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Higher kind type inference paper
by Edsko de Vries 07 Dec '06

07 Dec '06

Hi, Are there any papers that describe how higher kind type inference (and I really mean higher kind, not higher rank) is done? Thanks, Edsko

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ANN: hslogger
by John Goerzen 06 Dec '06

06 Dec '06

Hi, I'm pleased to announce the release of hslogger, a logging framework for Haskell. hslogger's features include: * Each log message has a priority and a source associated with it * Multiple log writers can be on the system * Configurable global actions based on priority and source * Extensible log writers (handlers) * Default handlers that write to the console, file handles, or * syslog * Easy to use operation You can find hslogger online at: http://software.complete.org/hslogger hslogger is the first package to be split off from the MissingH source tree. -- John

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cabal + haddock on Windows default paths working?
by Conal Elliott 05 Dec '06

05 Dec '06

When I configure libraries (monadLib, arrows) with default locations and then cabal-haddock my own library, I get this sort of warning: Warning: cannot use package arrows-0.2: HTML directory "C:\\Program Files\\Common Files\\arrows-0.2\\doc\\html" does not exist. That directory *does* exist, however, and contains the arrow library's haddock-generated files. Any ideas? Is any Windows haddock user *not* having this problem? Am I running into a bug in handling of path names with embedded spaces? Thanks, - Conal

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haddock not finding base lib docs -- $topdir ?
by Conal Elliott 05 Dec '06

05 Dec '06

I'm running haddock for the first time, via cabal. I get the following message when i do "runhaskell Setup.hs haddock" on monadLib: Warning: cannot use package base-2.0: HTML directory $topdir\html\libraries\base does not exist. I do have c:/ghc/ghc-6.6/doc/html/libraries/base/. Is there some way i can let cabal know how to find it? What is $topdir about? Thanks, - Conal

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GHC Error question
by nr＠eecs.harvard.edu 05 Dec '06

05 Dec '06

Without going too deep into the details of my type classes, I have written the following code (focusing on compile1): {-# OPTIONS -fglasgow-exts #-} compile1 :: (Builder b box) => t -> Name -> Ir.ANF -> b t compile1 f x body = do env <- compile body empty wire (Arg W) (env x) return f compile :: (Builder b box) => Ir.ANF -> Env box -> b (Env box) class (Monad b) => Builder b box where wire :: Source box -> Sink box -> b () ... type Env box = Name -> Sink box This program is rejected by GHC with the following message: Ccomp.hs:54:23: Could not deduce (Builder b box1) from the context (Builder b box) arising from use of `wire' at Ccomp.hs:54:23-42 Possible fix: add (Builder b box1) to the type signature(s) for `compile1' In the expression: wire (Arg W) (env x) In a 'do' expression: wire (Arg W) (env x) In the expression: do env <- compile body empty wire (Arg W) (env x) return f Note that compile1 has an explicit type signature much along the lines suggested by GHC. If I *remove* this type signature, the function compiles successfully, and ghci reporets this type for compile1: compile1 :: (Builder t box) => t1 -> Name -> Ir.ANF -> t t1 I believe this signature is isomorphic to the explicit signature I had attempted to use. Am I misusing the type-class system in some way, or should I be reporting a bug in GHC? Norman

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What guarantees (if any) do interruptible operations have in presence of asynchronous exceptions?
by Cat Dancer 05 Dec '06

05 Dec '06

>From the discussion of "Help needed interrupting accepting a network connection", what we have so far is: * To break out of an "accept" call, an asynchronous exception is needed. * The presence of asynchronous exceptions complicates the other code used to report if "accept" completed or was interrupted, whether that code is written using MVar's or STM. Thus the next question is what guarantees, if any, do interruptible operations possess? For example, suppose that inside of a "block", a putMVar operation was guaranteed to either interrupt and allow an asynchronous exception to be raised, or to complete the putMVar operation, but not both. If this were true, then if you caught an asynchronous exception from the putMVar operation, you'd know that a value was not put into the MVar by the operation. Then it would be easy to program with MVar's in the presence of asynchronous exceptions. When you caught an asynchronous exception, you could set a flag, and then redo the putMVar. The same question can be asked of other interruptible operations. For the "accept" call itself, is it guaranteed (inside of a "block") to either accept a connection, or be interrupted and allow an asynchronous exception to be raised, but not both? For STM, is "atomically" an interruptible operation? If it is, what guarantees does it offer in the presence of asynchronous exceptions?

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Safe forking question
by Chris Kuklewicz 05 Dec '06

05 Dec '06

In response to question by Cat Dancer <cat(a)catdancer.ws> I wrote a few tests of sending asynchronous signal to a thread using GHC 6.6 The goal was to run a child thread via forkIO and use handle or finally to respond to the thread's demise. Unfortunately, it seems that there is an irreducible window where this fails. The forkIO returns but any exception handlers such as block/handle/catch/finally are not in place yet. So this fails: > module Main where > > import Control.Exception > import Control.Concurrent > import Control.Concurrent.STM > > forever x = x >> forever x > > count tv = do > val <- takeTMVar tv > putTMVar tv $! (succ val) > return val > > -- Capture idiom of notifying a new MVar when a thread is finished > fork todo = do > doneMVar <- newEmptyMVar > tid <- block $ forkIO $ block $ handle (\e -> print ("Exception",e) >> throw e) > (finally (unblock todo) > (print "dying!" >> putMVar doneMVar ())) > return (doneMVar,tid) > > spawn = do > tv <- atomically (newTMVar 0) > fork . forever $ atomically (count tv) >>= print > > kill (mv,tid) = do > -- yield > print "killing.." > killThread tid > print "..checking corpse.." > readMVar mv > print "..confirmed dead" > > main = spawn >>= kill On my system, the above sends the killThread and destroys the thread before the handle or finally are setup, so the child thread never prints anything and never runs the "putMVar doneMVar ()". If I uncomment the yield statement then the child thread does start executing and the handle and finally work as desired. This makes it impossible to reliably do anything with the child thread. I cannot discern between a living and dead child thread at all, as there is way to know if it is waiting to be scheduled or if it has been killed waiting to be scheduled. The get/setUncaughtExceptionHandler does not seem to be inherited by the child thread, so this was not a useful guard. The best thing I can come up with is the ugly code: > fork todo = block $ do > doneVar <- atomically (newEmptyTMVar) > let putStarted = atomically (putTMVar doneVar False) > putStopped = atomically (tryTakeTMVar doneVar >> putTMVar doneVar True) > tid <- forkIO $ block $ (finally (putStarted >> unblock todo) putStopped) > yield > atomically $ do > value <- takeTMVar doneVar > when value (putTMVar doneVar True) > return (doneVar,tid) This does not return the ThreadId until the finally clause has started running. But if the thread is killed by any external force before getting that far then the main thread will hang on the "takeTMVar doneVar". Is there any remotely better way of forking a child with an exception handler? -- Chris

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How to use STM in GUI applications?
by Thorsten Seitz 05 Dec '06

05 Dec '06

Hello, I'm very intrigued by the concepts of STM but I'm having difficulties to see how a TVar based model can be used by a GUI application (e.g. Gtk2hs): For example, I'd like to update the GUI (e.g. changing widget sensitivity) upon changes in the model. But changing the sensitivity of a widget is an IO operation which cannot be initiated atomically by a model change which is an STM operation. Therefore the GUI change cannot be executed atomically together with the model change. This means for example that I can still press a button although the model has changed such that the button action is not allowed anymore. All I can do is check the condition of the model when the button is pressed and then ignore the button press which is not very satisfying for the user. Is it simply not appropriate to use TVar based models in a GUI application? Maybe STM should be more used like a service for MVar based GUI models? Thanks for any help! -Thorsten

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Haskell Weekly News: December 05, 2006
by dons＠cse.unsw.edu.au 04 Dec '06

04 Dec '06

--------------------------------------------------------------------------- Haskell Weekly News http://haskell.org/haskellwiki/HWN Issue 52 - December 05, 2006 --------------------------------------------------------------------------- Welcome to issue 52 of HWN, a weekly newsletter covering developments in the Haskell community. This week we see the 11th Haskell Communities and Activities Report released, Visual Haskell 0.2 is available, and a suite of new libraries and applications are announced. Announcements Communities and Activities Report. Andres Loeh [1]published the [2]Haskell Communities and Activities Report (11th edition, November 2006). The report is now available from the Haskell Communities home page in several formats. The goal of the report is to improve the communication between the increasingly diverse groups, projects and individuals working on, with, or inspired by Haskell. When we try for the next update, six months from now, you might want to report on your own work, project, research area or group as well. 1. http://article.gmane.org/gmane.comp.lang.haskell.general/14545 2. http://www.haskell.org/communities/ Visual Haskell prerelease 0.2. Krasimir Angelov [3]announced that there is a prerelease version of Visual Haskell [4]available. This is the first version that is: available for both VStudio 2003 and VStudio 2005, and distributed with a stable GHC version (6.6) 3. http://thread.gmane.org/gmane.comp.lang.haskell.general/14527/focus=14527 4. http://www.haskell.org/visualhaskell Haskell MIME library. Jeremy Shaw [5]announced the availability of a MIME processing library. This library is supposed to be able to parse emails and decode various attachments, and generate emails with attachments. [6]The library includes modules that implement portions of: RFC 2045, RFC 2046, RFC 2387 and RFC 2822. 5. http://article.gmane.org/gmane.comp.lang.haskell.general/14555 6. http://www.n-heptane.com/nhlab/repos/haskell-mime Core (ghc-base) library. Bulat Ziganshin [7]announced progress on the Core library project, to divide the Haskell base library into two parts: small compiler-specific one (the Core library proper) and the rest: new, compiler-independent Base library that uses only services provided by Core lib. 7. http://article.gmane.org/gmane.comp.lang.haskell.general/14546 hpodder 0.99.0. John Goerzen [8]announced hpodder 0.99.0, the first beta candidate for making an eventual 1.0.0 release of hpodder. [9]hpodder is a podcast downloader that happens to be written in Haskell. This version introduces two major new features: nicer apt-like output and multithreaded downloading. 8. http://article.gmane.org/gmane.comp.lang.haskell.general/14533/ 9. http://quux.org/devel/hpodder MissingH 0.16.3. John Goerzen [10]released MissingH 0.16.3. Including a new module MissingH.ProgressTracker which tracks the progress of long-running tasks, and MissingH.Quantity which renders numbers according to a quantification system such as SI or binary. 10. http://article.gmane.org/gmane.comp.lang.haskell.general/14532 The restricted IO monad. Stefan O'Rear [11]introduced RIO, an experimental library for extensible restricted IO in Haskell. 11. http://article.gmane.org/gmane.comp.lang.haskell.general/14526 Typed symbolic differentiation. Oleg Kiselyov [12]showed symbolic differentiation of a wide class of numeric functions without any interpretative overhead. The functions to symbolically differentiate can be given to us in a compiled form (in .hi files); their source code is not needed. We produce a (compiled, if needed) function that is an exact, algebraically simplified analytic derivative of the given function. Our approach is reifying code into its `dictionary view', intensional analysis of typed code expressions, and the use of staging to evaluate under lambda. 12. http://article.gmane.org/gmane.comp.lang.haskell.general/14566 Haskell' This section covers the [13]Haskell' standardisation process. * [14]Standard (core) libraries initiative: rationale * [15]Character literal question 13. http://hackage.haskell.org/trac/haskell-prime 14. http://article.gmane.org/gmane.comp.lang.haskell.prime/1919 15. http://article.gmane.org/gmane.comp.lang.haskell.prime/1942 Libraries This week's proposals and extensions to the [16]standard libraries. * [17]Data.List documentation improvements 16. http://haskell.org/haskellwiki/Library_submissions 17. http://thread.gmane.org/gmane.comp.lang.haskell.libraries/5902/focus=5902 Discussion The Data.Array.* hierarchy is unsafe: segfaulting for fun and profit. Spencer Janssen [18]revealed that a malicious Ix instance can be used to create segfaulting array programs in pure Haskell (under GHC or Hugs), without the use of anything marked 'unsafe'. The array operators trust the Ix instance to manage boundaries correctly. It was pointed out that the Haskell standard requires Ix instances obey an in-range side condition (just as Monad instances have side conditions). 18. http://article.gmane.org/gmane.comp.lang.haskell.libraries/5903/ Merging MissingH into the standard libraries. John Goerzen [19]proposed a [20]renaming plan to aid splitting, and merging the various MissingH modules into their own packages, and into base 19. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/16985/focus=16985 20. http://software.complete.org/missingh/wiki/TransitionPlanning Defining Cg, HLSL style vectors in Haskell. Slavomir Kaslev [21]asked about defining a couple of float2, float3, float4 Cg, HLSL style vectors in Haskell. 21. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/16898/focus=16898 Extracting sublists. Huazhi Gong [22]asked about the best way to extract sublists. Several solutions where suggested, including [23]a nice one by Oleg. 22. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/16976/focus=16976 23. http://article.gmane.org/gmane.comp.lang.haskell.cafe/16984 Generating 50 randoms. Huazhi Gong [24]also asked about how best to generate 50 random integers, leading to some lovely of creative writing... 24. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/17029/focus=17029 Blog noise [25]Haskell news from the blogosphere. * [26]Haskell Preliminaries: Implementations and Tools * [27]Simple Functions in Haskell * [28]Functions, Types, Function Types, and Type Inference * [29]Building Datatypes in Haskell (part 1) * [30]OOPSLA 2006 Summary * [31]Some Very Basic Haskell and Thoughts on Error Diagnosis * [32]Hello world! in Haskell * [33]Haskell - A valuable language * [34]On Functional Programming * [35]Listing files in Haskell * [36]Bits and Pieces of Haskell * [37]Typing like a Functional Programmer * [38]6 Languages * [39]Closures and bottom for Java * [40]A newbie in Haskell land * [41]Interesting parsers in Haskell * [42]Meep for developers (finite-difference time-domain simulation software in Haskell) * [43]Algorithmic Music in Real Time using Haskell * [44]Rank-2 polymorphism is a strange thing * [45]Scalable computer programming languages * [46]Parser with Writer monad * [47]Metacognitive processes involved in deriving a solution to unzip * [48]Python Decorators - not desirable in Haskell * [49]Tools for haskell problem solving * [50]Technology Time Warp: trying out Lisp and Haskell * [51]Haskell programming language assignment * [52]Building GHC in under 10 minutes * [53]Lets play a game * [54]foldr, foldl * [55]Monads in Ruby Part 1: Identity * [56]Monads In Ruby Part 1.5: Identity * [57]New Languages * [58]You have to love Haskell * [59]Polynomials as numbers * [60]Parser combinators * [61]Lisp, you're old (but we love you anyway) * [62]Will somebody please think of the Parens!? * [63]Diving into Haskell * [64]FourFours in Haskell 25. http://planet.haskell.org/ 26. http://scienceblogs.com/goodmath/2006/11/haskell_preliminaries_implemen.php 27. http://scienceblogs.com/goodmath/2006/11/simple_functions_in_haskell_1.php 28. http://scienceblogs.com/goodmath/2006/12/functions_types_function_types_1.p… 29. http://scienceblogs.com/goodmath/2006/12/building_datatypes_in_haskell_1.php 30. http://mark.santaniello.net/archives/274 31. http://praisecurseandrecurse.blogspot.com/2006/11/some-very-basic-haskell-a… 32. http://blog.milkfarmsoft.com/?cat=6 33. http://channel9.msdn.com/ShowPost.aspx?PostID=261386#261386 34. http://www.pbell.com/index.cfm/2006/11/26/On-Functional-Programming 35. http://therning.org/magnus/archives/228 36. http://pseudofish.com/blog/2006/11/28/bits-and-pieces-of-haskell/ 37. http://www.artima.com/weblogs/viewpost.jsp?thread=186200 38. http://sharp-blue.livejournal.com/14234.html 39. http://www.javac.info/closures-v03.html 40. http://www.alpheccar.org/en/posts/show/60 41. http://www-etud.iro.umontreal.ca/~laurieti/parser.html 42. http://shejun.blogspot.com/2006/11/meep-for-developers.html 43. http://hemiolesque.blogspot.com/2006/11/algorithmic-music-in-real-time.html 44. http://cucumariid.livejournal.com/36757.html 45. http://www.cs.caltech.edu/~mvanier/hacking/rants/scalable_computer_programm… 46. http://sequence.complete.org/node/231 47. http://sequence.complete.org/node/235 48. http://sequence.complete.org/node/232 49. http://sequence.complete.org/node/230 50. http://markagrant.blogspot.com/2006/12/technology-time-warp.html 51. http://ched.vox.com/library/photo/6a00c225264de2549d00cdf3a3d6fbcb8f.html#c… 52. http://cgi.cse.unsw.edu.au/~dons/blog/2006/12/03#build_ghc_fast 53. http://e-scribe.com/news/193#541 54. http://monad.carnalreason.org/wordpress/?p=19 55. http://meta-meta.blogspot.com/2006/12/monads-in-ruby-part-1-identity.html 56. http://meta-meta.blogspot.com/2006/12/monads-in-ruby-part-15-identity.html 57. http://www.travishartwell.net/blog/2006/12/02_1144 58. http://mikepence.wordpress.com/2006/10/31/you-have-to-love-haskell/ 59. http://www.quetzal.com/sambangu/2006/12/polynomials-as-numbers 60. http://a-preponderance-of-pondering.blogspot.com/2006/12/parser-combinators… 61. http://a-preponderance-of-pondering.blogspot.com/2006/12/lisp-youre-old-but… 62. http://foldr.org/~michaelw/log/programming/lisp/lispin 63. http://dm.zimmer428.net/index.php/archives/203 64. http://www.joachim-breitner.de/blog/archives/210-FourFours-in-Haskell.html Quotes of the Week * Andreas Rossberg: To the functional programmer's eye a loop is just a degenerate form of linear recursion written in an adhoc special syntax. * Cale: [on the discovery of a secret pi calculus monad] Someone apparently wasn't informed that integration into the collective Haskell hive-mind is mandatory * secretgeek: Because we've reached the limits of Moore's Law, the computers of the future will have many-core processors and all our programs will need to be written in a combination of Haskell and Lisp, that will itself be so powerful that the computers of the future will not be able to implement any of our ideas without creating time-travelling algorithms that borrow processing power from other computers that are further into the future. This sounds difficult, but in Lisp it isn't difficult at all. In Haskell this is a built-in feature and the way you implement it is just a no-brainer... * PaulPotts: Haskell is an even 'redder' pill than Lisp or Scheme * Pseudonym: Assuming an advanced compiler, your code will be sent to Oleg who will implement it in the type system. * cjeris: [wants ghc for wii] Build and deploy an army of functional dependencies and multi-parameter type classes to defend Hyrule from the menacing MUTATRON and his legion of Java programmers! * dons: My feeling is that the Java programmers will welcome us as liberators * greentea: To learn Monads, one must /become/ a Monad. * newsham: There are Just 1 types of people, Nothing and those who understand monads * vegai: In Soviet Russia, YOU blow up GHC's brain! Code Watch Wed Nov 29 11:24:21 PST 2006. simonpj. [65]Make SpecConstr work right for nullary constructors. For totally stupid reasons, SpecConstr didn't work for the (particularly easy) case of nullary constructors like True and False. I just had some equations in the wrong order, so that a Var case came first, which matches a nullary constructor, with the constructor-application case afterwards. The fix is easy. 65. http://article.gmane.org/gmane.comp.lang.haskell.cvs.ghc/18033 Fri Dec 1 04:53:04 PST 2006. Wolfgang Thaller. [66]Decouple -O from -fvia-C. Nowadays, there are situations where -fvia-C is definitely unwanted, such as when -fPIC is used on some platforms, so we do not want implicit -fvia-C any more. 66. http://article.gmane.org/gmane.comp.lang.haskell.cvs.ghc/18062 About the Haskell Weekly News Each week, new editions are posted to [67]the Haskell mailing list as well as to [68]the Haskell Sequence and [69]Planet Haskell. [70]RSS is also available, and headlines appear on [71]haskell.org. Headlines are available as PDF. The Haskell Weekly News is also [72]available in Spanish translation. To help create new editions of this newsletter, please see the [73]contributing information. Send stories to dons at cse.unsw.edu.au. The darcs repository is available at darcs get [74]http://www.cse.unsw.edu.au/~dons/code/hwn 67. http://www.haskell.org/mailman/listinfo/haskell 68. http://sequence.complete.org/ 69. http://planet.haskell.org/ 70. http://sequence.complete.org/node/feed 71. http://haskell.org/ 72. http://haskell.org/haskellwiki/HWN/es 73. http://haskell.org/haskellwiki/HWN 74. http://www.cse.unsw.edu.au/~dons/code/hwn

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ANNOUNCE: Another Haskell MIME Library
by Dominic Steinitz 04 Dec '06

04 Dec '06

What's the betting they all have Base64 in them. Base64 is like a virus; everyone seems to have their own copy. I put it in the crypto library (along with MD5 another virus-like module) in the hope that we would end up with one version of both. Clearly that hasn't worked. Any suggestions? Dominic.

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