January 2010 - Haskell-Cafe

on Happstack - "embedded HTML"
by Günther Schmidt 12 Jan '10

12 Jan '10

Hi, I'm just introducing myself to Happstack. I come across an example page where HTML and haskell code is mixed. Is this how happstack produces html, it's haskell code embedded in HTML? Günther

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Tokenizing and Parsec
by Günther Schmidt 12 Jan '10

12 Jan '10

Hi all, I've used Parsec to "tokenize" data from a text file. It was actually quite easy, everything is correctly identified. So now I have a list/stream of self defined "Tokens" and now I'm stuck. Because now I need to write my own parsec-token-parsers to parse this token stream in a context-sensitive way. Uhm, how do I that then? Günther a Token is something like: data Token = ZE String | OPS | OPSShort String | OPSLong String | Other String | ZECd String deriving Show

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hackage build errors
by Keith Sheppard 12 Jan '10

12 Jan '10

Hello Cafe, I noticed on my package's hackage page there is a build failure message: http://hackage.haskell.org/package/txt-sushi I don't know if these are real errors or not (I don't experience them on OS X and it's pure Haskell code) but I did poke around and noticed that some popular packages on hackage also have build failures: http://hackage.haskell.org/package/xmonad http://hackage.haskell.org/package/haddock http://hackage.haskell.org/package/pandoc http://hackage.haskell.org/package/alex http://hackage.haskell.org/package/hlint ... so what should I make of these errors? Are they useful in some way or just a problem with the build environment? (If that's the case I think they should probably be removed since they're confusing for potential users) Best Keith -- keithsheppard.name

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Haskell-iPhone mailing list?
by Tom Tobin 11 Jan '10

11 Jan '10

Is there a mailing list for efforts to get Haskell running on the iPhone? As I've been maintaining an iPhone app for my employer (and nearly foaming at the mouth with hatred for Objective-C), I'd love to see a more sane option in this space. I've seen the wiki page [1], but I'm not aware of any other space for collaboration here. [1] http://www.haskell.org/haskellwiki/IPhone

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ANN: chp-2.0.0, chp-plus-1.0.0
by Neil Brown 11 Jan '10

11 Jan '10

Hi, I've just released version 2.0.0 of the Communicating Haskell Processes (CHP) message-passing concurrency library onto Hackage. The main change from 1.x is that I have split the functionality into two libraries: the core functionality remains in the chp package (now version 2.0.0), while some of the additional capabilities have been moved out to the new chp-plus package (version 1.0.0). This should help in the long run by keeping the packages from becoming too large, and by allowing me to trim the dependencies of the core chp package. I've written more about the changes here, for those who it will affect: http://chplib.wordpress.com/2010/01/11/splitting-chp/ This will require small changes to your build system if you're using CHP and you upgrade to the new versions. The new versions should also fix build troubles with GHC 6.12 -- let me know if you have any troubles installing under 6.10 or 6.12. Hackage links: http://hackage.haskell.org/package/chp-2.0.0 http://hackage.haskell.org/package/chp-plus-1.0.0 Thanks, Neil.

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Update for type-level library (0.2.4)
by Seyed Hosein Attarzadeh Niaki 11 Jan '10

11 Jan '10

Dear Haskell developers, A new version of the type-level library for type-level programming is pushed into the development repository and uploaded to the Hackage database. This is only a minor update to fix the compatibility issues with base 4, GHC>=6.10 and cabal-install (not backward compatible). Bests, Hosein Attarzadeh

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Cabal bug?
by Henk-Jan van Tuyl 11 Jan '10

11 Jan '10

L.S., I had a strange response from Cabal: >cabal upgrade hlint Resolving dependencies... cabal: cannot configure containers-0.3.0.0. It requires base >=4.2 && <6 For the dependency on base >=4.2 && <6 there are these packages: base-4.2.0.0. However none of them are available. base-4.2.0.0 was excluded because of the top level dependency base -any So, base-4.2.0.0 cannot be used, because any version of base is OK? Is this a bug in Cabal? Regards, Henk-Jan van Tuyl -- http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html --

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Approaches to dependent types (DT)
by pat browne 11 Jan '10

11 Jan '10

Hi, I am trying to understand the concept of dependent types as described in Haskell literature (Hallgren 2001),(Hinze,Jeuring et al. 2006). I am particularly interested in understanding two flavours of dependency mentioned by Hinze et al: 1) Types depending on values called dependent types 2) Types depending on types called parametric and type-indexed types I think that ascending levels of abstraction are important here: values - types - classes (Hallgren 2001). Here is a Haskell example of the use of DT: class Named object name | object -> name where name :: object -> name instance (Eq name, Named object name) => Eq object where object1 == object2 = (name object1) == (name object2) I think that the intended semantics are: 1) Objects have names and can be compared for equality using these names. 2) Two objects are considered equal (identical) if they have the same name. 3) The type of a name depends on the type of the object, which gets expressed as a type dependency (object -> name). I am not sure about the last semantic it might be interpreted as "the named object depends on..". This may indicate a flaw in my understanding of DT. I am aware that dependent types can be used to express constraints on the size of lists and other collections. My understanding of Haskell's functional dependency is that object -> name indicates that fixing the type object should fix the type name (equivalently we could say that type name depends on type object). The class definition seems to show a *type-to-type* dependency (object to name), while the instance definition shows how a name value is used to define equality for objects which could be interpreted as a *value-to-type* dependency (name to object) in the opposite direction to that of the class. My questions are: Question 1: Is my understanding correct? Question 2: What flavour of DT is this does this example exhibit? Best regards, Pat Hallgren, T. (2001). Fun with Functional Dependencies. In the Proceedings of the Joint CS/CE Winter Meeting, Varberg, Sweden, January 2001. Hinze, R., J. Jeuring, et al. (2006). Comparing Approaches to Generic Programming in Haskell. Datatype-generic programming: international spring school, SSDGP 2006.

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mtl and transformers
by Günther Schmidt 11 Jan '10

11 Jan '10

Hi, when I cabal-installed the iteratee package, the transformers package was also installed as a dependency. Now when I run applications that import Control.Monad.Transformers I get this: Could not find module `Control.Monad.Trans': it was found in multiple packages: transformers-0.1.4.0 mtl-1.1.0.2 What's the work around for situations like this? Günther

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Haskell Weekly News: Issue 145 - January 10, 2010
by jfredett＠gmail.com 10 Jan '10

10 Jan '10

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20100110 Issue 145 - January 10, 2010 --------------------------------------------------------------------------- Welcome to issue 145 of HWN, a newsletter covering developments in the [1]Haskell community. Welcome back Haskellers to the HWN! After a short hiatus we return with a megaedition covering the last three weeks of Haskell news and discussion. Needless to say, lots of new package and event announcements, as well as some really great discussions (the discussion about 'lawless' uninstances of Functors was particularly interesting) over the holiday break. Hopefully everyone had a safe and happy holiday season, and is back now steadily working off the winter-weight (in accordance with resolutions or no) by tapping steadily at the keyboard, producing wonderful new packages for me to place in this, your Haskell Weekly News! Announcements Last CfP for TGC 2010: EXTENDED deadline Jan. 20 2010. Emilio Tuosto [2]announced an extension to the TGC'10 call for papers. SourceGraph-0.6.0.0 and Graphalyze-0.9.0.0. Ivan Lazar Miljenovic [3]announced new releases of the SourceGraph and Graphalyze packages. Streaming Component Combinators 0.4. Mario Blazevic [4]announced version 0.4 of Streaming Component Combinators (SCC). safer-file-handles-0.1. Bas van Dijk [5]announced a new member of the 'monadic regions' family -- safer-file-handles. This package provides safety features on top of System.IO for handling file handles and operations. HOR 2010 1st CALL FOR ABSTRACTS. Frederic Blanqui [6]1f4e gmane.comp.lang.haskell.general/17710 announced the first call for abstracts for the Fifth International Workshop on Higher-Order Rewriting. vty-4.2.1.0. Corey O'Connor [7]announced a new version of the vty package. stm-io-hooks-0.6.0. Peter Robinson [8]announced a new version of stm-io-hooks. This version includes major interface changes, including the elimation of the 'onRetry' combinator. hecc-0.2. Marcel Fourne [9]announced the second release of his elliptic curve cryptography package, hecc. This release includes a license change to the BSD3 license, speed improvements, and new algorithms for point multiplication, as well as other changes. ghc-6.12.1 binary package for OpenSolaris. Michael Lee [10]announced a binary package availability of ghc-6.12 for OpenSolaris. Final Call for Participation: TLDI'10. Andrew Kennedy [11]announced the final call for participation in TLDI'10, the Workshop on Types in Language Design and Implementation. Haskell Web News: Looking back on 2009. Don Stewart [12]announced The Haskell Web News, a monthly summary of the hottest news about the Haskell programming language. hakyll-0.4. Jasper Van der Jeugt [13]announced the release of hakyll, a static site generator library. system-uuid-1.2.0. Jason Dusek [14]announced a new release of system-uuid. New versions of ALL the monadic regions packages. Bas van Dijk [15]announced a new version of all of the monadic regions packages, explanation of what this means is best left to the original post, linked previously. tuntap-0.0.1. John Van Enk [16]announced a new package, extracted from his in-progress VPN project. This package provides access to the TUN/TAP device in Linux. CPython / libpython bindings. John Millikin [17]announced a package providing bindings to the C API for CPython/libpython. Discussion Typed Configuration Files. Sebastian Fischer [18]asked about whether there was an analogue to CmdArgs for config files. Review request for my permutations implementation. CK Kashyap [19]asked for a review of his implementation of a function for finding permutations -- in addition to a review he asked for earlier for code involving monads. The result is a nice pair of threads with introductory material about idiomatic, efficient Haskell code. lawless instances of Functor. Paul Brauner [20]asked about 'lawless' instances (Editor's Note: more so -- 'uninstances', since by definition, it cannot be a functor without satisfying the laws, even if it an instance can be written) of the `Functor` class. This is in an effort to better understand how Functors work in Haskell and in the wider theory. Blog noise [21]Haskell news from the [22]blogosphere. Blog posts from people new to the Haskell community are marked with >>>, be sure to welcome them! * Neil Mitchell: [23]Using .ghci files to run projects. * Tom Moertel: [24]A formal language for recipes: brain dump. * Mark Jason Dominus: [25]A monad for probability and provenance. * Alex Mason: [26]A small follow up. * Russell O'Connor: [27]Constructive Classical Completeness. * Galois, Inc: [28]GHC Nominated for Programming Language Award. * Neil Brown: [29]CHP vs CML, Forking and Picky Receivers. * Conal Elliott: [30]Is Haskell a purely functional language?. * Conal Elliott: [31]Can functional programming be liberated from the von Neumann paradigm?. * Real-World Haskell: [32]Real World Haskell in Japaneseâå®æŠã§åŠã¶é¢æ°åèšèªããã°ã©ãã³ã°. * Conal Elliott: [33]Garbage collecting the semantics of FRP. * Alex Mason: [34]Why I love Cereal. * Mark Jason Dominus: [35]A short bibliography of probability monads. * Conal Elliott: [36]Communication, curiosity, and personal style. * Epilogue for Epigram: [37]Bidirectional Basics. * Neil Mitchell: [38]Explaining Haskell IO without Monads. * Conal Elliott: [39]Is program proving viable and useful?. * Conal Elliott: [40]Why program with continuous time?. Quotes of the Week * fasta: Haskell is like an efficient version of Python and is used for the same things. * koeien37: there are the languages that everybody complains about, and the languages that nobody use * Pseudonym: Olegs can be warded off by adding a note claiming that it's impossible to implement in the type system. * elly: I leave for five minutes and godel numbering of endofunctors is invokved as a potential proof that the universe is simulated. Thanks, #haskell :P * kmc: a monad is like an invisible train filled with jello traveling backwards in time * conal: "Obviously" is one of those words that is most useful when it is least true * Jafet: Javascript is pretty much a DSL for making your web browser take up more CPU * copumpkin: limpac: in this channel we're all automata with no feelings, except for the occasional feeling of hate. * dmhouse: I'm not sure it's working correctly but it sure is fast... * Cale: We need a bunch of "Will it monad?" video clips, with "It monads!" at the end of each. * merijn: I run on HST (Hacker Standard Time) which is essentially current time zone -3 * ray: [about an anti-FP blog post] i came up with some contrived example, announced that it's impossible, and therefore FP sucks * hiredman: I used to think "dons" was a title, like people who were recognized as being really good at haskell were called dons About the Haskell Weekly News New editions are posted to [41]the Haskell mailing list as well as to [42]the Haskell Sequence and [43]Planet Haskell. [44]RSS is also available, and headlines appear on [45]haskell.org. To help create new editions of this newsletter, please see the information on [46]how to contribute. Send stories to jfredett . at . gmail . dot . com. The darcs repository is available at darcs get [47]http://patch-tag.com/r/jfredett/HWN2/pullrepo HWN2 . References 1. http://haskell.org/ 2. http://article.gmane.org/gmane.comp.lang.haskell.general/17716 3. http://article.gmane.org/gmane.comp.lang.haskell.general/17714 4. http://article.gmane.org/gmane.comp.lang.haskell.general/17713 5. http://article.gmane.org/gmane.comp.lang.haskell.general/17711 6. http://article.gmane.org/ 7. http://article.gmane.org/gmane.comp.lang.haskell.general/17705 8. http://article.gmane.org/gmane.comp.lang.haskell.general/17704 9. http://article.gmane.org/gmane.comp.lang.haskell.general/17701 10. http://article.gmane.org/gmane.comp.lang.haskell.general/17700 11. http://article.gmane.org/gmane.comp.lang.haskell.general/17699 12. http://article.gmane.org/gmane.comp.lang.haskell.general/17698 13. http://article.gmane.org/gmane.comp.lang.haskell.cafe/68864 14. http://article.gmane.org/gmane.comp.lang.haskell.cafe/68857 15. http://article.gmane.org/gmane.comp.lang.haskell.cafe/68827 16. http://article.gmane.org/gmane.comp.lang.haskell.cafe/68807 17. http://article.gmane.org/gmane.comp.lang.haskell.cafe/68388 18. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/68865 19. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/68811 20. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/68711 21. http://planet.haskell.org/ 22. http://haskell.org/haskellwiki/Blog_articles 23. http://neilmitchell.blogspot.com/2010/01/using-ghci-files-to-run-projects.h… 24. http://blog.moertel.com/articles/2010/01/08/a-formal-language-for-recipes-b… 25. http://blog.plover.com/prog/haskell/probmonad.html 26. http://random.axman6.com/blog/?p=139 27. http://r6.ca/blog/20100107T035902Z.html 28. http://www.galois.com/blog/2010/01/06/ghc-nominated-for-programming-languag… 29. http://chplib.wordpress.com/2010/01/06/chp-vs-cml-forking-and-picky-receive… 30. http://conal.net/blog/posts/is-haskell-a-purely-functional-language/ 31. http://conal.net/blog/posts/can-functional-programming-be-liberated-from-th… 32. http://www.realworldhaskell.org/blog/2010/01/05/real-world-haskell%e2%80%94… 33. http://conal.net/blog/posts/garbage-collecting-the-semantics-of-frp/ 34. http://random.axman6.com/blog/?p=124 35. http://blog.plover.com/prog/haskell/probmonad-refs.html 36. http://conal.net/blog/posts/communication-curiosity-and-personal-style/ 37. http://www.e-pig.org/epilogue/?p=283 38. http://neilmitchell.blogspot.com/2010/01/haskell-io-without-monads.html 39. http://conal.net/blog/posts/is-program-proving-viable-and-useful/ 40. http://conal.net/blog/posts/why-program-with-continuous-time/ 41. http://www.haskell.org/mailman/listinfo/haskell 42. http://sequence.complete.org/ 43. http://planet.haskell.org/ 44. http://sequence.complete.org/node/feed 45. http://haskell.org/ 46. http://haskell.org/haskellwiki/HWN 47. http://patch-tag.com/r/jfredett/HWN2/pullrepo%20HWN2

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