October 2013 - Haskell-Cafe

Weird issue with ConstraintKinds
by Jacques Carette 26 Oct '13

26 Oct '13

Consider the following (minimal, for illustration purposes) code: {-# LANGUAGE ConstraintKinds #-} module Weird where class A a where class B b where class C c where data X a = X a data Y a = Y a -- works fine, but can be verbose when things multiply instance (A a, B a) => C (X a) where -- So use ConstraintKinds: type D a = (A a, B a) instance D a => C (Y a) where ===== and now I get Variable `a' occurs more often than in the instance head in the constraint: D a (Use -XUndecidableInstances to permit this) In the instance declaration for `C (Y a)' Why?? Since D is an abbreviation, why would it behave differently than when I hand expand it? [This is with GHC 7.6.3] Jacques

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ANNOUNCE: Ajhc Haskell Compiler 0.8.0.9 Release
by Kiwamu Okabe 26 Oct '13

26 Oct '13

We are happy to announce Ajhc 0.8.0.9. This version is a point release to fix some BUGs, and support Android NDK platform. You can get Ajhc using "cabal install ajhc" command. The usage is found at Ajhc's project web site http://ajhc.metasepi.org/. The source code at https://github.com/ajhc/ajhc. Welcome sending any bugs or your ideas to https://github.com/ajhc/ajhc/issues. ## Support new platforms ### Android NDK http://developer.android.com/tools/sdk/ndk/index.html The demo movie at http://www.youtube.com/watch?v=n6cepTfnFoo. The touchable cube application is written with Haskell and compiled by Ajhc. In the demo, the application is breaked by ndk-gdb debugger when running GC. You could watch the demo source code at \url{https://github.com/ajhc/demo-android-ndk}. ## Other changes * Experimental support Mac OS X to compile Ajhc and use it. * Start to run buid test for Mac OS X on Travis CI. https://travis-ci.org/ajhc/ajhc/builds/12218394 * Depend on derive package. But near future use DrIFT again. For Haskell 98 compliant. https://github.com/ajhc/ajhc/issues/47 * Fix the bugs forgetting to clear memory. Enjoy! :) - - - Metasepi team

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Horray, a ghc bug!
by Andreas Abel 25 Oct '13

25 Oct '13

You do not get them often, but here is one... [208 of 254] Compiling Agda.TypeChecking.MetaVars.Occurs ( src/full/Agda/TypeChecking/MetaVars/Occurs.hs, dist/build/Agda/TypeChecking/MetaVars/Occurs.o ) ghc: panic! (the 'impossible' happened) (GHC version 7.4.1 for x86_64-unknown-linux): compiler/rename/RnSource.lhs:429:14-81: Irrefutable pattern failed for pattern Data.Maybe.Just (inst_tyvars, _, SrcLoc.L _ cls, _) Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug -- Andreas Abel <>< Du bist der geliebte Mensch. Theoretical Computer Science, University of Munich Oettingenstr. 67, D-80538 Munich, GERMANY andreas.abel(a)ifi.lmu.de http://www2.tcs.ifi.lmu.de/~abel/

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Trouble w/ ExitFailure 139 (Segmentation Fault), during Cabal installs.
by David Banas 25 Oct '13

25 Oct '13

Hi all, Prolog: I have installed GHC v7.6.3, from source, on Ubuntu 13.0.4. I think I've made my GHC v7.6.3 installation unstable, because I'm having lots of trouble with "ExitFailure 139"s when cabal installing. And I'm wondering if this is a pertinent clue: dbanas@dbanas-lap:~/prj/plush$ cabal unpack lifted-base Unpacking to lifted-base-0.2.1.0/ dbanas@dbanas-lap:~/prj/plush$ pushd lifted-base-0.2.1.0/ ~/prj/plush/lifted-base-0.2.1.0 ~/prj/plush ~ dbanas@dbanas-lap:~/prj/plush/lifted-base-0.2.1.0$ ghc --make Setup.hs [1 of 1] Compiling Main ( Setup.hs, Setup.o ) Linking Setup ... dbanas@dbanas-lap:~/prj/plush/lifted-base-0.2.1.0$ ./Setup configure Segmentation fault (core dumped) dbanas@dbanas-lap:~/prj/plush/lifted-base-0.2.1.0$ head Setup.hs #! /usr/bin/env runhaskell {-# LANGUAGE NoImplicitPrelude, UnicodeSyntax #-} module Main (main) where ------------------------------------------------------------------------------- -- Imports ------------------------------------------------------------------------------- dbanas@dbanas-lap:~/prj/plush/lifted-base-0.2.1.0$ chmod 755 Setup.hs dbanas@dbanas-lap:~/prj/plush/lifted-base-0.2.1.0$ ./Setup.hs configure Configuring lifted-base-0.2.1.0... Setup.hs: At least the following dependencies are missing: base-unicode-symbols >=0.1.1 && <0.3, monad-control ==0.3.*, transformers-base ==0.4.* dbanas@dbanas-lap:~/prj/plush/lifted-base-0.2.1.0$ What I'm attempting to call attention too, with the above excerpt, is this: *If I compile and run "Setup configure" I get a core dump, but if I just run "Setup.hs configure" (taking advantage of the fact that Setup.hs begins with a shebang line) it works fine.* Is this a clue as to how I've horked up my ghc installation? Thanks! -db

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to make a statically known sized non-empty list
by Markus Läll 25 Oct '13

25 Oct '13

Hi haskell-cafe, so for fun and profit I was trying to make a non-empty list data type, where the size of the non-empty prefix would be known for the type-system with the following code, but it fails with an error "No instance for (Functor (NonEmpty * a)) ..." for the last line. To me it seems that all should be known, but something must be missing.. data Nat = Zero | Succ Nat data family NonEmpty (n :: Nat) a data instance NonEmpty Zero a = Tail [a] data instance NonEmpty (Succ n) a = Head a (NonEmpty n a) instance Functor (NonEmpty Zero) where fmap f (Tail xs) = Tail (fmap f xs) instance Functor (NonEmpty (Succ a)) where fmap f (Head x xs) = Head (f x) (fmap f xs) -- Markus Läll

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Haskell Weekly News: Issue 284
by Daniel Santa Cruz 24 Oct '13

24 Oct '13

Welcome to issue 284 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of October 13 to 19, 2013. Quotes of the Week * Polarina: Once you get something with [lens] to work correctly, all other Haskell code looks like C++ in comparison. Top Reddit Stories * The GHC October status report - a nice summary of what to expect from 7.8 Domain: ghc.haskell.org, Score: 106, Comments: 51 On Reddit: [1] http://goo.gl/ZOYHI7 Original: [2] http://goo.gl/1PM4s7 * Haskell for all: An all-atom protein search engine powered by Haskell Domain: haskellforall.com, Score: 84, Comments: 51 On Reddit: [3] http://goo.gl/TwntnD Original: [4] http://goo.gl/4WbmZp * FSF - Recognizing an inspiring woman for Ada Lovelace Day: Haskell/Perl hacker Audrey Tang Domain: fsf.org, Score: 82, Comments: 15 On Reddit: [5] http://goo.gl/XbrLS7 Original: [6] http://goo.gl/zGTnbL * Automatic SIMD Vectorization for Haskell and ICFP 2013 Domain: dorchard.wordpress.com, Score: 50, Comments: 3 On Reddit: [7] http://goo.gl/ikP8nw Original: [8] http://goo.gl/kYCQyW * A Little Lens Starter Tutorial - School of Haskell Domain: fpcomplete.com, Score: 49, Comments: 43 On Reddit: [9] http://goo.gl/RQjC2m Original: [10] http://goo.gl/LA7fUo * Abstractions for the Functional Roboticist with Anthony Cowley at NY Haskell Domain: vimeo.com, Score: 43, Comments: 13 On Reddit: [11] http://goo.gl/f4j0CP Original: [12] http://goo.gl/pSZBNR * The "magic" of "join (,)" Domain: self.haskell, Score: 42, Comments: 18 On Reddit: [13] http://goo.gl/oVjtNL Original: [14] http://goo.gl/oVjtNL * A Taste of Agda with Francesco Mazzoli at NY Haskell Domain: vimeo.com, Score: 41, Comments: 15 On Reddit: [15] http://goo.gl/anu7xJ Original: [16] http://goo.gl/vIfDNM * [Demo movie] Ajhc Haskell compiler can create Haskell Apps running on Android NDK! Domain: youtube.com, Score: 36, Comments: 22 On Reddit: [17] http://goo.gl/HM4AJ3 Original: [18] http://goo.gl/j1Gmqi * Open question: help me design a new encoding API for aeson Domain: serpentine.com, Score: 31, Comments: 30 On Reddit: [19] http://goo.gl/CE6Q0I Original: [20] http://goo.gl/ikdO5O * Made some Haskell Wallpapers, thought I'd share Domain: imgur.com, Score: 30, Comments: 27 On Reddit: [21] http://goo.gl/A1LILP Original: [22] http://goo.gl/c054gN * How to use Travis-CI with multiple GHC versions Domain: github.com, Score: 30, Comments: 6 On Reddit: [23] http://goo.gl/hsH9a7 Original: [24] http://goo.gl/K2Eja5 * A Gentle Introduction to Parsec Domain: blog.barrucadu.co.uk, Score: 29, Comments: 1 On Reddit: [25] http://goo.gl/GHUil8 Original: [26] http://goo.gl/21xyq2 * Haskell.org committee is looking for self-nominations Domain: haskell.org, Score: 28, Comments: 2 On Reddit: [27] http://goo.gl/B3pc80 Original: [28] http://goo.gl/19dZX4 * Backpropogation, Steepest Descent, and Automatic Differentiation Domain: idontgetoutmuch.wordpress.com, Score: 23, Comments: 22 On Reddit: [29] http://goo.gl/izxqMg Original: [30] http://goo.gl/F2fjNk * Anonymous records. A solution to the problems of record-system. Domain: gist.github.com, Score: 22, Comments: 24 On Reddit: [31] http://goo.gl/CjdbT3 Original: [32] http://goo.gl/UY5OqT Top StackOverflow Questions * Is there a canonical haskell type for “One or Both”? votes: 15, answers: 3 Read on SO: [33] http://goo.gl/yn8MkJ * Show progress of Haskell program votes: 14, answers: 4 Read on SO: [34] http://goo.gl/IEegAQ * Does Data.ByteString.readFile block all threads? votes: 13, answers: 3 Read on SO: [35] http://goo.gl/5X805o * Am I abusing unsafePerformIO? votes: 11, answers: 3 Read on SO: [36] http://goo.gl/SnfOcK * GHCJS: How do I import a high order javascript function using FFI? votes: 10, answers: 1 Read on SO: [37] http://goo.gl/fCbPE2 * Simulating a spring/damper system in Yampa votes: 9, answers: 1 Read on SO: [38] http://goo.gl/zQ50Ao Until next time, [39]+Daniel Santa Cruz References 1. http://ghc.haskell.org/trac/ghc/wiki/Status/Oct13 2. http://www.reddit.com/r/haskell/comments/1opw22/the_ghc_october_status_repo… 3. http://www.haskellforall.com/2013/10/an-all-atom-protein-search-engine.html 4. http://www.reddit.com/r/haskell/comments/1oc5g5/haskell_for_all_an_allatom_… 5. https://www.fsf.org/blogs/community/recognizing-an-inspiring-woman-for-ada-… 6. http://www.reddit.com/r/haskell/comments/1oj2u4/fsf_recognizing_an_inspirin… 7. http://dorchard.wordpress.com/2013/10/14/automatic-simd-vectorization-for-h… 8. http://www.reddit.com/r/haskell/comments/1og58l/automatic_simd_vectorizatio… 9. https://www.fpcomplete.com/user/tel/a-little-lens-starter-tutorial 10. http://www.reddit.com/r/haskell/comments/1obpxb/a_little_lens_starter_tutor… 11. http://vimeo.com/rickasaurus/haskell-robots 12. http://www.reddit.com/r/haskell/comments/1oo2at/abstractions_for_the_functi… 13. http://www.reddit.com/r/haskell/comments/1olk04/the_magic_of_join/ 14. http://www.reddit.com/r/haskell/comments/1olk04/the_magic_of_join/ 15. https://vimeo.com/77168227 16. http://www.reddit.com/r/haskell/comments/1oq0dt/a_taste_of_agda_with_france… 17. http://www.youtube.com/watch?v=n6cepTfnFoo 18. http://www.reddit.com/r/haskell/comments/1oq22l/demo_movie_ajhc_haskell_com… 19. http://www.serpentine.com/blog/2013/10/14/open-question-help-me-design-a-ne… 20. http://www.reddit.com/r/haskell/comments/1ohe9m/open_question_help_me_desig… 21. http://imgur.com/a/DRdNi 22. http://www.reddit.com/r/haskell/comments/1om4ie/made_some_haskell_wallpaper… 23. https://github.com/hvr/multi-ghc-travis 24. http://www.reddit.com/r/haskell/comments/1os3f6/how_to_use_travisci_with_mu… 25. http://blog.barrucadu.co.uk/2013/05/27/a-gentle-introduction-to-parsec/ 26. http://www.reddit.com/r/haskell/comments/1ojsi4/a_gentle_introduction_to_pa… 27. http://www.haskell.org/pipermail/haskell-cafe/2013-October/110997.html 28. http://www.reddit.com/r/haskell/comments/1onnab/haskellorg_committee_is_loo… 29. http://idontgetoutmuch.wordpress.com/2013/10/13/backpropogation-is-just-ste… 30. http://www.reddit.com/r/haskell/comments/1odmk1/backpropogation_steepest_de… 31. https://gist.github.com/nikita-volkov/6977841 32. http://www.reddit.com/r/haskell/comments/1oicmu/anonymous_records_a_solutio… 33. http://stackoverflow.com/questions/19413438/is-there-a-canonical-haskell-ty… 34. http://stackoverflow.com/questions/19343695/show-progress-of-haskell-program 35. http://stackoverflow.com/questions/19347907/does-data-bytestring-readfile-b… 36. http://stackoverflow.com/questions/19371636/am-i-abusing-unsafeperformio 37. http://stackoverflow.com/questions/19415306/ghcjs-how-do-i-import-a-high-or… 38. http://stackoverflow.com/questions/19461227/simulating-a-spring-damper-syst… 39. https://plus.google.com/105107667630152149014/about

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mavericks + xcode 5 quick fix directions
by Carter Schonwald 24 Oct '13

24 Oct '13

Hey Everyone, I've synthesized some quick fix directions that let you use a preexisting GHC install despite the Mavericks upgrade https://gist.github.com/cartazio/7131371 please share with folks who are struggling with mavericks upgrade breaking everything :) thanks! -Carter

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Re: [Haskell-cafe] Why do I need UndecidableInstances?
by adam vogt 24 Oct '13

24 Oct '13

On Thu, Oct 24, 2013 at 11:23 AM, Stijn van Drongelen <rhymoid(a)gmail.com> wrote: > Hi, > > As far as I know, you're referring to the Paterson Conditions. Condition 'b' > kind of corresponds to "at least one of the types is shrinking", while > condition 'a' corresponds to "all the other are not growing", so the rules > are already formulated in the way you suggest. The Paterson Conditions are > also not the issue here; the Coverage Condition is. > >> The equivalent code using type families doesn't need >> undecidable instances in my ghc-7.6.2. > > > This is because (I think) type family instances are not subject to the > Coverage Condition. In principle, type families are to type classes as > functions are to relations. Functional dependencies may be used to make type > classes more 'function-like'. I guess(!) the Coverage Condition is trying to > capture this notion. > > I don't know why it's formulated in this way, because in Alejandro's > example, the classes really look 'function-like'. > > Kind regards, > > Stijn van Drongelen Hi Stijn, Thanks for clarifying the issue. There is an example of why the coverage condition is needed on line 512 of (github doesn't handle line numbers in lhs correctly unfortunately): https://github.com/ghc/ghc/blob/9a04e1c7257f93f0c1cadc56a5e2a177574f6871/co… It doesn't seem possible to write a type for that function with just a type family; you need a class Mul to get into an infinite loop while looking for instances. So it seems appropriate that the coverage condition does not apply to type families. Regards, Adam

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Better versions of this code
by Tyson Whitehead 24 Oct '13

24 Oct '13

Hey All, I wrote some quick code to change mediawiki preformatted text (adjacent lines begining with a ' ') to nowiki blocks (first line with a ' ' and rest enclosed in <nowiki>...</nowiki> quotes). http://lpaste.net/94688 I frequently find myself pretty unsatisfied with this type of code though. Anyone have any rewrites that would help teach me how to do this more elegantly in the future. Thanks! -Tyson PS: The mediawiki text I was using the above on has no existing <nowiki>...</nowiki> blocks.

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Why do I need UndecidableInstances?
by Alejandro Serrano Mena 24 Oct '13

24 Oct '13

Dear café, I'm trying to compile a set of simple examples using Functional Dependencies. One of the first ones is the case of natural numbers, which I've defined along with some operations as: data Zero data Succ a class Plus x y z | x y -> z instance Plus Zero x x instance Plus x y z => Plus (Succ x) y (Succ z) class Max x y z | x y -> z instance Max Zero y y instance Max (Succ x) Zero (Succ x) instance Max x y z => Max (Succ x) (Succ y) (Succ z) I thought the compiler would accept this only with the extensions EmptyDataDecls, MultiParamTypeClasses, FunctionalDependencies and FlexibleInstances. But, to my surprise, GHC is also asking me for UndecidableInstances. Why is this the case? Thanks in advance :)

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