July 2009 - Haskell-Cafe

Removing polymorphism from type classes (viz. Functor)
by George Pollard 13 Jul '09

13 Jul '09

Ok, so I have a small idea I'm trying to work on; call it a Prelude-rewrite if you want. For this I want to be able to have the hierarchy Functor → Applicative → Monad. For Functor, I would like to be able to implement it for a wider variety of types, as there are types which have aren't polymorphic which would also benefit from having an instance. My running example for this set of types is ByteString; the module contains the method: map ∷ (Word8 → Word8) → ByteString → ByteString However, we cannot use this for Functor because ByteString isn't polymorphic. To get around this, I devised the following: Introduce a type family which represents ‘points’ inside the type: type family Point f ∷ ★ For ByteString we have: type instance Point ByteString = Word8 For a polymorphic example (lists) we have: type instance Point [a] = a Now Functor becomes: class SimpleFunctor f where fmap ∷ (Point f → Point f) → (f → f) However, this doesn't allow for the existence of functions with the type (a → b). I need to introduce another type into the class: class Functor f g where fmap ∷ (Point f → Point g) → (f → g) But having two types isn't very nice (for one thing we can't introduce a fundep because for lists as it fails one of the coverage conditions), so introduce another type family to represent types which can be produced by giving a free variable: type Subst f a ∷ ★ type Subst [a] b = [b] type Subst ByteString b = ByteString class Functor f where fmap ∷ (Point f → Point (Subst f a)) → (f → Subst f a) I'm not sure how much of a hack this is, or if there is a better way. It seems to be OK... Now I want to implement Applicative. It would make sense to have ‘return’ be split out into a separate class, because this can be restricted in a similar way to Functor: class Pointed f where return ∷ Point f → f instance Pointed [a] where return x = [x] instance Pointed ByteString where return = BS.singleton Now, I want to be able to restrict Applicative to things which have [Pointed f, and forall a b. Point f ~ (a → b)]. At the moment I can't figure this out because I believe it would require something like the ‘quantified contexts’ proposal: class (Pointed f, ∀ a b. Point f ~ (a → b)) ⇒ Applicative f where ... I could have something like: class (Pointed f, Point f ~ (a → b)) ⇒ Applicative f a b where apply ∷ f → Subst f a → Subst f b This is still not very nice, because it requires two more type variables in the class, and the non-type-families version is far more straightforward... in fact, it makes sense for the Applicative class to have a polymorphic type because it must be able to have ‘return’ applied to arbitrary functions (remember [fmap f xs ≡ return f `apply` xs]). So back to: class Applicative f where apply ∷ f (a → b) → f a → f b But then ‘return’ cannot be added via a superclass restriction to Pointed! I seem to have painted myself into a corner. Does anyone see a better way to go about this? Thanks, - George

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Questions about haskell CPP macros
by Matthew Elder 13 Jul '09

13 Jul '09

Hello Cafe, I am trying to improve the error reporting in my sendfile library, and I know I can find out the current file name and line number with something like this: {-# LANGUAGE CPP #-} main = putStrLn (__FILE__ ++ ":" ++ show __LINE__) This outputs: test.hs:2 Unfortunately, if your file is in a hierarchy of folders, this flat file name doesn't give much context. Is there a macro to find out the current module? IE if I had a module Foo.Bar.Car.MyModule, I would like to be able to output something like this on error: Foo.Bar.Car.MyModule:2 Any help is appreciated! Thanks, Matt -- Need somewhere to put your code? http://patch-tag.com Want to build a webapp? http://happstack.com

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Are GADTs what I need?
by Kev Mahoney 13 Jul '09

13 Jul '09

Hi there, I'm currently writing an interpreter that I would like to be able to use with other haskell programs. I would like to be able to pass along arbitrary types though the interpreter. I've seen hints that GADTs can do this, but I am having trouble understanding them. So far, I've learnt you can do this: data Value where VInt :: Integer -> Value ... VWrapper :: a -> Value which can let you encode arbitrary 'dynamic' types into Value. I was hoping to be able to pattern match to get the value out again e.g. doSomething :: Value -> .... doSomething (VWrapper String s) = ..... Also, anything that can help me out with GADTs in general will be much appreciated. Thanks, Kevin.

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Re: [Haskell-cafe] Leaner Haskell.org frontpage
by Don Stewart 13 Jul '09

13 Jul '09

ttencate: > On Thu, Jul 9, 2009 at 18:33, Don Stewart<dons(a)galois.com> wrote: > > ttencate: > >> Are there any kind of hard statistics and analytics that we can base > >> this discussion upon? There is always room for improvement, but > >> stumbling around in the dark making blind guesses may not be the best > >> way to go. Although I personally feel that Lenny's proposed page is an > >> improvement, statistics could tell us what actual people actually use > >> the site for. > > Thanks Don, I should have thought of that. It's a start! No matter what we decide, I'd like to advocate for maintaining the RSS feed of Hackage uploads on the front page -- this is what makes the page seem alive and active, and was introduced in response to John Hughes complaining the page was always out of date. (Similar to the Arch Linux package updates: http://www.archlinux.org/) -- Don

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Type families and polymorphism
by Jeremy Yallop 13 Jul '09

13 Jul '09

Why does compiling the following program give an error? > {-# LANGUAGE TypeFamilies, RankNTypes #-} > > type family TF a > > identity :: (forall a. TF a) -> (forall a. TF a) > identity x = x GHC 6.10.3 gives me: Couldn't match expected type `TF a1' against inferred type `TF a' In the expression: x In the definition of `identity': identity x = x -- The University of Edinburgh is a charitable body, registered in Scotland, with registration number SC005336.

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Hylomorphisms (was: [Haskell-cafe] excercise - a completely lazy sorting algorithm)
by Raynor Vliegendhart 13 Jul '09

13 Jul '09

On 7/12/09, Heinrich Apfelmus <apfelmus(a)quantentunnel.de> wrote: > Raynor Vliegendhart wrote: > > On 7/9/09, Heinrich Apfelmus <apfelmus(a)quantentunnel.de> wrote: > >> Of course, some part of algorithm has to be recursive, but this can be > >> outsourced to a general recursion scheme, like the hylomorphism > >> > >> hylo :: Functor f => (a -> f a) -> (f b -> b) -> (a -> b) > >> hylo f g = g . fmap (hylo f g) . f > >> > > > > Is that definition of hylo actually usable? A few on IRC tried to use > > that definition for a few examples, but the examples failed to > > terminate or blew up the stack. > > The implementation of quicksort with hylo works fine for me, given > medium sized inputs like for example quicksort (reverse [1..1000]) . > > What were the examples you tried? > One of the examples I tried was: hylo (unfoldr (\a -> Just (a,a))) head $ 42 This expression fails to determinate. Here are two examples copumpkin tried on IRC: <copumpkin> > let hylo f g = g . fmap (hylo f g) . f in hylo (flip replicate 2) length 5 <lambdabot> 5 <copumpkin> > let hylo f g = g . fmap (hylo f g) . f in hylo (flip replicate 2) sum 5 <lambdabot> * Exception: stack overflow

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Why this cannot be compiled?
by Magicloud Magiclouds 13 Jul '09

13 Jul '09

Hi, The following code failed to compiled, with error: Attribute.hs:46:91: Couldn't match expected type `[t]' against inferred type `(a, String)' In the expression: (color_, rest_) In a case alternative: [(color_, rest_)] -> (color_, rest_) In the expression: case reads rest1 of { [(color_, rest_)] -> (color_, rest_) _ -> (Color 0 0 0, rest1) } 43 instance Read Attribute where 44 readsPrec _ str = [ (mkAttr attr_ color, rest) | (attr_, rest1) <- lex str 45 , (color, rest) <- case reads rest1 of 46 [(color_, rest_)] -> (color_, rest_) 47 _ -> (Color 0 0 0, rest1) ] 48 where mkAttr "AttrFgColor" color = AttrFgColor color 49 mkAttr "AttrBgColor" color = AttrBgColor color 50 mkAttr "AttrInverse" _ = AttrInverse 51 mkAttr "AttrWeak" _ = AttrWeak 52 mkAttr "AttrUnderline" _ = AttrUnderline -- 竹密岂妨流水过山高哪阻野云飞

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Could C libraries be part of Haskell Platform?
by Maurício 12 Jul '09

12 Jul '09

Probably not, but... There are stable and portable libraries that implement functionality missing in Haskell. It could be nice to be able to list then in a cabal file (using, say, pkgconfig-depends) and know for sure that any system with Haskell Platform will have then available for use with FFI. An example would be sqlite3. To me, this seems to be ugly in theory and good in practice. Best, Maurício

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ICFP09 Final Call for Participation
by Matthew Fluet (ICFP Publicity Chair) 12 Jul '09

12 Jul '09

===================================================================== Final Call for Participation The 14th ACM SIGPLAN International Conference on Functional Programming (ICFP 2009) http://www.cs.nott.ac.uk/~gmh/icfp09.html Edinburgh, Scotland, 31 August - 2 September 2009 ===================================================================== ***** Accommodation Deadline: July 20, 2009 ***** Due to the overlap with Edinburgh Festival, accommodations may be difficult to secure after the deadline. Reserve now! http://www.haskell.org/haskellwiki/ICFP_2009_Local_Arrangements ***** Early Registration Deadline: July 30, 2009 ***** http://www.regmaster.com/conf/icfp2009.html ICFP 2009 provides a forum for researchers and developers to hear about the latest work on the design, implementations, principles, and uses of functional programming. The conference covers the entire spectrum of work, from practice to theory, including its peripheries. Preliminary program: * Abstracts: + http://web.cecs.pdx.edu/~apt/icfp09_accepted_papers/accepted.html * Schedule: + http://web.cecs.pdx.edu/~apt/icfp09_preliminary_program.pdf * Invited speakers: + Guy Steele -- Organizing Functional Code for Parallel Execution: or, foldl and foldr Considered Slightly Harmful + Benjamin Pierce -- Lambda, the Ultimate TA: Using a Proof Assistant to Teach Programming Language Foundations + Dan Piponi -- Commutative Monads, Diagrams and Knots Schedule including related workshops: * Aug 30: ACM SIGPLAN Workshop on ML * Aug 30: ACM SIGPLAN Workshop on Generic Programming * Aug 31-Sep 2: ICFP09 * Sep 3: ACM SIGPLAN Haskell Symposium * Sep 3: ACM SIGPLAN Developer Tracks on Functional Programming * Sep 4: Commercial Users of Functional Programming * Sep 4: ACM SIGPLAN Workshop on Mechanizing Metatheory * Sep 4: ACM SIGPLAN Workshop on Approaches and Applications of Inductive Programming * Sep 5: ACM SIGPLAN Erlang Workshop * Sep 5: ACM SIGPLAN Developer Tracks on Functional Programming * Sep 5: ACM SIGPLAN Haskell Implementors Workshop Conference organizers: * General Chair: Graham Hutton (University of Nottingham) * Program Chair: Andrew Tolmach (Portland State University) * Local Arrangements Chairs: Philip Wadler (University of Edinburgh), Kevin Hammond (University of St Andrews), and Gregory Michaelson (Heriot-Watt University) * Workshop Co-Chairs: Christopher Stone (Harvey Mudd College), and Michael Sperber (DeinProgramm) * Programming Contest Chair: Andrew Gill (University of Kansas) * Publicity Chair: Matthew Fluet (Toyota Technological Institute at Chicago)

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Haskell Weekly News: Issue 125 - July 12, 2009
by Brent Yorgey 12 Jul '09

12 Jul '09

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20090712 Issue 125 - July 12, 2009 --------------------------------------------------------------------------- Welcome to issue 125 of HWN, a newsletter covering developments in the [1]Haskell community. Announcements Announcing the new Haskell Prime process, and Haskell 2010. Simon Marlow [2]announced the new and improved [3]Haskell Prime process, and a list of proposals which are currently under consideration to be accepted into the next (2010) revision of the standard. AspectAG 0.1.1. Marcos Viera [4]announced the release of [5]AspectAG, a library of strongly typed Attribute Grammars implemented using type-level programming. Colour tutorial. Russell O'Connor [6]announced a [7]tutorial wiki page for the [8]colour library. Haskell Hack Day, Edinburgh, 30 August 2009. Eric Kow [9]announced a [10]Haskell Hack Day to be held in Edinburgh, on Sunday 30 August, before ICFP. The Hack Day will be held at the ICFP conference venue, the Royal College of Physicians. sendfile-0.3. Matthew Elder [11]announced the release of [12]sendfile-0.3, which includes a more general interface, updated documentation, and more. Gtk2hsGenerics. Michael Dever [13]announced the release of [14]Gtk2hsGenerics, a package which contains utility functions for extracting from and adding to stores. darcs 2.3 beta 2. Petr Rockai [15]announced that darcs 2.3 beta 2 is available for testing. It can be installed using the [16]darcs-beta package on Hackage (be sure to 'cabal update' first). The new beta release adds index upgrade functionality to hashed-storage, and now uses an architecture-independent index format. hsparql, a SPARQL query generator/DSL and client. Jeff Wheeler [17]announced the first version of [18]hsparql, which makes it easy to query SPARQL-compliant servers using a relatively intuitive DSL and very simple client. SPARQL is a SQL-ish query language for stores of RDF data. Hayoo! beta 0.4. Timo B. [19]announced the next beta version 0.4 of [20]Hayoo!, the Haskell API search engine with find-as-you-type and suggestions. This release includes some major changes to the web interface, including the ability to display the full description of a function, and some example queries featured on the start page, as well as an updated search index. texmath 0.1.0.1 - conversion of LaTeX math to MathML. John MacFarlane [21]announced an early release of [22]texmath, a Haskell library for converting LaTeX math formulas to MathML. The package includes a standalone test program, testTeXMathML, and a CGI script, texmath-cgi, that can be used in web apps. You can see demos of the script [23]here and [24]here. BostonHaskell: Next meeting - July 16th at MIT CSAIL Reading Room (32-G882). Ravi Nanavati [25]announced the July meeting of the [26]Boston Area Haskell Users' Group, to be held Thursday, July 16th from 6:30pm - 8:30pm in the MIT CSAIL Reading Room. Scheduled talks include "An Introduction to GHC Hacking" by Alec Heller, and "Haskell on the iPhone" by Ryan Trinkle. There are still openings for Lightning Talks (5-minute talk, 2-minute Q&A). See the announcement for more details. AC-Vector, AC-Colour and AC-EasyRaster-GTK. Andrew Coppin [27]announced the release of three packages: [28]AC-Vector, which provides unboxed vectors of Doubles with arithmetic, dot product and cross product, and a few other useful items; [29]AC-Colour, which provides two simple RGB color types, optimized for simplicity and speed; and [30]AC-EasyRaster-GTK, a layer over Gtk2hs which provides easy pixel-twiddling functionality. Google Summer of Code Progress updates from participants in the 2008 [31]Google Summer of Code. Haddock improvements. Isaac Dupree has had a [32]good deal of success getting cross-package documentation to work, with a few remaining loose ends. space profiling. Gergely Patai's profile graphing application [33]is now capable of connecting to a profile relay server that can broadcast the heap profile of its associated process on the fly, so it is now possible to attach an observer to a program that was started earlier. He now plans to shift to working on a history manager. fast darcs. Petr Rockai has produced [34]another darcs 2.3 beta release, and has done more work on [35]designing an efficient storage system. Discussion Leaner Haskell.org frontpage. haskell [36]proposed a new design for the haskell.org front page. Bikeshedding ensues, news at 11. exercise - a completely lazy sorting algorithm. Petr Pudlak [37]asked whether it is possible to write a lazy sorting algorithm which allows access to the kth item of the sorted output in linear time, for all k. An interesting discussion and mind-expanding code followed. Implementing Las Vegas algorithms in Haskell. Matthias Görgens began a [38]discussion on implementing Las Vegas algorithms, which use a source of randomness but have results which are still deterministic. What should the type of such functions be? Is it OK to use unsafePerformIO in their definition? Blog noise [39]Haskell news from the [40]blogosphere. Blog posts from people new to the Haskell community are marked with >>>, be sure to welcome them! * Petr Rockai: [41]Designing Storage for darcs. * >>> haskelladdict: [42]Having fun sorting with Haskell. * Jeff Heard: [43]C2HS example: To save other people frustration. * JP Moresmau: [44]Mazes of Monad: 1.0.5 and (maybe) final version uploaded. * Sean Leather: [45]Fun and generic things to do with EMGM at the London HUG. * Greg Bacon: [46]Gyrigrams. * Magnus Therning: [47]Making a choice from a list in Haskell, Vty (part 2). * Jeff Heard: [48]A quick buster example. * Jeff Heard: [49]An ugly force-directed layout implementation. * Petr Rockai: [50]darcs 2.3 beta 2. * Isaac Dupree: [51]Cross-package documentation going well!. * >>> dasuxullebt: [52]I (don't) like Haskell. * Holumbus: [53]Some Hayoo! Improvements. * David Amos: [54]Conjugacy classes, part 2. * Michael Snoyman: [55]hack-handler-webkit. * Dan Piponi (sigfpe): [56]A Monad for Combinatorial Search with Heuristics. * Greg Bacon: [57]A programmable semicolon explained. * Holumbus: [58]Another Hayoo! Update. * Gergely Patai: [59]Remote profiling at your fingertips. * The GHC Team: [60]Visualising the Haskell package dependency graph. * Bryan O'Sullivan: [61]First steps with Haskell text API improvement. * Paul Hudak: [62]Haskell and the Arts. * >>> John Gunderman: [63]Haskell Cabal in Ubuntu. * Sebastian Sylvan: [64]Ray tracing signed distance functions. Quotes of the Week * SimonPJ: [re: class constraints on data declarations] In GHC's source code these contexts are consistently called stupid_theta. * JN: this tutorial just introduced _|_ and called it bottom. I assume that's because it looks like an ass? * uman: so you can pass functions around as objects... this sounds like JavaScript * Twey: Mr. Wing, sir, if you are listening: you have the I.Q. of a semolina pudding. And your homepage is full of <font> tags. I'm not sure which is the greater insult, but at least one is objectively true. * jmcarthur: #haskell: Overwhelmingly helpful. * lilac: class Monad m where / return and Kleisli compose / must form a monoid <lilac> that's my new monad tutorial haiku <jmcarthur> i think that is the best monad tutorial i have ever read * dcoutts: make sure happy is on your path and it'll all be ok * ski: my answer to that is : don't deduce, denote! * roco: guys i have problem, does anybody know programming ? * dhjdhj: All new features added to C++ are intended to fix previously new features added to C++ About the Haskell Weekly News New editions are posted to [65]the Haskell mailing list as well as to [66]the Haskell Sequence and [67]Planet Haskell. [68]RSS is also available, and headlines appear on [69]haskell.org. To help create new editions of this newsletter, please see the information on [70]how to contribute. Send stories to byorgey at cis dot upenn dot edu. The darcs repository is available at darcs get [71]http://code.haskell.org/~byorgey/code/hwn/ . References 1. http://haskell.org/ 2. http://www.haskell.org//pipermail/haskell/2009-July/021472.html 3. http://hackage.haskell.org/trac/haskell-prime/wiki/Process 4. http://www.haskell.org//pipermail/haskell/2009-July/021475.html 5. http://hackage.haskell.org/package/AspectAG 6. http://www.haskell.org//pipermail/haskell-cafe/2009-July/064065.html 7. http://www.haskell.org/haskellwiki/Colour 8. http://hackage.haskell.org/package/colour 9. http://article.gmane.org/gmane.comp.lang.haskell.general/17345 10. http://www.haskell.org/haskellwiki/Hac7 11. http://article.gmane.org/gmane.comp.lang.haskell.general/17343 12. http://hackage.haskell.org/package/sendfile 13. http://article.gmane.org/gmane.comp.lang.haskell.general/17336 14. http://hackage.haskell.org/package/Gtk2hsGenerics 15. http://article.gmane.org/gmane.comp.lang.haskell.cafe/61071 16. http://hackage.haskell.org/package/darcs%2Dbeta 17. http://article.gmane.org/gmane.comp.lang.haskell.cafe/61062 18. http://hackage.haskell.org/package/hsparql 19. http://article.gmane.org/gmane.comp.lang.haskell.cafe/61060 20. http://holumbus.fh-wedel.de/hayoo 21. http://article.gmane.org/gmane.comp.lang.haskell.cafe/61049 22. http://hackage.haskell.org/package/texmath 23. http://johnmacfarlane.net/texmath.xhtml 24. http://johnmacfarlane.net/texmath.html 25. http://article.gmane.org/gmane.comp.lang.haskell.cafe/61003 26. http://www.haskell.org/haskellwiki/Boston_Area_Haskell_Users%27_Group 27. http://article.gmane.org/gmane.comp.lang.haskell.cafe/60944 28. http://hackage.haskell.org/package/AC-Vector 29. http://hackage.haskell.org/package/AC-Colour 30. http://hackage.haskell.org/package/AC-EasyRaster-GTK 31. http://hackage.haskell.org/trac/summer-of-code/wiki/SoC2008 32. http://haddock2009.wordpress.com/2009/07/09/cross-package-documentation-goi… 33. http://just-bottom.blogspot.com/2009/07/remote-profiling-at-your-fingertips… 34. http://web.mornfall.net/blog/darcs_2.3_beta_2.html 35. http://web.mornfall.net/blog/designing_storage_for_darcs.html 36. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/61082 37. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/60981 38. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/60998 39. http://planet.haskell.org/ 40. http://haskell.org/haskellwiki/Blog_articles 41. http://web.mornfall.net/blog/designing_storage_for_darcs.html 42. http://haskelladdict.wordpress.com/2009/07/11/having-fun-sorting-with-haske… 43. http://vis.renci.org/jeff/2009/07/10/c2hs-example-to-save-other-people-frus… 44. http://jpmoresmau.blogspot.com/2009/07/mazes-of-monad-105-and-maybe-final.h… 45. http://feedproxy.google.com/~r/splonderzoek/~3/qnt94yp_Er4/fun-and-generic-… 46. http://feedproxy.google.com/~r/gbacon/~3/QKPjQ0Fxl_4/gyrigrams.html 47. http://therning.org/magnus/archives/689 48. http://vis.renci.org/jeff/2009/07/09/a-quick-buster-example/ 49. http://vis.renci.org/jeff/2009/07/09/an-ugly-force-directed-layout-implemen… 50. http://web.mornfall.net/blog/darcs_2.3_beta_2.html 51. http://haddock2009.wordpress.com/2009/07/09/cross-package-documentation-goi… 52. http://uxul.wordpress.com/2009/07/09/i-dont-like-haskell/ 53. http://holumbus.fh-wedel.de/blog/?p=24 54. http://haskellformaths.blogspot.com/2009/07/conjugacy-classes-part-2.html 55. http://blog.snoyman.com/2009/07/08/hack-handler-webkit/ 56. http://blog.sigfpe.com/2009/07/monad-for-combinatorial-search-with.html 57. http://feedproxy.google.com/~r/gbacon/~3/UGYWKr-huLY/programmable-semicolon… 58. http://holumbus.fh-wedel.de/blog/?p=23 59. http://just-bottom.blogspot.com/2009/07/remote-profiling-at-your-fingertips… 60. http://ghcmutterings.wordpress.com/2009/07/06/visualising-the-haskell-packa… 61. http://www.serpentine.com/blog/2009/07/06/first-steps-with-haskell-text-api… 62. http://www.infoq.com/presentations/haskell-arts-paul-hudak 63. http://sporkcode.wordpress.com/2009/07/05/haskell-cabal-in-ubuntu/ 64. http://sebastiansylvan.spaces.live.com/blog/cns!4469F26E93033B8C!173.entry 65. http://www.haskell.org/mailman/listinfo/haskell 66. http://sequence.complete.org/ 67. http://planet.haskell.org/ 68. http://sequence.complete.org/node/feed 69. http://haskell.org/ 70. http://haskell.org/haskellwiki/HWN 71. http://code.haskell.org/~byorgey/code/hwn/

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