (Maybe a). I've posted the source, including comment, below for reference. Following the logic of the counterexample and Conal's (and or Andy Gill's) comment, the problem hinges on the equality, Nothing = Just (zeroV :: a). I'm curious about the catigorical description of this situation. Does this mean that the category of Additive Groups and Maybe Additive Groups are equivalent as categories? I'm not proposing that this is somehow a resolution to vector-space, but rather trying to gain some insight into equivalence, naturalitity, adjunction, etc. My thinking as to why equivalence is that if we have the instance below defined using: zeroV = Just zeroV and making appropriate changes following that new decision. Nothing ^+^ Nothing = zeroV a ^+^ Nothing = a Nothing ^+^ b = b negateV Nothing = zeroV negateV a = fmap negateV then we pretty much defer the group structure of (Maybe a) to be based totally on the structure of a. And since `a` and `Maybe a` are not isomorphic then "hey, I just learned about this new thing so maybe its that." Its a horrible argument, hopefully the cafe will show some better thinking for me. Regards, Trevor -- Maybe is handled like the Maybe-of-Sum monoidinstance AdditiveGroup a => AdditiveGroup (Maybe a) where zeroV = Nothing Nothing ^+^ b' = b' a' ^+^ Nothing = a' Just a' ^+^ Just b' = Just (a' ^+^ b') negateV = fmap negateV{-Alexey Khudyakov wrote: I looked through vector-space package and found lawless instance. Namely Maybe's AdditiveGroup instance It's group so following relation is expected to hold. Otherwise it's not a group. > x ^+^ negateV x == zeroV Here is counterexample: > let x = Just 2 in x ^+^ negateV x == zeroV False I think it's not possible to sensibly define group instance for Maybe a at all.I see that the problem here is in distinguishing 'Just zeroV' fromNothing. I could fix the Just + Just line to use Nothing instead of JustzeroV when a' ^+^ b' == zeroV, although doing so would require Eq a andhence lose some generality. Even so, the abstraction leak would probablyshow up elsewhere.Hm.-}

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Haskell Weekly News: Issue 304
by Daniel Santa Cruz 11 Sep '14

11 Sep '14

Welcome to issue 305 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers from August 31 to September 06, 2014 Quotes of the Week * srhb: segfaults sure are easier to come by doing FFI stuff Top Reddit Stories * Pandoc author working with industry on Markdown spec Domain: standardmarkdown.com, Score: 68, Comments: 7 Original: [1] http://goo.gl/JBte8k On Reddit: [2] http://goo.gl/LWT42a * ICFP 2014 Talks Domain: youtube.com, Score: 56, Comments: 0 Original: [3] http://goo.gl/2zRte4 On Reddit: [4] http://goo.gl/NvWBBa * ICFP 2014 Workshops (Videos) Domain: youtube.com, Score: 53, Comments: 6 Original: [5] http://goo.gl/gpfI8T On Reddit: [6] http://goo.gl/Bj5SwB * Let's Build a Browser Engine in Haskell Domain: hrothen.github.io, Score: 45, Comments: 18 Original: [7] http://goo.gl/FkOM49 On Reddit: [8] http://goo.gl/rXM38x * Open type families are not modular Domain: blog.ezyang.com, Score: 41, Comments: 52 Original: [9] http://goo.gl/XO1bVK On Reddit: [10] http://goo.gl/ROxxUA * IDE for Haste projects Domain: haskell-web.blogspot.com.es, Score: 36, Comments: 5 Original: [11] http://goo.gl/uvjZUC On Reddit: [12] http://goo.gl/3Ra4dt * Real world one-liner examples Domain: github.com, Score: 33, Comments: 7 Original: [13] http://goo.gl/E5wFiW On Reddit: [14] http://goo.gl/9gipjb * Planning Yesod 1.4 Domain: yesodweb.com, Score: 32, Comments: 8 Original: [15] http://goo.gl/7Kul02 On Reddit: [16] http://goo.gl/erCRmZ * Practical Machines: A simple tutorial for the Machines streaming library Domain: statusfailed.com, Score: 32, Comments: 24 Original: [17] http://goo.gl/bmRQot On Reddit: [18] http://goo.gl/0QSdOL * Anecdotes and Advice on Hiring Junior or Summer/Co-Op Students for Haskell Work Domain: self.haskell, Score: 32, Comments: 29 Original: [19] http://goo.gl/M7NOJO On Reddit: [20] http://goo.gl/M7NOJO * Write your compiler passes in Coq for great good! Domain: megacz.com, Score: 30, Comments: 13 Original: [21] http://goo.gl/l7MU6n On Reddit: [22] http://goo.gl/oSqi3Z * IntelliJ IDEA haskell-idea-plugin + Hasklig Domain: self.haskell, Score: 28, Comments: 3 Original: [23] http://goo.gl/cNWkLh On Reddit: [24] http://goo.gl/cNWkLh * Upcast, a declarative cloud infrastructure orchestration tool that leverages Nix Domain: github.com, Score: 27, Comments: 12 Original: [25] http://goo.gl/vPpXcn On Reddit: [26] http://goo.gl/MHj0Le * Ray Marching Distance Fields with Haskell and GLSL Domain: github.com, Score: 27, Comments: 13 Original: [27] http://goo.gl/HZSjFq On Reddit: [28] http://goo.gl/w0IZ0w * A Pretty-Printer that Says What It Means (or, Why Idris's Pretty-Printer is a Functor) Domain: davidchristiansen.dk, Score: 25, Comments: 14 Original: [29] http://goo.gl/vfdISz On Reddit: [30] http://goo.gl/mHU49x * The Ivory Language is an eDSL for safe systems programming. You can think of Ivory as a safer C, embedded in Haskell. Domain: ivorylang.org, Score: 23, Comments: 3 Original: [31] http://goo.gl/xJJ68c On Reddit: [32] http://goo.gl/RLljxg * Pre-published the slide "Writing NetBSD Sound Drivers in Haskell" for Haskell Symposium 2014. #icfp2014 Domain: slideshare.net, Score: 23, Comments: 4 Original: [33] http://goo.gl/gsENVq On Reddit: [34] http://goo.gl/mWH3zH * Haskell 2014 talk: Reflection without Remorse: Revealing a hidden sequence to speed up monadic reflection Domain: youtube.com, Score: 23, Comments: 8 Original: [35] http://goo.gl/Dffhnx On Reddit: [36] http://goo.gl/UFPUu3 * Wadler's Blog: Howard on Curry-Howard Domain: wadler.blogspot.com, Score: 21, Comments: 0 Original: [37] http://goo.gl/Fz90FG On Reddit: [38] http://goo.gl/P5kemv Top StackOverflow Questions * Infinite lazy bitmap votes: 11, answers: 2 Read on SO: [39] http://goo.gl/4zDwFX * Correct terminology for continuations votes: 11, answers: 4 Read on SO: [40] http://goo.gl/QHVRWI * How do I make MonadRandom a Functor? votes: 9, answers: 1 Read on SO: [41] http://goo.gl/vvD7bR * Why does Haskell's foldr NOT stackoverflow while the same Scala implementation does? votes: 9, answers: 4 Read on SO: [42] http://goo.gl/zdxgAM Until next time, [43]+Daniel Santa Cruz References 1. http://standardmarkdown.com/ 2. http://www.reddit.com/r/haskell/comments/2fgtuu/pandoc_author_working_with_… 3. https://www.youtube.com/playlist?list=PL4UWOFngo5DVBqifWX6ZlXJ7idxWaOP69 4. http://www.reddit.com/r/haskell/comments/2f8o3x/icfp_2014_talks/ 5. http://www.youtube.com/playlist?list=PL4UWOFngo5DXuUiMCNumrFhaDfMx54QgV 6. http://www.reddit.com/r/haskell/comments/2f60hd/icfp_2014_workshops_videos/ 7. http://hrothen.github.io/2014/09/05/lets-build-a-browser-engine-in-haskell/ 8. http://www.reddit.com/r/haskell/comments/2flosm/lets_build_a_browser_engine… 9. http://blog.ezyang.com/2014/09/open-type-families-are-not-modular/ 10. http://www.reddit.com/r/haskell/comments/2fi4zc/open_type_families_are_not_… 11. http://haskell-web.blogspot.com.es/2014/09/ide-for-haste-projects.html 12. http://www.reddit.com/r/haskell/comments/2fk91p/ide_for_haste_projects/ 13. https://github.com/sjoerdvisscher/blog/blob/master/2014/2014-09-03%20real%2… 14. http://www.reddit.com/r/haskell/comments/2feen3/real_world_oneliner_example… 15. http://www.yesodweb.com/blog/2014/09/planning-yesod-1-4 16. http://www.reddit.com/r/haskell/comments/2f4kym/planning_yesod_14/ 17. http://statusfailed.com/blog/2014/09/02/practical-machines-in-60-seconds.ht… 18. http://www.reddit.com/r/haskell/comments/2fao00/practical_machines_a_simple… 19. http://www.reddit.com/r/haskell/comments/2fj0v5/anecdotes_and_advice_on_hir… 20. http://www.reddit.com/r/haskell/comments/2fj0v5/anecdotes_and_advice_on_hir… 21. http://www.megacz.com/berkeley/coq-in-ghc/ 22. http://www.reddit.com/r/haskell/comments/2f8bu9/write_your_compiler_passes_… 23. http://www.reddit.com/r/haskell/comments/2fb1j5/intellij_idea_haskellideapl… 24. http://www.reddit.com/r/haskell/comments/2fb1j5/intellij_idea_haskellideapl… 25. https://github.com/zalora/upcast 26. http://www.reddit.com/r/haskell/comments/2f2meq/upcast_a_declarative_cloud_… 27. https://github.com/blitzcode/ray-marching-distance-fields 28. http://www.reddit.com/r/haskell/comments/2ffmys/ray_marching_distance_field… 29. http://www.davidchristiansen.dk/2014/09/06/pretty-printing-idris/ 30. http://www.reddit.com/r/haskell/comments/2fntec/a_prettyprinter_that_says_w… 31. http://ivorylang.org/ 32. http://www.reddit.com/r/haskell/comments/2f6p2h/the_ivory_language_is_an_ed… 33. http://www.slideshare.net/master_q/writing-netbsd-sound-drivers-in-haskell 34. http://www.reddit.com/r/haskell/comments/2fd02w/prepublished_the_slide_writ… 35. https://www.youtube.com/watch?v=_XoI65Rxmss&index=31&list=UUP9g4dLR7xt6KzCY… 36. http://www.reddit.com/r/haskell/comments/2fnj6p/haskell_2014_talk_reflectio… 37. http://wadler.blogspot.com/2014/08/howard-on-curry-howard.html 38. http://www.reddit.com/r/haskell/comments/2f9v8e/wadlers_blog_howard_on_curr… 39. http://stackoverflow.com/questions/25621889/infinite-lazy-bitmap 40. http://stackoverflow.com/questions/25648332/correct-terminology-for-continu… 41. http://stackoverflow.com/questions/25622571/how-do-i-make-monadrandom-a-fun… 42. http://stackoverflow.com/questions/25622959/why-does-haskells-foldr-not-sta… 43. https://plus.google.com/105107667630152149014/about

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Re: [Haskell-cafe] api-tools questions
by Karsten Gebbert 10 Sep '14

10 Sep '14

Chris, thank you for the pointer, that is really helpful indeed! I see that you are using Haskell types for some fields, rather than types defined in your API. When I, for instance, try to use Map as field type, with String as both key and value type, I get an error: myrec :: MyRec // a map = record myfield :: Map String String Exception when trying to run compile-time code: Syntax error at line 345, column 20 (@8596): [(AlexPn 8596 345 20,TypeIden "String"),(AlexPn 8603 345 27,TypeIden "String")] I suspect this is only the case with types that are parameterized over others, as a type synonym seems to make this possible anyways. Is there a better way to deal with this than the one I mentioned? Thanks for your help, karsten Chris Dornan <chris(a)chrisdornan.com> writes: > You might also find the keystore[0] package useful as it makes use of > This technique in several places. See, for example the definition of > Pattern in the `Data.Keystore.Types.Schema`[1] and > `Data.Keystore.Types`[2]. > > Chris > > [0] http://hackage.haskell.org/package/keystore > [1] > https://github.com/cdornan/keystore/blob/master/src/Data/KeyStore/Types/Sch > ema.hs#L179 > [2] > https://github.com/cdornan/keystore/blob/master/src/Data/KeyStore/Types.hs# > L68 > [3] http://hackage.haskell.org/package/keystore-0.5.0.4 > > > On 09/09/2014 17:20, "Karsten Gebbert" <k(a)ioctl.it> wrote: > >>Excellent, thanks a lot Adam. I'll open a PR when I'm through with this >>project :) >> >>Adam Gundry <adam(a)well-typed.com> writes: >> >>> Hi Karsten, >>> >>> This isn't very well documented, but there is a (hidden) feature of >>> api-tools that should do what you want. If you say something like >>> >>> mt :: MyTime = basic string with inj_MyTime, prj_MyTime >>> >>> then you can define your own type MyTime and give conversion functions >>> inj_MyTime and prj_MyTime to convert back and forth from a >>> newtype-wrapped Text. (In fact, you could probably use UTCTime as >>> MyTime...). More precisely, api-tools will generate something like >>> >>> newtype REP__MyTime = REP__MyTime Text >>> >>> and you will need to implement >>> >>> inj_MyTime :: REP__MyTime -> ParserWithErrs MyTime >>> prj_MyTime :: MyTime -> REP__MyTime >>> >>> I hope this helps, and further questions or documentation contributions >>> are very welcome! >>> >>> Cheers, >>> >>> Adam >>> >>> >>> On 09/09/14 14:02, Karsten Gebbert wrote: >>>> Hi All, >>>> >>>> I have a question concerning the api-tools package. >>>> >>>> Its not clear from the tests, sources or the tutorial how I can use >>>> other date/time formats that the default `utc` type can handle. I'm >>>> trying to wrap a JSON API that is not under my control, so I have to >>>> adhere to whatever I get back from it. Could anybody with experience >>>> with the package point me to some example, relevant bits in the sources >>>> or a tip how to do it? >>>> >>>> I'm planning to create some more documentation around the package to >>>> contribute back once I figured out a few more details, because I think >>>> its quite a useful abstraction when dealing with (foreign) APIs IMO. >>>> >>>> Thanks already for any hints, >>>> >>>> karsten >>> >>> >>> -- >>> Adam Gundry, Haskell Consultant >>> Well-Typed LLP, http://www.well-typed.com/ >>_______________________________________________ >>Haskell-Cafe mailing list >>Haskell-Cafe(a)haskell.org >>http://www.haskell.org/mailman/listinfo/haskell-cafe

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Use Haskell shared library with foreign exports with dlopen/dlsym
by Tobias Neumann 10 Sep '14

10 Sep '14

Hello Cafe, I am trying to use a Haskell shared library with foreign exports from Haskell again via dlopen/dlsym. Sadly it segfaults, and the segfaults happen on dlclose during garbage collection points (as figured out by monochrom in #haskell). So right now I can only open a library once and may not dlclose it. Can someone point me to a mistake I made, or is this rather a ghc (7.8.3) bug? Please see attached minimal example. Regards, Tobias test.hs: module Main where import qualified System.Posix.DynamicLinker as DL import Foreign foreign import ccall "dynamic" mkTest :: FunPtr Int -> Int main = do DL.withDL ("./libtest.so") [DL.RTLD_NOW] $ \dl -> do dimPtr <- DL.dlsym dl "test" let test = mkTest dimPtr print test libtest.hs: module Test() where import Foreign foreign export ccall test :: Int test :: Int test = 124 build with: ghc --make -shared -dynamic -fPIC libtest.hs -o libtest.so ghc --make -dynamic test.hs

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Re: [Haskell-cafe] Categorical description for vector-space instance of AdditiveGroup (Maybe a)
by trevor cook 10 Sep '14

10 Sep '14

Here's that vector-space code, formatted better. -- Maybe is handled like the Maybe-of-Sum monoid instance AdditiveGroup a => AdditiveGroup (Maybe a) where zeroV = Nothing Nothing ^+^ b' = b' a' ^+^ Nothing = a' Just a' ^+^ Just b' = Just (a' ^+^ b') negateV = fmap negateV {- Alexey Khudyakov wrote: I looked through vector-space package and found lawless instance. Namely Maybe's AdditiveGroup instance It's group so following relation is expected to hold. Otherwise it's not a group. > x ^+^ negateV x == zeroV Here is counterexample: > let x = Just 2 in x ^+^ negateV x == zeroV False I think it's not possible to sensibly define group instance for Maybe a at all.I see that the problem here is in distinguishing 'Just zeroV' fromNothing. I could fix the Just + Just line to use Nothing instead of JustzeroV when a' ^+^ b' == zeroV, although doing so would require Eq a andhence lose some generality. Even so, the abstraction leak would probablyshow up elsewhere.Hm. -}

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cabal-install fails (osx 10.10)
by Michael Trotter 10 Sep '14

10 Sep '14

I'm having trouble getting cabal-install to work. I used homebrew (brew install cabal-install), which successfully installed the ghc dependency and then attempted to install cabal-install. The problem looks like it's either with my ghc install or maybe an issue with cabal-install and osx 10.10. This is my first haskell experience though, so I'm not really sure how to proceed. Here's the console output: setup: Error: Could not find module: Distribution.Compat.CreatePipe with any suffix: ["dyn_hi"] in the search path: ["dist/build"] cabal: Error: some packages failed to install: Cabal-1.20.0.2 failed during the final install step. The exception was: ExitFailure 1 cabal-install-1.20.0.3 depends on Cabal-1.20.0.2 which failed to install.

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LinkedIn job ad link for openings for several enthusiastic great Haskell developers (geoloc. globally) at Swedish spin-off Functor Group with subsidiaries + three Scala developer (geoloc. Sweden) with spear-edge / very strong qualifications and high-end industry salary for Functor Consulting AB
by Johan Glimming 10 Sep '14

10 Sep '14

Dear Haskell developers, type theory developers, Agda/Epigram/Idris/Coq developers/enthusiasts abound in this community, We are searching the planet for great Haskell and Scala programmers right now for a really interesting spin-off now growing and flourishing after 2.5 years since it was founded. If you are a functional programmer this might be the most exciting job around with e.g. a very exciting customer "as close as a space station as you can get", and industrially entirely unique heavily research-based technology that we have developed now for several years. » https://www.linkedin.com/jobs2/view/19207355 You would help us develop our products further, the new constructive programming paradigm based on type theory carrying languages including domain-specific languages as never done before in the software industry … ( And yes, we are entirely based on (Martin-Löf) type theory, that's what we are all about. And we use Haskell for our development internally almost entirely, with some Ocaml etc on top ... ) We look for three experienced Scala developers (for being based in Sweden, Stockholm-area) at Functor Group AB, Functor AB and Functor Consulting AB. These developers are recruited into Functor Consulting AB which is to become the spear-edge partner to Typesafe in Scandinavia, specifically Sweden, while interacting with our other two teams at Functor AB to a certain extent, focusing on DSL development, DDD and more. We bring type theory to the industry in the new constructive programming paradigm and with a new static analysis technique combined with testing and dependent types, all based on the same core technology … The three Scala developers, for extreme FP consultancy, are offered very competitive Swedish salary, plus added potential for also other benefits via Functor Group AB and the product company Functor AB. Experience from Scala development and documented skills with Scala and related technologies (such as Java) is must be considerable to be considered for these three positions. If you are not based in Sweden, we would assist as much as possible in moving to Stockholm/Uppsala, Sweden, where these three Scala spear-edge developers will be based, with a consultancy manager of considerable experience handling administration and co-ordination in what we expect to be an otherwise quite self-organised business targeting the hardest and most challenging projects at any given time. You can apply on LinkedIn — or, if you prefer, email your resumé, references, credentials, and cover letter to jobs(a)functorgroup.com. » https://www.linkedin.com/jobs2/view/19207355. Best wishes, Johan Glimming — Company website: http://www.functorgroup.com Twitter feeds to follow: @FunctorGroup @Functors @FunctorTech

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Persistent Journal/Event queue Implementation
by Arnaud Bailly 10 Sep '14

10 Sep '14

Hello Cafe, I am looking for a Haskell implementation (or library binding) to some persistent journal/event queue software, something like Apache Kafka. My google-fu is lacking so I was not able to find anything but I might have been doing it wrong. Thanks, -- Arnaud Bailly FoldLabs Associate: http://foldlabs.com

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Re: [Haskell-cafe] FFI question
by Vasili I. Galchin 10 Sep '14

10 Sep '14

On Tue, Sep 9, 2014 at 10:36 PM, Vasili I. Galchin <vigalchin(a)gmail.com> wrote: > Adam, > > Did a find/grep under my posix-realtime directory .. no > definition of MCL_CURRENT, MCL_FUTURE or MCL_CURRENTandFUTURE!! But If > I comment out the case arm MCL_CURRENTandFUTURE in my LockMem.hsc file > and run "cabal build" from the directory where my *.cabal file lives > .. clean compile ... Sigh!??!! > > Vasya > > On Mon, Sep 8, 2014 at 7:36 AM, adam vogt <vogt.adam(a)gmail.com> wrote: >> Hi Vasili >> >> Does the sys/mman.h or another included header actually define >> MCL_CURRENTandFUTURE? >> >> Regards, >> Adam >> >> On Sep 8, 2014 4:45 AM, "Vasili I. Galchin" <vigalchin(a)gmail.com> wrote: >>> >>> Following is a fragment generated by hsc2hs from LockedMem.hsc: >>> >>> #line 93 "LockedMem.hsc" >>> hsc_const (MCL_CURRENTandFUTURE); >>> hsc_fputs (")\n" >>> "", hsc_stdout()); >>> hsc_line (94, "LockedMem.hsc"); >>> hsc_fputs ("\n" >>> "foreign import ccall unsafe \"sys/mman.h mlockall\"\n" >>> " c_mlockall :: CInt -> IO CInt\n" >>> "\n" >>> "\n" >>> "-- | unlock all mapped pages of a process!\n" >>> "unlockAllMemory :: IO () \n" >>> "unlockAllMemory = do\n" >>> " throwErrnoIfMinus1 \"unlockAllMemory\" (c_munlockall)\n" >>> " return ()\n" >>> "\n" >>> "foreign import ccall unsafe \"sys/mman.h munlockall\"\n" >>> " c_munlockall :: IO CInt\n" >>> "", hsc_stdout()); >>> return 0; >>> >>> On Mon, Sep 8, 2014 at 2:11 AM, Vasili I. Galchin <vigalchin(a)gmail.com> >>> wrote: >>> > Hello, >>> > >>> > I need memory refreshing about FFI because I need to fix a bug >>> > (plus I got down a rat hole because of Ukraine war :-(). Following a >>> > snippet of something I wrote: >>> > >>> > -- | lock all of the memory space of a process >>> > lockAllMemory :: LockAllFlags -> IO () >>> > lockAllMemory flags = do >>> > throwErrnoIfMinus1 "lockAllMemory" (c_mlockall cflags) >>> > return () >>> > where >>> > cflags = case flags of >>> > CURRENT -> (#const MCL_CURRENT) >>> > FUTURE -> (#const MCL_FUTURE) >>> > CURRENTandFUTURE -> (#const MCL_CURRENTandFUTURE) >>> > >>> > >>> > it is a file called LockedMem.hsc and the third arm CURRENTandFUTURE I >>> > just added to fix the this bug .. >>> > >>> > MCL_CURRENTandFUTURE ends up in LockedMem_make.c(sp??) but I get a >>> > undeclared nasty message when I try to build LockedMem.hsc... my >>> > bad... . Please help so I can go back to help Ukraine. >>> > >>> > Kind regarsd, >>> > >>> > Vasya >>> _______________________________________________ >>> Haskell-Cafe mailing list >>> Haskell-Cafe(a)haskell.org >>> http://www.haskell.org/mailman/listinfo/haskell-cafe

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2014 haskell cabal update hangs on mac
by Madhu Babu 09 Sep '14

09 Sep '14

I initially installed haskell platform ( 2013 version; 7.6.3 ghc ) on my mac. Everything was working great. Just now saw the haskell platform website again and found new version was released ( Haskell Platform 2014.2.0.0 for Mac OS X, 64bit ). I installed it, and un-installed the older version using uninstall-hs. Now when i type "cabal" or "cabal update" on my terminal, it hangs. Actually when i look into Activity Monitor, i can see that it is invoking some "sh script & possibly some find command" infinitely. I initially guess may be it is building some indexes. but it has been running for an hr or so. I have Xcode 5, gcc and other command line tools installed properly Please advice. I cannot install any other package using cabal.

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