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The amount of CPP we have to use is getting out of hand
by Johan Tibell 09 Jan '15

09 Jan '15

Hi, (This was initially written as a Google+ post, but I'm reposting it here to raise awareness of the issue.) The amount of CPP we have to use in Haskell is getting a bit out of hand. Here are the number of modules, per library, that use CPP for some of the libraries I maintain: containers 18/18 hashable 4/5 unordered-containers 6/9 network 3/7 cassava 4/16 cabal/cabal-install 13/75 cabal/Cabal 7/78 ekg 1/15 If this doesn't look like a lot to you (I hope it does!) consider than some languages don't use CPP at all (e.g. Java). CPP really sucks from a maintenance perspective: * It's not Haskell, but this bizarre string concatenation language. * The code is harder to read, bitrots more easily, and is harder to test. * The code can't be compiled without using Cabal (which generates some of the CPP macros for us.) This hurts e.g. ad-hoc testing/benchmarking. There are a couple of reasons we use CPP, but the main one is breaking changes in GHC and libraries we depend on. We need to reduce these kind of breakages in the future. Dealing with breakages and maintaining the resulting CPP-ed code is costing us time we could spend on other things, such as improving our libraries or writing new ones. I for one would like to get on with writing applications instead of spending time on run-of-the-mill libraries. Often these breaking changes are done in the name of "making things cleaner". Breaking changes, no matter how well-intended, doesn't make code cleaner, it makes it less clean*. Users end up having to use *both* the old "unclean" API *and* the new "clean" API. The right way to move to evolve an new API is to add new functions and data types, not modify old ones, whenever possible. * It takes about 3 major GHC releases (~3 years) before you can remove the CPP, but since new things keep breaking all the time you always have a considerable amount of CPP. -- Johan

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PSA regarding Cabal-1.22 and ghc-mod
by Anthony Cowley 09 Jan '15

09 Jan '15

I was lucky to avoid this myself, but I keep seeing it crop up: ghc-mod doesn't work with GHC < 7.10 and Cabal-1.22. If you are still on GHC 7.8.3 or 7.8.4, you may want to hold off on updating cabal or cabal-install. This isn't really my news to report, but folks need to know. See this issue for more information: https://github.com/kazu-yamamoto/ghc-mod/issues/417#issuecomment-68819938 Anthony

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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...@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...@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...@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...@gmail.com<javascript:> ball...@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. 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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. 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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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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

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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

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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...@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...@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...@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...@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...@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...@haskell.org<javascript:> >>> http://www.haskell.org/mailman/listinfo/haskell-cafe >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskel...@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...@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.

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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