August 2008 - Haskell-Cafe

Haskell Weekly News: Issue 83 - August 30, 2008
by Brent Yorgey 31 Aug '08

31 Aug '08

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20080830 Issue 83 - August 30, 2008 --------------------------------------------------------------------------- Welcome to issue 83 of HWN, a newsletter covering developments in the [1]Haskell community. This is the "better late than never" edition. As an excuse I could tell you that my home internet service has been horrible (now fixed) and I was away from home for a few days with my wife celebrating our third wedding anniversary. But instead, I give you a link to the [2]Uncyclopedia entry on Haskell. If you haven't already seen it, you should give it a read, being sure not to drink any milk at the same time, or at least pointing your nose away from the keyboard if you insist on drinking milk. Community News If Dell sends John Goerzen (CosmicRay) one more catalog, it will [3]actually be a federal crime. Announcements LogFloat 0.9. wren ng thornton [4]announced a new official release of the [5]logfloat package for manipulating log-domain floating numbers. This release is mainly for those who are playing with Transfinite rather than LogFloat, but the interface changes warrant a minor version change. validating xml lib - need some guidance. Marc Weber [6]asked for help developing an xml generating library validating the result against a given DTD. gsl-random 0.1 and monte-carlo-0.1. Patrick Perry [7]announced that he has started on [8]bindings for the random number generators and random distributions provided by the gsl. He has also written a [9]monad and transformer for doing monte carlo computations that uses gsl-random internally. For a quick tutorial in the latter package, see [10]his blog. Wired 0.1.1. Emil Axelsson [11]announced the first release of the hardware description library [12]Wired. Wired can be seen as an extension to Lava that targets (not exclusively) semi-custom VLSI design. A particular aim of Wired is to give the designer more control over the routing wires' effects on performance. darcs weekly news #1. Eric Kow [13]sent out the first edition of the new Darcs Weekly News! zip-archive 0.0. John MacFarlane [14]announced the release of the [15]zip-archive library for dealing with zip archives. The Monad.Reader - Issue 11. Wouter Swierstra [16]announced a new issue of [17]The Monad.Reader, with articles by David Place, Kenn Knowles, and Doug Auclair. First Monad Tutorial of the Season. Hans van Thiel [18]announced a new monad tutorial, [19]The Greenhorn's Guide to becoming a Monad Cowboy. "Real World Haskell" hits a milestone. Bryan O'Sullivan [20]proudly announced that the draft manuscript of Real World Haskell is [21]complete! It is now available online in its entirety. The authors expect the final book to be published around the beginning of November, and to weigh in at about 700 pages. Mueval 0.5.1, 0.6, 0.6.1, 0.6.2, 0.6.3, 0.6.4. Gwern Branwen [22]announced a number of new releases of [23]Mueval. Lambdabot now uses mueval for all its dynamic Haskell evaluation needs. Hoogle Database Generation. Neil Mitchell (ndm) [24]announced that a new release of the [25]Hoogle command line is out, including bug fixes and additional features. Upgrading is recommended.Two interesting features of Hoogle 4 are working with multiple function databases (from multiple packages), and running your own web server. Blog noise [26]Haskell news from the [27]blogosphere. * Real-World Haskell: [28]Source handed over to production. * Douglas M. Auclair (geophf): [29]Earning \bot-Trophies. * Douglas M. Auclair (geophf): [30]Scanner-parsers II: State Monad Transformers. * Douglas M. Auclair (geophf): [31]Scanner-parsers I: lifting functions. * >>> software engineering radio: [32]Episode 108: Simon Peyton Jones on Functional Programming and Haskell. A podcast interview with Simon Peyton Jones. * Neil Mitchell: [33]Running your own Hoogle on a Web Server. * Braden Shepherdson: [34]Announcing xmonad-light. Braden is rolling out a new configuration framework for xmonad, providing an easier learning curve for those not wanting to learn Haskell right away, and an easy transition to a more powerful Haskell configuration when they want it. * Gabor Grief: [35]Category. Gabor is excited that base-3.0 will include Control.Category. * Douglas M. Auclair (geophf): [36]Ten = 1+2+3+4. Solving an arithmetic puzzle with Haskell, Prolog-style. * Paul R Brown: [37]perpubplat now on github. * Douglas M. Auclair (geophf): [38]"Lucky you!"?. Doug shares some secrets of his success in getting Haskell/Dylan/Mercury/Prolog jobs. * Neil Mitchell: [39]Hoogle Database Generation. Neil releases a new command-line version of Hoogle, including bug fixes and additional features -- multiple function databases and the ability to run your own Hoogle server. * Patrick Perry: [40]A Monte Carlo Monad for Haskell. * Mads Lindstroem: [41]Proposal: Adding composability to WxHaskell. * Tom Moertel: [42]PXSL Tools now on Hackage and GitHub. * Gabor Greif: [43]Uploaded. Gabor uploads his first contribution to Hackage, [44]thrist-0.1. * Muad`Dib (vixey): [45]Tail Call Optimization doesn't exist in Haskell. * >>> Hans van Thiel: [46]The Greenhorn's Guide to becoming a Monad Cowboy. YAMT * Dan Piponi (sigfpe): [47]Untangling with Continued Fractions: Part 2. Dan continues his excellent series on rational tangles, this time showing the connection between tangles and rational numbers. * >>> Praki Prakash: [48]Learning Haskell Redux. * Gabor Greif: [49]Ca(ni)balized. Gabor creates his first cabal package! * Real-World Haskell: [50]Our writing is now complete!. * Tom Moertel: [51]Thinking algebraically: a functional-programming dividend that pays during your imperative-programming day job. * Jason Dagit: [52]Darcs 2 Real-World Push Performance Evaluation. * Well-Typed.Com: [53]What's wrong with make?. A lot, apparently: it's too static, and makes it easy to write incorrect rules. Duncan (dcoutts) provides an analysis. * Dougal Stanton: [54]Lord of the Flies as a window onto monadic IO. * Real-World Haskell: [55]A tighter page count estimate. Real World Haskell is going to be about 700 pages!? * Real-World Haskell: [56]Real World Haskell tutorial next month at DEFUN 2008. Quotes of the Week * shepheb: #haskell isn't so much on-topic when discussing Haskell, but off-off-topic. * chrisdone: it's neat how you learn haskell because you are drawn in by the purely functional paradigm, and then you find loads more things like algebraic data types, monad abstractions, arrows and applicative, lack of objects... so that when people say "well, it's not haskell, but at least X is functional", it's just not the same at all * lambdabot: [tristes_tigres] @vixen unsafe [lambdabot] you're turning me on :) * b\6: sometimes i make variables f and ck and find some reason to multiply them like f*ck if i'm having a bad day. * waynemokane: wow... thanks everyone - it looks like I have a full day of reading type signatures ahead of me. * mauke: call/cthulhu * seydar: monads are just like saran wrap. * b\6: keep this info private, but you can actually overclock your brain. the technique i use is to loop mplayer playing oggs but increase the speed like -speed 1.25 for 125% normal. your brain's speed increases accordingly, allowing you to solve problems much more easily. About the Haskell Weekly News New editions are posted to [57]the Haskell mailing list as well as to [58]the Haskell Sequence and [59]Planet Haskell. [60]RSS is also available, and headlines appear on [61]haskell.org. To help create new editions of this newsletter, please see the information on [62]how to contribute. Send stories to byorgey at cis dot upenn dot edu. The darcs repository is available at darcs get [63]http://code.haskell.org/~byorgey/code/hwn/ . References 1. http://haskell.org/ 2. http://uncyclopedia.org/wiki/Haskell 3. http://changelog.complete.org/posts/741-From-Dell,-a-Uniquely-Terrible-Expe… 4. http://article.gmane.org/gmane.comp.lang.haskell.cafe/43829 5. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/logfloat 6. http://article.gmane.org/gmane.comp.lang.haskell.cafe/43825 7. http://article.gmane.org/gmane.comp.lang.haskell.cafe/43764 8. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/gsl-random 9. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/monte-carlo 10. http://quantile95.com/2008/08/27/a-monte-carlo-monad-for-haskell/ 11. http://article.gmane.org/gmane.comp.lang.haskell.cafe/43658 12. http://www.cs.chalmers.se/~emax/wired/index.html 13. http://article.gmane.org/gmane.comp.lang.haskell.cafe/43626 14. http://article.gmane.org/gmane.comp.lang.haskell.cafe/43539 15. http://johnmacfarlane.net/zip-archive/ 16. http://article.gmane.org/gmane.comp.lang.haskell.cafe/43491 17. http://www.haskell.org/haskellwiki/The_Monad.Reader 18. http://article.gmane.org/gmane.comp.lang.haskell.cafe/43403 19. http://www.muitovar.com/monad/moncow.xhtml 20. http://article.gmane.org/gmane.comp.lang.haskell.cafe/43401 21. http://www.realworldhaskell.org/blog/2008/08/22/our-writing-is-now-complete/ 22. http://article.gmane.org/gmane.comp.lang.haskell.cafe/43372 23. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/mueval 24. http://neilmitchell.blogspot.com/2008/08/hoogle-database-generation.html 25. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hoogle 26. http://planet.haskell.org/ 27. http://haskell.org/haskellwiki/Blog_articles 28. http://www.realworldhaskell.org/blog/2008/08/30/source-handed-over-to-produ… 29. http://logicaltypes.blogspot.com/2008/08/earning.html 30. http://logicaltypes.blogspot.com/2008/08/scanner-parsers-ii-state-monad.html 31. http://logicaltypes.blogspot.com/2008/08/scanner-parsers-i-lifting-function… 32. http://www.se-radio.net/podcast/2008-08/episode-108-simon-peyton-jones-func… 33. http://neilmitchell.blogspot.com/2008/08/running-your-own-hoogle-on-web-ser… 34. http://braincrater.wordpress.com/2008/08/28/announcing-xmonad-light/ 35. http://heisenbug.blogspot.com/2008/08/category.html 36. http://logicaltypes.blogspot.com/2008/08/ten-1234.html 37. http://mult.ifario.us/p/perpubplat-now-on-github 38. http://logicaltypes.blogspot.com/2008/08/lucky-you.html 39. http://neilmitchell.blogspot.com/2008/08/hoogle-database-generation.html 40. http://quantile95.com/2008/08/27/a-monte-carlo-monad-for-haskell/ 41. http://lindstroem.wordpress.com/2008/03/16/proposal-adding-composability-to… 42. http://blog.moertel.com/articles/2008/08/24/pxsl-tools-now-on-hackage-and-g… 43. http://heisenbug.blogspot.com/2008/08/uploaded.html 44. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/thrist 45. http://muaddibspace.blogspot.com/2008/08/tail-call-optimization-doesnt-exis… 46. http://www.muitovar.com/monad/moncow.xhtml 47. http://sigfpe.blogspot.com/2008/08/untangling-with-continued-fractions_23.h… 48. http://monadict.blogspot.com/2008/08/learning-haskell-redux-when-you-are.ht… 49. http://heisenbug.blogspot.com/2008/08/canibalized.html 50. http://www.realworldhaskell.org/blog/2008/08/22/our-writing-is-now-complete/ 51. http://blog.moertel.com/articles/2008/08/20/thinking-algebraically-a-functi… 52. http://blog.codersbase.com/2008/08/21/darcs-2-real-world-push-performance-e… 53. http://blog.well-typed.com/2008/08/whats-wrong-with-make/ 54. http://www.dougalstanton.net/blog/index.php/2008/08/21/lord-of-the-flies-as… 55. http://www.realworldhaskell.org/blog/2008/08/20/a-tighter-page-count-estima… 56. http://www.realworldhaskell.org/blog/2008/08/20/real-world-haskell-tutorial… 57. http://www.haskell.org/mailman/listinfo/haskell 58. http://sequence.complete.org/ 59. http://planet.haskell.org/ 60. http://sequence.complete.org/node/feed 61. http://haskell.org/ 62. http://haskell.org/haskellwiki/HWN 63. http://code.haskell.org/~byorgey/code/hwn/

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Named field syntax
by Maurício 30 Aug '08

30 Aug '08

Hi, I see that this works: data Test = Test Integer String This also works: data Test = Test {a::Integer,b::String} However, this doesn't: data Test = Test Integer {b::String} Is there some way to name only a single, or a few, of some data type fields? Thanks, Maurício

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ANN: LogFloat 0.9
by wren ng thornton 30 Aug '08

30 Aug '08

-------------------------------------------- -- Announcing: logfloat 0.9.1 -------------------------------------------- New official release of the logfloat package for manipulating log-domain floating numbers. This release is mainly for those who are playing with Transfinite rather than LogFloat, but the interface changes warrant a minor version change. If you're just using the LogFloat stuff you shouldn't notice any changes except that the constructors/destructors may be a bit pickier about being used when a polymorphic return type is expected. -------------------------------------------- -- Changes -------------------------------------------- * John Meacham pointed out that the Num and Ord superclasses on Transfinite may be onerous due to the growing disaffection with the Prelude. The Num requirement has been lifted, and the Ord requirement has been weakened to the new PartialOrd class. * Introduced a new class for partially ordered types: PartialOrd. * The transfinite-value respecting logarithm has been moved from Data.Number.LogFloat to Data.Number.Transfinite because it's more appropriate there. (It's still reexported by Data.Number.LogFloat.) * In the continuing battle between transfinite values and Rationals, I've added a new class RealToFrac for overloading the Prelude's realToFrac function. On GHC, primitive converters are used to improve both performance and correctness. For non-GHC compilers, only a few generic instances are given. For an older discussion on this topic, see: http://www.haskell.org/pipermail/haskell-prime/2006-February/000791.html * Due to all this development on trying to solve the issues of transfinite values, the "stability" of the package has been changed from stable to provisional. -------------------------------------------- -- Future Extensions -------------------------------------------- * A strict version. The current lazy version will move to Data.Number.LogFloat.Lazy, and the strict version will be the standard one in Data.Number.LogFloat * A signed version in Data.Number.LogFloat.Signed * A test suite -------------------------------------------- -- Links -------------------------------------------- Homepage: http://code.haskell.org/~wren/ Hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/logfloat Darcs: http://code.haskell.org/~wren/logfloat Haddock (Darcs version): http://code.haskell.org/~wren/logfloat/dist/doc/html/logfloat/ -- Live well, ~wren

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Syntax of 'do'
by Maurício 30 Aug '08

30 Aug '08

Hi, http://haskell.org/haskellwiki/Keywords says that: ------------- [do is a] syntactic sugar for use with monadic expressions. For example: do { x ; result <- y ; foo result } is shorthand for: x >> y >>= \result -> foo result ------------- I did some tests hiding Prelude.>> and Prelude.>>= and applying >> and >>= to non-monadic types, and saw that 'do' would not apply to them. So, I would like to add the following to that text: ------------- as long as proper types apply: x :: Prelude.Monad a y :: Prelude.Monad b foo :: b -> Prelude.Monad c ------------- Is that correct (Haskell and English)? Thanks, Maurício

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New Category Theory Intros
by Darrin Thompson 30 Aug '08

30 Aug '08

These category theory intros were new to me. Thought others here might care. http://golem.ph.utexas.edu/category/2008/08/new_structures_for_physics_i.ht… -- Darrin

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Haskell Speed Myth
by Isaac Gouy 30 Aug '08

30 Aug '08

> dons: > (Where I note GHC is currently in second place, though we've not > submitted any parallel programs yet). We might call that the thread-ring effect :-) > Also CC'd Isaac, Mr. Shootout. Isaac, is the quad core shootout > open for business? Should we rally the troops? iirc there was some discussion after the last GHC release about cleaning up the programs to make them less low-level given the improved capabilities of the compiler - I don't think that ever happened, and "low level" seems to be a common complaint against the Haskell programs shown in the benchmarks game. As Simon Peyton-Jones suggested we're certainly open for suggestions: http://groups.google.com/group/fa.haskell/browse_thread/thread/7eb82c689de8… However, we're operating new measurement scripts on both u64q (published) and gp4 (unpublished), and my focus is still on catching up to where we were with measurements from the old scripts, and installing third-party libraries on u64q.

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Re: [Haskell-cafe] Haskell Propeganda
by Richard A. O'Keefe 29 Aug '08

29 Aug '08

On 28 Aug 2008, at 8:34 am, Aaron Tomb wrote: > What type safety buys you, in my mind, is that Nothing is only a > valid value for explicit Maybe types. In cases where you don't use > Maybe, the "null" situation just can't occur. In languages with null > pointers, any pointer could possibly be null. This is not true of Eiffel. The ECMA Eiffel standard has ?T either a void reference or a reference to an instance of T !T a reference to an instance of T T same as !T in ECMA Eiffel; used to be same as ?T I suppose you could call the detachable type ?T an *implicit* Maybe. Needless to say, the switch in semantics of undecorated T helped to fork the Eiffel community...

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How to implement type level type equality?
by Marc Weber 29 Aug '08

29 Aug '08

I'm trying to write a validating XML library. I'm struggling with implementnig the choice alternative. Once piece of the code is Where st is the state representing the subelemnts which may still be attached to the current element. (Basically some tree containing Seq, Or, one or more, 0 or more, 0 or 1) Each xml tag is represented by it's own type. el is the element which should be added. (1): match of types, I'd like to return a C(onsumed) (2): no match, I'd like to return a F(ailure) class Consume st el r | st el -> r -- result is on of C,R,F -- element instance Consume ANY el C instance Consume (Elem el) el C -- (1) instance Consume (Elem el') el (F ExpectedButGot el' el) --(2) Of course this results in: || Functional dependencies conflict between instance declarations: Is there a nice way to implement this type level equality function? Of course I could give an instance for each possible comparison instance EQ TAG_A TAG_A HTrue instance EQ TAG_A TAG_DIV HFalse instance EQ TAG_A TAG_HTML HFalse which would work find but would also be a mess if a xml dtd has many elements. (100 elements means 100 * 100 comparisons = 10.000 *shrug*) I fear this slowing down compilation time a way too much. So my current idea is to assign numbers to all tags and do some type level number comparison. This will work resulting in something like this: data Zero data Succ a class Eq a b c | a b -> c instances: Eq (Succ a) Zero HFalse Eq Zero Zero HFalse Eq Zero (Succ a) HFalse Eq (Eq a b c) => (Succ b) (Succ a) c I want to know wether you know about a more elegant solution. Sincerly Marc Weber

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Re: [Haskell-cafe] Pure hashtable library
by Richard A. O'Keefe 29 Aug '08

29 Aug '08

On 28 Aug 2008, at 9:07 pm, Jules Bean wrote: > Insert for Data.Sequence is log(i) where i is the position of the > insertion; clearly bounded by log(n). toList is O(n) and index is > (at worst) log(i). > > I think the corresponding operations with tries are log(n), Let the key you want to insert have length L. Then insertion into a trie is O(L), independent of the size of the collection. If the "alphabet" the key's elements are drawn from is large, you can use a Ternary Search Tree, and insertion is then O(L.lg B) where B is the branching factor. There are fast Trie construction algorithms which are linear in the total size of the collection, \sum_{i=1}^{n}L_i. The worst case for Data.Sequence is where keys mostly vary at the end, in which case comparisons take O(L) time, and the cost is O(L.lg n), where n is usually much bigger than B. > So, it's all in the constants, isn't it? No.

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Haskell Propeganda
by Thomas Davie 29 Aug '08

29 Aug '08

Today I made an interesting discovery. We all know the benefits of a strong type system, and often tout it as a major advantage of using Haskell. The discovery I made, was that C programmer don't realise the implications of that, as this comment highlights: http://games.slashdot.org/comments.pl?sid=654821&cid=24716845 Apparently, no one realises that a SEGFAULT is a type error, just not a very helpful one. Bob

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