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To "cabal install cabal-install" on Windows ...
by KC 23 Nov '12

23 Nov '12

- completely remove the earlier Haskell Platform - reboot - defragment the partition/hard drive - reboot - install the newer Haskell Platform - reboot - defragment the partition/hard drive (recommended) - reboot - cabal update - cabal install cabal-install It seems like what is slowing down the install process (or hanging it) is the thrashing of your hard drive to find all the little file fragments it needs during the install Casey -- -- Regards, KC

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Call for Seattle Area Haskell Users Group
by Eli Frey 22 Nov '12

22 Nov '12

Greetings, (x-posted from /r/haskell) I am calling for any interested parties to come together and help me get a SEA HUG going. My boss has offered to provide facilities and foods. I don't think he's expecting much, maybe we can make him second guess the offer :P. I have put together a meetup <http://www.meetup.com/SEAHUG> group. I don't know what all Seattle area Haskellers are wanting from this kind of thing, so I thought our first meeting would be low key. Some time to shoot the bull and introduce ourselves. Some time to help each other on our stumbling blocks. Then talking about what we would want from further meetups. We'll need a central place for discussion and coordination, so aside from voicing your support, it would be great if you could take further commentary over to the meetup page. If that doesn't work for some, let me know. Hope to hear from y'all soon. - Eli

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QuickCheck Generators
by graham＠fatlazycat.com 22 Nov '12

22 Nov '12

I have data Tweet = Tweet { user :: String, text :: String, retweets :: Double } deriving (Show) data TweetSet = NoTweets | SomeTweets Tweet TweetSet TweetSet and trying to create some generators for testing, with instance Arbitrary Tweet where arbitrary = liftM3 Tweet arbitrary arbitrary arbitrary instance Arbitrary TweetSet where arbitrary = sized set' where set' 0 = return NoTweets set' n | n>0 = oneof[return NoTweets, liftM3 SomeTweets arbitrary subTweets subTweets] where subTweets = set' (n `div` 2) but wondering how I would go about generating a random TweetSet that contains a known random Tweet I later have reference to and I would also assume the known Tweet to be placed randomly. Then I could test a contains function. Thanks

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fixed-length bit-vectors
by Greg Fitzgerald 22 Nov '12

22 Nov '12

Hi all, My goal, eliminate the failure case in 'byte': https://gist.github.com/4128503 I don't want my 'byte' function to fail at runtime or return $ Left "vector not 8 bits". I want it to return a Word8 for an 8-bit bit-vector or not compile. Is there an existing library that offers fixed-length vectors as a thin veneer over the 'vector' package? I see 'vector-static', but it is unmaintained and hasn't been touched in years. Alternatively, there's 'VecN', but having already learned 'vector', I'd like to know if there is an existing solution that uses it. Thanks, Greg

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naming and packaging things
by Rico Moorman 22 Nov '12

22 Nov '12

Hi everyone, As anyone following this mailinglist surely noticed, there are a few issues regarding haskell packaging. Lot of discussion is going on about how cabal contributes to the problems and how things could be fixed by making our tools smarter (which is a great thing to do IMHO). I think though, that there might be other ways to deal with or at least reduce the problems. One of those ways would be expressing the influence of changes in a package's public API, using the name of a package instead of just version numbers. I mean, in case a major backwards incompatible/breaking change happens, one would change the name of the package/namespaces a.s.o. And with breaking I really mean that kind of "oh my ... now I have to rewrite this part of my application because of this". Simple additions to the API do not count as breaking because existing functionality is left alone. This way, incompatible versions of the "same" library can coexist without any problems right within the same program if one chose to do so even allowing for improved transition processes for larger application I presume. It would easily be possible to safely install newer versions of the same package without breaking things. There are more open source libraries/projects as it seems, which use this approach (version number in the package name) such as the kdelibs, gtk, beautifulsoup, apache and bind. One example: take an application which needs two libraries A and B which both depend on library C. Now lets assume that A uses a slightly older version of C and B uses a version with a major incompatibility. In the current versioning scheme, there could be major problems causing development delays or custom forks to emerge a.s.o. But following the other naming method, in case library C chooses to introduce a major breaking change in the public API, a new package C2 is born, which can be used alongside C (and effectively is a different library anyway I think). The choice for the new version of C (C2) is explicit within the name and all related things. I am aware of the fact that this would take more forethought on library design, but I do not think that this alone would be a bad idea anyway. Also it would be difficult to enforce this, but well, a good maintainer would be aware of this anyway, taking care of not introducing breaking changes to the public API I guess. Furthermore we would end up with more packages with different names (but probably the same idea behind it) which would be on hackage. Likely it would be nice to add some grouping or tagging features which would bind them together for everyone browsing the packages. But I think this is probably not a real problem but more a cosmetic one. I really would like to see this discussed here. What would be the good and the bad of promoting this kind of versioning. I am looking forward to hear what you think. Best regards, Rico Moorman

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Haskell Weekly News: Issue 251
by Daniel Santa Cruz 22 Nov '12

22 Nov '12

Welcome to issue 251 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of November 4 to 17, 2012. Quotes of the Week * Ralith: [why some people don't use conduits/pipes] also, I think at least some people are waiting for edwardk to write his version. * edwardk: f -| g means that f a -> b is isomorphic to a -> g b (for arrows in the appropriate categories) * edwardk: @type (^.) lambdabot: s -> Getting a s t a b -> a byorgey: I would not like to be getting a stab, thank you * edwardk: I mean we're so oop even our member accessors have diagrams explaining their member accessors. * shapr: hylo! That's the category theorist greeting. * shachaf: type AbstractAlgebra a = Abstract a -> a * nicoo: Yet many Writers concern themselves with issues of State, imaginary or real * rwbarton: when you learn C, watch out for the keyword "return", it doesn't do what you'd expect * Oleg: We show how to program with the law of excluded middle. We specifically avoid call/cc, which is overrated. Top Reddit Stories * Haskell Platform 2012.4.0.0 is out Domain: self.haskell, Score: 89, Comments: 21 On Reddit: [1] http://goo.gl/BPno4 Original: [2] http://goo.gl/BPno4 * A Khan Academy style tutorial on generalized algebraic data types Domain: apfelmus.nfshost.com, Score: 56, Comments: 7 On Reddit: [3] http://goo.gl/kXQ0a Original: [4] http://goo.gl/NWn3B * Cartesian Closed Comic #17: Typing Domain: ro-che.info, Score: 53, Comments: 24 On Reddit: [5] http://goo.gl/6c4Uy Original: [6] http://goo.gl/dZeDx * Cartesian Closed Comic #18: Equality Domain: ro-che.info, Score: 53, Comments: 8 On Reddit: [7] http://goo.gl/8Qf9d Original: [8] http://goo.gl/XCyuB * Natural Language Processing for the Working Programmer (in Haskell!) Domain: nlpwp.org, Score: 50, Comments: 17 On Reddit: [9] http://goo.gl/3yLTZ Original: [10] http://goo.gl/BleWX * Waiting for garbage collection can kill parallelism? Domain: ics.p.lodz.pl, Score: 50, Comments: 7 On Reddit: [11] http://goo.gl/iUPlD Original: [12] http://goo.gl/N0kiV * foldr is made of monoids Domain: byorgey.wordpress.com, Score: 47, Comments: 12 On Reddit: [13] http://goo.gl/Y0mz5 Original: [14] http://goo.gl/IxKov * Solving Cabal Hell: vetted packages, multiple hackages Domain: yesodweb.com, Score: 46, Comments: 67 On Reddit: [15] http://goo.gl/dkzMz Original: [16] http://goo.gl/lDjuj * Parallel falling sand game with Repa Domain: github.com, Score: 45, Comments: 16 On Reddit: [17] http://goo.gl/G6MGG Original: [18] http://goo.gl/cP1zH * Agda, Epigram or Idris. Which one to learn Domain: self.haskell, Score: 38, Comments: 61 On Reddit: [19] http://goo.gl/3PGVo Original: [20] http://goo.gl/3PGVo * Fun with Fay - A ring oscillator Domain: skybluetrades.net, Score: 38, Comments: 6 On Reddit: [21] http://goo.gl/mGvcZ Original: [22] http://goo.gl/bJxCI * Introduction to Category Theory 1: Course Overview Domain: youtube.com, Score: 37, Comments: 6 On Reddit: [23] http://goo.gl/fbfYR Original: [24] http://goo.gl/Nx5SH * HALO: Haskell to Logic through Denotational Semantics [PDF] Domain: research.microsoft.com, Score: 35, Comments: 6 On Reddit: [25] http://goo.gl/V0RWs Original: [26] http://goo.gl/oCzUw * How to write Python in Haskell: State and Either Domain: stackoverflow.com, Score: 34, Comments: 25 On Reddit: [27] http://goo.gl/Z80yb Original: [28] http://goo.gl/W933i * What is foldr made of? Domain: web.jaguarpaw.co.uk, Score: 30, Comments: 22 On Reddit: [29] http://goo.gl/LdS9X Original: [30] http://goo.gl/I2uzX * How to shoot yourself in the foot with Haskell Domain: ics.p.lodz.pl, Score: 29, Comments: 34 On Reddit: [31] http://goo.gl/CcvZB Original: [32] http://goo.gl/SJYvd * Happstack, Fay, & Acid-State: Shared Datatypes are Awesome Domain: happstack.com, Score: 29, Comments: 8 On Reddit: [33] http://goo.gl/AUtj6 Original: [34] http://goo.gl/cYQr1 * ANN: OpenGL packages update Domain: haskell.org, Score: 28, Comments: 7 On Reddit: [35] http://goo.gl/0SN7O Original: [36] http://goo.gl/PbtlT * How can I cleanly/consistently work with 3 different Exception-raising schemes? Domain: self.haskell, Score: 28, Comments: 22 On Reddit: [37] http://goo.gl/Xf4sq Original: [38] http://goo.gl/Xf4sq Top StackOverflow Questions * What are paramorphisms? votes: 39, answers: 1 Read on SO: [39] http://goo.gl/V6rXq * In pure functional languages, is there an algorithm to get the inverse function? votes: 35, answers: 9 Read on SO: [40] http://goo.gl/wBtqv * What are free monads? votes: 26, answers: 5 Read on SO: [41] http://goo.gl/NAx8n * What's the status of current Functional Reactive Programming implementations? votes: 23, answers: 3 Read on SO: [42] http://goo.gl/FmRYF * What does the “world” mean in functional programming world? votes: 22, answers: 7 Read on SO: [43] http://goo.gl/dyM8c * Type-safe difference lists votes: 20, answers: 1 Read on SO: [44] http://goo.gl/TlvF4 * Is it possible to annotate a function's special properties (e.g. surjectivity)? votes: 12, answers: 3 Read on SO: [45] http://goo.gl/uzanF * Under what circumstances are monadic computations tail-recursive? votes: 12, answers: 2 Read on SO: [46] http://goo.gl/lPJef * Should I use new Haskell language extensions in favour of the old ones? votes: 12, answers: 2 Read on SO: [47] http://goo.gl/xkc78 Until next time, Daniel Santa Cruz References 1. http://www.reddit.com/r/haskell/comments/12pxiq/haskell_platform_2012400_is… 2. http://www.reddit.com/r/haskell/comments/12pxiq/haskell_platform_2012400_is… 3. http://apfelmus.nfshost.com/blog/2010/06/01-gadts-video.html 4. http://www.reddit.com/r/haskell/comments/12ou1p/a_khan_academy_style_tutori… 5. http://ro-che.info/ccc/17.html 6. http://www.reddit.com/r/haskell/comments/12nm3w/cartesian_closed_comic_17_t… 7. http://ro-che.info/ccc/18.html 8. http://www.reddit.com/r/haskell/comments/138crz/cartesian_closed_comic_18_e… 9. http://nlpwp.org/book/ 10. http://www.reddit.com/r/haskell/comments/13a0ut/natural_language_processing… 11. http://ics.p.lodz.pl/~stolarek/blog/2012/11/waiting-for-garbage-collection-… 12. http://www.reddit.com/r/haskell/comments/13cpm3/waiting_for_garbage_collect… 13. http://byorgey.wordpress.com/2012/11/05/foldr-is-made-of-monoids/ 14. http://www.reddit.com/r/haskell/comments/12prpr/foldr_is_made_of_monoids/ 15. http://www.yesodweb.com/blog/2012/11/solving-cabal-hell 16. http://www.reddit.com/r/haskell/comments/1306wn/solving_cabal_hell_vetted_p… 17. https://github.com/tranma/falling-turnip 18. http://www.reddit.com/r/haskell/comments/133nja/parallel_falling_sand_game_… 19. http://www.reddit.com/r/haskell/comments/132kg0/agda_epigram_or_idris_which… 20. http://www.reddit.com/r/haskell/comments/132kg0/agda_epigram_or_idris_which… 21. http://www.skybluetrades.net/blog/posts/2012/11/13/fay-ring-oscillator/inde… 22. http://www.reddit.com/r/haskell/comments/134tr6/fun_with_fay_a_ring_oscilla… 23. http://www.youtube.com/watch?v=eu0rj5C2Otg 24. http://www.reddit.com/r/haskell/comments/12vvn2/introduction_to_category_th… 25. http://research.microsoft.com/en-us/people/dimitris/hcc-popl.pdf 26. http://www.reddit.com/r/haskell/comments/12wovt/halo_haskell_to_logic_throu… 27. http://stackoverflow.com/a/13432926/208257 28. http://www.reddit.com/r/haskell/comments/13cz6b/how_to_write_python_in_hask… 29. http://web.jaguarpaw.co.uk/~tom/blog/posts/2012-11-04-what-is-foldr-made-of… 30. http://www.reddit.com/r/haskell/comments/12mnhu/what_is_foldr_made_of/ 31. http://ics.p.lodz.pl/~stolarek/blog/2012/11/how-to-shoot-yourself-in-the-fo… 32. http://www.reddit.com/r/haskell/comments/12rlgt/how_to_shoot_yourself_in_th… 33. http://www.happstack.com/c/view-page-slug/15/happstack-fay-acid-state-share… 34. http://www.reddit.com/r/haskell/comments/1375q0/happstack_fay_acidstate_sha… 35. http://www.haskell.org/pipermail/haskell-cafe/2012-November/104329.html 36. http://www.reddit.com/r/haskell/comments/12mk7h/ann_opengl_packages_update/ 37. http://www.reddit.com/r/haskell/comments/12relk/how_can_i_cleanlyconsistent… 38. http://www.reddit.com/r/haskell/comments/12relk/how_can_i_cleanlyconsistent… 39. http://stackoverflow.com/questions/13317242/what-are-paramorphisms 40. http://stackoverflow.com/questions/13404208/in-pure-functional-languages-is… 41. http://stackoverflow.com/questions/13352205/what-are-free-monads 42. http://stackoverflow.com/questions/13341937/whats-the-status-of-current-fun… 43. http://stackoverflow.com/questions/13340458/what-does-the-world-mean-in-fun… 44. http://stackoverflow.com/questions/13346200/type-safe-difference-lists 45. http://stackoverflow.com/questions/13281316/is-it-possible-to-annotate-a-fu… 46. http://stackoverflow.com/questions/13379060/under-what-circumstances-are-mo… 47. http://stackoverflow.com/questions/13422610/should-i-use-new-haskell-langua…

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Common function names for '(.) . (.)', '(.) . (.) . (.)' ...?
by Thomas Bereknyei 21 Nov '12

21 Nov '12

I use .@. (sometimes ...) .#. .$. for the two three and four dot variations.

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Re: [Haskell-cafe] [Haskell] [ANN] gtk2hs-0.12.4
by Roman Cheplyaka 21 Nov '12

21 Nov '12

* wagnerdm(a)seas.upenn.edu <wagnerdm(a)seas.upenn.edu> [2012-11-21 13:48:35-0500] > Today, we welcome into the world version 0.12.4 of Gtk2Hs[1], a set > of Haskell bindings to many of the libraries included in the > Gtk+/Gnome platform. Gtk+ is an extensive and mature multi-platform > toolkit for creating graphical user interfaces. Great — thanks to you and John for keeping it alive. Roman

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AST Rewriting
by Steve Severance 21 Nov '12

21 Nov '12

Hi Everyone, I am trying to build a function to rewrite and AST. I have and AST which is designed to represent a computation graph. I will present a simplified version here designed to illustrate the problem. I have tried numerous ways of rewriting it including uniplate, recursion and Edward Kmett's implementation of plate in his lens package. My AST is defined using GADTs as follows: class (ReflectDescriptor a, Typeable a, Wire a) => ProtoBuf a data Expression a b where OpenTable :: (ProtoBuf b) => Int -> Table -> Expression () b OpenFile :: (ProtoBuf b) => Int -> String -> Expression () b WriteFile :: (Typeable a, ProtoBuf b) => Int -> String -> Expression a b -> Expression b () WriteTable :: (Typeable a, ProtoBuf b) => Int -> Table -> Expression a b -> Expression b () Map :: (ProtoBuf a, ProtoBuf b, ProtoBuf c) => Int -> (a -> b) -> Expression c a -> Expression a b LocalMerge :: (ProtoBuf a) => Int -> [Expression c a] -> Expression c a The user can create code inside a Monad Transformer like so: q <- query $ do table <- openTable myTable transform <- map someFunc table writeTable otherTable transform As part of this language the compiler I am building would need to for instance transform OpenTable into a series OpenFile nodes with a LocalMerge to merge the results together. So uniplate cannot work over GADTs if I recall correctly. I exchanged emails with Edward and he explained that for the lens case I would need something like an indexed lens family from his indexed package which is not implemented yet but which may be in the future. The issue with recursion is that as you recurse through the AST the a b on the Expression change and GHC cannot compile it because it wants the a b to be the same on each recursive call. My question to the Haskell community is how might one develop AST rewriting functionality. One possible solution is stripping the types away from GHC and doing all the type checking myself. That doesn't seem very good. Another possibility that I have looked at was using hoopl. It seems very compatible given that it is built for describing and optimizing data flow which I am doing however the learning curve looks quite steep. I have been reluctant so far to invest the time in it. Has anyone developed something similar? What recommendations do you have? Thanks. Steve

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Common function names for '(.) . (.)', '(.) . (.) . (.)' ...?
by Daniel Trstenjak 21 Nov '12

21 Nov '12

Greetings, Daniel

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