February 2011 - Haskell-Cafe

ANNOUNCE: htime-0.1
by José Pedro Magalhães 25 Feb '11

25 Feb '11

Hello all, I kept being annoyed at the fact that Windows doesn't come with anything similar to the unix "time" utility, so I created a small Haskell program that does something similar. I thought this could perhaps be useful to others, so it's now available on Hackage: http://hackage.haskell.org/package/htime-0.1 Cheers, Pedro

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Re: [Haskell-cafe] HANSEI in Haskell?
by oleg＠okmij.org 25 Feb '11

25 Feb '11

The topic of HANSEI in Haskell does come up from time to time. In fact, there was a Haskell edition of the first version of Hansei: http://okmij.org/ftp/kakuritu/probM.hs It was written to see how the code would look in Haskell, and how memoization (inherent in lazy evaluation of GHC) may help. The code contains some of the tests from the HANSEI distribution. The experience showed that lazy evaluation hurts -- a lot. The problem is not in lack of seq -- adding seq would make things even worse. There is a bit of explanation at the end of probM.hs. The gist of the problem is that approximate inference (sampling, to be precise) trades space for time. The search tree is way too large to fit into memory. Therefore, it is better to recompute the values (the branches of the tree) than to keep storing them. That is precisely the opposite to the trade-off of lazy evaluation. The end result is attempting to allocate gigabytes (really!), even for toy problems. We can get around the problem by using thunks explicitly. But then we lose the remaining benefits of Haskell syntax (the ones that are left after we have to switch to the monadic notation). OCaml becomes quite compelling then. It must be stressed that laziness in general is _crucial_ for solving even moderately complex problems. However, the needed laziness is NOT of the kind provided by GHC. We need memoization specific to a possible world, rather than the one that affects all possible worlds. GHC gives only the latter. The sort of laziness needed for non-deterministic programming calls for first-class store. It can be emulated, albeit imperfectly, for example, as was described in the ICFP09 paper. Hansei uses a quite similar idea. Ideally first-class memory is provided as built-in, since it requires interaction with garbage collection. Greg Morrisett has written extensively on this topic and done a few implementations; see for example, http://www.eecs.harvard.edu/~greg/papers/jgmorris-callcs.ps Sadly, the topic has been forgotten.

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GRIN and Urban Boquist's thesis
by John Lato 24 Feb '11

24 Feb '11

Hello, Does anyone have a current link to Urban Boquist's thesis? It's no longer available at the Chalmers site. On the LHC blog someone gave a link to a mirror[1], however that link is down for me as well. Thanks, John L. [1] http://lhc-compiler.blogspot.com/2010/06/mirroring-boquists-grin-papers.htm…

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Re: [Haskell-cafe] Haskell-Cafe Digest, Vol 90, Issue 48
by Louis Wasserman 24 Feb '11

24 Feb '11

> > Another common usage for Map is as a functional integer-indexed > random access array. > Once I implemented the standard algorithm for random shuffle > of a list using Data.Map Int. It was much nicer than the STArray > version, in my opinion. But when I tried switching to Data.IntMap, > hoping to make it faster, I was devastatingly disappointed. Now > I understand why. A couple thoughts: If you're going to provide an isSingleton method, provide getSingleton :: IntMap m -> Maybe (Int, a) which is trivial based on the current implementation. For my TrieMap package, I've been working on my WordMap implementation here<http://hackage.haskell.org/packages/archive/TrieMap/3.0.1/doc/html/src/Data…>. (Using Words as keys makes my life easier, and it doesn't take too much work to write a wrapper that provides an order-preserving bijection from Int to Word.) It provides O(1) size, but its design reflects my willingness to trade memory and code size for speed. ;) Louis Wasserman wasserman.louis(a)gmail.com http://profiles.google.com/wasserman.louis

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darcs hacking sprint #6 (March, April, or May 2011)
by Eric Kow 24 Feb '11

24 Feb '11

Hello Haskellers! We are starting to organising the next Darcs Hacking Sprint, most likely in Paris in this spring. If you fancy a weekend of hacking Haskell on a really neat real world project, come join us! If you'd like to attend, please add yourself to http://wiki.darcs.net/Sprints/2011-04 . We will fix the date by March 1st, and there's only about 10+ spaces left, so sign up now! If you want to know what is exactly a Darcs Sprint, have a look at: http://wiki.darcs.net/Sprints http://blog.darcs.net/search/label/sprints The event is open to everybody, even if you're new to Darcs or to Haskell. We have an ample supply of enthusiasm and ProbablyEasy bugs to help you get started. See http://wiki.darcs.net/Recruitment for some ideas and also have a look at http://wiki.darcs.net/Development to see how to start coding on Darcs. Looking forward to see you! Guillaume and Eric -- Eric Kow <http://www.nltg.brighton.ac.uk/home/Eric.Kow> For a faster response, try +44 (0)1273 64 2905 or xmpp:kowey@jabber.fr (Jabber or Google Talk only)

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Haskell Weekly News: Issue 170
by Daniel Santa Cruz 24 Feb '11

24 Feb '11

Welcome to issue 170 of the HWN, a newsletter covering developments in the [1]Haskell community. This release covers the week of February 13 - 19, 2011. Announcements Simon Michael [2]informed us of reviving of FunGEn as a community project. "FunGEn (Functional Game Engine) is a platform-independent, BSD-licensed, easy-to-install 2D game engine currently based on OpenGL and GLUT." Alexander McPhail [3]announced the release of vector-buffer. "It provides a buffer that can be turned into a Data.Vector.Storable. The mapM* functions map from the oldest element, not the first." Mark P. Jones [4]announce the availability of 2 openings for post-doctoral researches "to help design, develop, and apply a new strongly typed, pure functional language for systems programming." Trystan Spangler [5]made a second release of his Behavior Driven Development for Haskell, and the tool can now be found in Hackage! Jeremy Shaw [6]announce the release of Happstack 6. "We fully recomend that all Happstack users migrate to the new release." Gregory D. Weber [7]published a new release of Sifflet and sifflet-lib. "Sifflet is a visual, functional programming language and support system for students learning about recursion." Johan Tibell [8]announced the release of two new packages: unordered-containers (Efficient hashing-based container types), and hashable (Efficient hashing of common types). Quotes of the Week * kmc: i started to read the "tutorial" and it was incomprehensible. makes the Gentle Introduction to Haskell look like Teach Yourself PHP in 24 Hours * ConorMcBride: Sometimes it's easier to search for good programs in the space of well typed programs, rather than in the space of ascii turds. * monochrom: best practice haskell is "go with your heart" * accel: one day, it was raining -- I was walking home -- I saw Knuth biking home, wearing a helmet, in the rain -- and I freaked out -- rain was hitting him like any other mortal ... it wasn't bounching off his aura of awesomeness Top Reddit Stories * Type level natural numbers ... ready to merge into GHC mainline: From (hackage.haskell.org) scored 62 with 44 comments. Read on [9]reddit or the [10]original post. * Haskell Web Framework Happstack 6 Released: From (groups.google.com) scored 45 with 3 comments. Read on [11]reddit or the [12]original post. * Well-Typed is hiring: From (well-typed.com) scored 41 with 13 comments. Read on [13]reddit or the [14]original post. * Haskell is Better: slides from Mark Lentczner's talk at Ignite Silicon Valley: From (scribd.com) scored 40 with 43 comments. Read on [15]reddit or the [16]original post. * New, faster hashing-based containers library released: From (blog.johantibell.com) scored 40 with 6 comments. Read on [17]reddit or the [18]original post. * Disciple haskell dialect : strict evaluation as default , destructive update of data structures.: From (disciple.ouroborus.net) scored 35 with 25 comments. Read on [19]reddit or the [20]original post. * Yesod: Warp Speed Ahead: From (docs.yesodweb.com) scored 34 with 8 comments. Read on [21]reddit or the [22]original post. * Faster persistent data structures through hashing: From (blog.johantibell.com) scored 28 with 15 comments. Read on [23]reddit or the [24]original post. * Developing iPhone applications in Haskell â a tutorial: From (gergo.erdi.hu), scored 25 with 6 comments. Read on [25]reddit or the [26]original post. * An Introduction to Agda :: PDF: From (dl.dropbox.com) scored 24 with 4 comments. Read on [27]reddit or the [28]original post. Top StackOverflow Answers * [29]How to tell whether Haskell will cache a result or recompute it? votes: 17 primes, in your code, is not a function, but a constant, in haskellspeak known as a CAF. If it would take a parameter (say, ()), you would get two different versions of the same list back if calling it twice, but as it is a CAF, you get the exact same list back both times; As a top-level definition, primes never gets out of scope, thus the head of the list ... * [30]Specific type inference using uncurry function votes: 17 This has nothing to do with ghci. This is the monomorphism restriction being irritating. If you try to compile the following file: add = uncurry (+) main = do print $ add (1,2 :: Int) print $ add (1,2 :: Double) You will get an error. If you expand: main = do print $ uncurry (+) (1,2 :: Int) print $ uncurry (+) (1,2 :: Double) ... * [31]Haskell: Unexpected output for expression [0, 0.1 .. 1] votes: 15 This is a result of the imprecision of floating point values, it isn't particular to Haskell. If you can't deal with the approximation inherent in floating point then you can use Rational at a high performance cost: > import Data.Ratio Data.Ratio> [0,1%10.. 1%1] [0 % 1,1 % 10,1 % 5,3 % 10,2 % 5,1 % 2,3 % 5,7 % 10,4 % 5,9 % 10,1 % 1] Just to hammer ... * [32]valid haskell function names votes: 14 From the Haskell report: Haskell uses the Unicode character set. However, source programs are currently biased toward the ASCII character set used in earlier versions of Haskell. Recent versions of GHC seem to be fine with unicode (at least in the form of UTF-8):... * [33]Haskell range notation to generate list. Unexpected output votes: 13 The notation is meant to mimic the usual way to write simple sequences mathematics. The second element is not the step, but in fact the actual second element of the list. The rest is linearly extrapolated from there. Examples: [1,2..10] = [1,2,3,4,5,6,7,8,9,10] [1,3..10] = [1,3,5,7,9] [4,3..0] = [4,3,2,1,0] [0,5..] = [0,5,10,15,20,25,30,35... Top StackOverflow Questions * How to tell whether Haskell will cache a result or recompute it? (votes: 11, answers: 1) [34]read * How can one distribute Haskell programs to non-technical end users? (votes: 8, answers: 2) [35]read * Haskell range notation to generate list. Unexpected output (votes: 8, answers: 2) [36]read * Appropriate uses of Monad `fail` vs. MonadPlus `mzero` (votes: 8, answers: 2) [37]read * Why does Haskell not have an I Monad (for input only, unlike the IO monad)? (votes: 8, answers: 3) [38]read About the Haskell Weekly News To help create new editions of this newsletter, please send stories to dstcruz(a)gmail.com. Until next time, Daniel Santa Cruz References 1. http://haskell.org/ 2. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/86330 3. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/86371 4. http://www.haskell.org/pipermail/haskell/2011-February/022585.html 5. http://permalink.gmane.org/gmane.comp.lang.haskell.general/18510 6. http://www.mail-archive.com/haskell-cafe@haskell.org/msg87352.html 7. http://www.mail-archive.com/haskell@haskell.org/msg23319.html 8. http://article.gmane.org/gmane.comp.lang.haskell.general/18515 9. http://www.reddit.com/r/haskell/comments/fnc80/type_level_natural_numbers_r… 10. http://hackage.haskell.org/trac/ghc/ticket/4385 11. http://www.reddit.com/r/haskell/comments/fnit9/haskell_web_framework_happst… 12. http://groups.google.com/group/happs/browse_thread/thread/f72d7ff313ea9fa0 13. http://www.reddit.com/r/haskell/comments/flyyg/welltyped_is_hiring/ 14. http://www.well-typed.com/blog/50 15. http://www.reddit.com/r/haskell/comments/flajs/haskell_is_better_slides_fro… 16. http://www.scribd.com/doc/48773692/Ignite-SV-Haskell-is-Better 17. http://www.reddit.com/r/haskell/comments/fo9gm/new_faster_hashingbased_cont… 18. http://blog.johantibell.com/2011/02/new-faster-containers-library.html 19. http://www.reddit.com/r/haskell/comments/fojb4/disciple_haskell_dialect_str… 20. http://disciple.ouroborus.net/ 21. http://www.reddit.com/r/haskell/comments/flp87/yesod_warp_speed_ahead/ 22. http://docs.yesodweb.com/blog/warp-speed-ahead 23. http://www.reddit.com/r/haskell/comments/fm6gg/faster_persistent_data_struc… 24. http://blog.johantibell.com/2011/02/slides-from-my-hashing-based-containers… 25. http://www.reddit.com/r/haskell/comments/fkqe1/developing_iphone_applicatio… 26. http://gergo.erdi.hu/blog/2011-02-13-developing_iphone_applications_in_hask… 27. http://www.reddit.com/r/haskell/comments/fl0wr/an_introduction_to_agda_pdf/ 28. http://dl.dropbox.com/u/361503/Agda%20and%20dependent%20types.pdf 29. http://stackoverflow.com/questions/5032750/how-to-tell-whether-haskell-will… 30. http://stackoverflow.com/questions/4999020/specific-type-inference-using-un… 31. http://stackoverflow.com/questions/5048920/haskell-unexpected-output-for-ex… 32. http://stackoverflow.com/questions/5023485/valid-haskell-function-names/502… 33. http://stackoverflow.com/questions/5012185/haskell-range-notation-to-genera… 34. http://stackoverflow.com/questions/5032750/how-to-tell-whether-haskell-will… 35. http://stackoverflow.com/questions/5004902/how-can-one-distribute-haskell-p… 36. http://stackoverflow.com/questions/5012185/haskell-range-notation-to-genera… 37. http://stackoverflow.com/questions/5023969/appropriate-uses-of-monad-fail-v… 38. http://stackoverflow.com/questions/5032475/why-does-haskell-not-have-an-i-m…

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Performance difference between ghc and ghci
by C K Kashyap 23 Feb '11

23 Feb '11

Hi, Is there a runtime performance difference between a haskell program running under GHCI vs in its compiled form? Especially for a long running program - as in, ignoring the initial setup time. If I understand right, in both case tree reduction is what is happening and performance should be similar. Regards, Kashyap

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Having trouble with instance context
by Kurt Stutsman 23 Feb '11

23 Feb '11

I am trying to create an instance of a class for data types deriving from Enum. When I try to specify this through a context on the instance definition, I get an error. When I do something similar with a function, it seems to work fine. Why does the instance not work? {------------ Example: ---------------} class Test t where func :: t -> Int -- This instance does not compile instance (Enum e) => Test e where func x = fromEnum x -- This function does work newfunc :: (Enum e) => e -> Int newfunc x = fromEnum x Thanks, Kurt

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ANN: unordered-containers - a new, faster hashing-based containers library
by Johan Tibell 23 Feb '11

23 Feb '11

Hi all, I am delighted to announce the release of preview versions of two new packages: unordered-containers 0.1 Efficient hashing-based container types. http://hackage.haskell.org/package/unordered-containers hashable 1.1 Efficient hashing of common types. http://hackage.haskell.org/package/hashable These packages address a need for faster container types in Haskell, especially for string types like ByteString and Text. The package achieves better performance by using hashing and a Patricia trie (also known as a radix tree), instead of a balanced tree, in its implementation. The provided HashMap type is three times faster than Data.Map for lookups and twice as fast for inserts, for all key types I've tried, including ByteString, String, and Int. I'm calling this a preview release, even though the code hash been well tested both for correctness and performance, as the API is quite basic and doesn't provide a HashSet type yet. One thing you'll notice is that the API is split into a value-strict and a value-lazy version, making it easier to use in a strict setting. If you want to contribute, please get copies of the source trees from here: * git clone https://github.com/tibbe/unordered-containers.git * git clone https://github.com/tibbe/hashable.git Alternatively, just fork the projects on GitHub: http://github.com/tibbe As I mentioned in my talk on hashing-based containers earlier this week, I'm working on an even faster version, based on hash-array mapped tries, but it won't be ready until GHC 7.2. Johan

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Re: [Haskell-cafe] Cabal things
by Antoine Latter 23 Feb '11

23 Feb '11

On Wed, Feb 23, 2011 at 8:48 AM, Volker Wysk <pf3(a)volker-wysk.de> wrote: > Hello > > I'm new to Cabal, and I'm trying to Cabalize my library HsShellScript, with > help by Howard Golden. I'm using the Simple Build Infrastructure. > > 1. I'm irritated by the fact, that calling the Cabal with "runhaskell > configure; runhaskell build; runhaskell install" is _not_ sufficent in many > cases. You need additional arguments for "runhaskell configure", and > "runhaskell build" can't build everything, you need other actions in many > cases. I need haddock, and it can't be specified in the .cabal file, or run > by "runhaskell build". > End users installing packages by hand largely use the cabal-install[1] command-line tool, which can be configured to auto-generate documentation for installed packages and put them in a centralized location. It may even do this by default. [1]: http://hackage.haskell.org/trac/hackage/wiki/CabalInstall > > 2. I have some HTML pages of additional documentation, which need to be > installed, and need to be included in the (sdist) generated distribution > package. > > I have no clue how this should be done. > In the package description, you should be able to add an "Extra-Source-Files" section, see: http://www.haskell.org/cabal/release/cabal-latest/doc/users-guide/authors.h… That will get them into the source tarball. I'm not sure what else you need to do to get them installed with the package - there is also a "data-files" filed for files that are needed at run-time. > Cheers, Volker > > _______________________________________________ > Glasgow-haskell-users mailing list > Glasgow-haskell-users(a)haskell.org > http://www.haskell.org/mailman/listinfo/glasgow-haskell-users > >

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