- Haskell-Cafe - Haskell.org

Multi-parameter type class woes
by Mario Blažević 15 Dec '08

15 Dec '08

I have, for a change, a relatively simple problem with type classes. Can somebody explain to me, or point me to an explanation of the behaviour I see? Here is a short and useless example: {-# LANGUAGE MultiParamTypeClasses, FlexibleInstances #-} import Data.Maybe class Container x y where wrapper :: x -> Bool unwrap :: x -> y rewrap :: y -> x liftWrap :: Container x y => (y -> y) -> (x -> x) liftWrap f x = (if wrapper x then rewrap . f . unwrap else id) x instance Container (Maybe x) x where wrapper = isJust unwrap = fromJust rewrap = Just main = print (liftWrap (succ :: Int -> Int) (Just 1 :: Maybe Int)) GHC 6.10.1 refuses to typecheck the 'wrapper' function in definition of 'liftWrap', with the following error message: Could not deduce (Container x y) from the context (Container x y1) arising from a use of `wrapper' at Test.hs:11:22-30 Possible fix: add (Container x y) to the context of the type signature for `liftWrap' In the expression: wrapper x In the expression: (if wrapper x then rewrap . f . unwrap else id) x In the definition of `liftWrap': liftWrap f x = (if wrapper x then rewrap . f . unwrap else id) x Let me clarify that I'm aware that in this particular example a functional dependecy should be used. Also, I can think of a few workarounds for my actual problem, so I'm not asking for any solutions. I'm looking for an explanation. It bugs me that my intuition of how this type class should have worked is completely wrong. The error message does not help, to put it mildly. Where should I go, what should I read?

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ghc on CentOS 5 ?
by Steve Lihn 15 Dec '08

15 Dec '08

I recently got a CentOS server, but noticed that ghc rpm is not available from CentOS yum. After investigation, I found ghc is in Fedora repository, but does not make its way to EPEL, which yum on CentOS can use. ( http://download.fedora.redhat.com/pub/epel/5/x86_64/) Is there any plan to get it to EPEL? Or is there any missing piece preventing it? Thanks, Steve

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Origins of '$'
by George Pollard 14 Dec '08

14 Dec '08

Hello Haskell-Café: This is a little bit random, but I was just wondering if anyone knew where the $ low-precedence parenthesis-eliminating application operator originated. The Haskell Report doesn't mention anything, and I can't search for "$" on Google. So... who thought it up? Does it originate in an earlier language, or is it uniquely Haskellish? :) - George

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ANN: typehash version 1.3
by Lennart Augustsson 14 Dec '08

14 Dec '08

The typehash library allows you to produce a unique identifier (a cryptographic hash) for a type. This is useful if you save values of some type to a file (text, binary, whatever format you wish) and then when you read it back in again you want to verify that the type you want to read is the one you actually wrote. The type hash takes into account both the actual name of the type and the structure of the type. The type hash is a compact value that only allows comparing for equality. The library also supports type codes. A type code encodes the complete structure of a type and can be used for finer comparison than just equality, e.g., will the read function be able to read what show produced (in this case renaming types and adding constructors is allowed). The library is on hackage, of course. -- Lennart

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ANNOUNCE: uvector-algorithms 0.1
by Dan Doel 14 Dec '08

14 Dec '08

Hello, I've been sitting on this for a while, waiting for some changes to uvector to go in, but finally decided I should just release it, and fix it up if and when said changes go in. So, I'm announcing the first release of uvector- algorithms. What it is is a library of algorithms (mostly sorting) for the mutable arrays defined in uvector. It has several varieties of sorting, including introsort (quicksort which falls back on heapsort in bad cases), heapsort, a simple top- down merge sort and a radix sort (as well as insertion sort, and optimal sorting for length<=4 arrays, which probably won't get much use outside of the implementation of the other sorts). All modules attempt to export as uniform an interface as possible, so that one can substitute between them freely, say when trying to determine which algorithm is best suited for your particular datasets. Also exposed are the operations that allow you to use the arrays as heaps (which is the basis for implementing heapsort), and partial sorts and selects in heap and intro variety. Lastly, there is a combinator for safely using these mutable array algorithms to sort immutable arrays. All of these algorithms have been painstakingly profiled, and their generated core examined to do as little heap allocation and as much unboxing as possible, and to inline and specialize for maximum performance (unless, of course, I've managed to miss anything). I've been running a relatively simple benchmarking suite that tests various kinds of inputs, and in my experience, the sorts herein tend to beat glibc's sort, and do relatively well compared to GNU's STL sorts over vector<>s (taking 50% or so more time at present, although this library may well be better at heapsorting random arrays, if you want something to brag about :)). For future work, I've been working on an implementation of Python's timsort (which is a very fancy bottom-up merge sort), but it's not ready yet. I'd also like to introduce some combinators for easy Schwartzian transforms, but that again requires modifications to uvector. I may also look at moving some of my benchmark program into the package. Suggestions for other algorithms people would like to see are of course welcome (especially if accompanied by papers/references on how to implement them if they're not well known). The hackage page is here: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/uvector-algorith… Enjoy, and please let me know of any issues. -- Dan

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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 13 Dec '08

13 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 13 Dec '08

13 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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