December 2008 - Haskell-Cafe

A curios monad
by Andrew Coppin 14 Dec '08

14 Dec '08

Yesterday I wrote something slightly unusual: module Storage where data Storage x instance Monad Storage run :: Storage x -> x data Key v instance Eq (Key v) instance Ord (Key v) new_key :: v -> Storage (Key v) set_key :: Key v -> v -> Storage () get_key :: Key v -> Storage (Maybe v) delete_key :: Key v -> Storage () In other words, you can store a value (of arbitrary type) under a unique key. The monad chooses what key for you, and tells you the key so you can look up or alter the value again later. Under the covers, it uses Data.Map to store stuff. I used some trickery with existential quantification and unsafeCoerce (!!) to make it work. Notice the sneaky phantom type in the key, telling the type system what type to coerce the value back to when you read it. Neat, eh? ...until I realised that somebody that somebody could generate a key in one run and then try to use it in another run. o_O Oops! But hey, until then it was working quite well. And completely pure; no IO anywhere. Ah well, just thought I'd share... (You could of course do away with the monad, but then you'd be able to manipulate the dictionary directly, and key uniqueness would be even more fragile!)

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Small haskell practical course
by Marco Túlio Gontijo e Silva 13 Dec '08

13 Dec '08

Hello, I've presented a small practical course[0] in EMSL[1]. Any comments are welcome. 0: http://marcot.iaaeee.org/mini-curso.pdf (Portuguese only) 1: http://emsl.softwarelivre.org/ Greetings. -- marcot http://marcot.iaaeee.org/

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Haskell Weekly News: Issue 97 - December 13, 2008
by Brent Yorgey 13 Dec '08

13 Dec '08

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20081213 Issue 97 - December 13, 2008 --------------------------------------------------------------------------- Welcome to issue 97 of HWN, a newsletter covering developments in the [1]Haskell community. Lots of neat blog posts and funny quotes this week. Don't forget to keep adding [2]haiku to the wiki, and don't miss Alex McLean (yaxu)'s [3]streaming livecoding performance tonight! Announcements Spam on HaskellWiki. Ashley Yakeley [4]asked what people would like to do about the increasing amounts of spam on the Haskell wiki, and offered some suggestions. The Timber compiler 1.0.2. Johan Nordlander [5]announced the first public release of the [6]Timber compiler. Timber is a modern language for building event-driven systems, based around the notion of reactive objects. It is also a purely functional language derived from Haskell, although with a strict evaluation semantics. To try it out, just grab the [7]timberc package on Hackage. Retrospective on 2008?. Don Stewart [8]proposed the idea of a 2008 retrospective. How would you choose the 10 best new libraries, applications, blog posts, etc. of 2008? a haskell_proposals subreddit. Jason Dusek [9]announced a [10]subreddit for Haskell library proposals. The idea is that Web 2.0 will help us to allocate our collective talents more efficiently when it comes to extensions (and perhaps clue us in when our pet project is something people really want). permutation-0.2. Patrick Perry [11]announced a [12]new version of the permutation library, which includes data types for storing permutations. It implements pure and impure types, the latter which can be modified in-place. The main utility of the library is converting between the linear representation of a permutation to a sequence of swaps. This allows, for instance, applying a permutation or its inverse to an array with O(1) memory use. Data.List.Split. Brent Yorgey [13]announced the creation of a wiki page for [14]Data.List.Split, a hypothetical module containing implementations of every conceivable way of splitting lists known to man, so we no longer have to (1) argue about the 'one true' interface for a 'split' function, or (2) be embarrassed when people ask why there isn't a split function in the standard libraries. Please add code or comments! At some point it will be uploaded as a new module to Hackage. Announcing Haskell protocol-buffers version 1.2.2. Chris Kuklewicz [15]announced new versions of [16]protocol-buffers, [17]protocol-buffers-descriptor, and [18]hprotoc. Discussion A curious monad. Andrew Coppin [19]exhibited an interesting Storage monad, which (it turns out) is similar to ST. An enlightening discussion if you want to understand how ST works and the motivation behind it. Origins of '$'. George Pollard [20]asked about the origins of the $ operator (low-precedence function application) in the standard libraries, leading to some interesting history and general discussion about notation. Blog noise [21]Haskell news from the [22]blogosphere. * Jamie Brandon: [23]Zombified GMap. Jamie is determined to get his SoC generalized map library released! * Philip Wadler: [24]Informatics 1 - Fancy Dress and Competition. * >>> Eduard - Gabriel Munteanu: [25]Learning Haskell, part 2. * Martin Sulzmann: [26]Equality, containment and intersection among regular expressions via symbolic manipulation. * Neil Mitchell: [27]mapM, mapM_ and monadic statements. * Alson Kemp: [28]A HAML parser for Haskell. * Chris Done: [29]More Haskell blogging. * Twan van Laarhoven: [30]Knight in n, part 4: tensors. Part four of Twan's enlightening series on computing knight moves. * Alex McLean: [31]Saturday night stream. All the cool kids will be watching Alex's streaming livecoding performance TONIGHT, using (among other things) a tool implemented in Haskell. * David Sankel: [32]Synchronous Events. * Lennart Augustsson: [33]The OCaml code again. * Lennart Augustsson: [34]Abstracting on, suggested solutions. * Lennart Augustsson: [35]The abstraction continues. * Conal Elliott: [36]Functional interactive behavior. * Conal Elliott: [37]Trimming inputs in functional reactive programming. * Mikael Vejdemo Johansson (Syzygy-): [38]J, or how I learned to stop worrying and love the matrix. * Conal Elliott: [39]Why classic FRP does not fit interactive behavior. * Clifford Beshers: [40]Functional Programming Marketing. * Lennart Augustsson: [41]A somewhat failed adventure in Haskell abstraction. * >>> Joey Hess: [42]haskell and xmonad. * Andy Gill: [43]The Timber compiler 1.0.2. * Manuel M T Chakravarty: [44]Not a particularly good article, but.... * >>> Chris Double: [45]Random and Binary IO using Iteratees. * >>> Chris Double: [46]Not a Tutorial on HAppS. * "FP Lunch": [47]The new GHC API. * Luke Palmer: [48]Compact data types. * Neil Mitchell: [49]F# from a Haskell perspective. * Real-World Haskell: [50]RWH Now In Store. * >>> Sebastian Fischer: [51]Constraint Functional-Logic Programming. * >>> fhtr: [52]Hexagons with Haskell. * >>> Eduard - Gabriel Munteanu: [53]Learning Haskell. * Bryan O'Sullivan: [54]Functional programmers on Twitter. * Martin Sulzmann: [55]Parallel Join Patterns with Guards and Propagation. * Martin Sulzmann: [56]Multi-set rewrite rules with guards and a parallel execution scheme. * Martin Sulzmann: [57]STM with control: Communication for retrying transactions. * Martin Sulzmann: [58]Concurrent Goal-Based Execution of Constraint Handling Rules. * >>> talkingCode: [59]Haskell, GTK and Multi-Threading. * >>> Gianfranco Alongi: [60]QuickCheck(ing) the code I C - source. Quotes of the Week * quicksilver: Baughn: glFlush? the 80s called, they want your programs back? * gwern: the best way to optimize a program is to make it lazier or stricter. * ksf: Perl is obfuscated by design, haskell is designed by obfuscation. * conal: omg -- i can print right from emacs again. praise be to Linux! * mmorrow: [I] didn't realize what it really said until after i @remembered it * blackh: Haskell is great because of all the wonderful things you can't do with it. * JustinBogner: gitit's 46 dependencies convinced me to install cabal-install, and now I couldn't be happier! * Anonymous: I'd love to explain to you how to write hello world in Haskell, but first let me introduce you to basic category theory. * lilac: @type \o-> look at my muscles <lambdabot> forall t nice muscles. t -> nice -> muscles * ook: (:[]) * oink: <^(oo)^> * mmorrow: {-# RULES "HAI; CAN HAS STDIO?" id = unsafePerformIO (system "killall -9 breathingMachine && xeyes &" >> return id) #-} * gwern: We will be welcomed as liberators! I estimate that we will need 50000 haskellers at most and will be able to wind up the occupation quickly About the Haskell Weekly News New editions are posted to [61]the Haskell mailing list as well as to [62]the Haskell Sequence and [63]Planet Haskell. [64]RSS is also available, and headlines appear on [65]haskell.org. To help create new editions of this newsletter, please see the information on [66]how to contribute. Send stories to byorgey at cis dot upenn dot edu. The darcs repository is available at darcs get [67]http://code.haskell.org/~byorgey/code/hwn/ . References 1. http://haskell.org/ 2. http://haskell.org/haskellwiki/Haikus 3. http://yaxu.org/saturday-night-stream/ 4. http://article.gmane.org/gmane.comp.lang.haskell.general/16678 5. http://article.gmane.org/gmane.comp.lang.haskell.general/16672 6. http://timber-lang.org/ 7. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/timberc 8. http://article.gmane.org/gmane.comp.lang.haskell.cafe/48778 9. http://article.gmane.org/gmane.comp.lang.haskell.cafe/48721 10. http://www.reddit.com/r/haskell_proposals/ 11. http://article.gmane.org/gmane.comp.lang.haskell.cafe/48663 12. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/permutation 13. http://article.gmane.org/gmane.comp.lang.haskell.cafe/48868 14. http://haskell.org/haskellwiki/Data.List.Split 15. http://article.gmane.org/gmane.comp.lang.haskell.libraries/10406 16. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/protocol-buffers 17. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/protocol-buffers… 18. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hprotoc 19. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/48806 20. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/48622 21. http://planet.haskell.org/ 22. http://haskell.org/haskellwiki/Blog_articles 23. http://jamiiecb.blogspot.com/2008/12/zombified-gmap.html 24. http://wadler.blogspot.com/2008/12/informatics-1-functional-programming.html 25. http://eduard-munteanu.blogspot.com/2008/12/learning-haskell-part-2.html 26. http://sulzmann.blogspot.com/2008/12/equality-containment-and-intersection.… 27. http://neilmitchell.blogspot.com/2008/12/mapm-mapm-and-monadic-statements.h… 28. http://www.alsonkemp.com/programming/a-haml-parser-for-haskell/ 29. http://chrisdone.com/blog/2008/12/11/More-Haskell-blogging 30. http://twan.home.fmf.nl/blog/haskell/Knight4.details 31. http://yaxu.org/saturday-night-stream/ 32. http://netsuperbrain.com/blog/posts/synchronous-events/ 33. http://augustss.blogspot.com/2008/12/ocaml-code-again-im-posting-slight.html 34. http://augustss.blogspot.com/2008/12/abstracting-on-suggested-solutions-i.h… 35. http://augustss.blogspot.com/2008/12/abstraction-continues-i-got-several.ht… 36. http://conal.net/blog/posts/functional-interactive-behavior/ 37. http://conal.net/blog/posts/trimming-inputs-in-functional-reactive-programm… 38. http://blog.mikael.johanssons.org/archive/2008/12/j-or-how-i-learned-to-sto… 39. http://conal.net/blog/posts/why-classic-frp-does-not-fit-interactive-behavi… 40. http://cliffordbeshers.blogspot.com/2007/11/functional-programming-marketin… 41. http://augustss.blogspot.com/2008/12/somewhat-failed-adventure-in-haskell.h… 42. http://kitenet.net/~joey/blog/entry/haskell_and_xmonad/ 43. http://blog.unsafeperformio.com/?p=32 44. http://justtesting.org/post/63809222 45. http://www.bluishcoder.co.nz/2008/12/random-and-binary-io-using-iteratees.h… 46. http://www.bluishcoder.co.nz/2008/12/not-tutorial-on-happs.html 47. http://sneezy.cs.nott.ac.uk/fplunch/weblog/?p=168 48. file://localhost/home/brent/hacking/hwn/20081213.html 49. http://neilmitchell.blogspot.com/2008/12/f-from-haskell-perspective.html 50. http://www.realworldhaskell.org/blog/2008/12/06/rwh-now-in-store/ 51. http://www-ps.informatik.uni-kiel.de/~sebf/projects/cflp.html 52. http://fhtr.blogspot.com/2008/12/hexagons-with-haskell.html 53. http://eduard-munteanu.blogspot.com/2008/12/learning-haskell.html 54. http://www.serpentine.com/blog/2008/12/05/functional-programmers-on-twitter/ 55. http://sulzmann.blogspot.com/2008/12/parallel-join-patterns-with-guards-and… 56. http://sulzmann.blogspot.com/2008/10/multi-set-rewrite-rules-with-guards-an… 57. http://sulzmann.blogspot.com/2008/12/stm-with-control-communication-for.html 58. http://sulzmann.blogspot.com/2008/12/concurrent-goal-based-execution-of.html 59. http://talkingcode.co.uk/2008/12/02/haskell-gtk-and-multi-threading/ 60. http://writert.blogspot.com/2008/12/quickchecking-code-i-c-source.html 61. http://www.haskell.org/mailman/listinfo/haskell 62. http://sequence.complete.org/ 63. http://planet.haskell.org/ 64. http://sequence.complete.org/node/feed 65. http://haskell.org/ 66. http://haskell.org/haskellwiki/HWN 67. http://code.haskell.org/~byorgey/code/hwn/

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Numerics & implementing different instances of the same class
by George Pollard 12 Dec '08

12 Dec '08

Is there a good way of doing this? My running example is Monoid: > class Monoid a where > operation :: a -> a -> a > identity :: a With the obvious examples on Num: > instance (Num a) => Monoid a where > operation = (+) > identity = 1 > > instance (Num a) => Monoid a where > operation = (*) > identity = 0 Of course, this won't work. I could introduce a newtype wrapper: > newtype (Num a) => MulNum a = MulNum a > newtype (Num a) => AddNum a = AddNum a > > instance (Num a) => Monoid (MulNum a) where > operation (MulNum x) (MulNum y) = MulNum (x * y) > identity = MulNum 1 > > instance (Num a) => Monoid (AddNum a) where ... -- etc However, when it comes to defining (e.g.) a Field class you have two Abelian groups over the same type, which won't work straight off: > class Field a where ... > instance (AbelianGroup a, AbelianGroup a) => Field a where ... Could try using the newtypes again: > > instance (AbelianGroup (x a), AbelianGroup (y a) => Field a where ... ... but this requires undecidable instances. I'm not even sure if it will do what I want. (For one thing it would also require an indication of which group distributes over the other, and this may restore decidability.) I'm beginning to think that the best way to do things would be to drop the newtype wrappers and include instead an additional parameter of a type-level Nat to allow multiple definitions per type. Is this a good way to do things? Has anyone else done something similar? I've taken a look at the Numeric Prelude but it seems to be doing things a bit differently. (e.g. there aren't constraints on Ring that require Monoid, etc) - George

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wlan library
by Cetin Sert 12 Dec '08

12 Dec '08

Hi *^o^*, I am writing a network manager <http://sert.homedns.org/hnm/> as a replacement for some broken, already existing knetworkmanager for a friend's computer. I was looking for some haskell libraries that provide access to wlan cards but could not find any on hackage. Maybe I missed something. Currently my tool is using linux the commands: modprobe, ifconfig, iwconfig, iwlist, dhclient etc.. Can a library calling external stand-alone linux commands be considered good enough as is? Or should one definitely write a wrapper around a more low-level, stable, POSIX-compliant foreign library to access and configure, control WLAN cards? I'd love to hear your comments. Best Regards, Cetin Sert

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Parsec and (then?) type-check
by Greg Fitzgerald 12 Dec '08

12 Dec '08

Parser gurus, When you write a parser with a library like Parsec, do you typically type-check while parsing, or afterward in a separate pass? The latter is more modular, but it means labeling every element in the AST with the parser position so that you can give good error messages. Do you find the added modularity worth the hassle or just pack type-checking into the parser pass? Thanks, Greg

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Memoization-question
by Mattias Bengtsson 12 Dec '08

12 Dec '08

The program below computes (f 27) almost instantly but if i replace the definition of (f n) below with (f n = f (n - 1) * f (n -1)) then it takes around 12s to terminate. I realize this is because the original version caches results and only has to calculate, for example, (f 25) once instead of (i guess) four times. There is probably a good reason why this isn't caught by the compiler. But I'm interested in why. Anyone care to explain? > main = print (f 27) > > f 0 = 1 > f n = let f' = f (n-1) > in f' * f' (compiled with ghc --make -O2) Mattias

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Data.Array.Storable vs GC
by David F. Place 12 Dec '08

12 Dec '08

If you have a giant unboxed array that will never become garbage, it would be nice to put it somewhere where the GC won't bother with it. Since Data.Array.Storable arrays are allocated in the C heap, I thought it would be a good choice. However, I am getting very poor performance due to the GC copying 6G in each run. The only explanation that I can think of is that it is copying my giant array. Am I wrong in my understanding that Data.Array.Storable arrays should not be copied by the GC? BTW, I am using GHC 6.10.1 on linux. Thanks for reading.

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Multi-parameter Type Class
by Paul Keir 12 Dec '08

12 Dec '08

Hi all, I've been trying to refactor my tree conversion code to make better use of type classes; and I've discovered multi-parameter type classes and functional dependencies. I have a class with a function a2b, and I'd like "map" to be used when it's a list of type a. I've created a simple failing example: data Foo = Foo Bar deriving(Show) data Bar = Bar String deriving(Show) class ZOT a b | a -> b where zot :: a -> b instance ZOT Foo Integer where zot x = 17 instance ZOT Bar String where zot x = "Eighteen" instance ZOT [x] [y] where -- This bit zot xs = map zot xs -- fails main = do print $ zot $ Foo $ Bar "Blah" print $ zot $ Bar "Blah" print $ zot $ [Bar "Blah", Bar "Blah"] -- No map here please I know this would work if the third instance of zot explicitly took [Bar] and [String]. Can I not instead generalise for all the ADTs in my tree in the way I've outlined? Must I instantiate for the type of each list pair? Cheers, Paul

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Re: problems with hs-plugins load-eval (possible bug)
by Alberto G. Corona 11 Dec '08

11 Dec '08

The duplicate definition error appears when I compile Main.hs and execute it. But when I run it with runghc the behaviour is different. It works well: > runghc Main.hs 3 Any idea? is this a bug of hs-plugins? it is just something expected?? 2008/12/6 Alberto G. Corona <agocorona(a)gmail.com> > I have a web server which load server extensions. these extensions > eval-uate configuration files that contains code (user-editable workflow > descriptions). The problem is that I need common definitions (inside > imported modules) for the extensions and for the configuration files. This > is not permitted by ha-plugins. > > The minimal code example are the files below. main loads eval.hs , that > evaluate a expression. The common definitions are in Include.hs. The error > is: > > *GHCi runtime linker: fatal error: I found a duplicate definition for > symbol > Include_sum1_srt > whilst processing object file > /home/magocoal/haskell/devel/votesWorkflow/src/unused/tests/Include.o > > This could be caused by: > * Loading two different object files which export the same symbol > * Specifying the same object file twice on the GHCi command line > * An incorrect `package.conf' entry, causing some object to be > loaded twice. > GHCi cannot safely continue in this situation. Exiting now. Sorry. > * > > > ** > Do you kno how to solve the problem while maintaining the functionality? > > -------Include.hs------- > module Include where > > sum [x,y]= x+y > > > ------Main.hs----- > module Main > where > > import Include > import System.Plugins > > main= do > s <-loadExec "eval.o" "mainc" > print s > > loadExec:: String-> String->IO String > loadExec file method = do > > mv <- load file ["."] [] method > case mv of > LoadSuccess mod v -> v :: IO String > LoadFailure msg -> return $ concat msg > > > > ------------Eval.hs-------- > > module Eval(mainc) where > import System.IO.Unsafe > import System.Eval.Haskell > > mainc= do i <- unsafeEval_ "sum1 [1,2]" ["Include"] [] []["."] :: IO > (Either [String] Int) > return $ show i > > > > >

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