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Re: [Haskell-cafe] Installing ihaskell on nixos
by info＠rotnetix.com 09 Jan '15

09 Jan '15

Thank you for the tip. I did that but then the ihaskell notebook failed to actually load haskell. so I added the following to the config file: c = get_config() c.KernelManager.kernel_cmd = [exe, 'kernel', '{connection_file}'] c.Session.key = b'' c.Session.keyfile = b'' # Syntax highlight properly in Haskell notebooks. c.NbConvertBase.default_language = "haskell" # Where to look for templates. template_path = "/".join(__file__.split("/")[:-1] + ["templates"]) c.TemplateExporter.template_path = [template_path] And every time I run the ihaskell notebook / console it also adds: exe = '/nix/store/hlkvkavkgm3hj08ffdlziq30r7zyr060-haskell-ihaskell-ghc7.8.3-0.4.3.0-shared/bin/IHaskell'.replace(' ', '\\ ') So I now have a whole list of these at the top. However now the kernel keeps crashing with : 2015-01-09 03:47:51.673 [NotebookApp] KernelRestarter: restarting kernel (1/5) WARNING:root:kernel 0d8057d4-b51e-4acc-a262-ceee9a84edee restarted IHaskell: Failed to load interface for ‘IHaskell.Display’ Use -v to see a list of the files searched for. Failed to load interface for ‘IHaskell.Display’ Use -v to see a list of the files searched for. Failed to load interface for ‘IHaskell.IPython.Stdin’ Use -v to see a list of the files searched for. Any ideas? On Thursday, January 8, 2015 at 8:09:03 PM UTC+11, Utku Demir wrote: > > I reproduced the issue, had ~/.ipython/profile_haskell directory, but not > ipython_config.py inside it after installation. > > I just created an empty file with `touch > ~/.ipython/profile_haskell/ipython_config.py` and then iHaskell started > successfully. > > On Thu Jan 08 2015 at 10:07:36 EET <in...(a)rotnetix.com <javascript:>> > wrote: > >> I am trying to set up ihaskell on EC2. I start with the NixOs EC2 >> instance (from http://nixos.org/nixos/download.html) and then basically >> install the ihaskell package using nix-env -i ihaskell. The installation >> runs fine but when I try to start the notebook with ihaskell notebook i get >> the following: >> >> Updating IPython profile. >> Loading profile from >> /nix/store/hlkvkavkgm3hj08ffdlziq30r7zyr060-haskell-ihaskell-ghc7.8.3-0.4.3.0-shared/share/x86_64-linux-ghc-7.8.3/ihaskell-0.4.3.0/profile/profile.tar >> IHaskell: >> Ran commands: >> mkdir -p /root/.ihaskell >> mkdir -p /root/.ihaskell/notebooks >> mkdir -p /root/.ihaskell >> /nix/store/hnmgn0jrpzlp5d91m5xawpr2iqsjxh7j-ipython-2.3.1/bin/ipython >> locate profile haskell >> which >> /nix/store/hnmgn0jrpzlp5d91m5xawpr2iqsjxh7j-ipython-2.3.1/bin/ipython >> /nix/store/hnmgn0jrpzlp5d91m5xawpr2iqsjxh7j-ipython-2.3.1/bin/ipython >> locate profile haskell >> which >> /nix/store/hnmgn0jrpzlp5d91m5xawpr2iqsjxh7j-ipython-2.3.1/bin/ipython >> >> Exception: /root/.ipython/profile_haskell/ipython_config.py: openFile: >> does not exist (No such file or directory) >> >> I poked around a bit and the root/.ipython/profile_haskell/ contains none >> of the expected profile files... Could not find anything by googling >> around except the suggestion to run the ihaskell setup command which seems >> to have been discontinued >> >> Can anyone suggest what I could try? >> >> Thanks >> Riaan >> >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskel...(a)haskell.org <javascript:> >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >

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Re: [Haskell-cafe] typeclass woes...how to constain a typeclass to be "closed" under an operation....
by Nicholls, Mark 08 Jan '15

08 Jan '15

Reading around this more i think the extreme case (of what im interested in) is type arithemetic, ie the set of type "inside" the container is not necessarily finite in general, but yes, i see how you construct works. So in think for my specific purposes the associated type route is the first place for me to investigate Excuse the spelling, sent from a phone with itty bitty keys, it like trying to sow a button on a shirt with a sausage. On 7 Jan 2015, at 20:58, Akash Ganesan <akaberto(a)gmail.com<mailto:akaberto@gmail.com>> wrote: If I understand the issue at hand, you want the container, which is basically a collection of types to be closed under our operation. Since we are dealing with set of different typed objects, which is to be closed under a given operation, why not create a new type that captures the essence of the set (ie., a new data type)? Something like the following. data S = SInt Int | SChar Char | STuple (Char, Char) deriving (Show, Eq) class MyFoo a where op :: a -> a And the instance goes something like this. instance MyFoo S where op (SInt x) = SInt $ toEnum x op (SChar x) = STuple (x,x) op (STuple (x,y)) = SInt $ fromEnum x On Tuesday, 6 January 2015 23:13:59 UTC+5:30, Nicholls, Mark wrote: This is reposted from the beginnners….I’ve not done Haskell for a while and was struggling to get anything to work, I’ve hopefully refound my feet. Its quite common in maths to have operations in a theory that are (set) closed, i just want to translate that notion to a typeclass I have a suggestion that does work class Foo m where op :: m a -> m (S a) That is closed, but now were working on types of kind • -> • (S can be a type family or a data type…lets say it’s a type family….probably an associated type (if I know what that means)) Is this the idiom/pattern i should follow? Or can the closure contraint be expressed directly in a typeclass any other way? From: Haskell-Cafe [mailto:haskell-ca...@haskell.org<javascript:>] On Behalf Of Brandon Allbery Sent: 06 January 2015 1:59 PM To: Rob Leslie Cc: Haskell Cafe Subject: Re: [Haskell-cafe] Haskeline and asynchronous messages On Tue, Jan 6, 2015 at 3:27 AM, Rob Leslie <r...(a)mars.org<javascript:>> wrote: Does anyone have a suggestion on using Haskeline in an environment where another thread may be writing messages to the terminal -- is it possible to somehow print incoming messages above the input line without disturbing the input line? Terminals don't really work that way; you need to be looking at something like curses or vty. -- brandon s allbery kf8nh sine nomine associates allb...(a)gmail.com<javascript:> ball...(a)sinenomine.net<javascript:> unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net CONFIDENTIALITY NOTICE This e-mail (and any attached files) is confidential and protected by copyright (and other intellectual property rights). If you are not the intended recipient please e-mail the sender and then delete the email and any attached files immediately. Any further use or dissemination is prohibited. While MTV Networks Europe has taken steps to ensure that this email and any attachments are virus free, it is your responsibility to ensure that this message and any attachments are virus free and do not affect your systems / data. Communicating by email is not 100% secure and carries risks such as delay, data corruption, non-delivery, wrongful interception and unauthorised amendment. 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Installing ihaskell on nixos
by info＠rotnetix.com 08 Jan '15

08 Jan '15

I am trying to set up ihaskell on EC2. I start with the NixOs EC2 instance (from http://nixos.org/nixos/download.html) and then basically install the ihaskell package using nix-env -i ihaskell. The installation runs fine but when I try to start the notebook with ihaskell notebook i get the following: Updating IPython profile. Loading profile from /nix/store/hlkvkavkgm3hj08ffdlziq30r7zyr060-haskell-ihaskell-ghc7.8.3-0.4.3.0-shared/share/x86_64-linux-ghc-7.8.3/ihaskell-0.4.3.0/profile/profile.tar IHaskell: Ran commands: mkdir -p /root/.ihaskell mkdir -p /root/.ihaskell/notebooks mkdir -p /root/.ihaskell /nix/store/hnmgn0jrpzlp5d91m5xawpr2iqsjxh7j-ipython-2.3.1/bin/ipython locate profile haskell which /nix/store/hnmgn0jrpzlp5d91m5xawpr2iqsjxh7j-ipython-2.3.1/bin/ipython /nix/store/hnmgn0jrpzlp5d91m5xawpr2iqsjxh7j-ipython-2.3.1/bin/ipython locate profile haskell which /nix/store/hnmgn0jrpzlp5d91m5xawpr2iqsjxh7j-ipython-2.3.1/bin/ipython Exception: /root/.ipython/profile_haskell/ipython_config.py: openFile: does not exist (No such file or directory) I poked around a bit and the root/.ipython/profile_haskell/ contains none of the expected profile files... Could not find anything by googling around except the suggestion to run the ihaskell setup command which seems to have been discontinued Can anyone suggest what I could try? Thanks Riaan

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Haskell Weekly News: Issue 314
by Daniel Santa Cruz 08 Jan '15

08 Jan '15

Welcome to issue 314 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers from December 7, 2014 to January 3, 2015 I know... I know. It's been a while Quotes of the Week * arianvp__: after 5 hours of hacking my code works || now its 3 am || Types make a lot more sense when I'm tired for some reason * dmwit: "Any coder worth his salt can do the impossible." * hodapp: Math is too hard. Let's use JavaScript. * Zemyla: I imagine if [Edward Kmett] had found C++, he would be writing template definitions that would make the faces of the Boost team melt off. Top Reddit Stories * Kronos Haskell Notebook (Mac App) Domain: kronosnotebook.com, Score: 114, Comments: 21 Original: [1] http://goo.gl/nOnLuz On Reddit: [2] http://goo.gl/SNLxjo * The "What Are Monads?" Fallacy Domain: two-wrongs.com, Score: 112, Comments: 93 Original: [3] http://goo.gl/Beu6wO On Reddit: [4] http://goo.gl/CxcK1f * Learn You Some Algebras for Glorious Good! - A fun, easy-to-read math textbook. Domain: self.haskell, Score: 109, Comments: 106 Original: [5] http://goo.gl/3623n6 On Reddit: [6] http://goo.gl/3623n6 * GHC illustrated [PDF] Domain: takenobu-hs.github.io, Score: 108, Comments: 30 Original: [7] http://goo.gl/yj7ySL On Reddit: [8] http://goo.gl/sro7qH * The Haskell Cast #9 - Conal Elliott on FRP and Denotational Design Domain: haskellcast.com, Score: 94, Comments: 34 Original: [9] http://goo.gl/tls8K3 On Reddit: [10] http://goo.gl/NIJZ1X * GHC 7.10.1 will support 'static values', allowing you to create stable references to values across programs/machines for serialization, RPC, etc... Domain: phabricator.haskell.org, Score: 91, Comments: 26 Original: [11] http://goo.gl/bsfROl On Reddit: [12] http://goo.gl/weGkIq * 24 Days of GHC Extensions: Thanks! Domain: ocharles.org.uk, Score: 82, Comments: 11 Original: [13] http://goo.gl/6ee2Jb On Reddit: [14] http://goo.gl/CYoai5 * git clone in Haskell from the bottom up Domain: stefan.saasen.me, Score: 81, Comments: 9 Original: [15] http://goo.gl/nWSXV On Reddit: [16] http://goo.gl/ULS3YO * A commentary on 24 days of GHC extensions Domain: augustss.blogspot.com, Score: 78, Comments: 5 Original: [17] http://goo.gl/C4nKNN On Reddit: [18] http://goo.gl/cGBdss * Haskell for all: A very general API for relational joins Domain: haskellforall.com, Score: 77, Comments: 29 Original: [19] http://goo.gl/WVGtQl On Reddit: [20] http://goo.gl/8McEkB * Elm 0.14 - Simpler Core, Better Tools Domain: elm-lang.org, Score: 76, Comments: 38 Original: [21] http://goo.gl/7Oq4Xe On Reddit: [22] http://goo.gl/e9wbAm * Rethinking webservice and APIs in Haskell: servant 0.2 Domain: alpmestan.com, Score: 74, Comments: 59 Original: [23] http://goo.gl/Jl2CMS On Reddit: [24] http://goo.gl/HsB5zi * ANNOUNCE: GHC 7.10.1 Release Candidate 1 Domain: permalink.gmane.org, Score: 74, Comments: 4 Original: [25] http://goo.gl/I9ezar On Reddit: [26] http://goo.gl/SRNHG9 * 24 Days of GHC Extensions: Rank N Types Domain: ocharles.org.uk, Score: 69, Comments: 27 Original: [27] http://goo.gl/1OuyNg On Reddit: [28] http://goo.gl/5m79zT * A Haskell Implementation Reading List Domain: stephendiehl.com, Score: 66, Comments: 0 Original: [29] http://goo.gl/YnFyTs On Reddit: [30] http://goo.gl/3TudIF * Learning Curves for different programming languages (including Haskell) Domain: github.com, Score: 66, Comments: 106 Original: [31] http://goo.gl/SxsT10 On Reddit: [32] http://goo.gl/KNsLqh * Functional Education Domain: bitemyapp.com, Score: 66, Comments: 10 Original: [33] http://goo.gl/f4EI02 On Reddit: [34] http://goo.gl/wDxjkj * Snowdrift.coop hiring lead programmer (Haskell/Yesod) Domain: snowdrift.coop, Score: 65, Comments: 8 Original: [35] http://goo.gl/Nh0GL2 On Reddit: [36] http://goo.gl/8tLsy8 * 24 Days of GHC Extensions: Functional Dependencies Domain: ocharles.org.uk, Score: 58, Comments: 4 Original: [37] http://goo.gl/mBkf8n On Reddit: [38] http://goo.gl/uv3Mhs * So you want to learn type theory... but where to start? Here are some suggestions. Domain: purelytheoretical.com, Score: 58, Comments: 9 Original: [39] http://goo.gl/z73MMM On Reddit: [40] http://goo.gl/9AuJJl * 24 Days of Hackage: Static Pointers (guest post by Mathieu Boespflug) Domain: ocharles.org.uk, Score: 57, Comments: 32 Original: [41] http://goo.gl/4mhlNL On Reddit: [42] http://goo.gl/jsSYr9 * 24 Days of GHC Extensions: Type Families Domain: ocharles.org.uk, Score: 56, Comments: 11 Original: [43] http://goo.gl/uwLBsA On Reddit: [44] http://goo.gl/ykn06l * 24 Days of GHC Extensions: Nullary Type Classes Domain: ocharles.org.uk, Score: 54, Comments: 23 Original: [45] http://goo.gl/4k5bQw On Reddit: [46] http://goo.gl/crIJbb * 24 Days of GHC Extensions: DeriveGeneric Domain: ocharles.org.uk, Score: 54, Comments: 27 Original: [47] http://goo.gl/gKijr1 On Reddit: [48] http://goo.gl/SRMNuR * 24 Days of GHC Extensions: Existential Quantification (guest post by Roman Cheplyaka) Domain: ocharles.org.uk, Score: 54, Comments: 0 Original: [49] http://goo.gl/wXwsDU On Reddit: [50] http://goo.gl/vLaS92 * 24 Days of GHC Extensions: Multi-parameter Type Classes Domain: ocharles.org.uk, Score: 52, Comments: 11 Original: [51] http://goo.gl/2sWnsC On Reddit: [52] http://goo.gl/h4mRBb * A demo written in Haskell got 3rd place in Experience Domain: youtube.com, Score: 48, Comments: 37 Original: [53] http://goo.gl/9l7B6h On Reddit: [54] http://goo.gl/8Np97P * 24 Days of GHC Extensions: Overloaded Strings Domain: ocharles.org.uk, Score: 48, Comments: 23 Original: [55] http://goo.gl/u6aiAi On Reddit: [56] http://goo.gl/Xtww0S * Intro to Machines & Arrows (Part 1: Stream and Auto) Domain: blog.jle.im, Score: 48, Comments: 25 Original: [57] http://goo.gl/MwouKm On Reddit: [58] http://goo.gl/934frV * 24 Days of GHC Extensions: Recursive Do Domain: ocharles.org.uk, Score: 47, Comments: 7 Original: [59] http://goo.gl/iejmP8 On Reddit: [60] http://goo.gl/H8uEQS * 24 Days of GHC Extensions: Scoped Type Variables (guest post by Tim Docker) Domain: ocharles.org.uk, Score: 47, Comments: 18 Original: [61] http://goo.gl/sWS0dT On Reddit: [62] http://goo.gl/jFPZAC * ANNOUNCE: GHC version 7.8.4 Domain: permalink.gmane.org, Score: 47, Comments: 19 Original: [63] http://goo.gl/0ygVbv On Reddit: [64] http://goo.gl/2eWUsJ * Categories Great and Small (From Bartosz Milewski's Categories for Programmers series) Domain: bartoszmilewski.com, Score: 46, Comments: 8 Original: [65] http://goo.gl/NfpA7X On Reddit: [66] http://goo.gl/ghq472 * Denotational design does not work Domain: ro-che.info, Score: 45, Comments: 39 Original: [67] http://goo.gl/8hOm0A On Reddit: [68] http://goo.gl/tf4JVc Top StackOverflow Questions * What is happening when I compose * with + in Haskell? votes: 35, answers: 5 Read on SO: [69] http://goo.gl/S9h6Xi * Difference in performance of compiled accelerate code ran from ghci and shell votes: 22, answers: 0 Read on SO: [70] http://goo.gl/zxojbX * Why does this Haskell program diverge? votes: 20, answers: 1 Read on SO: [71] http://goo.gl/NDasIA * Is it possible to skip the nursery? votes: 20, answers: 1 Read on SO: [72] http://goo.gl/Qk4K5i * Haskell performance implementing unix's “cat” program with Data.ByteString votes: 19, answers: 3 Read on SO: [73] http://goo.gl/ekoeor * showsPrec and operator precedences votes: 15, answers: 1 Read on SO: [74] http://goo.gl/ifxaUB * Why does let y = 1 + y compile, and what does it mean? votes: 15, answers: 1 Read on SO: [75] http://goo.gl/vp1Q02 * Type Family Shenanigans in GHCi votes: 13, answers: 1 Read on SO: [76] http://goo.gl/fvJzzO * Quirkiness in Haskell 7.8.3 votes: 13, answers: 1 Read on SO: [77] http://goo.gl/KprcEM * What is the difference between value constructors and tuples? votes: 11, answers: 2 Read on SO: [78] http://goo.gl/5ybmYY * Efficient version of 'inits' votes: 11, answers: 2 Read on SO: [79] http://goo.gl/Qwoirp * Working out the details of a type indexed free monad votes: 11, answers: 7 Read on SO: [80] http://goo.gl/oMjYQn * How to model a currencies, money, and banks that exchange money between currencies? votes: 10, answers: 3 Read on SO: [81] http://goo.gl/svZY2T * Why are these functions differently evaluated votes: 10, answers: 2 Read on SO: [82] http://goo.gl/nrD5pA * Why does a more general type affect runtime in Haskell? votes: 10, answers: 2 Read on SO: [83] http://goo.gl/oVeQKg * List based on right Kan extension votes: 9, answers: 2 Read on SO: [84] http://goo.gl/VxPhv0 * How to inject the result of an IO action into a non-IO monadic computation votes: 9, answers: 3 Read on SO: [85] http://goo.gl/WzrvDP * Why I get the “class Num a where” instead of the “class (Eq a, Show a) => Num a”? votes: 9, answers: 4 Read on SO: [86] http://goo.gl/glKEeK * How to avoid default return value when accessing a non-existent field with lenses? votes: 9, answers: 2 Read on SO: [87] http://goo.gl/RROrVM * How much does Haskell/GHC memoize? votes: 9, answers: 1 Read on SO: [88] http://goo.gl/PuUKt4 Until next time, [89]+Daniel Santa Cruz References 1. http://www.kronosnotebook.com/haskell 2. http://www.reddit.com/r/haskell/comments/2r43q0/kronos_haskell_notebook_mac… 3. http://two-wrongs.com/the-what-are-monads-fallacy 4. http://www.reddit.com/r/haskell/comments/2r2cpe/the_what_are_monads_fallacy/ 5. http://www.reddit.com/r/haskell/comments/2qu7w8/learn_you_some_algebras_for… 6. http://www.reddit.com/r/haskell/comments/2qu7w8/learn_you_some_algebras_for… 7. http://takenobu-hs.github.io/downloads/haskell_ghc_illustrated.pdf 8. http://www.reddit.com/r/haskell/comments/2qxcmz/ghc_illustrated_pdf/ 9. http://www.haskellcast.com/episode/009-conal-elliott-on-frp-and-denotationa… 10. http://www.reddit.com/r/haskell/comments/2pdhe9/the_haskell_cast_9_conal_el… 11. https://phabricator.haskell.org/rGHCfc45f32491313d2a44e72d8d59cdf95b1660189d 12. http://www.reddit.com/r/haskell/comments/2otl0f/ghc_7101_will_support_stati… 13. https://ocharles.org.uk/blog/posts/2014-12-24-conclusion.html 14. http://www.reddit.com/r/haskell/comments/2qbfpe/24_days_of_ghc_extensions_t… 15. http://stefan.saasen.me/articles/git-clone-in-haskell-from-the-bottom-up/ 16. http://www.reddit.com/r/haskell/comments/2okt2e/git_clone_in_haskell_from_t… 17. http://augustss.blogspot.com/2014/12/a-commentary-on-24-days-of-ghc.html 18. http://www.reddit.com/r/haskell/comments/2pdu4a/a_commentary_on_24_days_of_… 19. http://www.haskellforall.com/2014/12/a-very-general-api-for-relational-join… 20. http://www.reddit.com/r/haskell/comments/2oji35/haskell_for_all_a_very_gene… 21. http://elm-lang.org/blog/announce/0.14.elm 22. http://www.reddit.com/r/haskell/comments/2ovgw4/elm_014_simpler_core_better… 23. http://alpmestan.com/posts/2014-12-09-rethinking-webservices-apis-haskell.h… 24. http://www.reddit.com/r/haskell/comments/2orb8t/rethinking_webservice_and_a… 25. http://permalink.gmane.org/gmane.comp.lang.haskell.ghc.devel/7529 26. http://www.reddit.com/r/haskell/comments/2q6eei/announce_ghc_7101_release_c… 27. https://ocharles.org.uk/blog/guest-posts/2014-12-18-rank-n-types.html 28. http://www.reddit.com/r/haskell/comments/2pqtxk/24_days_of_ghc_extensions_r… 29. http://www.stephendiehl.com/posts/essential_compilers.html 30. http://www.reddit.com/r/haskell/comments/2pbt0e/a_haskell_implementation_re… 31. https://github.com/Dobiasd/articles/blob/master/programming_language_learni… 32. http://www.reddit.com/r/haskell/comments/2qjnho/learning_curves_for_differe… 33. http://bitemyapp.com/posts/2014-12-31-functional-education.html 34. http://www.reddit.com/r/haskell/comments/2r0q9h/functional_education/ 35. https://snowdrift.coop/p/snowdrift/w/en/jobs 36. http://www.reddit.com/r/haskell/comments/2r6nke/snowdriftcoop_hiring_lead_p… 37. https://ocharles.org.uk/blog/posts/2014-12-14-functional-dependencies.html 38. http://www.reddit.com/r/haskell/comments/2pal4v/24_days_of_ghc_extensions_f… 39. http://purelytheoretical.com/sywtltt.html 40. http://www.reddit.com/r/haskell/comments/2pzczc/so_you_want_to_learn_type_t… 41. https://ocharles.org.uk/blog/guest-posts/2014-12-23-static-pointers.html 42. http://www.reddit.com/r/haskell/comments/2q85zp/24_days_of_hackage_static_p… 43. https://ocharles.org.uk/blog/posts/2014-12-12-type-families.html 44. http://www.reddit.com/r/haskell/comments/2p3mrm/24_days_of_ghc_extensions_t… 45. https://ocharles.org.uk/blog/posts/2014-12-10-nullary-type-classes.html 46. http://www.reddit.com/r/haskell/comments/2ovvnk/24_days_of_ghc_extensions_n… 47. https://ocharles.org.uk/blog/posts/2014-12-16-derive-generic.html 48. http://www.reddit.com/r/haskell/comments/2pirdg/24_days_of_ghc_extensions_d… 49. https://ocharles.org.uk/blog/guest-posts/2014-12-19-existential-quantificat… 50. http://www.reddit.com/r/haskell/comments/2pu0e8/24_days_of_ghc_extensions_e… 51. https://ocharles.org.uk/blog/posts/2014-12-13-multi-param-type-classes.html 52. http://www.reddit.com/r/haskell/comments/2p792t/24_days_of_ghc_extensions_m… 53. https://www.youtube.com/watch?v=-qISxjnikJ4 54. http://www.reddit.com/r/haskell/comments/2oqo33/a_demo_written_in_haskell_g… 55. https://ocharles.org.uk/blog/posts/2014-12-17-overloaded-strings.html 56. http://www.reddit.com/r/haskell/comments/2pmon2/24_days_of_ghc_extensions_o… 57. http://blog.jle.im/entry/intro-to-machines-arrows-part-1-stream-and 58. http://www.reddit.com/r/haskell/comments/2qirmb/intro_to_machines_arrows_pa… 59. https://ocharles.org.uk/blog/posts/2014-12-09-recursive-do.html 60. http://www.reddit.com/r/haskell/comments/2osh3v/24_days_of_ghc_extensions_r… 61. https://ocharles.org.uk/blog/guest-posts/2014-12-20-scoped-type-variables.h… 62. http://www.reddit.com/r/haskell/comments/2pwecw/24_days_of_ghc_extensions_s… 63. http://permalink.gmane.org/gmane.comp.lang.haskell.ghc.devel/7526 64. http://www.reddit.com/r/haskell/comments/2q6oln/announce_ghc_version_784/ 65. http://bartoszmilewski.com/2014/12/05/categories-great-and-small/ 66. http://www.reddit.com/r/haskell/comments/2phex1/categories_great_and_small_… 67. http://ro-che.info/articles/2014-12-31-denotational-design-does-not-work 68. http://www.reddit.com/r/haskell/comments/2qvbhp/denotational_design_does_no… 69. http://stackoverflow.com/questions/27664213/what-is-happening-when-i-compos… 70. http://stackoverflow.com/questions/27541609/difference-in-performance-of-co… 71. http://stackoverflow.com/questions/27421598/why-does-this-haskell-program-d… 72. http://stackoverflow.com/questions/27630833/is-it-possible-to-skip-the-nurs… 73. http://stackoverflow.com/questions/27463669/haskell-performance-implementin… 74. http://stackoverflow.com/questions/27471937/showsprec-and-operator-preceden… 75. http://stackoverflow.com/questions/27557380/why-does-let-y-1-y-compile-and-… 76. http://stackoverflow.com/questions/27490352/type-family-shenanigans-in-ghci 77. http://stackoverflow.com/questions/27663435/quirkiness-in-haskell-7-8-3 78. http://stackoverflow.com/questions/27476446/what-is-the-difference-between-… 79. http://stackoverflow.com/questions/27672585/efficient-version-of-inits 80. http://stackoverflow.com/questions/27676294/working-out-the-details-of-a-ty… 81. http://stackoverflow.com/questions/27408873/how-to-model-a-currencies-money… 82. http://stackoverflow.com/questions/27568791/why-are-these-functions-differe… 83. http://stackoverflow.com/questions/27683108/why-does-a-more-general-type-af… 84. http://stackoverflow.com/questions/27381133/list-based-on-right-kan-extensi… 85. http://stackoverflow.com/questions/27391293/how-to-inject-the-result-of-an-… 86. http://stackoverflow.com/questions/27561604/why-i-get-the-class-num-a-where… 87. http://stackoverflow.com/questions/27567664/how-to-avoid-default-return-val… 88. http://stackoverflow.com/questions/27570371/how-much-does-haskell-ghc-memoi… 89. 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1 0

hxt: How to Unpickle Multiple Adjacent Elements
by Jimmy Lu 08 Jan '15

08 Jan '15

Hi, With xml element having DTD like following: <!ELEMENT entry ((art*, formula?, table?), hw, hsl?, (pr | altpr)?, (ahw, hsl?, (pr | altpr)?)*, vr?, fl?, lb*, in*, ((dx) | (cx?, def?))?, dro*, dxnl*, uro*, syns*)> I want to unpickle the sequence hw, hsl?, (pr | altpr)?, (ahw, hsl?, (pr | altpr)?)* into Haskell data type [Headword] where data Headword = Hw { hwText :: String , hwSl :: Maybe String , hwPr :: Maybe Pronunciation } However in the hxt pickle API I cannot found a way to extract a sequence of adjacent elements. Is there any one knowing how to do it? Thanks, Jimmy Lu

3 2

Study GUI and FRP sequentially or concurrently? Superimposing graphics and text.
by Jeffrey Brown 07 Jan '15

07 Jan '15

Hi! I want to write a mindmapping app, like Freeplane, which involves drawing rectangles containing text and arrows connecting them. For that purpose, today I installed reactive-banana, wxWidgets, wxHaskell and reactive-banana-wx. (It's the first complex install for which I didn't have to write the list for help!) Am I correct that the question of whether graphics can be superimposed over text depends only on wxHaskell, and not reactive-banana? Would you recommend first writing code that uses only wxHaskell, to become familiar with the library, or jumping right in by using reactive-banana at the same time? Regards, Jeff

5 6

Re: [Haskell-cafe] typeclass woes...how to constain a typeclass to be "closed" under an operation....
by Nicholls, Mark 07 Jan '15

07 Jan '15

(Caveat) In haskell specifically, i cant say, i rarely use it, and not in anger, im trying to pick it up again. Set closure under an operation is common, and in maths is axiomatic to many theories eg group theory, in fact at some level its probably axiomatic to all maths eg model theory Many useful operations are trivially closed eg arithmetic. Operatosn that are closed to the domain of the operation can be generalised to N applications, eg because + on integer is closed i can apply it N times and derive * if it werent then 6*5 may be defined but 6*6 may not be! (Undefined throws a spanner in the works, but i'll ignore it) I think in programming you rarely explicitly think about it, but you use it (arguably) everywhere Closure under the typeclass instance type of kind *->* (i expect thats not the right way to say it) seems common in haskell, monoid, monad, num etc...... I wanted to see if there was a way to do it using type families over types of kind *, as in some ways this seems less onerous on the user of the class and more prescriptive.....ie fill in this type family def, write the operation definitoon and your done Excuse the spelling, sent from a phone with itty bitty keys, it like trying to sow a button on a shirt with a sausage. On 7 Jan 2015, at 07:24, Dmin <dgomez1092(a)gmail.com<mailto:dgomez1092@gmail.com>> wrote: hi, i'm sorry to interject but this post caught my attention. i understand how both classes help constrain the type ops >> class Foo_ (S a) => Foo a where >> type S a >> op :: a -> (S a) but what is really on my mind is for what purpose do you want to understand "closing" a type signature in a class. As a beginner myself I would venture to say this is common in haskell, no? Is this a good analogy to the set-theory paradigm? Which I believe was the original question, right? On Tuesday, January 6, 2015 11:18:29 AM UTC-8, Nicholls, Mark wrote: In fact i had tried somethjng very much like this but without the Foo_ and haskell wasnt happy with the cyclic definition So maybe this is the answer! I'll let you know Excuse the spelling, sent from a phone with itty bitty keys, it like trying to sow a button on a shirt with a sausage. > On 6 Jan 2015, at 19:14, Nicholls, Mark <nichol...(a)vimn.com<javascript:>> wrote: > > Oooo > > I'll have a go with this tomorrow > > Thanks > > Excuse the spelling, sent from a phone with itty bitty keys, it like trying to sow a button on a shirt with a sausage. > > >> On 6 Jan 2015, at 19:06, adam vogt <vogt...(a)gmail.com<javascript:>> wrote: >> >> Hi, >> >> You can constrain the result type to be in the same class by writing >> something like: >> >> {-# LANGUAGE UndecidableInstances, FlexibleInstances #-} >> {-# LANGUAGE TypeFamilies, FlexibleContexts #-} >> >> class Foo_ a -- just to prevent a cycle in superclass constraints >> instance Foo a => Foo_ a >> >> class Foo_ (S a) => Foo a where >> type S a >> op :: a -> (S a) >> >> -- and an example where you get a compile error if "op x" has an instance, >> -- but "op (op x)" does not have an instance. >> instance Foo Int where >> type S Int = Char >> op = toEnum >> >> instance Foo Char where >> type S Char = (Char,Char) >> op x = (x,x) >> >> instance Foo (Char,Char) where >> type S (Char,Char) = Int >> op (x,y) = fromEnum x >> >> >>> On Tue, Jan 6, 2015 at 1:53 PM, Nicholls, Mark <nichol...(a)vimn.com<javascript:>> wrote: >>> I will post the question again properly tomorrow but your example indeed is almost a good example, but is trivially closed >>> >>> If the operation on monoid was >>> >>> [a]->[b]->[c] >>> >>> That is also closed; ie [c] is also a monoid.... And the typeclass declaration follows the idiom ive suggested >>> >>> This only works for types of kind *->* >>> >>> If i wanted to do this over a type of kind * (ignoring the trivial a->a) >>> >>> Can i express this in a typeclass? >>> >>> Please ignore until i resubmit this question >>> >>> Ps i hate phones >>> >>> Excuse the spelling, sent from a phone with itty bitty keys, it like trying to sow a button on a shirt with a sausage. >>> >>> >>>>> On 6 Jan 2015, at 18:10, Tom Ellis <tom-lists-has...(a)jaguarpaw.co.uk<javascript:>> wrote: >>>>> >>>>> On Tue, Jan 06, 2015 at 05:43:47PM +0000, Nicholls, Mark wrote: >>>>> Its quite common in maths to have operations in a theory that are (set) >>>>> closed, i just want to translate that notion to a typeclass >>>> >>>> Do you really need a typeclass (or indeed any way) of doing this? I suspect >>>> it will not work. If you consider the multiplication for the monoid of >>>> concatenation of lists >>>> >>>> (++) :: [a] -> [a] -> [a] >>>> >>>> you see that its type already implies that it is "closed". >>>> >>>> Tom >>>> _______________________________________________ >>>> Haskell-Cafe mailing list >>>> Haskel...(a)haskell.org<javascript:> >>>> http://www.haskell.org/mailman/listinfo/haskell-cafe >>> CONFIDENTIALITY NOTICE >>> >>> This e-mail (and any attached files) is confidential and protected by copyright (and other intellectual property rights). If you are not the intended recipient please e-mail the sender and then delete the email and any attached files immediately. Any further use or dissemination is prohibited. >>> >>> While MTV Networks Europe has taken steps to ensure that this email and any attachments are virus free, it is your responsibility to ensure that this message and any attachments are virus free and do not affect your systems / data. >>> >>> Communicating by email is not 100% secure and carries risks such as delay, data corruption, non-delivery, wrongful interception and unauthorised amendment. If you communicate with us by e-mail, you acknowledge and assume these risks, and you agree to take appropriate measures to minimise these risks when e-mailing us. >>> >>> MTV Networks International, MTV Networks UK & Ireland, Greenhouse, Nickelodeon Viacom Consumer Products, VBSi, Viacom Brand Solutions International, Be Viacom, Viacom International Media Networks and VIMN and Comedy Central are all trading names of MTV Networks Europe. MTV Networks Europe is a partnership between MTV Networks Europe Inc. and Viacom Networks Europe Inc. Address for service in Great Britain is 17-29 Hawley Crescent, London, NW1 8TT. >>> >>> _______________________________________________ >>> Haskell-Cafe mailing list >>> Haskel...(a)haskell.org<javascript:> >>> http://www.haskell.org/mailman/listinfo/haskell-cafe >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskel...(a)haskell.org<javascript:> >> http://www.haskell.org/mailman/listinfo/haskell-cafe CONFIDENTIALITY NOTICE This e-mail (and any attached files) is confidential and protected by copyright (and other intellectual property rights). If you are not the intended recipient please e-mail the sender and then delete the email and any attached files immediately. Any further use or dissemination is prohibited. While MTV Networks Europe has taken steps to ensure that this email and any attachments are virus free, it is your responsibility to ensure that this message and any attachments are virus free and do not affect your systems / data. Communicating by email is not 100% secure and carries risks such as delay, data corruption, non-delivery, wrongful interception and unauthorised amendment. If you communicate with us by e-mail, you acknowledge and assume these risks, and you agree to take appropriate measures to minimise these risks when e-mailing us. MTV Networks International, MTV Networks UK & Ireland, Greenhouse, Nickelodeon Viacom Consumer Products, VBSi, Viacom Brand Solutions International, Be Viacom, Viacom International Media Networks and VIMN and Comedy Central are all trading names of MTV Networks Europe. MTV Networks Europe is a partnership between MTV Networks Europe Inc. and Viacom Networks Europe Inc. Address for service in Great Britain is 17-29 Hawley Crescent, London, NW1 8TT. _______________________________________________ Haskell-Cafe mailing list Haskel...(a)haskell.org<javascript:> http://www.haskell.org/mailman/listinfo/haskell-cafe CONFIDENTIALITY NOTICE This e-mail (and any attached files) is confidential and protected by copyright (and other intellectual property rights). If you are not the intended recipient please e-mail the sender and then delete the email and any attached files immediately. Any further use or dissemination is prohibited. While MTV Networks Europe has taken steps to ensure that this email and any attachments are virus free, it is your responsibility to ensure that this message and any attachments are virus free and do not affect your systems / data. Communicating by email is not 100% secure and carries risks such as delay, data corruption, non-delivery, wrongful interception and unauthorised amendment. If you communicate with us by e-mail, you acknowledge and assume these risks, and you agree to take appropriate measures to minimise these risks when e-mailing us. MTV Networks International, MTV Networks UK & Ireland, Greenhouse, Nickelodeon Viacom Consumer Products, VBSi, Viacom Brand Solutions International, Be Viacom, Viacom International Media Networks and VIMN and Comedy Central are all trading names of MTV Networks Europe. MTV Networks Europe is a partnership between MTV Networks Europe Inc. and Viacom Networks Europe Inc. Address for service in Great Britain is 17-29 Hawley Crescent, London, NW1 8TT.

2 1

threepenny-gui: garbage collection
by Tom Ellis 07 Jan '15

07 Jan '15

Suppose that each time I make a particular threepenny-gui widget, call it a `Foo`, I register an event handler using `on` so that the `Foo` can change its appearance based on some `Event`. Suppose I create, show, and hide a billion `Foo`s, each one never to be shown again. Can they be garbage collected? Presumably there is something storing the callback that was registered for the `Event` and the callback refers to the `Foo`. How then can the `Foo`s be freed? `Reactive.Threepenny.register` talks about an "unregister" action but says "FIXME: Unregistering event handlers does not work yet.". But how could it be known anyway when to unregister? Is there not the possibility of a space leak regardless? Thanks, Tom

2 5

windows crashes (bsod) during link phase
by Han Joosten 07 Jan '15

07 Jan '15

Hi, When I build ampersand (sources from https://github.com/AmpersandTarski/ampersand ) everything compiles correctly. However, during link time, windows crashes and I get a blue screen of death. A colleque of mine has exactly the same experience. However, when I do this on another windows 7 machine, I get a working executable. I would say that any program that compiles correctly cannot cause the linker to fail. I am not very familiar with the build system, but this is a serious blocking issue for me working on that project. Any help is very much appreciated. Thanks!

1 0

GHC 7.8.4 for Windows x86
by Anton Dessiatov 07 Jan '15

07 Jan '15

Hello, Haskell Café! Does anybody know what’s the reason for the lack of GHC 7.8.4 windows x86 build? I could only find x64 version on the official website and it seems like minghc project doesn’t offer x86 build too. Best regards, Anton.

1 0

Significant compilation slowdown when using Data.CaseInsensitive.mk
by Brendan Hay 06 Jan '15

06 Jan '15

Hi, I've recently run into an issue whereby changing textual case comparison to use the Data.CaseInsensitive.mk <http://hackage.haskell.org/package/case-insensitive-1.2.0.3/docs/Data-CaseI…> smart constructor causes substantial differences to outputted core language and a related compilation speed regression. The code which seems to cause this is here: https://github.com/brendanhay/amazonka/blob/release/0.1.4/core/src/Network/… With the downstream project having about 68 call sites for the above function starting here: https://github.com/brendanhay/amazonka/blob/release/0.1.4/amazonka-ec2/gen/… For comparison, the previous version used Attoparsec.Text.takeText <http://hackage.haskell.org/package/attoparsec-0.12.1.2/docs/Data-Attoparsec…> and Text values. Here are some hand wavy points of interest: * Before (Case Sensitive) Function: takeText = Attoparsec.Text.takeText Time (real): 2m27.735s amazonka-ec2/gen/Network/AWS/EC2/Types.hs (simpl): {terms: 242,688, types: 310,463, coercions: 31,215} * After (Case Insensitive) Function: takeCI = Data.CaseInsensitive.mk <$> Attoparsec.Text.takeText Time (real): 11m31.937s amazonka-ec2/gen/Network/AWS/EC2/Types.hs (simpl): {terms: 1,384,779, types: 465,656, coercions: 33,510} I'm finding the outputted core rather unwieldy to analyse, but the main difference seems to be the inclusion of huge swathes of case statements like: https://gist.github.com/brendanhay/5a57f073d5a7d196a1ad It would be appreciated if someone could help me to understand in simple terms why the use of case-insensitive causes this.

1 1

typeclass woes...how to constain a typeclass to be "closed" under an operation....
by Nicholls, Mark 06 Jan '15

06 Jan '15

This is reposted from the beginnners….I’ve not done Haskell for a while and was struggling to get anything to work, I’ve hopefully refound my feet. Its quite common in maths to have operations in a theory that are (set) closed, i just want to translate that notion to a typeclass I have a suggestion that does work class Foo m where op :: m a -> m (S a) That is closed, but now were working on types of kind • -> • (S can be a type family or a data type…lets say it’s a type family….probably an associated type (if I know what that means)) Is this the idiom/pattern i should follow? Or can the closure contraint be expressed directly in a typeclass any other way? From: Haskell-Cafe [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Brandon Allbery Sent: 06 January 2015 1:59 PM To: Rob Leslie Cc: Haskell Cafe Subject: Re: [Haskell-cafe] Haskeline and asynchronous messages On Tue, Jan 6, 2015 at 3:27 AM, Rob Leslie <rob(a)mars.org<mailto:rob@mars.org>> wrote: Does anyone have a suggestion on using Haskeline in an environment where another thread may be writing messages to the terminal -- is it possible to somehow print incoming messages above the input line without disturbing the input line? Terminals don't really work that way; you need to be looking at something like curses or vty. -- brandon s allbery kf8nh sine nomine associates allbery.b(a)gmail.com<mailto:allbery.b@gmail.com> ballbery(a)sinenomine.net<mailto:ballbery@sinenomine.net> unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net CONFIDENTIALITY NOTICE This e-mail (and any attached files) is confidential and protected by copyright (and other intellectual property rights). If you are not the intended recipient please e-mail the sender and then delete the email and any attached files immediately. Any further use or dissemination is prohibited. While MTV Networks Europe has taken steps to ensure that this email and any attachments are virus free, it is your responsibility to ensure that this message and any attachments are virus free and do not affect your systems / data. Communicating by email is not 100% secure and carries risks such as delay, data corruption, non-delivery, wrongful interception and unauthorised amendment. If you communicate with us by e-mail, you acknowledge and assume these risks, and you agree to take appropriate measures to minimise these risks when e-mailing us. MTV Networks International, MTV Networks UK & Ireland, Greenhouse, Nickelodeon Viacom Consumer Products, VBSi, Viacom Brand Solutions International, Be Viacom, Viacom International Media Networks and VIMN and Comedy Central are all trading names of MTV Networks Europe. MTV Networks Europe is a partnership between MTV Networks Europe Inc. and Viacom Networks Europe Inc. Address for service in Great Britain is 17-29 Hawley Crescent, London, NW1 8TT.

4 7

Haskeline and asynchronous messages
by Rob Leslie 06 Jan '15

06 Jan '15

Greetings, Does anyone have a suggestion on using Haskeline in an environment where another thread may be writing messages to the terminal -- is it possible to somehow print incoming messages above the input line without disturbing the input line? Thanks for any advice. -- Rob Leslie rob(a)mars.org

3 2

Postgresql Intervals with Opaleye
by info＠rotnetix.com 06 Jan '15

06 Jan '15

Another opaleye question: What type should I use for the postgresql interval type? Thanks Riaan

2 4

Installing grapefruit: "lexical error at character 'i'"
by Jeffrey Brown 06 Jan '15

06 Jan '15

Dear list, I am trying to install grapefruit on Kubuntu 14.04. Most of the dependencies seemed to install without incident, but I am running into the following error: jeff@jeff-Lenovo:~$ cabal install grapefruit-ui-gtk grapefruit-examples Resolving dependencies... Configuring grapefruit-ui-0.1.0.5... Building grapefruit-ui-0.1.0.5... Preprocessing library grapefruit-ui-0.1.0.5... src/Internal/UIItem.hs-boot:12:2: lexical error at character 'i' Failed to install grapefruit-ui-0.1.0.5 cabal: Error: some packages failed to install: grapefruit-examples-0.1.0.5 depends on grapefruit-ui-0.1.0.5 which failed to install. grapefruit-ui-0.1.0.5 failed during the building phase. The exception was: ExitFailure 1 grapefruit-ui-gtk-0.1.0.5 depends on grapefruit-ui-0.1.0.5 which failed to install. jeff@jeff-Lenovo:~$ I wanted to find the offending 'i' character, but the file "src/Internal/UIItem.hs-boot" I was unable to find anywhere: jeff@jeff-Lenovo:/$ pwd / jeff@jeff-Lenovo:/$ sudo find . -name *UIItem.hs* [sudo] password for jeff: jeff@jeff-Lenovo:/$ Any ideas? Thanks, Jeff

4 7

Concurrency problems with mysql bindings?
by Elliot Cameron 05 Jan '15

05 Jan '15

Hello Fellow Haskellers, I am not experienced with the FFI, but I have suspicions that Haskell's "mysql" package falls short in terms of its support for concurrency. A tiny bit of discussion has occurred here: https://github.com/bos/mysql/issues/11 Since it appears that "mysql" is the de-facto binding for MySQL (see reverse dependencies at http://www.stackage.org/package/mysql ), it seems that this lack has far-reaching effects on the Haskell ecosystem at large. What attracts many to Haskell is it's strengths in writing parallel /concurrent code, but it seems we lose this benefit when interacting with MySQL. I for one have a beautiful Haskell program using MySQL which I would love to push into production, but it's performance pales in comparison to a Python script that builds up calls to bash and actually achieves concurrency. At this point I have not been able to gain traction for this problem, and since I'm not an experienced Haskell developer, my contributions are slow and tedious. I'm hoping that I can garner some help in the following: 1) Confirm or deny my suspicions as represented in the issue linked above 2) If they are confirmed to any degree, I hope for some help in working through the solution Please let me know if you would like to help in either of these ways! Even the smallest comments will be highly valued! Elliot Cameron Covenant Eyes Software Developer elliot.cameron(a)covenanteyes.com 810-771-8322

1 0

New Functional Programming Job Opportunities
by Functional Jobs 05 Jan '15

05 Jan '15

Here are some functional programming job opportunities that were posted recently: Senior Software Engineer at McGraw-Hill Education http://functionaljobs.com/jobs/8775-senior-software-engineer-at-mcgraw-hill… Cheers, Sean Murphy FunctionalJobs.com

1 0

Class constraints on associated types ...
by aditya siram 05 Jan '15

05 Jan '15

Hi all, I'd like to be able to constrain an associated type. I used to have an instance that looked like: class Dispatch a b c | a b -> c where runF :: a -> b -> c instance (C a) => Dispatch D1 D2 ( a -> IO ()) where runF d1 d2 = (\_ -> return ()) Since I upgraded to 7.8 from 7.5 that instance declaration is no longer accepted is no longer accepted since it violates FD's. I have been updating my code to use type families like so: class Dispatch a b where type Impl a b :: * runF :: a -> b -> Impl a b instance (C a) => Dispatch D1 D2 where type Impl D1 D2 = a -> IO () runF d1 d2 = (\_ return ()) Unfortunately the `type Impl ...` line in the instance is rejected because it uses `a` on the RHS. In this one case I could just package it up into a newtype or something but I would ideally like to be able to constrain any number of arguments like: instance (C a, C b ... C z) => Dispatch D1 D2 where type Impl D1 D2 = a -> b -> ... -> z -> IO () ... This was something I could do in 7.6 (although I realize this is way safer). How do I go about getting that constraint back? Thanks! -deech

3 6

Small language with parser in Haskell and Faker
by Alex Gaziev 05 Jan '15

05 Jan '15

Hi all! As for beginner – it was pretty hard to me to parse YAML or JSON. Too much information, too much functions for such a simple task (in Ruby it is really simple, because of duck typing). So I wrote mine super simple language for data files – Giml – and two parsers for it – in Ruby and in Haskell. Aim of this language - to be fast and so simple that you need only couple pages of code of any language without heavy dependencies to implement parser. Here is the manifest: https://github.com/gazay/giml. Parser in Haskell: https://github.com/gazay/gimlh It was written mostly for my library Faker, which was nicely discussed on reddit ( http://www.reddit.com/r/haskell/comments/2rb8ym/haskell_library_for_generat…) so I have a lot of plans what to do in future with Faker. But I didn't present Giml anywhere and it will be really great if someone can take a look (there only couple pages of code) and tell me what can be improved, what can be rewritten in better way and so on. I didn't use Parsec for a reason in this library as I wanted to write my parser as clean of dependencies as possible, but maybe it will improve performance or something else?

1 0

ANN: git-vogue - a pre-commit check framework for haskell code quality
by Christian Marie 05 Jan '15

05 Jan '15

There are no standards in the Haskell community when it comes to IDEs and editors. As such we've found ourselves without a common benchmark for linting, formatting and code quality checks. Within our development team at Anchor, we wanted a tool that would: * Install in one command * Require nothing to be learned for a new developer * Tell that developer what they need to fix in the code that they modify. We wanted to to satisfy these needs, whilst saving developers from the drudgery of installing, configuring and running a suite of tools independently. Let's face it, if it's difficult to run the formatters and linters and not a part of your established work flow, some developers just aren't going to bother. We built such a tool: http://anchor.github.io/git-vogue/ To install and configure, just: cabal install git-vogue # Wait 15 minutes cd your-project git vogue init All staged files will now be checked on pre-commit. If you wish to check the whole repo: git vogue check --all You can automatically apply stylish-haskell fixes to staged files: git vouge fix Or the whole repo: git vogue fix --all Thank you to thsutton, groberts, oswynb, glasnt, tranma, barneydesmond and sio for your development efforts, and everyone who has taken the time to test and provide feedback.

1 0

hxt: How to Unpickle Multiple Adjacent Elements
by Jimmy Lu 04 Jan '15

04 Jan '15

Hi, With xml element having DTD like following: <!ELEMENT entry ((art*, formula?, table?), hw, hsl?, (pr | altpr)?, (ahw, hsl?, (pr | altpr)?)*, vr?, fl?, lb*, in*, ((dx) | (cx?, def?))?, dro*, dxnl*, uro*, syns*)> I want to unpickle the sequence hw, hsl?, (pr | altpr)?, (ahw, hsl?, (pr | altpr)?)* into Haskell data type [Headword] where data Headword = Hw { hwText :: String , hwSl :: Maybe String , hwPr :: Maybe Pronunciation } However in the hxt pickle API I cannot found a way to extract a sequence of adjacent elements. Is there any one knowing how to do it? Thanks, Jimmy Lu

1 0

ANN: Cabal 1.22 and cabal-install 1.22
by Johan Tibell 04 Jan '15

04 Jan '15

On behalf of the cabal developers, I'm happy to announce the Cabal 1.22 release! To install: cabal update && cabal install Cabal-1.22.0.0 cabal-install-1.22.0.0 Please report bugs at https://github.com/haskell/cabal/issues. Pre-built binaries should be available with a week or so. Non-exhaustive list of changes: * Support GHC 7.10. * Experimental DWARF debug info output on GHC 7.10 (--enable-debug-info). * Preliminary support for relocatable packages. * Cabal can now be used inside a cabal exec environment. * Fix solver bug where the user's manual flag choice was ignored. * Add GHCJS support. * Improved command line documentation. * Build C sources with -fPIC when GHC is using dynamic libraries. * Preliminary support for package keys (a step on the way to NixOS-like packages). * Add a 'user-config' command to help users manage ~/.cabal/config. * HPC support improved. Here are all the contributors to this release, ordered by number of commits: 181 Mikhail Glushenkov 48 Johan Tibell 35 Edward Z. Yang 34 Christiaan Baaij 32 Thomas Tuegel 24 Ben Armston 19 Ian D. Bollinger 17 Lennart Spitzner 12 Luite Stegeman 11 Herbert Valerio Riedel 10 Duncan Coutts 9 Erik de Castro Lopo 8 Andres Loeh 5 Chris Wong 5 Iain Nicol 5 Heather 5 Jake Wheat 4 Daniel Trstenjak 4 Tuncer Ayaz 4 Neil Vice 4 Rudy Matela 2 David Feuer 2 Bertram Felgenhauer 2 Fujimura Daisuke 2 Iustin Pop 2 John Chee 2 Keshav Kini 2 Maxwell Swadling 2 Michael Snoyman 2 Owen Stephens 2 Sergey Vinokurov 2 Thomas Miedema 2 Vincent Hanquez 2 jake 1 Brent Yorgey 1 Maximilian Tagher 1 Anton Dessiatov 1 phischu 1 Bob Ippolito 1 Miëtek Bak 1 Bardur Arantsson 1 Nikita Karetnikov 1 tchakkazulu 1 Peter Trško 1 Austin Seipp 1 Ryan Mulligan 1 RyanGlScott 1 Samuel Gélineau 1 Sergei Trofimovich 1 jpmoresmau 1 Simon Hengel 1 Thomas M. DuBuisson 1 Harry Garrood 1 Isamu Mogi 1 kosmikus 1 David Fox 1 Curtis Gagliardi 1 Antonio Nikishaev 1 Karel Gardas 1 Travis Cardwell 1 Chris Allen 1 Matthew William Cox

1 0

[ANN] Haskell client for Event Store
by Yorick Laupa 04 Jan '15

04 Jan '15

Hi, I'd like to announce my Event Store TCP client. It licensed under BSD3. You can find the source on Github <http://github.com/YoEight/eventstore>. Package is available on Hackage <http://hackage.haskell.org/package/eventstore-0.5.0.0>. Here's a list of supported features: * Single event writes. * Batch writes. * Transactional writes. * Stream deletion. * Single event reads. * Range reads from regular streams and from $all stream (backward and forward). * Volatile subscriptions (regular stream and $all). * Catch-up subscriptions (regular stream and $all). * Authenticated connection. Will be implemented: * Persistent subscriptions. * SSL connection. I originally wrote that library because I need it for a real project. I only implemented features I need first, so feel free to open a ticket (or better a pull request) on Github for something you need or a bug you've experienced. I'll be as fast as I can. Few notes on the code itself. This implementation uses a Functional Reactive Programming (FRP) library named Sodium <https://hackage.haskell.org/package/sodium>. Connection bookkeeping (operation and subscription management for instance) was amazingly easy to implement with it. I only tested that library with EventStore 3.0.1. Hope you enjoy it. Regards, Yorick

1 0

ECMA262 (Javascript) interpreter
by Fabian Bergmark 04 Jan '15

04 Jan '15

I have been implementing an ECMA-262 (Javascript) interpreter over the last few weeks. I have been following http://www.ecma-international.org/ecma-262/5.1 and tried to keep my implementation as close to the specification as possible. The code can be found here https://github.com/fabianbergmark/ECMA-262 So far I have implemented the core language, but has yet to write all the specification functions (Array.splice, String.substring etc.). Before I do this and release it on hackage, I would appreciate feedback on my implementation. The issues I see at the time are: * The parser is slow for deeply nested code * Strict mode is not implemented * Interpret.hs is ~ 6k lines * SubType.hs uses IncoherentInstances, and a better implementation would be nice. At the moment it works but for one case. To test the code, install the cabal package and run ecma262 on a .js file. One host object, console.log is provided and prints to stdout. Fabian Bergmark

4 7

servant-pandoc
by Matthew Pickering 04 Jan '15

04 Jan '15

Dear list, As a quick and easy way to render API documentation to many more formats I thought it wise to create a new pandoc renderer. For those who don't know, pandoc is a multi-format document conversion tool which currently supports 24 different output formats.[1] The rendering is quite faithful to the current `markdown` renderer, if people find this package useful then I can look to fine tune the representation. Note: The package will not build until servant-docs 0.3 is released which will hopefully be tomorrow or the day after. Below are some examples of the rendering and links to the package. examples: https://github.com/mpickering/servant-pandoc/tree/master/examples github: https://github.com/mpickering/servant-pandoc hackage: http://hackage.haskell.org/package/servant-pandoc [1]: http://pandoc.org/ Matt

1 1

Updating tar for ghc 7.10 FAM compatibility
by Darren Grant 03 Jan '15

03 Jan '15

Hi all, Does anyone know who I can reach out to in order to get the tar hackage updated for ghc 7.10? Unfortunately, Duncan's email bounced. Cheers, Darren Grant

2 1

Proof that scanl can be written as foldr?
by Cristian Ontivero 03 Jan '15

03 Jan '15

Hello, I'm trying to solve the exercise 4.6.5 of Bird's "Introduction to functional programming using Haskell". Let me quote: "Prove that scanl f e can be written as a foldr, if f is associative with unit e. Can scanl f e be written as a foldl without any assumptions on f and e?" So far reasoning by myself and googling a bit I reached to the following scanl definition using foldl (without any assumptions afaik): scanl f e = foldl (\xs next -> xs ++ [f (last xs) next]) [e] Furthermore, previously on the same chapter, it says that: scanl f e = foldr g [e] where g x xs = e : map (f x) xs Using this definition, I tried proving it by induction, but since I got stuck, I tried seeing if I could find some way to use the first duality theorem (with the foldl definition), or fusion, but to no avail. The induction proof I wrote is the following: scanl f e = foldr g [e] where g y ys = e : map (f y) ys By the principle of extensionality, this is equivalent to: scanl f e xs = foldr g [e] xs where g y ys = e : map (f y) ys We prove this equality by induction on xs. Case([]). For the left-hand side we reason scanl f e [] = {scanl.1} [e] For the right-hand side foldr g [e] [] = {foldr.1} [e] Case(x:xs). For the left-hand side we reason scanl f e (x:xs) = {scanl.2} e : scanl f (f e x) xs = {f has unit e} e : scanl f x xs = {induction hypothesis} e : foldr g [x] xs For the right-hand side foldr g [e] (x:xs) = {foldr.2} g x (foldr g [e] xs) = {definition of g} e : map (f x) (foldr g [e] xs) = {definition of map} e : foldr ((:) . (f x)) [] (foldr g [e] xs) I'm not even sure the last step is useful, but I thought maybe I could combine both foldrs into a single one (that's why I thought about the fusion law). Anyway, any help or pointer that can lead me in the right direction is very much appreciated. Thanks in advance.

1 0

Handling Postgresql array types
by Riaan 03 Jan '15

03 Jan '15

This is a question about database query libraries. I am trying to do some analytics on a Postgresql database and up to now have been using Database.HDBC for most of my querying. One of the columns on the database is a two dimensional float8 array. I tried using Database.PostgreSQL.Simple but got stuck on this array as I could not figure out how to extend the built in conversion functions to cater for something like this. So went back to HDBC. But now my queries are starting to get fairly long and I have been looking at libraries like Persistent with Esqualeto, HaskellDB and Groundhog to make my queries a little more composable and type safe. However I have not been able to find any examples of any of these libraries using postgres arrays and given my postgresql-simple experience I worried that I might similarly be unable, because of my low level of haskell ability, to extend the conversion functions. So to my question. Does anyone have experience with one of these libraries in dealing with postgresql arrays and could someone perhaps send me a simple example that I could use as a basis. Failing that, can anyone advise me on which of these libraries have the most haskell newbie (LYAH trained, but still struggling with RWH) friendly approach to writing new conversion functions? Thanks Riaan

6 16

Re: [Haskell-cafe] Yet Another Forkable Class
by oleg＠okmij.org 02 Jan '15

02 Jan '15

Perhaps effect libraries (there are several to choose from) could be a better answer to Fork effects than monad transformers. One lesson from the recent research in effects is that we should start thinking what effect we want to achieve rather than which monad transformer to use. Using ReaderT or StateT or something else is an implementation detail. Once we know what effect to achieve we can write a handler, or interpreter, to implement the desired operation on the World, obeying the desired equations. And we are done. For example, with ExtEff library with which I'm more familiar, the Fork effect would take as an argument a computation that cannot throw any requests. That means that the parent has to provide interpreters for all child effects. It becomes trivially to implement: > Another example would be a child that should not be able to throw errors as > opposed to the parent thread. It is possible to specify which errors will be allowed for the child thread (the ones that the parent will be willing to reflect and interpret). The rest of errors will be statically prohibited then. > instance (Protocol p) => Forkable (WebSockets p) (ReaderT (Sink p) IO) where > fork (ReaderT f) = liftIO . forkIO . f =<< getSink This is a good illustration of too much implementation detail. Why do we need to know of (Sink p) as a Reader layer? Would it be clearer to define an Effect of sending to the socket? Computation's type will make it patent the computation is sending to the socket. The parent thread, before forking, has to provide a handler for that effect (and the handler will probably need a socket). Defining a new class for each effect is possible but not needed at all. With monad transformers, a class per effect is meant to hide the ordering of transformer layers in a monad transformer stack. Effect libraries abstract over the implementation details out of the box. Crutches -- extra classes -- are unnecessary. We can start by writing handlers on a case-by-case basis. Generalization, if any, we'll be easier to see. From my experience, generalizing from concrete cases is easier than trying to write a (too) general code at the outset. Way too often, as I read and saw, code that is meant to be reusable ends up hardly usable.

9 12

Modifying a product type generically
by Michael Orlitzky 02 Jan '15

02 Jan '15

I've been playing with generic conversions between simple product types. For example, converting (Bar Int Int) back and forth between (Int,Int). See, https://www.haskell.org/pipermail/haskell-cafe/2014-December/117489.html These save me a *lot* of boilerplate. But naturally, I've gotten greedy. Another transformation I need to make takes one data type, and sticks a new field on the beginning of it. So I'd like to be able to take a, Foo Int Int and construct a, BigFoo String Int Int with as little code as possible. In my real problem, Foo may have 30 fields, and BigFoo 31 -- if you know of a better way, stop me now. And, I actually managed to do it using generics-sop: > prepend "Hello" (Foo 1 2) :: BigFoo BigFoo "Hello" 1 2 But, I had to do something dirty; I used an incomplete pattern match. I don't know enough about what's going on here to handle the other case for prepend_sop. I basically guessed the existing type signatures because it wouldn't work without them. I would like to either, a) Get prepend_sop working for sums of products (the missing case), not just a single product (the one I've got working) b) Restrict prepend_sop to the case that I've got working via the type system, somehow In other words, I don't want it to be crashy, but I'm stuck. Here's the complete working code for the example above. {-# LANGUAGE DataKinds #-} {-# LANGUAGE DeriveGeneric #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE TypeOperators #-} module Prepend where import qualified GHC.Generics as GHC import Generics.SOP data Foo = Foo Int Int deriving (Show, GHC.Generic) instance Generic Foo -- A "copy" of Foo, with a String in front of it. data BigFoo = BigFoo String Int Int deriving (Show, GHC.Generic) instance Generic BigFoo prepend_sop :: (xss ~ (x ': xs), (yss ~ ((a ': x) ': xs))) => a -> SOP I xss -> SOP I yss prepend_sop z (SOP (Z rest)) = SOP $ Z ((I z) :* rest) --prepend_sop z (SOP (S rest)) = ??? prepend :: (Generic a, Generic c, Code a ~ (x ': xs), Code c ~ ((b ': x) ': xs)) => b -> a -> c prepend z = to . (prepend_sop z) . from

3 6

Type-directed functions
by Julian Arni 02 Jan '15

02 Jan '15

Hi all, I've been playing around with what might be described as type-directed functions. One example is a list-like structure of phantom-typed values {-# LANGUAGE TypeOperators #-} {-# LANGUAGE DataKinds #-} {-# LANGUAGE PolyKinds #-} import GHC.TypeLits infixr 6 ::: data a ::: b = a ::: b deriving (Show, Eq) data Tag b = Tag String deriving (Show, Eq) ex1 :: Tag 5 ::: Tag 3 ::: Tag 7 ex1 = Tag "Alice" ::: Tag "Bob" ::: Tag "Carol" And then sorting 'ex1' based on the Nats, such that sort ex1 :: Tag 3 ::: Tag 5 ::: Tag 7 sort ex1 = Tag "Bob" ::: Tag "Alice" ::: Tag "Carol" Notice how it's the types, not the values, that determine the result, but that the value-level also changes. I know how to do this using classes, but it's a little excruciating - it's like programming in a verbose and very restricted Prolog. With type families it's much easier to get the result *type* (pattern matching is simple, recursive calls are natural, and it all looks a lot more like Haskell), but I haven't yet seen a way of effectively using type families to direct the value-level component of the calculation. Are there any examples of how this might be done? Or are there other alternatives to using type-classes that I am missing? Or, alternatively, are there libraries to reduce the boilerplate of this type-class code? Thanks, Julian

3 3

Re: [Haskell-cafe] Negable constraints on boolean normal forms
by Oleg Grenrus 02 Jan '15

02 Jan '15

I’ll try to find time to experiment with API and make a PoC in the beginning of the next week. Currently I have no idea what the API could be. It would be nice if it will turn out so you could do `CNF a -> NNF a` and `DNF a -> NFF a` transformations without any restrictions on `a`. Yet you still need `Negable a` to do reversed transformations as well as `CNF a <-> DNF a` Let’s see how it turns out. I’ll try to come back with something before Epiphany (i.e. on Monday). > On 01 Jan 2015, at 18:04, Yitzchak Gale <gale(a)sefer.org> wrote: > > Yes, sounds great! > > In practice, what I really need right now is to be > able to convert between normal forms without > contraints on the value type. If that will do it - > I'm happy. > > But beyond that, this is a nice library that is > well thought out. Grounded in good theory, but > still easily understandable and usable. Any > further development in that direction would of > course be appreciated! > > In terms of specifics - would we need replacements > for CoBoolean and CoBoolean1 if we move to > a richer lattice-based organization of the types? > Or would we then be able to move to simpler API like > toLattice, fromLattice, toNormalForm, etc. for the > conversions, that would just work? > > Thanks, > Yitz > > On Thu, Jan 1, 2015 at 5:45 PM, Oleg Grenrus <oleg.grenrus(a)iki.fi> wrote: >> First of all: nice to hear that someone is using boolean-normal-forms! >> >> I totally feel your pain. Indeed there is >> “Monoid is used, where Semigroup would be enough” issue. >> >> `Boolean` is too strong class there.. I’d like to rework >> the classes to have hierarchy: >> Lattice => Boolean >> Complementable >> Maybe also `Heyting` and `Negable`. >> >> Then NNF would be free Lattice, and DNF and CNF are also just lattices. >> >> With more granular classes, the requirements for transformations could be >> more precise. >> >> How does this sound? >> >> Cheers, >> Oleg >> >>> On 01 Jan 2015, at 17:06, Yitzchak Gale <gale(a)sefer.org> wrote: >>> >>> I've been using Oleg Genrus' wonderful boolean-normal-forms >>> library lately, and enjoying it. However, I'm running into >>> some trouble with overly constrained conversions. >>> >>> In theory, it's possible to convert between any of the >>> normal forms without requiring a Negable instance on >>> the value type. But the conversion mechanism in the >>> library routes everything via the Boolean type class, >>> and that ends up slapping a Negable constraint on everything. >>> >>> You can work around that by embedding your value type >>> in the free Negable, conveniently provided by the library. >>> But in practice that causes a lot of trouble. You end >>> up being required to deal with Neg cases in your result type >>> that are actually impossible to occur and make no sense >>> in context. You have to throw bottoms into provably total >>> functions, or write code that produces meaningless results >>> that you promise in human-readable comments will never >>> happen in practice. You also make the code less efficient >>> by requiring extra passes over the data to "verify" that Neg >>> isn't there. It's similar to what happens when you try to use >>> a Monoid where a Semigroup is really needed by bolting on >>> an artificial "empty" constructor. >>> >>> One thing you can do to begin with is to remove the >>> Negable constraints on the NormalForm instances; >>> I don't see that constraint ever being used. But that still >>> doesn't solve the main issue. >>> >>> Perhaps one approach would be to define a >>> Boolean "bi-monoid" class, where a Boolean bi-monoid >>> is a like Boolean algebra except without negation. >>> All of the normal forms are instances of Boolean bi-monoid >>> even without a Negable constraint on the value type. >>> >>> If you then relax the constraints on the methods of >>> CoBoolean and CoBoolean1 to Boolean bi-monoid instead >>> of Boolean, all of the existing conversion implementations >>> work without modification and without requiring the Negable >>> constraint. >>> >>> I'm not sure if that creates other problems for some other >>> use cases of this library though. >>> >>> Any thoughts? >>> >>> Thanks, >>> Yitz >>> >> >

1 0

GHC(STG,Cmm,asm) illustrated for hardware persons
by Takenobu Tani 02 Jan '15

02 Jan '15

Dear Haskellers, I love haskell. I'm studying and playing around with Haskell, GHC, STG, Cmm, asm... When I started studying them, I wanted some figures. I've not have some mental models because I am a hardware person. So I am drawing few illustrations for hardware persons. Here is: GHC(STG,Cmm,asm) illustrated for hardware persons http://takenobu-hs.github.io/downloads/haskell_ghc_illustrated.pdf https://github.com/takenobu-hs/haskell-ghc-illustrated If there are some misunderstandings or errors, please teach me. I'll correct or remove them. Thank you :-) I wish you a happy new year, Takenobu

10 11

Re: [Haskell-cafe] Weird reachability issue with haskell.org my my Italian ISP (fastweb)
by Alfredo Di Napoli 02 Jan '15

02 Jan '15

I ended up using a machine running privoxy (using aws or similar) and that works, but I'm not sure I want to pay for the idiocity of my ISP. Happy new year all, anyway. Alfredo Di Napoli > On 01/gen/2015, at 11:53, Giacomo Cosenza <mimmo.cosenza(a)gmail.com> wrote: > > Same thing here too. few weeks ago there were no problems. > mimmo > >> On Tuesday, December 30, 2014 8:26:15 PM UTC+1, george watkins wrote: >> Lo stesso, same thing here . >> I am using this mirror if for hoogle it can help https://www.fpcomplete.com/hoogle. >> At the beginnig i thought it was my dns improperly configured or something wrong with my network >> >> Il giorno martedì 23 dicembre 2014 18:55:00 UTC+1, Alfredo Di Napoli ha scritto: >>> >>> Hello Cafè, >>> >>> let me start with the email I've sent to Rackspace, which will help you understand the context: >>> >>>> Hi there! >>> >>>> I'm trying to understand how come I can't reach www.haskell.org from my ISP here in Italy (fastweb). In order to troubleshoot the issue, I tried dig on it, revealing it's hosted by you: >>> >>>> dig haskell.org >>> >>>> >>>> ; <<>> DiG 9.8.3-P1 <<>> haskell.org >>>> ;; global options: +cmd >>>> ;; Got answer: >>>> ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 63629 >>>> ;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0 >>>> ;; QUESTION SECTION: >>>> ;haskell.org.INA >>>> ;; ANSWER SECTION: >>>> haskell.org. 299 IN A 23.253.242.70 >>>> ;; Query time: 139 msec >>>> ;; SERVER: 8.8.8.8#53(8.8.8.8) >>>> ;; WHEN: Tue Dec 23 18:33:00 2014 >>>> ;; MSG SIZE rcvd: 45 >>> >>>> Trying a whois reveals the subnet is the entire 23.253.0.0 one: >>> >>>> Rackspace Cloud Servers RACKS-8-1387376775861482 (NET-23-253-80-0-1) 23.253.80.0 - 23.253.95.255 >>>> Rackspace Hosting RACKS-8-NET-15 (NET-23-253-0-0-1) 23.253.0.0 - 23.253.255.25 >>> >>>> If I try to ping the IP directly, it obviously doesn't work, so I have tried contiguous IP addresses in the same subnet to see if the subnet itself was reachable at all.. trying 23.253.89.73 indeed worked! It resolved to >>>> http://www.papamurphys.com/ >>>> This means my ISP can reach that subnet, but doesn't explain why it can't connect to 23.253.242.70. >>> >>> >>> That's very weird indeed, as every other website is reachable, only haskell.org is not! How funny my fate is! I'm one of the few Haskellers in Italy and yet I cannot access the website.. perhaps is the consequence of "Avoid success at all costs"?? :) :) >>> >>> Jokes aside, do you guys know (I'm asking mainly to Austin here) if rackspace uses some weird setup for their website deployment or if haskell.org is hosted "the usual way" and listening on port 80? >>> >>> Thanks in advance for the 1% who is concerned, for all the other 99%, merry Xmas ;) >>> >>> Alfredo

1 0

Porting setlocale to Windows
by Sven Bartscher 02 Jan '15

02 Jan '15

Greetings, A while ago I took over the packages setlocale. I now would like to make it portable to windows, but came across the problem that LC_MESSAGES isn't defined on mingw. I thought about these options, for solving this: - Not defining LC_MESSAGES, which would mean, every package using LC_MESSAGES wouldn't be portable to windows. - Defining LC_MESSAGES as some an error, which would basically have the same problem as above. I don't like either of these solutions, but don't know any better. I would be happy about suggestions. Regards Sven

2 5

Re; ordNub
by Doug McIlroy 01 Jan '15

01 Jan '15

> I think the following computable function shows that it is always > possible (it chooses an order during queries): > Maintain a table, initially emtpy > As soon as (a <= b) is requested, see if a and b are already in > the table (using the computatble equality function) , if so, use > their ordering in the table. If an element is not in the table, add it. > Hence the table gives a consistent total order (it depends on which > ordering queries are requested, but that is not relevant?) This method has more than Gedanken value. Amusingly, I used it in "A killer adversary to quicksort" [Software - Practice and Experience 29 (1999) 341-344, http://www.cs.dartmouth.edu/~doug/mdmspe.pdf] to drive essentially any quicksort (illustrated with the C standard sort interface) into quadratic behavior. Doug McIlroy

1 0

Re: [Haskell-cafe] ordNub
by oleg＠okmij.org 01 Jan '15

01 Jan '15

Tom Ellis wrote: > Is there a type with an Eq instance for which on Ord instance could not also > be provided (i.e. I'm interested in Clark's "converse")? Of course. One of the very many examples is Complex. One can say we can compare complex numbers, by their absolute value. That is true; however we end up in a position where compare x y returns EQ but x==y returns False. In general, Ord represents total order. There are many structures on which total order cannot be established. Take nodes in the tree, the Tree data type. One can compare nodes by taking a parent to be less than any of its children. But then two brothers are not comparable. Searching for "partial orders" and pre-orders gives many more examples.

4 16

Negable constraints on boolean normal forms
by Yitzchak Gale 01 Jan '15

01 Jan '15

I've been using Oleg Genrus' wonderful boolean-normal-forms library lately, and enjoying it. However, I'm running into some trouble with overly constrained conversions. In theory, it's possible to convert between any of the normal forms without requiring a Negable instance on the value type. But the conversion mechanism in the library routes everything via the Boolean type class, and that ends up slapping a Negable constraint on everything. You can work around that by embedding your value type in the free Negable, conveniently provided by the library. But in practice that causes a lot of trouble. You end up being required to deal with Neg cases in your result type that are actually impossible to occur and make no sense in context. You have to throw bottoms into provably total functions, or write code that produces meaningless results that you promise in human-readable comments will never happen in practice. You also make the code less efficient by requiring extra passes over the data to "verify" that Neg isn't there. It's similar to what happens when you try to use a Monoid where a Semigroup is really needed by bolting on an artificial "empty" constructor. One thing you can do to begin with is to remove the Negable constraints on the NormalForm instances; I don't see that constraint ever being used. But that still doesn't solve the main issue. Perhaps one approach would be to define a Boolean "bi-monoid" class, where a Boolean bi-monoid is a like Boolean algebra except without negation. All of the normal forms are instances of Boolean bi-monoid even without a Negable constraint on the value type. If you then relax the constraints on the methods of CoBoolean and CoBoolean1 to Boolean bi-monoid instead of Boolean, all of the existing conversion implementations work without modification and without requiring the Negable constraint. I'm not sure if that creates other problems for some other use cases of this library though. Any thoughts? Thanks, Yitz

1 0

ordNub
by Niklas Hambüchen 01 Jan '15

01 Jan '15

tldr: nub is abnormally slow, we shouldn't use it, but we do. As you might know, Data.List.nub is O(n²). (*) As you might not know, almost *all* practical Haskell projects use it, and that in places where an Ord instance is given, e.g. happy, Xmonad, ghc-mod, Agda, darcs, QuickCheck, yesod, shake, Cabal, haddock, and 600 more (see https://github.com/nh2/haskell-ordnub) I've taken the Ord-based O(n * log n) implementation from yi using a Set: ordNub :: (Ord a) => [a] -> [a] ordNub l = go empty l where go _ [] = [] go s (x:xs) = if x `member` s then go s xs else x : go (insert x s) xs and put benchmarks on http://htmlpreview.github.io/?https://github.com/nh2/haskell-ordnub/blob/1f… (compare `nub` vs `ordNub`). `ordNub` is not only in a different complexity class, but even seems to perform better than nub for very small numbers of actually different list elements (that's the numbers before the benchmark names). (The benchmark also shows some other potential problem: Using a state monad to keep the set instead of a function argument can be up to 20 times slower. Should that happen?) What do you think about ordNub? I've seen a proposal from 5 years ago about adding a *sort*Nub function started by Neil, but it just died. (*) The mentioned complexity is for the (very common) worst case, in which the number of different elements in the list grows with the list (alias you don't have an N element list with always only 5 different things inside).

23 49

Proposed change to a couple Data.Tree functions
by David Feuer 01 Jan '15

01 Jan '15

Since you folks seem to know a lot about Data.Tree, I thought maybe you could take a look at https://github.com/haskell/containers/pull/126 and give me your thoughts. Does it look correct? Might it be efficient? David Feuer

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HaskellNet: maintainer(s) needed!
by Jonathan Daugherty 31 Dec '14

31 Dec '14

Hi, TLDR: HaskellNet needs a new maintainer! I am the current maintainer of HaskellNet, a library for accessing POP, IMAP, and SMTP services. Because I do not have the time to be the kind of maintainer this library needs, I need help. If you use HaskellNet in production, care about providing this service to the community, and/or feel qualified to provide technical support for this library, please let me know! I am looking for someone interested enough to take ownership of this package. Other than that, some kinds of help desired include: - improve documentation - improve test suite - resolve GitHub support tickets - test on Windows https://github.com/jtdaugherty/HaskellNet http://hackage.haskell.org/package/HaskellNet Thanks, -- Jonathan Daugherty Software Engineer Galois, Inc.

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Haskell Package for Fedora 21?
by Dominick Samperi 31 Dec '14

31 Dec '14

It appears that there are some problems using the Haskell Package with Fedora Linux 21. After following the instructions for Debian Linux, an attempt to start ghci yields this: $ ghci Loading packatge ghc-prim ... linking done. Loading package integer-gmp .. <command line>: can't load .so/.DLL for: libgmp.so (cannot open shared object). Adding /usr/lib64 or /usr/lib64/openssl/engines to LD_LIBRARY_PATH doesn't help, even though libgmp.so is located in the latter directory. Are there work-arounds for this? Are there best practices for Fedora 21? Compile from source? Thanks, Dominick

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Haskell Platform for Fedora 21?
by Dominick Samperi 31 Dec '14

31 Dec '14

The thread with title "Haskell Package for Fedora 21?" should have title "Haskell Platform for Fedora 21?" Sorry about the noise...

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Cabal 1.22 RC ready to test
by Johan Tibell 30 Dec '14

30 Dec '14

Hi all, I've now prepared release candidate for Cabal/cabal-install 1.22. Please test them and report bugs here and/or on the bug tracker ( https://github.com/haskell/cabal/issues) To install: cabal install http://johantibell.com/files/Cabal-1.22.0.0-rc.tar.gz http://johantibell.com/files/cabal-install-1.22.0.0-rc.tar.gz Cabal is now such a big project that's used in so many context (e.g. different OSes) that we need your help to help us test, to make sure that new releases work for you. -- Johan

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User-Controlled Defaulting Proposals?
by Andrew Gibiansky 30 Dec '14

30 Dec '14

Hey all, I wish Haskell gave typeclass authors more control over defaulting. I see there are a number of proposals here: https://ghc.haskell.org/trac/haskell-prime/wiki/Defaulting Are any of these being actively pursued? If not, why not, and if so, what proposals will potentially make it into Haskell eventually? Just curious, Andrew

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Encoding for flow charts
by Richard Wallace 30 Dec '14

30 Dec '14

Hello, I've been working to encode a fairly complex flow chart. At first I took the simple approach of having a single data type for the possible states data S = S0 | S1 | S2 and then a simple recursive function to step through it loop :: Monad m => S -> StateT s m Result loop S0 = testS0 >>= \r -> if r then loop S1 else loop S2 loop S1 = s1Result loop S2 = s2Result This works well enough, but with the size of the flow chart I'm working with (50+ decision points) it quickly becomes clear that it's probably not the most best way to do it. One problem that I'd like to solve is being able to test each decision point in isolation, making sure that decision point `m` will go to decision point `n` when it is supposed to, without having to run through the whole rest of the flow chart that would otherwise follow. Something else I'd like to achieve is being able to organize the decisions into separate modules - much better than having just the one big function above. Here's what I've come up with as an alternative so far. data Step a b where TrueFlow :: Decision a d e => a -> Step a b FalseFlow :: Decision b f g => b -> Step a b Done :: Result -> Step a b class Decision a b c | a -> b, a -> c where decide :: (Functor m, Monad m) => a -> StateT s m (Step b c) data S0 = S0 deriving (Show, Eq) instance Decision S0 S1 S2 where decide _ = bool (FalseFlow S2) (TrueFlow S1) <$> testS0 data S1 = S1 deriving (Show, Eq) instance Decision S1 () () where decide _ = s1Result data S2 = S2 deriving (Show, Eq) instance Decision S2 () () where decide _ = s2Result decision :: (Functor m, Monad m, Decision a b c) => a -> StateT s m Result decision s = decide s >>= \case TrueFlow b -> decision b FalseFlow c -> decision c Done s -> return s With this implementation it is easy to test a single decision point, - just use `evalStateT (decide S0) s` and compare the resulting Step to make sure it is either S1 or S2. The logic can also be easily separated and organized into modules. The biggest drawback of this new approach is the need to create a new `Step` record at every decision point. In the end, this may be worth it for the added design benefits, but would love to be able to get rid of it and find a solution that gives the added benefits without incurring any performance penalties. Any suggestions anyone can give on improvements would be greatly appreciated. Thanks! Rich

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macosx-app on Yosemite
by Jeffrey Brown 30 Dec '14

30 Dec '14

Dear list, I tried to install wxHaskell, following these instructions: https://www.haskell.org/haskellwiki/WxHaskell/Mac It lists some configurations known to work, but I am running OS X 10.10,which is not among them. "brew install wxmac --devel" reported no errors. Next, "cabal install wx cabal-macosx" reported no errors. Next, "ghc --make HelloWorld.hs" reported no errors. But when I try to run "macosx-app HelloWorld", I get "-bash: macosx-app: command not found". Below is a transcript of the installation process, tab-indented for ease of viewing. Many thanks, Jeff ---- this worked: Jefferys-MBP:~ jbb$ brew install wxmac --devel ==> Installing dependencies for wxmac: jpeg, libpng, libtiff ==> Installing wxmac dependency: jpeg ==> Downloading https://downloads.sf.net/project/machomebrew/Bottles/jpeg-8d.yosemi ######################################################################## 100.0% ==> Pouring jpeg-8d.yosemite.bottle.2.tar.gz [image: 🍺] /usr/local/Cellar/jpeg/8d: 18 files, 776K ==> Installing wxmac dependency: libpng ==> Downloading https://downloads.sf.net/project/machomebrew/Bottles/libpng-1.6.15. ######################################################################## 100.0% ==> Pouring libpng-1.6.15.yosemite.bottle.tar.gz [image: 🍺] /usr/local/Cellar/libpng/1.6.15: 17 files, 1.3M ==> Installing wxmac dependency: libtiff ==> Downloading https://downloads.sf.net/project/machomebrew/Bottles/libtiff-4.0.3. ######################################################################## 100.0% ==> Pouring libtiff-4.0.3.yosemite.bottle.tar.gz [image: 🍺] /usr/local/Cellar/libtiff/4.0.3: 254 files, 3.8M ==> Installing wxmac ==> Downloading https://downloads.sf.net/project/machomebrew/Bottles/wxmac-3.0.2.yo ######################################################################## 100.0% ==> Pouring wxmac-3.0.2.yosemite.bottle.9.tar.gz [image: 🍺] /usr/local/Cellar/wxmac/3.0.2: 777 files, 41M Jefferys-MBP:~ jbb$ next, this worked: Last login: Sun Dec 28 15:38:02 on ttys000 Jefferys-MBP:~ jbb$ cabal install wx cabal-macosx Resolving dependencies... Downloading fgl-5.4.2.4... Configuring wxc-0.91.0.0... Configuring fgl-5.4.2.4... Building fgl-5.4.2.4... Building wxc-0.91.0.0... Installed fgl-5.4.2.4 Downloading cabal-macosx-0.2.3... Configuring cabal-macosx-0.2.3... Building cabal-macosx-0.2.3... Installed cabal-macosx-0.2.3 Installed wxc-0.91.0.0 Downloading wxcore-0.91.0.0... Configuring wxcore-0.91.0.0... Building wxcore-0.91.0.0... Installed wxcore-0.91.0.0 Downloading wx-0.91.0.0... Configuring wx-0.91.0.0... Building wx-0.91.0.0... Installed wx-0.91.0.0 Updating documentation index /Users/jbb/Library/Haskell/share/doc/index.html Jefferys-MBP:~ jbb$ next, this failed: Jefferys-MBP:wxHaskell jbb$ date Mon Dec 29 16:56:05 PST 2014 Jefferys-MBP:wxHaskell jbb$ pwd /Users/jbb/Mine/fun/computer/Haskell/wxHaskell Jefferys-MBP:wxHaskell jbb$ ls HelloWorld.hs Jefferys-MBP:wxHaskell jbb$ ghc --make HelloWorld.hs [1 of 1] Compiling Main ( HelloWorld.hs, HelloWorld.o ) Linking HelloWorld ... Jefferys-MBP:wxHaskell jbb$ macosx-app HelloWorld -bash: macosx-app: command not found Jefferys-MBP:wxHaskell jbb$

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Do you know the author of the paper "Semantic Lego" ?
by Hidekazu IWAKI 29 Dec '14

29 Dec '14

Dear Sirs. I want to introduce the monad theory as a modular denotational semantics. So I am translating a really nice paper "Semantic Lego" into Japanese. But I have not be able to get the agreement of translating and publishing on the web. I already sent e-mails to two contact address. I know them from his homepage http://groups.csail.mit.edu/mac/users/dae/ However the addresses are not use both them. So anyone know him and his e-mail address, could you please forward the following mail to the author Dr. David Espinosa ? It should be noted he seems to work at Google. Sincerely. ------------------ My name is IWAKI Hidekazu and I am an amateur researcher of computer science and a Japanese Wikipedian. I am a Japanese living in Japan. https://ja.wikipedia.org/wiki/%E5%88%A9%E7%94%A8%E8%80%85:I.hidekazu I am concerning the methods of modular denotational semantics(the theory of monads). But for example Lian,Hudak or Wadler's paper is so difficult or not clear. so I can not explain the theory. Your Ph.D thesis "Semantic Lego" is so nice paper for learning the methods. It is very important for every computer scientists and functional programmer interesting in the theory of Monad. I want to translate this paper to Japanese. So could you give me the agreement ofJapanese translation? I am looking forward to hearing from you. Faithfully yours. -- *:..｡o○☆*ﾟ¨ﾟﾟ･*:..｡o○☆*ﾟ¨ﾟﾟ･*:..｡o○☆*ﾟ¨ﾟﾟﾟ *岩城　秀和（いわき　ひでかず） *メール: i.hidekazu at gmail.com *:..｡o○☆*ﾟ¨ﾟﾟ･*:..｡o○☆*ﾟ¨ﾟﾟ･*:..｡o○☆*ﾟ¨ﾟﾟﾟ

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Is it possible to zip sequences any faster?
by David Feuer 28 Dec '14

28 Dec '14

containers-0.5.6 has a new implementation of zipWith in Data.Sequence that, like the old one, offers O(n) zipping, but unlike the old one offers immediate O((log(min{i,n-i}))^2) access to the elements, where i is the requested index. I haven't been able to think of a way to bring that down from polylogarithmic to logarithmic. The difficulty is that t splits one of the sequences repeatedly, so reaching an element the first time is kind of like pushing a snow shovel all the way down the driveway, rather than throwing snow off to either side. Does anyone have an idea for doing it better, or a good argument that it can't be done? Thanks, David Feuer

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How to call hUpdateAtHNat with a value known only at runtime
by Phil Darnowsky 28 Dec '14

28 Dec '14

Hello all, I'm working on something that involves HLists where I'm making use of the hUpdateAtHNat function for array-like semantics, which works great if you know the index of the element you want at compile time: Prelude> :module +Data.HList Prelude Data.HList> let hl = hEnd $ hBuild "foobar" 123 True Loading [a bunch of packages] Prelude Data.HList> hl H["foobar", 123, True] Prelude Data.HList> hUpdateAtHNat hZero 90909 hl H[90909, 123, True] Prelude Data.HList> hUpdateAtHNat (hSucc hZero) 90909 hl H["foobar", 90909, True] Prelude Data.HList> hUpdateAtHNat (hSucc $ hSucc hZero) 90909 hl H["foobar", 123, 90909] So far so good. What's giving me trouble is figuring out how to update at an arbitrary index, where that index is only known at runtime, because hZero and hSucc are of different types: Prelude Data.HList> :t hZero hZero :: Proxy 'HZero Prelude Data.HList> :t hSucc hZero hSucc hZero :: Proxy ('HSucc 'HZero) What I'd ideally like is some kind of function intToHNatProxy that would take an Int and return a Proxy of the appropriate type, but I so far only kinda sorta understand how proxies and lifted types work, so I haven't yet succeeded in constructing one. Any advice would be very welcome. Thanks, Phil

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