- Haskell-Cafe - Haskell.org

Class constraints with "free" type variables and fundeps
by Francesco Mazzoli 29 Sep '12

29 Sep '12

I would expect this to work, maybe with some additional notation (a la ScopedTypeVariables) {-# LANGUAGE FunctionalDependencies #-} {-# LANGUAGE MultiParamTypeClasses #-} class Foo a b | a -> b class Foo a b => Bar a where foo :: a -> b -> c The type family equivalent works as expected: {-# LANGUAGE TypeFamilies #-} class Foo a where type T a :: * class Bar a where foo :: a -> T a -> c I can't use type families because the `Foo' I'm using is in an external library. Is there any way to achieve what I want without adding `b' to `Bar'? -- Francesco * Often in error, never in doubt

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Discovery of unnecessary build-depends
by Jason Whittle 29 Sep '12

29 Sep '12

Is there a tool available that will tell me if the cabal file for my library or application has any unnecessary build-depends? I have a habit of developing new modules within an application and then moving them out to separate libraries. For instance, I might move module Foo out of my application and into a new or existing library package. If Foo was the only module in my application to explicitly import any modules from the Bar package, I would like to know that I can now remove the dependency on Bar from my application's build-depends in the cabal file. My strategy right now would be to search my whole application for other imports of any of the modules from the Bar package, or just remove Bar from build-depends and check if the application still compiles. Neither solution is particularly scalable or satisfying. Cheers, Jason Whittle

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Re: [Haskell-cafe] Class constraints with "free" type variables and fundeps
by Francesco Mazzoli 29 Sep '12

29 Sep '12

CCing the list back. At Fri, 28 Sep 2012 13:30:52 -0700, Alexander Solla wrote: > What is the problem, exactly? It looks to me like UndecidableInstances and > ScopedTypeVariables (on foo, or its arguments) would be enough. I'm not sure what you mean. I don't see the need for UndecidableInstances, and there is no way I can see to bring the `b' into scope in the type sig for foo, ScopedTypeVariables or not - unless I'm missing something. > Also note that as stated, foo's type "is a bottom" (more specifically, is a > function onto bottom, since c is free in the class, and so foo must be > parametrically polymorphic in its return type, and so is devoid of "real" > Haskell values). Hopefully that is just an artefact of the translation to > Foo's and Bar's. Yeah the type for `foo' is irrelevant, I just needed to put something there. -- Francesco * Often in error, never in doubt

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ticketimer - haskell project on Gruender-Garage contest!
by Anton Kholomiov 28 Sep '12

28 Sep '12

I'd like to announce ticketimer.com -- it's a website that is not done yet. With ticketimer you can choose films for your local cinema. People can buy tickets in advance and promote the films they like. Do we need to eat this blockbuster stuff all the time? See how to vote for a change on http://www.indiegogo.com/ticketimer?a=1301421 We are going to use Yesod as a main weapon. Anton

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GHC 6.13 and GHC 7.6 in parallel on Linux
by spacestation＠venussociety.org 28 Sep '12

28 Sep '12

Hello to everyone, First of all sorry if my question posted to this list is maybe OFF TOPIC. Myself is using Haskell from time to time on WIN XP and OpenSuse system. So far without a problem using RWH book as reference and 6.12 version with a lot of modules installed via cabal. Also a Hugs install from source on my OpenSuse system only caused minor problems.setting a symlink solved the minor problem. But what surprised me totally was that Hugs/Cabal and (or ) GHC are using shared libraries and Hugs install interfered with cabal/ghc. I thought that both systems are totally independent from each other.Hugs a Haskell code interpreter GHV a Haskell code compiler. But now my question : How is it possible to run 2 different versions of GHC on the same computer ( OS , in my case OpenSuse 6.1x and hopefully 7.6 ) Any search for "running GHC versions in parallel only returned results for the DPH extensions wiki but not how to run two GHC versions on the same OS. http://www.haskell.org/haskellwiki/GHC/Data_Parallel_Haskell http://www.haskell.org/haskellwiki/Parallel So any feedback welcome Rgds Gottfried

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An easy way to represent and modify graphs?
by Takayuki Muranushi 27 Sep '12

27 Sep '12

tl;dr: I would like to apply genetic algorithm to directed acyclic graphs that represents arithmetic operations. Thank you for hosts, speakers and all participants in ICFP2012. I had chance to make a lot of questions (c.f. [prize.jpg]) and receive lots of stimulating ideas. I have been working on automated parallel code generation and automated tuning for hydrodynamics http://iopscience.iop.org/1749-4699/5/1/015003 ; one of my next target is model acquisition in robotics: http://creativemachines.cornell.edu/sites/default/files/moriguchi11.pdf ; both deal with computations represented in forms of directed acyclic graphs c.f. [graph-1.png] [graph-2.png]. We would like to describe them easily in Haskell, but also to mutate (add random changes) and crossover (create a new graph by mixing other two graphs.) In my previous work, I used Monad to generate graphs and used the Functional Graph Library to explicitly modify the graphs. Now I have learned that there are more elegant methods to describe the computations with sharing. But are those method also enables easy editing of the graphs? That's my question. In [data-reify.hs] I tried Observed Sharing by Gill09. Apparently, sharing is lost once we try to modify the graph. $ runhaskell data-reify.hs let [(1,GraphMul 2 2),(2,GraphAdd 3 3),(3,GraphMul 4 5),(5,GraphVar "X"),(4,GraphVal 40)] in 1 let [(1,GraphMul 2 9),(9,GraphAdd 10 13),(13,GraphMul 14 15),(15,GraphVar "X"),...,(5,GraphVar "X"),(4,GraphVal 42)] in 1 In [GenericMain.hs] I tried PHOAS by Oliveria & Cook 2012. $ runhaskell GenericMain.hs Mu ( a => Fork "Add"(Fork "Sub"(b) (Empty)) (c) b => Fork "Mul"(c) (c) c => Fork "40"(Empty) (Empty) d => Fork "X"(Empty) (Empty) ) Mu ( a => Fork "Add"(Fork "Sub"(b) (Empty)) (c) b => Fork "Mul"(c) (c) c => Fork "42"(Empty) (Empty) d => Fork "X"(Empty) (Empty) ) This time, I can modify a node without loosing the sharing. How about edges? Can you write operations such as "Choose one of the edges that point to (c) and make it point to (d)" ? Is there any good methods to describe and modify structures that has sharing (if it helps, I don't need loops.)? Or, after all is the best way explicit manipulation of the graph structure? Both information are helpful to me. Thanks in advance, -- Takayuki MURANUSHI The Hakubi Center for Advanced Research, Kyoto University http://www.hakubi.kyoto-u.ac.jp/02_mem/h22/muranushi.html

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haddock changes between 2.10 and 2.12?
by Edward Kmett 27 Sep '12

27 Sep '12

Was there some significant change/bug introduced to haddock made between 2.10 and 2.12? When I look at the haddocks for kan-extensions 3.1: Data.Functor.Yoneda<http://hackage.haskell.org/packages/archive/kan-extensions/3.1/doc/html/Dat…> which purports to have been built by haddock 2.12 it shows all sorts of spurious superclass constraints on almost every instance that should have simplified out, but going back to kan-extensions-2.7: Data.Functor.Yoneda<http://hackage.haskell.org/packages/archive/kan-extensions/2.7/doc/html/Dat…> none of this extra noise is present -- and the code in question hasn't been changed in the interim, so I'm pretty sure its not on my end. -Edward

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Can not configure haskell platform
by s9gf4ult＠gmail.com 27 Sep '12

27 Sep '12

> 13:53 razor@localhost /home/razor/tmp/haskell-platform-2012.2.0.0 % ./configure --prefix=/home/razor/.haskell > ************************************************** > * > * Haskell Platform Source Installer > * > * For the Haskell Platform 2012.2.0.0 and GHC 7.4.1 > * > ************************************************** > checking build system type... scripts/config.guess: line 156: 34:LIBC=gnu: command not found > Invalid configuration `x86_64-unknown-linux-': machine `x86_64-unknown-linux' not recognized > configure: error: /bin/sh scripts/config.sub x86_64-unknown-linux- failed ghc is available in PATH, google does not google. How to fix ?

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Haskell Weekly News: Issue 245
by Daniel Santa Cruz 26 Sep '12

26 Sep '12

Welcome to issue 245 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of September 16 to September 22, 2012. Quotes of the Week * dmwit: You have your intuition correct, despite all the pedantry in the room. * Cale: THIS CODE IS EXCELLENT CODE, I AM AN EXCELLENT ENTERPRISE PROGRAMMER ACCOMPLISHING REAL WORLD TASKS USING PATTERNS LIKE I READ ABOUT IN ALL THE JAVA BLOGS * byorgey: HASKELL is a pure, functional, strongly typed language featuring a name with all capitals, so it is suitable for beginners * ciaranm: a comonad is just a comonoid in the category of endofunctors. what's the coproblem? * dmwit: ISAAC -- ignore success at all costs * merijn: If you're not gonna use the types [by writing them] you might as well just write python * Cale: The real reason for the installed size of GHC being so large is that in the background it secretly downloads a video of SPJ riding a unicycle. Top Reddit Stories * First commercial Haskell-powered (game logic) computer game? (Nikki and the Robots Story Episodes) Domain: buy.joyridelabs.de, Score: 57, Comments: 27 On Reddit: [1] http://goo.gl/hngXz Original: [2] http://goo.gl/Y5Oax * Common misconceptions about Safe Haskell Domain: blog.ezyang.com, Score: 49, Comments: 23 On Reddit: [3] http://goo.gl/NBSx7 Original: [4] http://goo.gl/43i6n * The MonadTrans class is missing a method Domain: haskellforall.com, Score: 44, Comments: 32 On Reddit: [5] http://goo.gl/okxAX Original: [6] http://goo.gl/GnvzB * The Haskell Implementors' Workshop videos and slides available online Domain: haskell.org, Score: 43, Comments: 4 On Reddit: [7] http://goo.gl/LuORg Original: [8] http://goo.gl/pEvww * Writing javascript games in haskell : breakout Domain: jshaskell.blogspot.com, Score: 39, Comments: 10 On Reddit: [9] http://goo.gl/Dv8QK Original: [10] http://goo.gl/lXxsS * HIW 2012. Simon Peyton Jones: GHC Status Update. - YouTube Domain: youtube.com, Score: 37, Comments: 12 On Reddit: [11] http://goo.gl/3GEZK Original: [12] http://goo.gl/j7al9 * Comparing Snap's and Yesod's Template Languages Heist and Hamlet Domain: gist.github.com, Score: 36, Comments: 33 On Reddit: [13] http://goo.gl/AzaVV Original: [14] http://goo.gl/StRaF * Request: Tutorial on debugging using stack trace features of GHC 7.4 Domain: self.haskell, Score: 31, Comments: 6 On Reddit: [15] http://goo.gl/DcR1C Original: [16] http://goo.gl/DcR1C * A kind for regions Domain: monoidal.blogspot.com, Score: 29, Comments: 27 On Reddit: [17] http://goo.gl/4aBe7 Original: [18] http://goo.gl/tBDDW * [screencast] Quick introduction to Haskell using Gtk and the web Domain: vimeo.com, Score: 28, Comments: 4 On Reddit: [19] http://goo.gl/oi2IB Original: [20] http://goo.gl/LOkcf * Warp optimization - Sending header and body at once Domain: yesodweb.com, Score: 27, Comments: 17 On Reddit: [21] http://goo.gl/wstwx Original: [22] http://goo.gl/opsHb * Runtime Codegeneration in Haskell: Harpy vs. LLVM Domain: blog.wien.tomnetworks.com, Score: 24, Comments: 10 On Reddit: [23] http://goo.gl/aD0hE Original: [24] http://goo.gl/jU3nf * Show off your Vim/Emacs config! Domain: self.haskell, Score: 24, Comments: 7 On Reddit: [25] http://goo.gl/jV6Sa Original: [26] http://goo.gl/jV6Sa * Elm 0.4: Graphics Upgrade! Markdown, better for games, new optimizations, minification, ... Domain: elm-lang.org, Score: 22, Comments: 1 On Reddit: [27] http://goo.gl/SJW29 Original: [28] http://goo.gl/2Q5Fg * Making your zipper disappear with Zippo Domain: brandon.si, Score: 22, Comments: 6 On Reddit: [29] http://goo.gl/MQg77 Original: [30] http://goo.gl/Ua8a9 * [ANNOUNCE] Juicy.Pixels 2.0 - With Gif file decoding Domain: hackage.haskell.org, Score: 20, Comments: 12 On Reddit: [31] http://goo.gl/kEKrE Original: [32] http://goo.gl/EVvBh * Videos from this the CUFP 2012 in Copenhagen are now up Domain: cufp.org, Score: 17, Comments: 2 On Reddit: [33] http://goo.gl/mpe8Z Original: [34] http://goo.gl/aCB9I * Haskell developer jobs at StanChart - London, Singapore and Tianjin Domain: haskellers.com, Score: 17, Comments: 6 On Reddit: [35] http://goo.gl/t7axk Original: [36] http://goo.gl/wZrV5 * New GHCJS Hello World (can you find the hidden game?) Domain: ghcjs.github.com, Score: 17, Comments: 10 On Reddit: [37] http://goo.gl/c4BeA Original: [38] http://goo.gl/Aqh1i Top StackOverflow Questions * “What part of Milner-Hindley do you not understand?” votes: 73, answers: 4 Read on SO: [39] http://goo.gl/Nossl * Matching on type level Nat in GHC 7.6 votes: 17, answers: 1 Read on SO: [40] http://goo.gl/YijoA * Threading extra state through a parser in Scala votes: 16, answers: 1 Read on SO: [41] http://goo.gl/8cYMQ * find all types that are instances of a typeclass votes: 13, answers: 1 Read on SO: [42] http://goo.gl/ywHUN * When to use various language pragmas and optimisations? votes: 13, answers: 2 Read on SO: [43] http://goo.gl/qqtID * What are these explicit “forall”s doing? votes: 12, answers: 1 Read on SO: [44] http://goo.gl/fO0kf * How does the ST monad work? votes: 11, answers: 2 Read on SO: [45] http://goo.gl/xxntD * What are the pros and cons of adding additional functions/combinators to a typeclass definition? votes: 10, answers: 2 Read on SO: [46] http://goo.gl/HhDgT * Type parameters constraints for instances of typeclasses with kind * -> * votes: 9, answers: 2 Read on SO: [47] http://goo.gl/bGEQW * Theoretically, is this a valid comonad instance for a list? votes: 9, answers: 2 Read on SO: [48] http://goo.gl/z4DI4 * In haskell how can I uppercase a unicode character with respect to current locale votes: 9, answers: 1 Read on SO: [49] http://goo.gl/enNwK * Does haskell erase types? votes: 8, answers: 2 Read on SO: [50] http://goo.gl/YP8Nn * Is there an Iteratee-like concept which pulls data from multiple sources? votes: 8, answers: 4 Read on SO: [51] http://goo.gl/AOV0Y Until next time, Daniel Santa Cruz References 1. http://buy.joyridelabs.de/ 2. http://www.reddit.com/r/haskell/comments/106ocf/first_commercial_haskellpow… 3. http://blog.ezyang.com/2012/09/common-misconceptions-about-safe-haskell/ 4. http://www.reddit.com/r/haskell/comments/10242c/common_misconceptions_about… 5. http://www.haskellforall.com/2012/09/the-monadtrans-class-is-missing-method… 6. http://www.reddit.com/r/haskell/comments/106xwf/the_monadtrans_class_is_mis… 7. http://www.haskell.org/haskellwiki/HaskellImplementorsWorkshop/2012#Program… 8. http://www.reddit.com/r/haskell/comments/102x4p/the_haskell_implementors_wo… 9. http://jshaskell.blogspot.com/2012/09/breakout.html 10. http://www.reddit.com/r/haskell/comments/109hsb/writing_javascript_games_in… 11. http://www.youtube.com/watch?v=2Pucbf8_hGQ&feature=plcp 12. http://www.reddit.com/r/haskell/comments/102t49/hiw_2012_simon_peyton_jones… 13. https://gist.github.com/3757918 14. http://www.reddit.com/r/haskell/comments/107v79/comparing_snaps_and_yesods_… 15. http://www.reddit.com/r/haskell/comments/104xa2/request_tutorial_on_debuggi… 16. http://www.reddit.com/r/haskell/comments/104xa2/request_tutorial_on_debuggi… 17. http://monoidal.blogspot.com/2012/09/a-kind-for-regions.html 18. http://www.reddit.com/r/haskell/comments/102v1x/a_kind_for_regions/ 19. https://vimeo.com/49918847 20. http://www.reddit.com/r/haskell/comments/10az3o/screencast_quick_introducti… 21. http://www.yesodweb.com/blog/2012/09/header-body 22. http://www.reddit.com/r/haskell/comments/104sx0/warp_optimization_sending_h… 23. http://blog.wien.tomnetworks.com/2012/09/17/runtime-codegeneration-in-haske… 24. http://www.reddit.com/r/haskell/comments/1014ct/runtime_codegeneration_in_h… 25. http://www.reddit.com/r/haskell/comments/10b20f/show_off_your_vimemacs_conf… 26. http://www.reddit.com/r/haskell/comments/10b20f/show_off_your_vimemacs_conf… 27. http://elm-lang.org/blog/announce/version-0.4.0.elm 28. http://www.reddit.com/r/haskell/comments/102zqr/elm_04_graphics_upgrade_mar… 29. http://brandon.si/code/making-your-zipper-disappear-with-zippo/ 30. http://www.reddit.com/r/haskell/comments/103rhu/making_your_zipper_disappea… 31. http://hackage.haskell.org/package/JuicyPixels-2.0 32. http://www.reddit.com/r/haskell/comments/zziwm/announce_juicypixels_20_with… 33. http://cufp.org/videos 34. http://www.reddit.com/r/haskell/comments/102rer/videos_from_this_the_cufp_2… 35. http://www.haskellers.com/jobs/19 36. http://www.reddit.com/r/haskell/comments/106wat/haskell_developer_jobs_at_s… 37. http://ghcjs.github.com/bin/ghcjs-hello.jsexe 38. http://www.reddit.com/r/haskell/comments/10b6h3/new_ghcjs_hello_world_can_y… 39. http://stackoverflow.com/questions/12532552/what-part-of-milner-hindley-do-… 40. http://stackoverflow.com/questions/12463474/matching-on-type-level-nat-in-g… 41. http://stackoverflow.com/questions/12487827/threading-extra-state-through-a… 42. http://stackoverflow.com/questions/12524034/find-all-types-that-are-instanc… 43. http://stackoverflow.com/questions/12534145/when-to-use-various-language-pr… 44. http://stackoverflow.com/questions/12519878/what-are-these-explicit-foralls… 45. http://stackoverflow.com/questions/12468622/how-does-the-st-monad-work 46. http://stackoverflow.com/questions/12515655/what-are-the-pros-and-cons-of-a… 47. http://stackoverflow.com/questions/12497542/type-parameters-constraints-for… 48. http://stackoverflow.com/questions/12537149/theoretically-is-this-a-valid-c… 49. http://stackoverflow.com/questions/12537377/in-haskell-how-can-i-uppercase-… 50. http://stackoverflow.com/questions/12468722/does-haskell-erase-types 51. http://stackoverflow.com/questions/12496654/is-there-an-iteratee-like-conce…

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Haskell Programmers for Distributed Systems
by Richard Lyman 26 Sep '12

26 Sep '12

Hi folks, Parallel Scientific is looking for Haskell developers for distributed systems work. Parallel Scientific, Inc. is a Boulder, CO based funded startup company working in the area of scalable parallelization. We seek experienced Haskell programmers to build a sophisticated, ultra-scalable high availability framework to be used in conjunction with scientific applications, big data applications and I/O systems. We are primarily seeking candidates in Northern England, central France and/or Germany but candidates in other locations will be considered. Relocation to these places can be discussed. Parallel Scientific was founded by Dr Peter Braam in 2010. Peter formerly taught mathematics and computer science at Oxford and Carnegie Mellon. Then he contributed file systems to Linux and invented Lustre (which provides storage to 9 of the top 10 systems in the world). He ran several successful startups, and Parallel Scientific is run by an very experienced management team and board. Successful candidates will work in a modern virtual environment in their homes. Some travel may be required. We provide training in advanced processes for software design and implementation and domain specific knowledge. Required skills: Capability to design and implement sophisticated distributed algorithms (quorum, liveness, lock management, barriers) Very strong background in computer science or mathematics Experience with Haskell, or network and cluster programming in other functional languages (e.g. Erlang) Knowledge of systems programming and operating systems functionality Experience in one or more of the following areas is desirable: Experience in high availability clustering projects Experience with internals of sophisticated distributed I/O and/or database systems To apply, please send a resume to jobs(a)parsci.com. Thanks... Rich -- Rich Lyman richard.lyman(a)parsci.com

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