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Haskell Weekly News: Issue 230
by Daniel Santa Cruz 06 Jun '12

06 Jun '12

Welcome to issue 230 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of May 27 to June 02, 2012. Announcements Mark Lentczner had great news for us this week, announcing the release of the Haskell Platform 2012.2.0.0. Many many thanks to the platform team for getting this release out the door! [1] http://goo.gl/XX9P6 Top Reddit Stories * Conduit bugs Domain: haskellforall.com, Score: 45, Comments: 25 On Reddit: [2] http://goo.gl/hQCbs Original: [3] http://goo.gl/3wR5W * Response to "Conduit Bugs" Domain: yesodweb.com, Score: 43, Comments: 53 On Reddit: [4] http://goo.gl/TV5i3 Original: [5] http://goo.gl/N8uMM * Haskell fully async Chan/TChan vs. Go's channels Domain: plus.google.com, Score: 42, Comments: 14 On Reddit: [6] http://goo.gl/UJBXN Original: [7] http://goo.gl/K0zzA * Most Important GHC extensions to learn/use? Domain: permalink.gmane.org, Score: 38, Comments: 11 On Reddit: [8] http://goo.gl/i6ofz Original: [9] http://goo.gl/x7Mxg * What's so bad about Template Haskell? Domain: stackoverflow.com, Score: 35, Comments: 8 On Reddit: [10] http://goo.gl/fdiUm Original: [11] http://goo.gl/Vg4AT * stylish-haskell: A simple tool to clean up Haskell code Domain: github.com, Score: 32, Comments: 74 On Reddit: [12] http://goo.gl/QT9br Original: [13] http://goo.gl/TlZum * Understanding Haskell's monomorphism restriction Domain: self.haskell, Score: 27, Comments: 38 On Reddit: [14] http://goo.gl/tb1uC Original: [15] http://goo.gl/tb1uC * Pipes 2.0 vs pipes-core Domain: paolocapriotti.com, Score: 26, Comments: 40 On Reddit: [16] http://goo.gl/lmNeD Original: [17] http://goo.gl/najMM * FP Complete and Well-Typed delivered a two-day hands-on Haskell training at Qualcomm Domain: haskell.org, Score: 26, Comments: 2 On Reddit: [18] http://goo.gl/R3Ufl Original: [19] http://goo.gl/sC3O3 * ICFP'12 Accepted Papers (with links, where available!) Domain: github.com, Score: 26, Comments: 2 On Reddit: [20] http://goo.gl/361wt Original: [21] http://goo.gl/3ACvz Top StackOverflow Questions * What's so bad about Template Haskell? votes: 67, answers: 3 Read on SO: [22] http://goo.gl/Vg4AT * Which Haskell (GHC) extensions should users use/avoid? votes: 23, answers: 1 Read on SO: [23] http://goo.gl/s0TNH * Space leak with redundant use of seq in GHC interpreter votes: 17, answers: 2 Read on SO: [24] http://goo.gl/no97P * Is there a list of GHC extensions that are considered 'safe'? votes: 15, answers: 1 Read on SO: [25] http://goo.gl/10SJc * What are lenses used/useful for? votes: 14, answers: 4 Read on SO: [26] http://goo.gl/kn8uj * Haskell: Can I perform several folds over the same lazy list without keeping list in memory? votes: 12, answers: 2 Read on SO: [27] http://goo.gl/VuWQ7 * What are the advantages and disadvantages of using lenses? votes: 11, answers: 2 Read on SO: [28] http://goo.gl/kGX4B * Calculating work done by f x = (x,x) votes: 11, answers: 1 Read on SO: [29] http://goo.gl/838C2 * Why is function definition for all types at once not allowed in Haskell? votes: 10, answers: 5 Read on SO: [30] http://goo.gl/eoDXp * Generic variant of bi f a b = (f a, f b) votes: 9, answers: 4 Read on SO: [31] http://goo.gl/NPDMU * Unicode console I/O in Haskell on Windows votes: 9, answers: 1 Read on SO: [32] http://goo.gl/AFCQh Until next time, Daniel Santa Cruz References 1. http://comments.gmane.org/gmane.comp.lang.haskell.platform/1964 2. http://www.haskellforall.com/2012/05/conduit-bugs.html 3. http://www.reddit.com/r/haskell/comments/u7vyx/conduit_bugs/ 4. http://www.yesodweb.com/blog/2012/05/response-conduit-bugs 5. http://www.reddit.com/r/haskell/comments/u8fe6/response_to_conduit_bugs/ 6. https://plus.google.com/109566665911385859313/posts/FAmNTExSLtz 7. http://www.reddit.com/r/haskell/comments/ubmz7/haskell_fully_async_chantcha… 8. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/98574 9. http://www.reddit.com/r/haskell/comments/ufenx/most_important_ghc_extension… 10. http://stackoverflow.com/questions/10857030/whats-so-bad-about-template-has… 11. http://www.reddit.com/r/haskell/comments/ugd94/whats_so_bad_about_template_… 12. https://github.com/jaspervdj/stylish-haskell/blob/master/README.markdown 13. http://www.reddit.com/r/haskell/comments/u8u1y/stylishhaskell_a_simple_tool… 14. http://www.reddit.com/r/haskell/comments/udnx2/understanding_haskells_monom… 15. http://www.reddit.com/r/haskell/comments/udnx2/understanding_haskells_monom… 16. http://paolocapriotti.com/blog/2012/05/29/pipes-2.0-vs-pipes-core/ 17. http://www.reddit.com/r/haskell/comments/uav9d/pipes_20_vs_pipescore/ 18. http://www.haskell.org/pipermail/haskell-cafe/2012-June/101558.html 19. http://www.reddit.com/r/haskell/comments/ug4wq/fp_complete_and_welltyped_de… 20. https://github.com/technogeeky/icfp12-paper-links/blob/master/README.md 21. http://www.reddit.com/r/haskell/comments/uhhd2/icfp12_accepted_papers_with_… 22. http://stackoverflow.com/questions/10857030/whats-so-bad-about-template-has… 23. http://stackoverflow.com/questions/10845179/which-haskell-ghc-extensions-sh… 24. http://stackoverflow.com/questions/10793371/space-leak-with-redundant-use-o… 25. http://stackoverflow.com/questions/10830757/is-there-a-list-of-ghc-extensio… 26. http://stackoverflow.com/questions/10788261/what-are-lenses-used-useful-for 27. http://stackoverflow.com/questions/10803221/haskell-can-i-perform-several-f… 28. http://stackoverflow.com/questions/10774614/what-are-the-advantages-and-dis… 29. http://stackoverflow.com/questions/10825933/calculating-work-done-by-f-x-x-x 30. http://stackoverflow.com/questions/10811657/why-is-function-definition-for-… 31. http://stackoverflow.com/questions/10772121/generic-variant-of-bi-f-a-b-f-a… 32. http://stackoverflow.com/questions/10779149/unicode-console-i-o-in-haskell-…

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HaL7: Haskell-Treffen in Halle/Saale, Freitag 2012-07-13
by Henning Thielemann 06 Jun '12

06 Jun '12

For our German readers: Es ist wieder so weit: Am Freitag, 13. Juli laden wir zum 7. Haskell-Treffen HaL ein, diesmal nach Halle an der Saale. Wir bieten eine bunte Mischung aus Tutorien zum Mitmachen, Vortraegen zum Anhoeren und Gelegenheiten zum Fachsimpeln, theoretisches wie praktisches, ernstes wie heiteres. Das Programm im Schnelldurchlauf ist: vormittags 6 Tutorien 09:30 - 11:00 snap repa schule 11:30 - 13:00 reactive generics liveseq nachmittags 4 Vortraege 14:30 - 15:15 Dorn: Schule 15:15 - 16:00 Voigtlaender: ADP 16:30 - 17:15 Goergens: XenClient 17:15 - 18:00 Eijkel: Hommage sonstiges: 09:00 - 11:30 Installationshilfen 11:00 - 11:30 Kaffeepause 13:00 - 14:30 Mittagspause 16:00 - 16:30 Kaffeepause 19:00 - Ende Abendessen und Fachsimpelei im Restaurant Die Details finden Sie hier: http://iba-cg.de/hal7.html und die Anmeldung da: http://sim.mathematik.uni-halle.de:8080/hal7/ Bitte melden Sie sich bis zum 4. Juli an. Wir freuen uns auf Ihr zahlreiches Erscheinen! für das Programmkomitee Henning Thielemann

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Typing Judgments for Haskell2010++
by Shayan Najd Javadipour 05 Jun '12

05 Jun '12

Hi Haskell Cafe! To develop Haskell-Type-Exts (HTE) [1,2], I need to have typing judgments for each language construct in Haskell 2010 with two extensions: RankNTypes and local assumptions (GADTs, Type Functions and etc). Since I could not find any academic paper describing a type system supporting both local assumptions [3] and RankNTypes [4], we tried to combine the two systems. We also tried to extend the type system to support Patterns and some other constructs. The result (in a very sketchy state) is posted at: http://cleantypecheck.wordpress.com/2012/06/06/typing-judgment-rules-14/ Now before I start to implement these rules in HTE, I need to make sure of their validity. Therefore, I'd highly appreciate if experts would take a closer look at these judgement rules and comment. Considering the fact that HTE is a Google SoC project and me being just a motivated student (limited knowledge/experience), the success of this project strongly relies on Haskell community's support. Thanks! /Shayan [1] http://www.google-melange.com/gsoc/project/google/gsoc2012/shayannajd/18001 [2] http://cleantypecheck.wordpress.com/2012/05/03/kickoff/ [3] D. Vytiniotis, S. Peyton Jones, T. Schrijvers, M. Sulzmann. OutsideIn(X) – Modular type inference with local assumptions [4] S. Peyton Jones, D. Vytiniotis, S. Weirich, and M. Shields. Practical type inference for arbitrary-rank types.

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[ANNOUNCE] cereal-0.3.5.2
by Trevor Elliott 05 Jun '12

05 Jun '12

Hi Everyone, I'm happy to announce version 0.3.5.2[1] of the cereal serialization library. This is a bugfix release, mostly improving performance characteristics of the library. One noteable change is with runGetLazy, which will now perform as expected when a parser attempts to consume more input than is available. Happy hacking! --trevor [1] http://hackage.haskell.org/package/cereal-0.3.5.2

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converting functional dependencies to type families
by Henning Thielemann 05 Jun '12

05 Jun '12

Hi all, when I reported a typechecker performance problem related to functional dependencies http://hackage.haskell.org/trac/ghc/ticket/5970 I promised to try to convert from functional dependencies to type families. Thus I converted my code and the llvm package to type-families: http://code.haskell.org/~thielema/llvm-tf/ Here are some of my experiences: == Advantages of TypeFamilies == * Speed For what I did the type families solution was considerably faster than the functional dependencies code at least in GHC-7.4.1. Thus the bug in ticket 5970 does no longer hurt me. (In GHC-6.12.3 the conversion to type families made the compilation even slower.) * Anonymous type function values One of the most annoying type classes of the llvm package was the IsSized class: class (LLVM.IsType a, IsPositive size) => IsSized a size | a -> size where size is a type-level decimal natural number. Many llvm functions require that an LLVM type has a size where the particular size is not important. However, I always have to name the size type. I also cannot get rid of it using a subclass, like class (IsSized a size) => IsAnonymouslySized a where The 'size' type is somehow sticky. The conversion of this type class to type families is straightforward: class (IsType a, PositiveT (SizeOf a)) => IsSized a where type SizeOf a :: * Now I have to use SizeOf only if needed. I can also easily define sub-classes like class (IsSized a) => C a where * No TypeSynonymInstances At the right hand side of a 'type instance' I can use type synonyms like type instance F T = String without the TypeSynonymInstance extension. This feels somehow more correct than refering to a type synonym in a class instance head like in instance C T String where The compiler does not need to analyze String in order to find the correct instance. * No FlexibleInstances The same applies to instance C (T a) (A (B a)) which is a flexible instance that is not required for type instance F (T a) = A (B a) * No MultiParamTypeClass, No UndecidableInstances I have some type classes that convert a type to another type and a tuple of types to another tuple of types where the element types are converted accordingly. With functional dependencies: class MakeValueTuple haskellTuple llvmTuple | haskellTuple -> llvmTuple where instance (MakeValueTuple ha la, MakeValueTuple hb lb) => MakeValueTuple (ha,hb) (la,lb) The class is a multi-parameter type class and the instance is undecidable. This is much simpler with type families: class MakeValueTuple haskellTuple where type ValueTuple haskellTuple :: * instance (MakeValueTuple ha, MakeValueTuple hb) => MakeValueTuple (ha,hb) where type ValueTuple (ha,hb) = (ValueTuple ha, ValueTuple hb) Thus summarized: Type families may replace several other type extensions. If I ignore the associated type functions then many classes become Haskell 98 with Haskell 98 instances. This is good because those instances prevent instance conflicts with other non-orphan instances. == Disadvantage of TypeFamilies == * Redundant instance arguments I have to write the type arguments both in the instance head and in the function argument. This is especially annoying in the presence of multi-parameter type classes with bidirectional dependencies. E.g. class (a ~ Input parameter b, b ~ Output parameter a) => C parameter a b where type Input parameter b :: * type Output parameter a :: * process :: Causal p (parameter, a) b instance (...) => C (FilterParam a) v (FilterResult v) where type Input (FilterParam a) (FilterResult v) = v type Output (FilterParam a) v = FilterResult v With functional dependencies it was: class C parameter a b | parameter a -> b, parameter b -> a where process :: Causal p (parameter, a) b instance (...) => C (FilterParam a) v (FilterResult v) where * Bidirectional dependencies In GHC-6.12.3 it was not possible to write class (a ~ Back b, b ~ Forth a) => C a b where Fortunately, this is now allowed in GHC-7. But bidirectional dependencies are still cumbersome to work with as shown in the example above. * Equality constraints are not supported for newtype deriving Not so important, just for completeness: http://hackage.haskell.org/trac/ghc/ticket/6088 == Confusions == * Upper case type function names Why are type function names upper case, not lower case? They are not constructors after all. Maybe this is one reason, why I forget from time to time that type functions are not injective. Sure, lower-case type variables are implicitly forall quantified in Haskell 98. In the presence of lower-case type functions we would need explicit forall quantification. * Why can associated types not be exported by C(AssocType) syntax? Why must they be exported independently from the associated class? * FlexibleContexts The context (Class (TypeFun a)) requires FlexibleContexts extension, whereas the equivalent (TypeFun a ~ b, Class b) does not require FlexibleContexts. Best, Henning

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Re: [Haskell-cafe] Tests by properties: origin?
by Janis Voigtländer 05 Jun '12

05 Jun '12

Am 01.06.2012 12:00, schrieb Yves: > Out of curiosity, does someone know if QuickCheck was the first test > framework working through test by properties associated with random > generation or if it drew the idea from something else? > > Because the idea has be retaken by a lot of frameworks in several languages > (seehttp://en.wikipedia.org/wiki/Quickcheck) but I can't find what was > QuickCheck inspiration. How about reading the original paper introducing QuickCheck? If the authors drew inspiration from elsewhere, the paper is for sure where they would tell you, first hand. :-) Best, Janis. -- Jun.-Prof. Dr. Janis Voigtländer http://www.iai.uni-bonn.de/~jv/

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attoparsec double precision, quickCheck and aeson
by Warren Harris 05 Jun '12

05 Jun '12

The double parser provided by Data.Attoparsec.ByteString.Char8 looses precision around the 13-15th decimal place (http://hackage.haskell.org/packages/archive/attoparsec/0.10.2.0/doc/html/Da…) Unfortunately this reeks havoc with my attempts to write a quickCheck test that validates print-read equivalence for a program that uses Aeson. I have tried compensating for this round-off error in my quickCheck generator using a function like this: roundDouble :: Double -> Double roundDouble d = let Right v = A8.parseOnly A8.double (C.pack (show d)) in v which helps in many cases, but for some the parsing seems bi-stable, alternating between two imprecise double values and causing the test to fail. I was wondering if anyone could suggest a better work-around for this problem, or explain why Attoparsec's double parser can't be isomorphic to haskell's. Thanks, Warren

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Extending constraints
by Bas van Dijk 05 Jun '12

05 Jun '12

Hello, I have the following program: -------------------------------------------------------------------------- {-# LANGUAGE ConstraintKinds #-} {-# LANGUAGE ExistentialQuantification #-} {-# LANGUAGE ScopedTypeVariables #-} import Data.Proxy (Proxy) import Data.Typeable (Typeable, TypeRep, typeOf) data ProxyWrapper constraint = forall a. constraint a => ProxyWrapper (Proxy a) typeOfInnerProxy :: ProxyWrapper constraint -> TypeRep typeOfInnerProxy (ProxyWrapper p) = typeOfArg p typeOfArg :: forall t a. Typeable a => t a -> TypeRep typeOfArg _ = typeOf (undefined :: a) -------------------------------------------------------------------------- Type checking this gives the following expected type error: ProxyWrapper.hs:12:37: Could not deduce (Typeable a) arising from a use of `typeOfArg' from the context (constraint a) bound by a pattern with constructor ProxyWrapper :: forall (constraint :: * -> Constraint) a. (constraint a) => Proxy a -> ProxyWrapper constraint, in an equation for `typeOfInnerProxy' Is there a way to extend the 'constraint' with the 'Typeable' constraint in the type signature of 'typeOfInnerProxy'? Regards, Bas

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Install a script with Cabal?
by Ketil Malde 05 Jun '12

05 Jun '12

Hi, I have a small project that installs a couple of Haskell tools and a script that uses these. Cabal will of course build and install the Haskell programs, but how can I get Cabal to install the script as well? There's a host of UserHooks available¹, but it'd probably be better to have an example than to try to experiment with the right configuration. -k ¹ http://www.haskell.org/cabal/release/cabal-latest/doc/API/Cabal/Distributio… -- If I haven't seen further, it is by standing in the footprints of giants

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Installation of the New Haskell Platform
by Victor Miller 05 Jun '12

05 Jun '12

Before I install it (on on Mac OS X Lion) is there anything, manually, that I need to do in order to keep all of the cabal packages that I've installed, or are the reinstalled automatically? Victor

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