- Haskell-Cafe - Haskell.org

Ghci dynamic linking (Was: C++ libraries and GHCI)
by Paolo Losi 29 Sep '09

29 Sep '09

Hi all, I would really appreciate if anyone could give a look to the following problem. Thanks in advance!!! Paolo ---------- Forwarded message ---------- From: Paolo Losi <paolo.losi(a)gmail.com> Date: Mon, Sep 14, 2009 at 10:33 AM Subject: C++ libraries and GHCI To: glasgow-haskell-users(a)haskell.org Hi all, I'm working on a tentative implementation of haskell binding to libsvm [1]: http://hackage.haskell.org/package/HSvm The package includes libsvm C++ sources. While it works perfectly with ghc --make (try to install the package and compile http://bitbucket.org/pao/hsvm/raw/6cf7ca91f1e5/test/test.hs) it fails with ghci: (hsvm)(env)paolo@moltrasio:~/trash$ ghci test.hs GHCi, version 6.10.4: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Ok, modules loaded: Main. Prelude Main> main Loading package syb ... linking ... done. Loading package array-0.2.0.0 ... linking ... done. Loading package containers-0.2.0.1 ... linking ... done. Loading package HSvm-0.1.0.2.89 ... linking ... <interactive>: /home/paolo/trash/hsvm/lib/HSvm-0.1.0.2.89/ghc-6.10.4/HSHSvm-0.1.0.2.89.o: unknown symbol `_ZTV6Kernel' ghc: unable to load package `HSvm-0.1.0.2.89' Please note that the symbol is available in libHSHSvm-0.1.0.2.89.a $ nm HSHSvm-0.1.0.2.89.o |grep _ZTV6Kernel 00000000 V _ZTV6Kernel $ nm libHSHSvm-0.1.0.2.89.a |grep _ZTV6Kernel 00000000 V _ZTV6Kernel Can anyone give a look to the problem? Thanks Paolo [1] http://www.csie.ntu.edu.tw/~cjlin/libsvm/ -- Paolo Losi e-mail: paolo(a)enuan.com mob: +39 348 7705261 ENUAN Srl Via XX Settembre, 12 - 29100 Piacenza

1 0

ANNOUNCE: SourceGraph-0.5.0.0
by Ivan Lazar Miljenovic 29 Sep '09

29 Sep '09

SourceGraph [1] is a tool to statically analyse your Haskell code by applying graph-theoretic techniques on the call graph. It utilised the Graphalyze [2] library to do so, both of which were originally written as part of my Honours thesis last year [3]. [1] http://hackage.haskell.org/package/SourceGraph [2] http://hackage.haskell.org/package/Graphalyze [3] http://ivanmiljenovic.wordpress.com/2008/11/03/graph-theoretic-analysis-of-… I have been meaning to update SourceGraph since last year, but was waiting for improvements to the graphviz [4] library first. Now that these are done and I have fewer excuses, I've started to improve upon it. [4] http://hackage.haskell.org/package/graphviz When I first released SourceGraph last year, several people (e.g. Gwern) wanted support for CPP and command-line options. Unfortunately, I haven't yet gotten around to that; what I have done is added support for type classes and data structures, something I originally said I wasn't going to do. This support isn't perfect, especially when dealing with type classes from outside the scope of the code base, but in most cases works (except that the virtual instance functions don't always get positioned in the correct spots for some reason). The produced graphs are now also much prettier, with various colours and shapes being used. The useful features that SourceGraph offers include: * Visualisation of each module, the relations between modules and the entire code base. * Finds functions that are exported from a module that isn't in exposed in the .cabal file. * Alternate module splits. Please note that SourceGraph is _not_ a refactoring tool: it is designed to give you the programmer more information about what is in your code. I'd appreciate it if people could give SourceGraph a whirl and at least check how well the conversion from Haskell code to the graph is, as I'm not that sure how well I've converted haskel-src-ext's [5] internal state to SourceGraph's state. Please note that there are already various aspects that I know don't work properly; these can be found in the ParsingProblems.txt file (KnownProblems.txt contains other non-parsing problems that I'm aware of). In particular, TH, etc. aren't supported. [5] http://hackage.haskell.org/package/haskell-src-exts-1.1.4 To use SourceGraph after it's been installed, at a prompt do the following: > SourceGraph path/to/project.cabal The resulting report will then be found in a directory called "SourceGraph" in the same directory as the cabal file. -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

3 4

A thought about liberating Haskell's syntax
by George Pollard 29 Sep '09

29 Sep '09

Dear all, Here is a note concerning a thought I just had. I have written a rough sketch/outline and would like feedback upon this. The beginning thought: I would like to be able to write new bracket-like operators without having to rewrite GHC. (Case in point: applicative brackets.) First idea is to make them Template Haskell so we can mess with the internals (write custom list/set comprehension implementations). However a type (String → Exp) is not good enough; parsing the String is too much work! (Think precedence, etc.) We want to be able to have something (Exp → Exp). For this, we probably also need Template Haskell versions of normal (infix) operators, so that we can have something like: template infixr , (-1) -- lower precedence than $ a, b = if isList b then a : b else a : [b] Problem with this; [x, [y,z]] would have type :: [α] Lists constructed with ',' should have a different type from normal lists; call it QList. , :: Exp → Exp → Exp a, b@(AppE (ConE 'QCons) _) = a `qcons` b a, b = a `qcons` (b `qcons` QNil) where qcons a b = (ConE 'QCons) `AppE` a `AppE` b {- NB: we also need to provide implementations for type and pattern contexts, see next section. -} I believe that with this we can have the tuple and list syntax not hard-coded in. An example: -- note we need to provide the context for TH: Pattern, Expression, or Type -- no thought to sections here -- how to add tuple sections? -- perhaps relax operator rules for templates so that a, , , b is legitimate template outfix ( ) expression = (\x → case x of (QCons _ _) → TupE $ fromQList x _ → x template outfix ( ) pattern = (\x → case x of (QCons _ _) → TupP $ fromQList x _ → x template outfix ( ) type = (\x → case x of (QCons _ _) → foldl (\z x → z `AppT` x) (TupleT (length x)) x _ → x Anyway, we could then have easily-written syntax for; - sets - applicative brackets - parallel list comprehensions wouldn't have to be hardcoded (provide something like the above ',' but '|' returning something else. Perhaps QList should be tagged by a phantom type.) - statically-lengthed vectors Problems I can see: - QList (or other use-only-for-x types) could turn up in an actual program; l = a, b :: QList α -- is this actually a problem? - "Hidden" rewriting/macros not really the Template Haskell way, this is more scheme-like. A further (possibly evil) extension: template outfix do {-outdent-} expression = ... >:) -- would require thinking about EOL handling/semi-colon insertion

4 7

Re: [Haskell-cafe] unicode text libraries
by Don Stewart 29 Sep '09

29 Sep '09

tittoassini: > 2009/9/28 Don Stewart <dons(a)galois.com>: > > titto: > >> Hi, > >> > >> I am looking for an unicode strings library, I found on hackage: > >> > >> http://hackage.haskell.org/package/compact-string > >> > >> http://hackage.haskell.org/package/text > >> > >> They both look solid and functionally complete so ... I don't know which > >> one to use :-) > >> > >> As I am sure I am not the first one facing this choice, may I ask > >> which one you preferred and why? > > > Data.Text > > Thanks , but .. why? Sorry, was on the way out the door. Data.Text has growing use, is well designed, and builds on the pedigree of bytestring and the vector* series of fusion libraries. I trust that code. -- Don

7 7

Haskell Weekly News: Issue 131 - Semptember 25, 2009
by Joe Fredette 29 Sep '09

29 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

2 1

Re: [Haskell-cafe] unicode text libraries, and the winner is ...
by Pasqualino "Titto" Assini 29 Sep '09

29 Sep '09

By unanimous opinion the "text" library is the man. Thanks to all who answered. titto

1 0

ANNOUNCE: Graphalyze-0.7.0.0
by Ivan Lazar Miljenovic 29 Sep '09

29 Sep '09

Graphalyze [1] is a library for using graph-theoretic techniques to analyse the relationships inherent within discrete data. It was originally written for my Honours thesis [2] last year, and I have now started updating it. [1] http://hackage.haskell.org/package/Graphalyze [2] http://ivanmiljenovic.wordpress.com/2008/11/03/graph-theoretic-analysis-of-… Graphalyze provides helper functions to import discrete data, analyse it using various algorithms (a dodgy term, I know, but I couldn't think of a better one) and then create a report with the results. The main changes since the previous version are: * The ability to have graphs with edge labels. * More of a focus on applying changes to the overall information state of the data rather than just extracting the graph and applying a function to it. * Usage of the updated features in the graphviz library (http://hackage.haskell.org/package/graphviz) to visualise graphs. Changes to come: * Re-do the reporting framework to use more of a pretty-printing approach and make it more customisable. -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

2 2

So I wrote this performance measurement library thingy
by Bryan O'Sullivan 29 Sep '09

29 Sep '09

Which I will not blather on too long about here, but point you at a blog posting instead: http://www.serpentine.com/blog/2009/09/29/criterion-a-new-benchmarking-libr…

2 1

ANNOUNCE: graphviz-2999.6.0.0
by Ivan Lazar Miljenovic 29 Sep '09

29 Sep '09

I'm pleased to announce version 2999.6.0.0 [1] of the graphviz library, which provides bindings to the GraphViz [2] suite of tools for drawing graphs. [1] http://hackage.haskell.org/package/graphviz-2999.6.0.0 [2] http://www.graphviz.org/ Changes since the previous version are: * Remove some Shape aliases and change capitalisation of others. * Properly parse and print IDs of clusters. * Allow NodeCluster values have node types different from the LNode they come from. Since the node type is only used for passing to the function to create the appropriate Attributes for that node, this can mean avoiding having to apply a transformation function before getting the attributes if you have a composite type for node value/cluster. -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

1 0

QuickCheck Questions
by Yusaku Hashimoto 28 Sep '09

28 Sep '09

Hello, I recently worked with QuickCheck for a while, But I still can't handle it well, And a few questions come to my mind. 1. How to find properties In QuickCheck examples on the codes or the papers, they find good properties easily. How did they find these properties? What property can make us believed its implementation is collect? 2. Property driven developments But finally, I could find some patterns of the properties. For example, occasionally, their shape is like unF . f == id or unF . f == Just I thought it could be applied to writing parser, and lead me to "Property Driven Developments." I tried writing S-expression parser in this way. First, I defined my property as parseSexpr . prettySexpr == Just where parseSexpr :: String -> Maybe Sexpr, prettySexpr :: Sexpr -> String. As you see, I should write printer before parser. Fortunately writing such printer for S-expr is easy, but I don't know if what I did is right. Do you think I wasted times? Have you ever tried PDD? And has it worked? If you have experience with TDD, how do you think about PDD? If you have any answers in any questions above, please tell me them. Thanks in advance. Cheers -nwn

8 11

GADT pattern match in non-rigid context
by João Paulo 28 Sep '09

28 Sep '09

Dear all, The following is a (I'm afraid too large!) fragment of a program implementing a GADT-based generic zipper: ********************************************** data Zipper path where Zipper :: hole -> Context (Up (left, hole, right) up) -> Zipper (Up (left, hole, right) up) data Context path where ContTop :: Context (Up (Top, a, Top) Top) Cont :: Left l (h -> r) -> Right r h_parent -> Context (Up (l_parent, h_parent, r_parent) path) -> Context (Up (l, h, r) (Up (l_parent, h_parent, r_parent) path)) data Up a b data Top data Left contains expects where LeftUnit :: a -> Left Top a LeftCons :: Left c (b -> a) -> b -> Left (c -> b) a data Right provides final where RightNull :: Right final final RightCons :: b -> Right a final -> Right (b -> a) final data Erase c a = forall b. (Typeable b) => Erase (c b a) ff :: (Typeable l, Data h, Typeable r, Typeable up) => Zipper (Up (l, h, r) up) -> Erase Left h ff (Zipper h c) = (gfoldl erased_left_cons erased_left_unit h) gg :: (Typeable l_down, Typeable h_down, Data h) => Erase Left h -> Maybe (Left (l_down -> h_down) h) gg (Erase l) = cast l move_down :: (Typeable l_down, Typeable h_down, Typeable l, Data h, Typeable r, Typeable up) => Zipper (Up (l, h, r) up) -> Maybe (Zipper (Up (l_down, h_down, h) (Up (l, h, r) up))) move_down z@(Zipper h c) = case gg (ff z) of # Just (LeftCons l' h_down) -> Just (Zipper h_down (Cont l' RightNull c)) Nothing -> Nothing instance Typeable Top where typeOf _ = mkTyConApp (mkTyCon "Top") [] instance Typeable2 Left where typeOf2 _ = mkTyConApp (mkTyCon "Left") [] instance Typeable2 Up where typeOf2 _ = mkTyConApp (mkTyCon "Up") [] erased_left_cons :: (Typeable b) => Erase Left (b -> a) -> b -> Erase Left a erased_left_cons (Erase c) b = Erase (LeftCons c b) erased_left_unit :: a -> Erase Left a erased_left_unit a = Erase (LeftUnit a) ********************************************** Compiling it with GHC 6.10.4 yelds the error: *** GADT pattern match in non-rigid context for `LeftCons' Solution: add a type signature In the pattern: LeftCons l' h_down [marked with # in the code] *** The suggestion is quite clear :D (in fact, I think this compiler suggestion is the result of previous interactions in this mailing list) The thing is that, in order to add a type signature to the suggested pattern, I believe I have to use the type variables in the signature of function 'move_down' (which I don't think is possible); Can anyone help me with this? I believe this code has compiled before, on previous GHC versions... Thank you very much, João -- João Paulo Fernandes Universidade do Minho www.di.uminho.pt/~jpaulo

2 1

16 bit floating point data in Haskell?
by Olex P 28 Sep '09

28 Sep '09

Hi guys, Do we have anything like half precision floats in Haskell? Maybe in some non standard libraries? Or I have to use FFI + OpenEXR library to achieve this? Cheers, Oleksandr.

8 10

unicode text libraries
by Titto Assini 28 Sep '09

28 Sep '09

Hi, I am looking for an unicode strings library, I found on hackage: http://hackage.haskell.org/package/compact-string http://hackage.haskell.org/package/text They both look solid and functionally complete so ... I don't know which one to use :-) As I am sure I am not the first one facing this choice, may I ask which one you preferred and why? Thanks titto -- Pasqualino "Titto" Assini, Ph.D. http://quicquid.org/

3 2

ANN: ListTree 0.1
by yairchu＠gmail.com 28 Sep '09

28 Sep '09

Hi, I am pleased to announce the release of ListTree. ListTree is a package for combinatorial search and pruning of trees, and should be useful for problems such as those in Google Code Jam (where I, and possibly others* could make use of it), but possibly could even be useful for real applications! It offers BFS, DFS, Best-First-Search, Branch-And-Bound pruning, and more. The trees it works on are not those from Data.Tree, but rather monadic lists of a list monad (ListT []), which is an alternative that has the advantages of working with standard MonadPlus or List operations like takeWhile etc, and where consumption is a monadic action, which allows keeping state required for branch-and-bound (the bound) etc. This would be best explained with an example. Google Code Jam 2009 Round 2 Problem C: Given the prices at 25 time points for 16 stocks (in the small input), split the stocks into several groups, where in each group, the line- plots of its stocks' prices do not intersect or touch. Your mission is to find the minimum number of groups necessary. I will present an inefficient (best one I could come up with during the compo) solution that can solve their small input (it cannot solve the large input of a 100 stocks): * Search the space of all possible splits into groups of the stocks * Prune this search by cutting branches where the plots of two stocks in the same group intersect * Use "Branch and Bound". For each node, calculate a lower and upper bound. Lower bound is number of groups so far, and large bound is number of groups plus number of remaining items to place in group (each one may require a new group). Keep the lowest upper bound encountered, and prune all subtrees with a lower bound larger or equal to it. Code for this solution below. Using ListTree, the code is as modular as the algorithm description above, and there's a function to perform branch-and-bound. One problem with my package (which I'll attempt fixing), however, is speed. I haven't used it during the competition, and the quick and dirty, less modular code, that I coded in the competition, which performs exactly the same algorithm, runs a 100 times faster! Both are fast enough, but this is still troubling. I guess I should look into "Stream Fusion" to try and make my package faster. And the example's code below: import Control.Monad.Identity import Control.Monad.ListT -- from "List", not mtl import Control.Monad.State import Control.Monad.Trans import Data.Array import Data.List.Class (cons, execute) import Data.List.Tree import Data.Maybe -- search tree for all possible splits to separate groups -- each node has the same groups of its parent with a new element added to a group -- or as the sole element of a new group -- the new element is the first element of the first group searchTree :: [a] -> [[a]] -> ListT [] [[a]] searchTree [] groups = return groups searchTree (x : xs) groups = do i <- lift [0 .. length groups] let (pre, group : post) = splitAt i ([] : groups) cur = (x : group) : dropWhile null (pre ++ post) cons groups $ searchTree xs cur getWords :: Read a => IO [a] getWords = fmap (map read . words) getLine main :: IO () main = do numCases <- readLn :: IO Int forM_ [1..numCases] $ \i -> do [n, _] <- getWords stocks <- replicateM n getWords :: IO [[Int]] let friends = listArray rng . map isFriends . range $ rng rng = ((0, 0), (n-1, n-1)) isFriends (a, b) = f (>) a b || f (<) a b f op a b = and $ zipWith op (stocks !! a) (stocks !! b) res = fromJust . snd . runIdentity . (`runStateT` Nothing) . execute . execute . branchAndBound bnds . prune p $ searchTree [0 .. n-1] [] p ((x : xs) : _) = all ((friends !) . ((,) x)) xs p _ = True bnds [] = (Just 0, Just n) bnds groups = ( Just (length groups) , Just (length groups + n - 1 - head (head groups)) ) putStrLn $ "Case #" ++ show i ++ ": " ++ show res * possibly others - currently only 2 out of the 500 "surviving" contestants of codejam use Haskell (http://www.go-hero.net/jam/09/lang/ Haskell). so by others I mean Reid from the US. Good luck Reid!

2 2

Re: [Haskell-cafe] 80 bit floating point data in Haskell?
by Andrew Coppin 28 Sep '09

28 Sep '09

wren ng thornton wrote: > Andrew Coppin wrote: >> While we're on the subject... I understand that certain FPUs support >> 80-bit precision. Is there any way to get at that? Or is this going >> to require FFI too? > > Perhaps you want Foreign.C.Types.CLDouble ? > > http://hackage.haskell.org/packages/archive/base/latest/doc/html/Foreign-C-… > > http://en.wikipedia.org/wiki/Long_double > > (Or do you really want to ensure that it's 80-bits regardless of > platform and the whims of the C compiler?) > Yep, that'll do. :-)

2 1

A proposals
by Louis Wasserman 28 Sep '09

28 Sep '09

I'd like to see something resembling as-patterns in type signatures. Specifically, there are cases where I'm inclined to use (m ~ pat) in a type context when m isn't otherwise constrained, just so I can use m as an abbreviation for pat. To reduce these cases, I'd like to see the syntax m@pat allowed for use in type signatures, with m becoming simply an alias for pat. Thoughts? I've added a ticket here <http://hackage.haskell.org/trac/ghc/ticket/3545>. Louis Wasserman wasserman.louis(a)gmail.com http://profiles.google.com/wasserman.louis

6 6

ANN: JSONb-0.0.2
by Jason Dusek 28 Sep '09

28 Sep '09

JSONb is a ByteString parser for JSON, yielding a simple JSON type composed of Rationals, ByteString tries and strict ByteStrings. http://hackage.haskell.org/package/json-b-0.0.2 An example application program, `json-schema`, is provided. It derives schema for JSON input. When first looking at a web API, for example, you can feed it a few thousand lines of input and see what you got. A sample schema, derived from a stream of the Twitter API, is here: http://wiki.github.com/jsnx/JSONb/schema-example It's worth noting that the schema has a lone object at the bottom that has exactly the format of the "user" property of the schema ahead of it -- why is that? Well, `json-schema` uses a fault tolerant parsing approach; the very last Tweet was cut off and the "user" property's value was among the things that could be salvaged. -- Jason Dusek

1 0

Re: [Hs-Generics] how to automatically create and install documentations of a package?
by Sean Leather 28 Sep '09

28 Sep '09

Hi Daneel, On Sat, Sep 19, 2009 at 13:58, Daneel Yaitskov wrote: > > Recently I've found wonderful thing cabal can install packages itself! It > can even install those packages which need for final one. But I'm disturbed > cabal doesn't create the documentation of a package by default. It manually > makes "runhaskell Setup haddock". > > Who knows? > I believe you can do 'cabal haddock', but I don't know why it doesn't do it by default. BTW, the Haskell Café is typically more suitable for questions such as these. The Generics list is more specifically for generic programming discussions while the Haskell Café serves a broader audience. I've CC'd haskell-cafe@ to see if anybody there knows the answer to your question. Regards, Sean

5 5

frag game-compiling error
by selahaddin gerdan 28 Sep '09

28 Sep '09

Hi there, when I try to install frag,I get this error: .cabal/bin/cabal install frag Resolving dependencies... Downloading frag-1.1.2... Configuring frag-1.1.2... Preprocessing executables for frag-1.1.2... Building frag-1.1.2... [ 1 of 39] Compiling IdentityList ( src/IdentityList.hs, dist/build/frag/frag-tmp/IdentityList.o ) [ 2 of 39] Compiling Quaternion ( src/Quaternion.hs, dist/build/frag/frag-tmp/Quaternion.o ) src/Quaternion.hs:22:27: Couldn't match expected type `GLfloat' against inferred type `Float' In the expression: (r00 :: GLfloat) In the second argument of `newMatrix', namely `[(r00 :: GLfloat), r01, r02, r03, ....]' In the expression: newMatrix ColumnMajor [(r00 :: GLfloat), r01, r02, r03, ....] cabal: Error: some packages failed to install: frag-1.1.2 failed during the building phase. The exception was: exit: ExitFailure 1 Why?

2 1

F# mailing list?
by Günther Schmidt 28 Sep '09

28 Sep '09

Hi guys, is there a mailing list for haskellers that defected to F#? Not that I was I going to, just asking, absolutely hypothetically. Uhm. Günther

3 3

Edinburgh Hackathon (Hac7) summary
by Dougal Stanton 27 Sep '09

27 Sep '09

It's been nearly a month now since the Edinburgh Hackathon (29 & 30 August), organised to coincide with the convergence of Haskellers arriving for the ICFP. My apologies for this very late summary! Eric Kow was the driving force behind this Hackathon, though I agreed to help out locally because I live in Edinburgh. We had trouble organising a good venue through a combination of university holidays and Edinburgh International Festival. We spent the first day camping out in a cafe. On the second day Graeme Hutton arranged for us to use an empty room in the bowels of the ICFP venue. If I remember well the first day was heavily Darcs-oriented, and there were interesting conversations on version control representation, conflicts, and the future of Darcs over breakfast. The staff at Henderson's Restaurant didn't seem to mind our impromptu geeky enclave, and we kept buying coffee to pay for our wifi usage. I'll let the other Haskellers explain in their own words what they were up to for the rest of the time. Maybe some of them have updates on where these projects have gone since then? "At the hackathon I started working on a small extension to the GHCi debugger. The goal of the extension is to allow the debugger to show the term representation of thunks where possible. Presently, to find out the value of a thunk in the debugger you must force its evaluation. However, this is not always desirable, especially if the thunk contains breakpoints, or if it diverges. In some cases it ought to be possible to extract source code information from a thunk and show that to the user. I didn't get very far in the implementation because I spent most of the time re-learning how the term printer in the debugger works (and chatting to other people)." -- Bernie Pope "I was continuing to work on my camp correctness proofs. In particular, investigating how best to handle symmetric lemmas, and refactoring the existing definitions and proofs." -- Ian Lynagh "I worked on fixing Solaris/SPARC port of GHC. It turned out to be a tedious header file related issue, not very interesting." -- Ben Lippmeier "I worked on the formlets (again), I released a version that doesn't add any functionality, it only solves a bug." -- Chris Eidhof "I worked on a feature to let darcs users more conveniently upgrade to the hashed repository format. I also had lots of interesting discussions with darcs users and hackers :-)" -- Eric Kow "I worked on analysing some darcs performance problems, fixing a couple of GHC bugs, and some discussion about packages." -- Simon Marlow "I (along with Benedikt Huber) worked on adding a 'cabal init' command which automates much of the work of cabalising a project (creating a .cabal file, a Setup.hs file, a LICENSE file, etc). It's not quite done yet but should be finding its way into the cabal-install HEAD soon! -- Brent Yorgey "I worked on the new hackage-server a bit. darcs get http://code.haskell.org/hackage-server/ live instance at http://sparky.haskell.org:8080/" -- Duncan Coutts Thanks everyone who took part and who supplied an elegant summary of their work that weekend. I hope you all had a good time! Cheers, D -- Dougal Stanton dougal(a)dougalstanton.net // http://www.dougalstanton.net

1 0

(mostly OT) Strange patterns of commas
by Matthias Kilian 27 Sep '09

27 Sep '09

Fibonacci numbers are always surprising. Try this on a terminal that's 159 characters wide (using a properly sized xterm(1) may be a good idea): let f = 1 : 1 : zipWith (+) f (tail f) in take 780 f Ciao, Kili -- There are two ways of constructing a software design: One way is to make it so simple that there are obviously no deficiencies, and the other way is to make it so complicated that there are no obvious deficiencies. The first method is far more difficult. -- C.A.R. Hoare

2 1

ANN: rss2irc 0.4 released
by Simon Michael 27 Sep '09

27 Sep '09

I have released rss2irc 0.4, with some improvements from the field: - fix a problem connecting with irc.quakenet.org (Radoslav Dorcik) - feed polling now recovers from transient failures - can poll a local file: uri as well as remote uris - more robust new item detection, with some alternate strategies available - --idle option, waits for channel inactivity before announcing - limit individual field sizes, preventing too many irc messages per item - misc. output improvements - logging improvements - --delay now takes minutes, not seconds See hackage for more detail. Note the new contributor! Thanks Radoslav. home: http://hackage.haskell.org/package/rss2irc darcs repo: http://joyful.com/darcsweb/darcsweb.cgi?r=rss2irc I am currently using this to run hackagebot, darcscommitbot and other announce-bots on freenode. A tip: if you experiment, avoid joining the irc server too frequently; once per minute may be ok. Feedback and patches welcome. Best, -Simon

1 1

Re: [Haskell-cafe] 16 bit floating point data in Haskell?
by Olex P 27 Sep '09

27 Sep '09

Hi, Yes, I mean "sizeOf 2". It's useful not only on GPUs but also in "normal" software. Think of huge data sets in computer graphics (particle clouds, volumetric data, images etc.) Some data (normals, density, temperature and so on) can be easily represented as float 16 making files 200 GB instead of 300 GB. Good benefits. Cheers, Oleksandr. On Sun, Sep 27, 2009 at 9:19 PM, Peter Verswyvelen <bugfact(a)gmail.com>wrote: > He meant 16-bit floats, which have sizeOf 2 > On GPUs this is common and implemented in hardware (at least on the old > GPUs). > > On DPSs you commonly had 24-bit floats too. > > But these days I guess 32-bit is the minimum one would want to use? Most of > the time I just use double anyway :) > > On Sun, Sep 27, 2009 at 9:47 PM, Ross Mellgren <rmm-haskell(a)z.odi.ac>wrote: > >> What about the built-in Float type? >> >> Prelude Foreign.Storable> sizeOf (undefined :: Float) >> 4 >> Prelude Foreign.Storable> sizeOf (undefined :: Double) >> 8 >> >> Or maybe you mean something that can be used with FFI calls to C, in which >> case Foreign.C.Types (CFloat). >> >> Both instance the Floating, RealFloat, RealFrac, etc, classes so should >> operate largely the same as (modulo precision) a Double. >> >> -Ross >> >> >> On Sep 27, 2009, at 2:42 PM, Olex P wrote: >> >> Hi guys, >>> >>> Do we have anything like half precision floats in Haskell? Maybe in some >>> non standard libraries? Or I have to use FFI + OpenEXR library to achieve >>> this? >>> >>> Cheers, >>> Oleksandr. >>> _______________________________________________ >>> Haskell-Cafe mailing list >>> Haskell-Cafe(a)haskell.org >>> http://www.haskell.org/mailman/listinfo/haskell-cafe >>> >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> > >

2 2

Processing EXIF data with Haskell
by Colin Paul Adams 27 Sep '09

27 Sep '09

I'm writing web software for a photograph gallery. I want to be able to display selective fields from the EXIF data. I thought I would be able to use the GD and exif libraries from Hackage to do this. However: Calling Graphics.GD.loadJpegByteString followed by Graphics.GD.saveJpegFile does not preserve the EXIF fields of the uploaded file, but instead writes some new ones (many fewer fields than the original, and nothing of interest). Graphics.Exif.allTags shows an interesting subset of the original EXIF data, but nothing from the version saved by GD. Can anyone shed any light on any of this? -- Colin Adams Preston Lancashire

2 2

yi-editor: Duplicate instance declarations
by Marcelo Sousa 27 Sep '09

27 Sep '09

Hey guys, I'm trying to install yi using cabal but I got this error. Any ideas how to solve it?! I'm using ghc-6.10.1 and cabal-install version 0.6.2 using version 1.6.0.2 of the Cabal library. Thanks, Marcelo -- Code -- $ cabal install yi Resolving dependencies... Configuring yi-0.6.1... Preprocessing library yi-0.6.1... Preprocessing executables for yi-0.6.1... Building yi-0.6.1... [ 1 of 120] Compiling System.FriendlyPath ( System/FriendlyPath.hs, dist/build/System/FriendlyPath.o ) [ 2 of 120] Compiling Shim.ProjectContent ( Shim/ProjectContent.hs, dist/build/Shim/ProjectContent.o ) [ 3 of 120] Compiling Parser.Incremental ( Parser/Incremental.hs, dist/build/Parser/Incremental.o ) [ 4 of 120] Compiling Data.Trie ( Data/Trie.hs, dist/build/Data/Trie.o ) [ 5 of 120] Compiling Data.DelayList ( Data/DelayList.hs, dist/build/Data/DelayList.o ) [ 6 of 120] Compiling Data.Rope ( Data/Rope.hs, dist/build/Data/Rope.o ) [ 7 of 120] Compiling Data.Prototype ( Data/Prototype.hs, dist/build/Data/Prototype.o ) [ 8 of 120] Compiling HConf.Utils ( HConf/Utils.hs, dist/build/HConf/Utils.o ) [ 9 of 120] Compiling HConf.Paths ( HConf/Paths.hs, dist/build/HConf/Paths.o ) [ 10 of 120] Compiling Paths_yi ( dist/build/autogen/Paths_yi.hs, dist/build/Paths_yi.o ) [ 11 of 120] Compiling HConf ( HConf.hs, dist/build/HConf.o ) [ 12 of 120] Compiling Yi.Char.Unicode ( Yi/Char/Unicode.hs, dist/build/Yi/Char/Unicode.o ) [ 13 of 120] Compiling Yi.UI.Common[boot] ( Yi/UI/Common.hs-boot, dist/build/Yi/UI/Common.o-boot ) [ 14 of 120] Compiling Yi.String ( Yi/String.hs, dist/build/Yi/String.o ) [ 15 of 120] Compiling Yi.Monad ( Yi/Monad.hs, dist/build/Yi/Monad.o ) [ 16 of 120] Compiling Yi.Keymap.Completion ( Yi/Keymap/Completion.hs, dist/build/Yi/Keymap/Completion.o ) [ 17 of 120] Compiling Yi.Editor[boot] ( Yi/Editor.hs-boot, dist/build/Yi/Editor.o-boot ) [ 18 of 120] Compiling Yi.Debug ( Yi/Debug.hs, dist/build/Yi/Debug.o ) [ 19 of 120] Compiling Yi.Prelude ( Yi/Prelude.hs, dist/build/Yi/Prelude.o ) Yi/Prelude.hs:182:9: Duplicate instance declarations: instance Category Accessor.T -- Defined at Yi/Prelude.hs:182:9-38 instance Category Accessor.T -- Defined in data-accessor-0.2.1:Data.Accessor.Private cabal: Error: some packages failed to install: yi-0.6.1 failed during the building phase. The exception was: exit: ExitFailure 1

3 2

Quadratic complexity though use of STArrays
by Dan Rosén 27 Sep '09

27 Sep '09

Dear haskell-cafe users, I am constructing a shuffle function: given an StdGen and a list, return the list permuted, with all permutations of equal probability. There is the simlpe recursive definition: generate a number from 1 to length list, take this element out from the list, call the function recursively on the remaining list and then cons the element on the shuffled list. A more imperative approach is to make the list an array, and traverse the array in reverse, swapping the iterated element with an arbitrary element less than or equal to the iterator. These functions are implemented as shuffleRec and shuffleArr, respectively. What complexity does these functions have? I argue that the shuffleArr function should be O(n), since it only contains one loop of n, where each loop does actions that are O(1): generating a random number and swapping two elements in an array. I argue that the shuffleRec function should be O(n^2), since for each call, it creates a new list in O(n), with the drop and take calls, and calls itself recursively. This yields O(n^2). However, they both have the same runnig time (roughly), and through looking at the plot it _very_ much looks quadratic. I am compiling with GHC and I guess there is something in the lazy semantics that confuses me about the complexities, and maybe I have misunderstood how STArrays work. Any pointers to what's going in is greatly appreciated! Best regards, Dan Rosén, Sweden Here is the code: module Main where import Control.Monad import Control.Monad.ST import Data.Array.ST import Data.STRef import System.Random import Time import CPUTime shuffleArr :: StdGen -> [a] -> [a] shuffleArr g list = runST $ do let n = length list gref <- newSTRef g arr <- listToArray list forM_ [n,n-1..2] $ \p -> do m <- rand (1,p) gref swap arr m p getElems arr where rand range gref = do g <- readSTRef gref let (v,g') = randomR range g writeSTRef gref g' return v swap a n m = do [n',m'] <- mapM (readArray a) [n,m] mapM (uncurry $ writeArray a) [(m,n'),(n,m')] listToArray :: [a] -> ST s (STArray s Int a) listToArray list = let n = length list in newListArray (1,n) list shuffleRec :: StdGen -> [a] -> [a] shuffleRec g list = x:shuffleArr g' xs where (n,g') = randomR (0,length list-1) g (x:xs') = drop n list xs = take n list ++ xs' -- A somewhat lame attempt to derive the complexities through testing, -- prints the times for the different functions in a table main :: IO () main = do let times = take 30 $ iterate (+30000) 10000 answers <- mapM test times sequence_ [ putStrLn $ concatMap ((++ "\t"). show) [toInteger t,arr,rec] | (t,(arr,rec)) <- zip times answers ] -- Perform a test of size n, and return the cycles it took for the different -- algorithms in a pair. Evaluation is enforced by seq on length of the list. test :: Int -> IO (Integer,Integer) test n = do let list = [1..n] [g1,g2] <- replicateM 2 newStdGen length list `seq` do s <- doTime ("shuffleArr " ++ show n) $ (length $ shuffleArr g1 list) `seq` return () s' <- doTime ("shuffleRec " ++ show n) $ (length $ shuffleRec g2 list) `seq` return () return (s,s') -- This is taken from GenUtil from the JHC creator's homepage doTime :: String -> IO a -> IO Integer doTime str action = do start <- getCPUTime x <- action end <- getCPUTime let time = (end - start) `div` 1000000 -- `div` cpuTimePrecision -- putStrLn $ "Timing: " ++ str ++ " " ++ show time return time

5 5

Hurray! I hit my 1st true lazy IO problem
by Günther Schmidt 27 Sep '09

27 Sep '09

and now I'll really have to figure out Iteratee / Enumerator to solve it. Can I call myself a Haskeller now? Günther. PS: The problem domain is crawling over a website.

4 3

Snow Leopard Breaks GHC
by David Leimbach 27 Sep '09

27 Sep '09

Just thought I'd point out that my old GHC install is now broken by the update to Snow Leopard. Dave

9 21

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

1 0

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

1 0

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

1 0

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

2 2

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 > >

1 0

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

6 6

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.

4 11

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)" -}

3 2

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

2 3

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)

3 2

ANN: epoll bindings 0.2
by Toralf Wittner 25 Sep '09

25 Sep '09

Hi, I am pleased to announce the release of epoll bindings 0.2 available from: http://hackage.haskell.org/package/epoll 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. -Toralf

1 0

How does one delare a 2D STUArray in Haskell?
by Casey Hawthorne 25 Sep '09

25 Sep '09

How does one delare a 2D STUArray in Haskell? I see the following from a diffusion program segment: module Diffusion where import Data.Array import Data.List (sortBy) type VFieldElem = Float type VField = Array (Int,Int) VFieldElem <snip> zeros = listArray ((1,1),(imax,jmax)) (repeat 0) From Real World Haskell to declare a 1D array (I changed some of the value names) there is the following: import Data.Array.ST (STUArray) import Data.Array.Unboxed (UArray) import Data.Word (Word32) data PlayingField1D a = PF1D { pf1DState :: (a -> [Word32]) , pf1DArray :: UArray Word32 Bool } data MutPlayingField1D s a = MPF1D { mpf1DState :: (a -> [Word32]) , mutpf1DArray :: STUArray s Word32 Bool } But I cannot see how to declare a 2D array. Although, it is not "strictly" necessary, pun intended, since one can reframe the 1D array as 2D array by using row/column mapping functions. -- Regards, Casey

4 6

Haddock and literate Haskell: annotations must be marked as source?
by Andy Gimblett 25 Sep '09

25 Sep '09

Hi all, I've developed a bit of a taste for literate Haskell lately, being a verbose sort of guy. Unfortunately, it doesn't seem to interact with Haddock in the way I'd like/expect. I just wanted to check that my understanding of the situation is correct before I (regretfully) give up on LHS. It seems to me that Haddock only picks up annotation comments (ie those that should be included in the produced documentation) from literate Haskell source when those annotations are themselves marked up as source - it seems not to recognise them in the general text. For example, I was expecting/hoping that from the point of view of Haddock, the following literate Haskell code with an annotation-marked paragraph (starts with a vertical bar) in the non-source text. | Turn a foo into a bar. > foo :: Foo -> Bar > foo b = ... ought to be equivalent to the following "illiterate" Haskell: -- | Turn a foo into a bar. foo :: Foo -> Bar foo b = ... ie, it should produce Haddock docs where the definition of foo is annotated with that comment. Sadly, that appears not to be the case. It appears the literate version needs to look like this: > -- | Turn a foo into a bar > foo :: Foo -> Bar > foo b = ... which to my mind rather defeats the purpose of being literate. So: am I right that this is the intended/expected behaviour? If not, how does one get round it? If so, could someone perhaps comment on the prospects/complexity of implementing this - or the reasons why it is in fact a bad idea? Many thanks! -Andy

3 6

Monad Tutorial in C++
by AdrianMay 25 Sep '09

25 Sep '09

Just wrote yet another of the above if anybody wants one: http://nuerd.blogspot.com/2009/09/monad-tutorial-in-c.html Adrian. -- View this message in context: http://www.nabble.com/Monad-Tutorial-in-C%2B%2B-tp25606090p25606090.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

1 1

Referring to Prelude.(++)
by Terry Hayes 25 Sep '09

25 Sep '09

I'd like to redefine (++) so that it works on a more general class of "lists" (ListOf a). To do this, I found that I can import the Prelude hiding the definition of (++). Then I want to make [] an instance of ListOf, and have the (++) function call the built-in Prelude.(++). My problem is that I can't figure out how to call the built-in function. Just using "Prelude.(++)" doesn't seem to work in the way that "Prelude.foldl" would (for example). Any ideas? Terry

2 2

ANN: diagrams 0.2.1, and planned major improvements
by Brent Yorgey 24 Sep '09

24 Sep '09

Hi all, I'm pleased to announce version 0.2.1 of the diagrams library, available now on Hackage [1]. This is a minor release which fixes a few bugs and adds a few new combinators, most notably a grid layout combinator contributed by Ganesh Sittampalam. For a full list of the features new to 0.2.1, see the CHANGES file [2]. The real reason for the release is to get existing new features out the door before gearing up for a planned major rewrite of the backend to use a constraint-solving layout engine. This will allow for much greater elegance and flexibility, as well as a number of features (such as arrows connecting different parts of the diagram) which would be difficult or impossible to implement in the current framework. My ultimate vision is for the diagrams library to become a viable alternative to declarative drawing systems such as MetaPost [3] and Asymptote [4], with the distinct advantages that it will be (1) *purely* declarative, and (2) an *embedded* DSL, providing the full power of Haskell and its ecosystem, as opposed to the ad-hoc specialized languages used by MetaPost and Asymptote. If this sounds exciting to you, I hope you'll join me, either by trying out diagrams for your projects and providing feedback, or by contributing some code. If you're interested in helping with the rewrite itself, let me know; I also plan to set up a core/contrib model like that of xmonad, so there should also be plenty of opportunities for contributing independent add-on modules which enhance the core functionality. -Brent [1] http://hackage.haskell.org/package/diagrams [2] http://code.haskell.org/diagrams/CHANGES [3] http://www.tug.org/metapost.html [4] http://asymptote.sourceforge.net/

2 1

source locations (was: best practices for monad transformers and exceptions?)
by Evan Laforge 24 Sep '09

24 Sep '09

I forgot to mention this, but control-monad-exception reminded me: Is there any interest in a way for ghc to report source locations? I have a preprocessor that basically works and is really useful but also awkward and not scalable. control-monad-exception has TH which is more scalable but requires manual annotation at every call site. The loch package also requires annotation at call sites. I searched on trac, but it's hard to know what to search for and I only found a feature request to make 'undefined' report line numbers. Some way to have the callee get the caller's location would be quite handy. Perhaps something like: -- | (calling_function, calling_filename, line_number) type SrcPos = Maybe (String, String, Int) {-# SRCPOS_ANNOTATE foo, foo_srcpos #-} foo :: Arg -> Result foo = foo_srcpos Nothing foo_srcpos :: SrcPos -> Arg -> Result foo_srcpos srcpos arg = do State.modify $ \st -> st { state_stack = srcpos : state_stack st } -- or whatever Then ghc will rewrite calls to 'foo' as calls to 'foo_srcpos' and add a SrcPos arg. If the extension isn't present, plain 'foo' will be called which will pass Nothing. This is basically what my preprocessor does, only it doesn't understand modules and importing, so it relies on me having a consistent import convention. I suppose the preprocessor could be extended with the ability to understand import lines... but ghc could put the annotation in .hi and wouldn't have to scan all the source files. My preprocessor wouldn't work with libraries. Getting a complete call stack would be even nicer, and I remember seeing something a while ago about how it would require keeping track of the calls separately from the real (possibly heavily inlined or tail recursive) call stack but would be technically possible especially with some more detailed source code information which I suppose may be in by now...

1 1

Parallel graphics
by Andrew Coppin 24 Sep '09

24 Sep '09

I have a number of compute-bound graphics programs written in Haskell. (Fractal generators, ray tracers, that kind of thing.) GHC offers several concurrency and parallelism abstractions, but what's the best way to use these to get images rendered as fast as possible, using the available compute power? (OK, well the *best* way is to use the GPU. But AFAIK that's still a theoretical research project, so we'll leave that for now.) I've identified a couple of common cases. You have a 2D grid of points, and you want to compute the value at each point. Eventually you will have a grid of /pixels/ where each value is a /colour/, but there may be intermediate steps before that. So, what cases exist? 1. A point's value is a function of its coordinates. 2. A point's value is a function of its previous value from the last frame. 3. A point's value is a function of /several/ points from the last frame. How can we accelerate this? I see a few options: - Create a spark for every point in the grid. - Create several explicit threads to populate non-overlapping regions of the grid. - Use parallel arrays. (Does this actually works yet??) I'm presuming that sparking every individual point is going to create billions of absolutely tiny sparks, which probably won't give great performance. We could spark every line rather than every point? Using explicit threads has the nice side-effect that we can produce progress information. Few things are more frustrating than staring at a blank screen with no idea how long it's going to take. I'm thinking this method might also allow you to avoid two cores tripping over each other's caches. And then there's parallel arrays, which presumably are designed from the ground up for exactly this type of task. But are they usable yet? Any further options?

9 8

Re: [Haskell-cafe] best practices for monad transformers and exceptions?
by Evan Laforge 24 Sep '09

24 Sep '09

On Thu, Sep 24, 2009 at 8:11 AM, pepe <pepeiborra(a)gmail.com> wrote: > Exceptions in the ErrorT monad are not compositional. > It's the first time I see the approach of stacking > several ErrorT monads. It seems slightly better than > having a 'global' exception type, although the explicit > lifting looks quite painful. > > I really don't know whether there is a way to write > catch_abort, sorry! Even if there is, I don't think you > want to use it. A much better option is to use an extensible > exception type. Yeah, I had forgotten about Tyeable implementations, which is silly since that's how the IO exceptions now work. Your extensible error framework looks pretty nice! I just made the exceptions global (i.e. put them all in the lowest level type) since I only have two at this point, but if I had more I would definitely check out Control.Monad.Exception. Ideally I would like to be able to package up a set of "services" as a transformer and plug it in compositionally without worrying about its implementation, but it seems monad transformers are only part of the way there. You can get some hiding with a newtype, and then you can get unlifted access with classes as mtl does (but then you need a bunch of instance boilerplate), but then if the one of the services the transformer needs is "early return", then you have to pull the ErrorT out and put it into the "final" monad, and handle all the exceptions in the final monad's "run". It would be nice someday to have more satisfactory way of combining effects. I dream of a way to effortlessly create encapsulated transformers that easily compose with other transformers...

1 0

ANN: Workflow-0.5.5, TCache-0.6.4 RefSerialize-0.2.4
by Alberto G. Corona 24 Sep '09

24 Sep '09

Hi I'm proud to announce Workflow 0.5.5. Workflow has 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 excution is resumed at the last action that was logged. The process continues at the same state as if not interruption took place. Any feedback will be appreciated. Besides state logging and recovery, there are a number of communication primitives that are aware of persistence across reinitiations such are persistent queues, persistent timeouts, or wait for events in the STM monad. These primitives permits inter-woikflow communications and communications with external threads. I hope that this package would be useful for very long computations, either the programs that are CPU intensive and produce valuable intermediate data or programs that wait for actions from users and others processes during days or weeks. That is typical in web applications. Such programs can be defined in a single monadic procedure transparently, without regards of saving intermediate results or reinitiations at the correct point. This new version is not restricted to handle a single type. Every intermediate data must be an instance of Read and Show. For complex data types, other persistence mechanisms can be used (see documentation). The package is at: http://hackage.haskell.org/package/Workflow http://hackage.haskell.org/packages/archive/Workflow/0.5.5/Workflow-0.5.5.t… The tar archive has the documentation and some examples. Among them, a simulation of workflow for the creation and approval of documents, with two levels of approval and approval timeouts, It uses most of the features of the package. NOTE: cabal install reports Tar checksum errors when installed, however, such errors do not appear by downloading the tar.gz archive and decompressing it with the unix or windows tools. I do not know why cabal install behaves as such. Here is a simple example: This is a counter that shows a sequence of numbers, one a second: *module Main where import Control.Concurrent(threadDelay) import System.IO (hFlush,stdout) count n= do putStr (show n ++ " " ) >> hFlush stdout >> threadDelay 1000000 count (n+1) main= count 0 * This is the same program, with the added feature of remembering the last count after interruption (sequence.hs): *module Main where import Control.Workflow import Control.Concurrent(threadDelay) import System.IO (hFlush,stdout) mcount n= do step $ putStr (show n ++ " " ) >> hFlush stdout >> threadDelay 1000000 mcount (n+1) main= do registerType :: IO () registerType :: IO Int let start= 0 :: Int startWF "count" start [("count", mcount)] :: IO () * This is the execution log: *Worflow-0.5.5demos>runghc sequence.hs 0 1 2 3 4 5 6 7 sequence.hs: win32ConsoleHandler sequence.hs: sequence.hs: interrupted Worflow-0.5.5demos> Worflow-0.5.5demos>runghc sequence.hs 7 8 9 10 11 .... * This package uses TCache and RefSerialize. I also uploaded new versions of these packages with extensive documentation and examplles included in the tar.gz archives ( cabal install also repor checksum errors, but are OK when downloading and installing by hand): http://hackage.haskell.org/package/TCache <http://hackage.haskell.org/package/TCache> http://hackage.haskell.org/packages/archive/TCache/0.6.4/TCache-0.6.4.tar.gz http://hackage.haskell.org/package/RefSerialize <http://hackage.haskell.org/package/RefSerialize> http://hackage.haskell.org/packages/archive/RefSerialize/0.2.4/RefSerialize…

1 0

An encoding of parametricity in Agda
by Eugene Kirpichov 24 Sep '09

24 Sep '09

Hi, I've discovered the following post on the internets: http://gelisam.blogspot.com/2009/09/samuels-really-straightforward-proof-of… It contains an ingenious (to my mind) encoding of parametricity in Agda that makes the Free Theorems a trivial consequence. I've tried to formalize it in Coq, but, as of now, with no success. -- Eugene Kirpichov Web IR developer, market.yandex.ru

4 4