September 2009 - Haskell-Cafe

Sorry about the triple post
by Joe Fredette 26 Sep '09

26 Sep '09

Trying to work out how to make sendmail do what I want, my computer is a fickle beast. Also- it should be HWN issue 133. But I'm an idiot, and just copied the value without thinking. Someday, when I automate all the uploading/sending to the list business, this will not be an issue... :/ /Joe

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Haskell Weekly News: Issue 131 - Semptember 25, 2009
by Joe Fredette 26 Sep '09

26 Sep '09

This week is an experiment, I'm eliminating the mail client from the sendout, and simply using sendmail directly. If this doesn't fix the line ending problem, then I'm pretty sure it's a software issue, and not a client issue. If you notice problems, please email me at this address, put [HWN] in the subject line so my filters will catch it. Please _don't_ email me if there are no problems. Thanks, /Joe --------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20090926 Issue 133 - September 26, 2009 --------------------------------------------------------------------------- Welcome to issue 133 of HWN, a newsletter covering developments in the [1]Haskell community. This week, we have a few new libraries, some interesting discussion about EDSLs, a comment from Oleg, and dons extolling the virtues of SCIENCE! On the new HWN software front, I've decided to jump right into something I had planned for far further down the development chain. Specifically, rather than scraping GMane for messages, I've been working on a way to grab the messages directly from the mailing-lists. I'm not entirely sure how I'm going to create links as they are now for the messages, but one crisis at a time. Till next week, here's the Haskell Weekly News! Announcements epoll bindings 0.2. Toralf Wittner [2]announced the release of epoll bindings 0.2 available [3]here. Epoll is an I/O event notification facility for Linux similar to poll but with good scaling characteristics. This release adds a buffer abstraction on top of the existing low-level bindings, so client code can write and read to buffers without having to deal directly with the underlying epoll event handling. diagrams 0.2.1, and planned major improvements. Brent Yorgey [4]announced version 0.2.1 of the diagrams library, available now on [5]Hackage. This minor release which fixes a few bugs and adds a few new combinators, most notably a grid layout combinator contributed by Ganesh Sittampalam. Workflow-0.5.5, TCache-0.6.4 RefSerialize-0.2.4. Alberto G. Corona [6]announced Workflow 0.5.5. Workflow provides a monad transformer that encapsulates any monad in a state monad that bring automatic state logging and recovery. A workflow can be viewed as a thread that persist across planeed or unplanned application shutdowns. When recovering the execution is resumed at the last action that was logged. The process continues at the same state as if not interruption took place. graphviz-2999.5.1.1. Ivan Lazar Miljenovic [7]announced version 2999.5.1.1 of the [8]graphviz library. This is another bug-fix release, fixing the problem spotted by Kathleen Fisher where Dot keywords need to be explicitly quoted if used as labels, etc. There is no change to the API. histogram-fill, library for creating histograms. Khudyakov Alexey [9]announced [10]histogram-fill. histogram-fill provides a generic and convenient API for making histograms. Features include, multiple simultaneous histogram creation, Immutable histograms, and Serialization to and from human readable text. Darcs Hacking Sprint - 14-15 November Vienna. Eric Kow [11]announced the third Darcs Hacking Sprint. Which will take place 14-15 November, 2009 at the University of Technology, Vienna, Austria. Anybody who wants to hack on Darcs (or Camp, Focal, SO6, etc) -- Beginners especially -- are welcome! 2nd CFP: TLDI 2010. Andrew Kennedy [12]announced a second call for papers for TLDI2010, the Types in Language Design and Implementation Workshop. darcs 2.3.1: better docs, fewer bugs. Reinier Lamers [13]announced a new stable version of darcs, with bugfixes from 2.3.0, improved documentation, and removal of the old autoconf build system. TFM09: Call for Participation (FMWeek, Eindhoven, November 2009). J.N. Oliveira [14]announced a Call for Participation in TFM2009 2nd Int. FME Conference on Teaching Formal Methods Friday, November 6th 2009, co-located with FM2009 : 16th Int. Symposium on Formal Methods Eindhoven, the Netherlands, November 2 - November 6, 2009. Discussion Monad Tutorial in C++. Adrian May [15]wrote a tutorial about monads in some other niche language... Beginning of a meta-Haskell. Oleg -- [16]as if he needs any introduction -- commented on things far above my ability to understand. Evidently, however, it involves extensible, modular interpreters in the ``tagless final'' style. It was a reply to an earlier thread [17]here. An issue with EDSLs in the ``finally tagless'' tradition. Brad Larsen [18]talked about his run in with [19]the expression problem while experimenting with EDSLs. Blog noise [20]Haskell news from the [21]blogosphere. Blog posts from people new to the Haskell community are marked with >>>, be sure to welcome them! * Bryan O'Sullivan: [22]Riddle me this. * David Amos: [23]Finite geometries, part 3: Points in PG(n,Fq). David's continuing series on Finite Geometries. * Neil Brown: [24]Concurrent Pearl: The Expanding Prime Pipeline. * Mikael Vejdemo Johansson (Syzygy-): [25][MATH198] Lecture 1 now online. Mikael's first Category Theory Lecture is up online. * Brent Yorgey: [26]diagrams 0.2.1, and future plans. * Alex McLean: [27]hackpact week 4. Part of the continuing series on Alex's hackpact progress. * Manuel M T Chakravarty: [28]Heads Up: GHC devs on Macs - GHC's testsuite crashes spotlight indexer on SL. * Clint Moore: [29]8 Cores of Awesome. * Bryan O'Sullivan: [30]Video of my CUFP keynote. * Chris Smith: [31]Thoughts on Hackage and the Haskell Platform. * Manuel M T Chakravarty: [32]Haskell Bindings to C -> c2hs. * Neil Brown: [33]Functions into processes, using arrows. * Brent Yorgey: [34]Functional MetaPost. * Malcolm Wallace: [35]Haskell Symposium 2009 - videos now online. * DEFUN 2009: [36]DEFUN and CUFP 2009 registration are now open!. * Chris Smith: [37]Type Classes With An Easier Example. * Darcs: [38]darcs weekly news #41. * Greg Bacon: [39]Haskell craps. * Bryan O'Sullivan: [40]A new pseudo-random number generator for Haskell. * Thomas M. DuBuisson: [41]HacPDX is Coming. * Dan Piponi (sigfpe): [42]More Parsing With Best First Search. * Osfameron: [43]Coin Tricks. Quotes of the Week * lilac: ponders whether unsafePerformIO would be better as simonSaysPerformIO * bos: [On the type signature of hPrintf] This makes me a sad Irish panda. * ksf: (But if (on the other hand)) (I think only a number in general (whether it be five or a hundred)) (this thought is rather the representation of a method (whereby a multiplicity (for instance a thousand) may be represented (in an image in conformity with a certain concept)) than the image itself. * dons: ah, via the magic of SCIENCE * dobblego: many of my colleagues used to be [fond of ruby] as well until I was let loose on them * dons: (on whether a library is wanted) *yes* put it on Hackage! * BMeph: (about parsec) 'Cause it's light-years ahead of the competition! * switch: Comeon people! You make the news! * ray: I think programmers make the worst programmers, also the worst people, and I'm saying this having not looked at programming reddit in a while. * Orclev: ... a lot of haskell still looks greek to me, and I'm not talking about lambdas. * Jason Dusek: "Some day, we're going to need a short, catchy name for Cabal packages. Let's call them cabbages." [see http://thread.gmane.org/gmane.comp.lang.haskell.cafe/63649] * Reinier Lamers: If we keep up the current pace of performance hacking, darcs will be complete before you even hit the enter key in a few years * Trent Buck: [To Reiner Lamers] With the appropriate (ie unbuffered) terminal, this is already the case for interactive prompts. About the Haskell Weekly News New editions are posted to [44]the Haskell mailing list as well as to [45]the Haskell Sequence and [46]Planet Haskell. [47]RSS is also available, and headlines appear on [48]haskell.org. To help create new editions of this newsletter, please see the information on [49]how to contribute. Send stories to jfredett . at . gmail . dot . com. The darcs repository is available at darcs get [50]http://patch-tag.com/r/HWN2/home . References 1. http://haskell.org/ 2. http://article.gmane.org/gmane.comp.lang.haskell.cafe/63815 3. http://hackage.haskell.org/package/epoll 4. http://article.gmane.org/gmane.comp.lang.haskell.cafe/63791 5. http://hackage.haskell.org/package/diagrams 6. http://article.gmane.org/gmane.comp.lang.haskell.cafe/63786 7. http://article.gmane.org/gmane.comp.lang.haskell.cafe/63770 8. http://hackage.haskell.org/package/graphviz-2999.5.1.1 9. http://article.gmane.org/gmane.comp.lang.haskell.cafe/63640 10. http://hackage.haskell.org/package/histogram-fill 11. http://article.gmane.org/gmane.comp.lang.haskell.general/17511 12. http://article.gmane.org/gmane.comp.lang.haskell.general/17508 13. http://article.gmane.org/gmane.comp.lang.haskell.general/17507 14. http://article.gmane.org/gmane.comp.lang.haskell.general/17506 15. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/63800 16. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/63761 17. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/63751 18. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/63751 19. http://www.daimi.au.dk/~madst/tool/papers/expression.txt 20. http://planet.haskell.org/ 21. http://haskell.org/haskellwiki/Blog_articles 22. http://www.serpentine.com/blog/2009/09/25/riddle-me-this/ 23. http://haskellformaths.blogspot.com/2009/09/finite-geometries-part-3-points… 24. http://chplib.wordpress.com/2009/09/24/the-expanding-prime-pipeline/ 25. http://blog.mikael.johanssons.org/archive/2009/09/math198-lecture-1-now-onl… 26. http://byorgey.wordpress.com/2009/09/24/diagrams-0-2-1-and-future-plans/ 27. http://yaxu.org/hackpact-week-3-2/ 28. http://justtesting.org/post/195668539 29. http://www.l2mlogistics.com/2009/09/8-cores-of-awesome.html 30. http://www.serpentine.com/blog/2009/09/23/video-of-my-cufp-keynote/ 31. http://cdsmith.wordpress.com/2009/09/23/thoughts-on-hackage-and-the-haskell… 32. http://justtesting.org/post/194790750 33. http://chplib.wordpress.com/2009/09/22/functions-into-processes-using-arrow… 34. http://byorgey.wordpress.com/2009/09/21/functional-metapost/ 35. http://feedproxy.google.com/~r/malcolm/~3/wxsdwfUArJ8/haskell-symposium-200… 36. http://www.defun2009.info/blog/2009/06/defun-2009-registration-is-now-open/ 37. http://cdsmith.wordpress.com/2009/09/20/side-computations-via-type-classes/ 38. http://blog.darcs.net/2009/09/darcs-weekly-news-41.html 39. http://feedproxy.google.com/~r/gbacon/~3/QILx5EtQN4o/haskell-craps.html 40. http://www.serpentine.com/blog/2009/09/19/a-new-pseudo-random-number-genera… 41. http://tommd.wordpress.com/2009/09/19/hacpdx-is-coming/ 42. http://blog.sigfpe.com/2009/09/language-nomonomorphismrestrictiongener.html 43. http://greenokapi.net/blog/2009/09/19/coin-tricks/ 44. http://www.haskell.org/mailman/listinfo/haskell 45. http://sequence.complete.org/ 46. http://planet.haskell.org/ 47. http://sequence.complete.org/node/feed 48. http://haskell.org/ 49. http://haskell.org/haskellwiki/HWN 50. http://patch-tag.com/r/HWN2/home

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Haskell Weekly News: Issue 131 - Semptember 25, 2009
by Joe Fredette 26 Sep '09

26 Sep '09

This week is an experiment, I'm eliminating the mail client from the sendout, and simply using sendmail directly. If this doesn't fix the line ending problem, then I'm pretty sure it's a software issue, and not a client issue. If you notice problems, please email me at this address, put [HWN] in the subject line so my filters will catch it. Please _don't_ email me if there are no problems. Thanks, /Joe --------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20090926 Issue 133 - September 26, 2009 --------------------------------------------------------------------------- Welcome to issue 133 of HWN, a newsletter covering developments in the [1]Haskell community. This week, we have a few new libraries, some interesting discussion about EDSLs, a comment from Oleg, and dons extolling the virtues of SCIENCE! On the new HWN software front, I've decided to jump right into something I had planned for far further down the development chain. Specifically, rather than scraping GMane for messages, I've been working on a way to grab the messages directly from the mailing-lists. I'm not entirely sure how I'm going to create links as they are now for the messages, but one crisis at a time. Till next week, here's the Haskell Weekly News! Announcements epoll bindings 0.2. Toralf Wittner [2]announced the release of epoll bindings 0.2 available [3]here. Epoll is an I/O event notification facility for Linux similar to poll but with good scaling characteristics. This release adds a buffer abstraction on top of the existing low-level bindings, so client code can write and read to buffers without having to deal directly with the underlying epoll event handling. diagrams 0.2.1, and planned major improvements. Brent Yorgey [4]announced version 0.2.1 of the diagrams library, available now on [5]Hackage. This minor release which fixes a few bugs and adds a few new combinators, most notably a grid layout combinator contributed by Ganesh Sittampalam. Workflow-0.5.5, TCache-0.6.4 RefSerialize-0.2.4. Alberto G. Corona [6]announced Workflow 0.5.5. Workflow provides a monad transformer that encapsulates any monad in a state monad that bring automatic state logging and recovery. A workflow can be viewed as a thread that persist across planeed or unplanned application shutdowns. When recovering the execution is resumed at the last action that was logged. The process continues at the same state as if not interruption took place. graphviz-2999.5.1.1. Ivan Lazar Miljenovic [7]announced version 2999.5.1.1 of the [8]graphviz library. This is another bug-fix release, fixing the problem spotted by Kathleen Fisher where Dot keywords need to be explicitly quoted if used as labels, etc. There is no change to the API. histogram-fill, library for creating histograms. Khudyakov Alexey [9]announced [10]histogram-fill. histogram-fill provides a generic and convenient API for making histograms. Features include, multiple simultaneous histogram creation, Immutable histograms, and Serialization to and from human readable text. Darcs Hacking Sprint - 14-15 November Vienna. Eric Kow [11]announced the third Darcs Hacking Sprint. Which will take place 14-15 November, 2009 at the University of Technology, Vienna, Austria. Anybody who wants to hack on Darcs (or Camp, Focal, SO6, etc) -- Beginners especially -- are welcome! 2nd CFP: TLDI 2010. Andrew Kennedy [12]announced a second call for papers for TLDI2010, the Types in Language Design and Implementation Workshop. darcs 2.3.1: better docs, fewer bugs. Reinier Lamers [13]announced a new stable version of darcs, with bugfixes from 2.3.0, improved documentation, and removal of the old autoconf build system. TFM09: Call for Participation (FMWeek, Eindhoven, November 2009). J.N. Oliveira [14]announced a Call for Participation in TFM2009 2nd Int. FME Conference on Teaching Formal Methods Friday, November 6th 2009, co-located with FM2009 : 16th Int. Symposium on Formal Methods Eindhoven, the Netherlands, November 2 - November 6, 2009. Discussion Monad Tutorial in C++. Adrian May [15]wrote a tutorial about monads in some other niche language... Beginning of a meta-Haskell. Oleg -- [16]as if he needs any introduction -- commented on things far above my ability to understand. Evidently, however, it involves extensible, modular interpreters in the ``tagless final'' style. It was a reply to an earlier thread [17]here. An issue with EDSLs in the ``finally tagless'' tradition. Brad Larsen [18]talked about his run in with [19]the expression problem while experimenting with EDSLs. Blog noise [20]Haskell news from the [21]blogosphere. Blog posts from people new to the Haskell community are marked with >>>, be sure to welcome them! * Bryan O'Sullivan: [22]Riddle me this. * David Amos: [23]Finite geometries, part 3: Points in PG(n,Fq). David's continuing series on Finite Geometries. * Neil Brown: [24]Concurrent Pearl: The Expanding Prime Pipeline. * Mikael Vejdemo Johansson (Syzygy-): [25][MATH198] Lecture 1 now online. Mikael's first Category Theory Lecture is up online. * Brent Yorgey: [26]diagrams 0.2.1, and future plans. * Alex McLean: [27]hackpact week 4. Part of the continuing series on Alex's hackpact progress. * Manuel M T Chakravarty: [28]Heads Up: GHC devs on Macs - GHC's testsuite crashes spotlight indexer on SL. * Clint Moore: [29]8 Cores of Awesome. * Bryan O'Sullivan: [30]Video of my CUFP keynote. * Chris Smith: [31]Thoughts on Hackage and the Haskell Platform. * Manuel M T Chakravarty: [32]Haskell Bindings to C -> c2hs. * Neil Brown: [33]Functions into processes, using arrows. * Brent Yorgey: [34]Functional MetaPost. * Malcolm Wallace: [35]Haskell Symposium 2009 - videos now online. * DEFUN 2009: [36]DEFUN and CUFP 2009 registration are now open!. * Chris Smith: [37]Type Classes With An Easier Example. * Darcs: [38]darcs weekly news #41. * Greg Bacon: [39]Haskell craps. * Bryan O'Sullivan: [40]A new pseudo-random number generator for Haskell. * Thomas M. DuBuisson: [41]HacPDX is Coming. * Dan Piponi (sigfpe): [42]More Parsing With Best First Search. * Osfameron: [43]Coin Tricks. Quotes of the Week * lilac: ponders whether unsafePerformIO would be better as simonSaysPerformIO * bos: [On the type signature of hPrintf] This makes me a sad Irish panda. * ksf: (But if (on the other hand)) (I think only a number in general (whether it be five or a hundred)) (this thought is rather the representation of a method (whereby a multiplicity (for instance a thousand) may be represented (in an image in conformity with a certain concept)) than the image itself. * dons: ah, via the magic of SCIENCE * dobblego: many of my colleagues used to be [fond of ruby] as well until I was let loose on them * dons: (on whether a library is wanted) *yes* put it on Hackage! * BMeph: (about parsec) 'Cause it's light-years ahead of the competition! * switch: Comeon people! You make the news! * ray: I think programmers make the worst programmers, also the worst people, and I'm saying this having not looked at programming reddit in a while. * Orclev: ... a lot of haskell still looks greek to me, and I'm not talking about lambdas. * Jason Dusek: "Some day, we're going to need a short, catchy name for Cabal packages. Let's call them cabbages." [see http://thread.gmane.org/gmane.comp.lang.haskell.cafe/63649] * Reinier Lamers: If we keep up the current pace of performance hacking, darcs will be complete before you even hit the enter key in a few years * Trent Buck: [To Reiner Lamers] With the appropriate (ie unbuffered) terminal, this is already the case for interactive prompts. About the Haskell Weekly News New editions are posted to [44]the Haskell mailing list as well as to [45]the Haskell Sequence and [46]Planet Haskell. [47]RSS is also available, and headlines appear on [48]haskell.org. To help create new editions of this newsletter, please see the information on [49]how to contribute. Send stories to jfredett . at . gmail . dot . com. The darcs repository is available at darcs get [50]http://patch-tag.com/r/HWN2/home . References 1. http://haskell.org/ 2. http://article.gmane.org/gmane.comp.lang.haskell.cafe/63815 3. http://hackage.haskell.org/package/epoll 4. http://article.gmane.org/gmane.comp.lang.haskell.cafe/63791 5. http://hackage.haskell.org/package/diagrams 6. http://article.gmane.org/gmane.comp.lang.haskell.cafe/63786 7. http://article.gmane.org/gmane.comp.lang.haskell.cafe/63770 8. http://hackage.haskell.org/package/graphviz-2999.5.1.1 9. http://article.gmane.org/gmane.comp.lang.haskell.cafe/63640 10. http://hackage.haskell.org/package/histogram-fill 11. http://article.gmane.org/gmane.comp.lang.haskell.general/17511 12. http://article.gmane.org/gmane.comp.lang.haskell.general/17508 13. http://article.gmane.org/gmane.comp.lang.haskell.general/17507 14. http://article.gmane.org/gmane.comp.lang.haskell.general/17506 15. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/63800 16. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/63761 17. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/63751 18. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/63751 19. http://www.daimi.au.dk/~madst/tool/papers/expression.txt 20. http://planet.haskell.org/ 21. http://haskell.org/haskellwiki/Blog_articles 22. http://www.serpentine.com/blog/2009/09/25/riddle-me-this/ 23. http://haskellformaths.blogspot.com/2009/09/finite-geometries-part-3-points… 24. http://chplib.wordpress.com/2009/09/24/the-expanding-prime-pipeline/ 25. http://blog.mikael.johanssons.org/archive/2009/09/math198-lecture-1-now-onl… 26. http://byorgey.wordpress.com/2009/09/24/diagrams-0-2-1-and-future-plans/ 27. http://yaxu.org/hackpact-week-3-2/ 28. http://justtesting.org/post/195668539 29. http://www.l2mlogistics.com/2009/09/8-cores-of-awesome.html 30. http://www.serpentine.com/blog/2009/09/23/video-of-my-cufp-keynote/ 31. http://cdsmith.wordpress.com/2009/09/23/thoughts-on-hackage-and-the-haskell… 32. http://justtesting.org/post/194790750 33. http://chplib.wordpress.com/2009/09/22/functions-into-processes-using-arrow… 34. http://byorgey.wordpress.com/2009/09/21/functional-metapost/ 35. http://feedproxy.google.com/~r/malcolm/~3/wxsdwfUArJ8/haskell-symposium-200… 36. http://www.defun2009.info/blog/2009/06/defun-2009-registration-is-now-open/ 37. http://cdsmith.wordpress.com/2009/09/20/side-computations-via-type-classes/ 38. http://blog.darcs.net/2009/09/darcs-weekly-news-41.html 39. http://feedproxy.google.com/~r/gbacon/~3/QILx5EtQN4o/haskell-craps.html 40. http://www.serpentine.com/blog/2009/09/19/a-new-pseudo-random-number-genera… 41. http://tommd.wordpress.com/2009/09/19/hacpdx-is-coming/ 42. http://blog.sigfpe.com/2009/09/language-nomonomorphismrestrictiongener.html 43. http://greenokapi.net/blog/2009/09/19/coin-tricks/ 44. http://www.haskell.org/mailman/listinfo/haskell 45. http://sequence.complete.org/ 46. http://planet.haskell.org/ 47. http://sequence.complete.org/node/feed 48. http://haskell.org/ 49. http://haskell.org/haskellwiki/HWN 50. http://patch-tag.com/r/HWN2/home

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Re: [Haskell-cafe] An issue with EDSLs in the ``finally tagless'' tradition
by Brad Larsen 26 Sep '09

26 Sep '09

Bruno, On Thu, Sep 24, 2009 at 1:20 AM, Bruno Oliveira <bruno(a)ropas.snu.ac.kr> wrote: > Hello Brad, > > I believe that the problem you encountered is not quite the expression > problem (which is about adding new constructors and functions modularly), > but rather about refining *existing* constructs with more specific types. > One could argue that they are related though but, for your own sake, you may > want to use a term that more directly points to the problem in question. [...] Indeed, for finding existing approaches to this problem, it is prudent to know what others refer to it as. If you squint a little, this looks like an instance of the expression problem: type classes are (families of) constructors, and instances of those type classes (i.e., interpretations) are functions on those constructors. Sincerely, Brad

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Type parametrized modules (was Strong duck typing)
by Peter Verswyvelen 26 Sep '09

26 Sep '09

Thanks a lot for the answers. Another feature I really miss is type parameterized modules. For example, suppose I have a module that defines simple geometry / linear algebra math, with types like Vector a, Matrix a, Point a, Size a, Rectangle a, etc... Usually when you import such a module, you will be picking one particular type for "a" across the whole module or even program, e.g. Double, Float, Complex Double. I see such a type more like a "module global type", so instead of writing import qualified MyGeometry (Vector, Matrix, Point, Size, Rectangle) as G type Vector = G.Vector Float type Matrix = G.Matrix Float type Point = G.Point Float etc I would like to write import (MyGeometry Float) or even module (MyGame real) import (MyGeometry real) and then I can just use Vector, Matrix, etc, without specifying the type. So it's as if (MyGeometry Float) performs a "partial type application", just like ((+) 1) does "partial value application" However, when writing import MyGeometry one would have to use the parametrized version of Vector etc again. I'm not sure if this makes any sense, or if it is even worth considering :-) On Sat, Sep 26, 2009 at 6:30 AM, Sebastian Sylvan < sebastian.sylvan(a)gmail.com> wrote: > > > On Fri, Sep 25, 2009 at 10:55 PM, Casey Hawthorne <caseyh(a)istar.ca> wrote: > >> On Fri, 25 Sep 2009 23:25:21 +0200, you wrote: >> >> >On Fri, Sep 25, 2009 at 8:14 PM, Job Vranish <jvranish(a)gmail.com> wrote: >> > >> >> Supposedly OCaml has an OO feature that does this but I haven't tried >> it >> >> out. >> >> >> > >> >Indeed, OCaml has stuctural polymorphism, it's a wonderful feature. >> > >> >*# let f myobj = myobj#foo "Hi !";; >> >val f : < foo : string -> 'a; .. > -> 'a = <fun>* >> > >> >IIRC, there has been work on Template Haskell for structural >> polymorphism. >> >> Structural subtyping/polymorphism: >> >> Pros: >> - an object can be coerced to any compatible type, the types do not >> have to be specified ahead of time, that is at compile time. >> >> Cons: >> - may be overly permissive; some coercions might not make sense >> semantically. >> >> I wonder how Haskell will minimize the cons, since it is strongly >> typed. >> >> > I kind of think there's no real problem here. If you say that you can > accept any record with a given set of fields, then you have to make sure you > make no other assumptions. This is, in principle, no different from passing > a speed Double to a function that expects a mass Double (e.g. it may produce > garbage for negative values). In both cases a function that requires extra > invariants can enforce it by using a newtype that's constructed and > manipulated in a way which preserves the extra semantic rules. > > -- > Sebastian Sylvan > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > >

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Strong duck typing / structural subtyping / type class aliases / ??? in Haskell
by Peter Verswyvelen 26 Sep '09

26 Sep '09

Haskell's records are a bit annoying, and type-classes often group together too many methods, which means you make early decisions about future unknown requirements, and IMO you always get it wrong :-) After having read an email in the cafe about the Noop language & Self language, I realized that what I really would like to have is "strong duck typing" on records (or is it called structural subtyping? or prototype-based-objects? or something like that) For example (silly example full of inaccuracies, but you get the picture): class HasPosition a where position :: a -> Point withPosition :: Point -> a -> a class HasVelocity a where velocity :: a -> Vector withVelocity :: Vector -> a -> a which we really should write as field HasPosition :: Point field HasVelocity :: Vector And then record IsKinetic :: HasPosition HasVelocity suppose we write a function like kineticEulerStep dt k = withPosition (position k .+^ dt *^ velocity k) k kineticEulerStep will work on any type a that HasPosition and HasVelocity, and would get inferred signature kineticEulerStep :: IsKinetic a => Float -> a -> a which is identical to kineticEulerStep :: (HasPosition a, HasVelocity a) => Float -> a -> a So basically kineticEulerStep accepts anything that HasPosition and HasVelocity, whatever it is. So if it walks like a duck and ..., then it is a duck, but statically known... We could also do field HasForce :: Vector field HasMass :: Float record IsDynamic :: IsKinetic HasForce HasMass acceleration d = force d ^/ mass d withAcceleration a d = withForce (a ^* mass d) d dynamicEulerStep dt d = withVelocity (velocity d ^+^ dt *^ acceleration d) Of course you would also need type families to be really correct since Vector, Point, etc should also be parametrized. And really kineticEulerStep might also work on something that HasVelocity and HasAcceleration (since the code in dynamicEulerStep is almost the same as kineticEulerStep), so better abstraction might be needed. I'm not sure what kind of overhead a system like this would have in Haskell, since I suspect the many dictionaries are often not optimized away. I think for Haskell prime, something like this was suggested<http://repetae.net/recent/out/classalias.html>, but is was rejected? Languages like OCaml and haXe <http://haxe.org/manual/2_types> also provide a similar feature? I would like to collect ways of doing this in Haskell, without boilerplate, and preferably without runtime overhead. I remember reading OOHaskell a while time ago, and while I didn't understand a lot of it, I recall it also was doing a similar thing, but since the compiler lacks native support, the error messages you get most likely make it impossible to figure out what is going wrong. I think Grapefruit's Records, HList, Data.Accessor, etc.. might also work. Any guidelines and comments regarding "strong duck typing"/"structural subtyping" are very welcome, since the lack of this is the only reason why I would prefer a dynamic language over a static one. Thanks a lot, Peter Verswyvelen

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An issue with EDSLs in the ``finally tagless'' tradition
by Brad Larsen 26 Sep '09

26 Sep '09

I seem to have run into an instance of the expression problem [1], or something very similar, when experimenting with ``finally tagless'' EDSLs, and don't see a good way to work around it. I have been experimenting with embedded DSLs, using the techniques described in a couple recent papers [2,3]. The idea is this: implement an embedded DSL using type classes, rather than ADTs or GADTs. This allows one to define analyses, queries, and manipulations of EDSL programs independently, as class instances. Furthermore, by using type classes rather than data types, there is no interpretive overhead in the analyses, queries, and manipulations on the EDSL programs. Finally, using type classes permits greater modularity, as an EDSL can be defined as the combination of several simpler EDSLs [3]. Suppose we have a type class for simple integer arithmetic expressions: > class IntArithExpr exp where > integer :: Integer -> exp Integer > add :: exp Integer -> exp Integer -> exp Integer We can write an evaluator for these expressions like this: > newtype E a = E { eval :: a } > > instance IntArithExpr E where > integer = E > add e1 e2 = E (eval e1 + eval e2) > > -- eval $ add (integer 20) (integer 22) <==> 42 The trouble comes in when defining a more general arithmetic expression type class. Suppose we want polymorphic arithmetic expressions: > class PolyArithExpr exp where > constant :: a -> exp a > addP :: exp a -> exp a -> exp a We then try to define an evaluator: > -- instance PolyArithExpr E where > -- constant = E > -- addP e1 e2 = E (eval e1 + eval e2) -- bzzt! The instance definition for `addP' is not type correct: Could not deduce (Num a) from the context () arising from a use of `+' at /home/blarsen/mail.lhs:42:20-36 One way to remedy this is to change the class definition of PolyArithExpr so that `addP' has a Num constraint on `a': > class PolyArithExprFixed exp where > pae_constant :: a -> exp a > pae_add :: Num a => exp a -> exp a -> exp a which allows us to define an evaluator: > instance PolyArithExprFixed E where > pae_constant = E > pae_add e1 e2 = E (eval e1 + eval e2) I find this ``fix'' lacking, however: to define a new interpretation of the EDSL, we may be forced to change the DSL definition. This is non-modular, and seems like an instance of the expression problem. (There may be a multiparameter type class solution for this.) How can one define the polymorphic arithmetic EDSL without cluttering up the class definitions with interpretation-specific constraints, and still write the desired interpretations? Sincerely, Bradford Larsen References: [1] Philip Wadler. The Expression Problem. November 12, 1998. http://www.daimi.au.dk/~madst/tool/papers/expression.txt. [2] Jacques Carette, Oleg Kiselyov, and Chung-chieh Shan. Finally Tagless, Partially Evaluated: Tagless Staged Interpreters for Simpler Typed Languages. APLAS 2007. [3] Robert Atkey, Sam Lindley, and Jeremy Yallop. Unembedding Domain-Specific Languages. ICFP Haskell Symposium 2009.

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Beginning of a meta-Haskell [was: An issue with the ``finally tagless'' tradition]
by oleg＠okmij.org 25 Sep '09

25 Sep '09

The topic of an extensible, modular interpreter in the tagless final style has come up before. A bit more than a year ago, on a flight from Frankfurt to San Francisco I wrote two interpreters for a trivial subset of Haskell or ML (PCF actually), just big enough for Power, Fibonacci and other classic functions. The following code is a fragment of meta-Haskell. It defines the object language and two interpreters: one is the typed meta-circular interpreter, and the other is a non-too-pretty printer. We can write the expression once: > power = > fix $ \self -> > lam $ \x -> lam $ \n -> > if_ (n <= 0) 1 > (x * ((self $$ x) $$ (n - 1))) and interpret it several times, as an integer > -- testpw :: Int > testpw = (unR power) (unR 2) ((unR 7)::Int) > -- 128 or as a string > -- testpwc :: P.String > testpwc = showQC power {- "(let self0 = (\\t1 -> (\\t2 -> (if (t2 <= 0) then 1 else (t1 * ((self0 t1) (t2 - 1)))))) in self0)" -} The code follows. It is essentially Haskell98, with the exception of multi-parameter type classes (but no functional dependencies, let alone overlapping instances). {-# LANGUAGE NoMonomorphismRestriction, NoImplicitPrelude #-} {-# LANGUAGE MultiParamTypeClasses, FlexibleInstances #-} -- A trivial introduction to `meta-Haskell', just enough to give a taste -- Please see the tests at the end of the file module Intro where import qualified Prelude as P import Prelude (Monad(..), (.), putStrLn, IO, Integer, Int, ($), (++), (=<<), Bool(..)) import Control.Monad (ap) import qualified Control.Monad.State as S -- Definition of our object language -- Unlike that in the tagless final paper, the definition here is spread -- across several type classes for modularity class QNum repr a where (+) :: repr a -> repr a -> repr a (-) :: repr a -> repr a -> repr a (*) :: repr a -> repr a -> repr a negate :: repr a -> repr a fromInteger :: Integer -> repr a infixl 6 +, - infixl 7 * class QBool repr where true, false :: repr Bool if_ :: repr Bool -> repr w -> repr w -> repr w class QBool repr => QLeq repr a where (<=) :: repr a -> repr a -> repr Bool infix 4 <= -- Higher-order fragment of the language class QHO repr where lam :: (repr a -> repr r) -> repr (a -> r) ($$) :: repr (a -> r) -> (repr a -> repr r) fix :: (repr a -> repr a) -> repr a infixr 0 $$ -- The first interpreter R -- which embeds the object language in -- Haskell. It is a meta-circular interpreter, and so is trivial. -- It still could be useful if we wish just to see the result -- of our expressions, quickly newtype R a = R{unR :: a} instance P.Num a => QNum R a where R x + R y = R $ x P.+ y R x - R y = R $ x P.- y R x * R y = R $ x P.* y negate = R . P.negate . unR fromInteger = R . P.fromInteger instance QBool R where true = R True false = R False if_ (R True) x y = x if_ (R False) x y = y instance QLeq R Int where R x <= R y = R $ x P.<= y instance QHO R where lam f = R $ unR . f . R R f $$ R x = R $ f x fix f = f (fix f) -- The second interpreter: pretty-printer -- Actually, it is not pretty, but sufficient newtype S a = S{unS :: S.State Int P.String} instance QNum S a where S x + S y = S $ app_infix "+" x y S x - S y = S $ app_infix "-" x y S x * S y = S $ app_infix "*" x y negate (S x) = S $ (return $ \xc -> "(negate " ++ xc ++ ")") `ap` x fromInteger = S . return . P.show app_infix op x y = do xc <- x yc <- y return $ "(" ++ xc ++ " " ++ op ++ " " ++ yc ++ ")" instance QBool S where true = S $ return "True" false = S $ return "False" if_ (S b) (S x) (S y) = S $ do bc <- b xc <- x yc <- y return $ "(if " ++ bc ++ " then " ++ xc ++ " else " ++ yc ++ ")" instance QLeq S a where S x <= S y = S $ app_infix "<=" x y newName stem = do cnt <- S.get S.put (P.succ cnt) return $ stem ++ P.show cnt instance QHO S where S x $$ S y = S $ app_infix "" x y lam f = S $ do name <- newName "t" let xc = name bc <- unS . f . S $ return xc return $ "(\\" ++ xc ++ " -> " ++ bc ++ ")" fix f = S $ do self <- newName "self" let sc = self bc <- unS . f . S $ return sc return $ "(let " ++ self ++ " = " ++ bc ++ " in " ++ sc ++ ")" showQC :: S a -> P.String showQC (S m) = S.evalState m (unR 0) -- ------------------------------------------------------------------------ -- Tests -- Perhaps the first test should be the power function... -- The following code can be interpreted and compiled just as it is... power = fix $ \self -> lam $ \x -> lam $ \n -> if_ (n <= 0) 1 (x * ((self $$ x) $$ (n - 1))) -- The interpreted result -- testpw :: Int testpw = (unR power) (unR 2) ((unR 7)::Int) -- 128 -- The result of compilation. -- testpwc :: P.String testpwc = showQC power {- "(let self0 = (\\t1 -> (\\t2 -> (if (t2 <= 0) then 1 else (t1 * ((self0 t1) (t2 - 1)))))) in self0)" -}

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mapping large structures into memory
by Warren Harris 25 Sep '09

25 Sep '09

I've dabbled in haskell, but am by no means an expert. I was hoping someone here could help me settle this debate so that we can more seriously consider haskell for a next version of an application we're building.... I would like to understand better what its capabilities are for directly mapping and managing memory. For instance, I would like mmap many large files into memory and mutate their internals directly... without needing to reallocate them (or chunks of them) in the haskell heap, and without resorting to a byte-array and byte-offset representation. Furthermore, I might also like to map intrinsic haskell data structures into this mmap'd memory such that standard library functions can manipulate them (perhaps in a purely functional way, e.g. treating them as haskell arrays of smaller foreign structures). I understand that the foreign function interface has the ability to marshall/unmarshall C structs, but I'm unsure of the memory implications of using this mechanism. Our application has a very large footprint, and reallocating some or all of these mapped files is a non- starter. Thanks, Warren

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Proof in Haskell
by pat browne 25 Sep '09

25 Sep '09

Hi, Below is a function that returns a mirror of a tree, originally from: http://www.nijoruj.org/~as/2009/04/20/A-little-fun.html where it was used to demonstrate the use of Haskabelle(1) which converts Haskell programs to the Isabelle theorem prover. Isabelle was used to show that the Haskell implementation of mirror is a model for the axiom: mirror (mirror x) = x My question is this: Is there any way to achieve such a proof in Haskell itself? GHC appears to reject equations such has mirror (mirror x) = x mirror (mirror(Node x y z)) = Node x y z Regards, Pat =================CODE===================== module BTree where data Tree a = Tip | Node (Tree a) a (Tree a) mirror :: Tree a -> Tree a mirror (Node x y z) = Node (mirror z) y (mirror x) mirror Tip = Tip (1)Thanks to John Ramsdell for the link to Haskabelle: http://www.cl.cam.ac.uk/research/hvg/Isabelle/haskabelle.html)

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