- Haskell-Cafe - Haskell.org

Poll: Do you want a mascot?
by heathmatlock 01 Dec '11

01 Dec '11

Question: Do you want a mascot? Answers: Yes No -- This is an attempt to figure out if this idea is going anywhere.

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Ann: Netwire 3.0.0
by Ertugrul Söylemez 01 Dec '11

01 Dec '11

Hello there, Netwire 3.0.0 is out: <http://hackage.haskell.org/package/netwire> Overview -------- Netwire is a library for high performance functional reactive programming, which calls its signal functions 'wires'. It solves the classic problems of FRP through a number of simple concepts: * It is based on an automaton arrow (similar to Yampa), so performance problems like time leaks become local. Getting high FRP performance is the art of writing good primitive automata, which is simple enough. * Wire inhibition (ArrowZero), combination (ArrowPlus) and choice (ArrowChoice) make switching combinators entirely obsolete. * Events are based on inhibition, so non-happened events take no CPU time. * Very simple internal representation: Writing custom primitive wires is easy. Since Netwire is developed mainly with libraries in mind, its internals are exposed entirely. If you need a wire that is not predefined, just write an addon. * Finally Netwire comes with a large set of predefined wires. This new version comes along with a major API change (hence the new major version number) -- hopefully the last one. The main change is that the Wire type is now a data family. This gives the flexibility of Netwire 2 with the high performance of Netwire 1. We are back to 600k FPS now for moderately complicated wires. =) Changes ------- Major changes and new features include: * Wire is now a data family. Most wire features are now addons through type classes, and every arrow can provide its own version of them, which is optimized for their respective internal representation. Mainly you get back the high performance monad-based wires from Netwire 1. * Nondeterministic wires (see below). * New wire transformers for supplying a clock, concurrent wires, and memoization/caching. * Improved ArrowLoop instance: Inhibition is now allowed inside of 'rec', as long as it doesn't break data dependencies. If it does, you get a useful error message now instead of a fatal pattern match failure. Minor changes: * Measuring FPS without disturbing performance: avgFpsInt. * The 'execute' wire is back. * More convenient wire stepping and testing functions. * Often you don't care about an event's value: gotEvent. Nondeterministic wires ---------------------- Now that Wire is a data family, you can have efficient nondeterministic wires by using a Kleisli-wrapped logic monad as the base arrow: import Control.Monad.Logic type MyWire = Wire e (Kleisli (LogicT IO)) Now you can use branching in a MyWire computation: x <- swallow branch -< [1..100] This will split the wire session into 101 branches at the first instant (it will always branch into a single inhibiting wire). Suppose you have an HTTP bot. You can now read URLs from a file and branch into an own wire for each URL: url <- swallow (branch <<< execute) -< fmap lines (readFile "urls.txt") quitWith <<< myBot -< url Performance ----------- Netwire 3 easily outperforms Netwire 2 by a factor of 10-30. Usually you will work with wire arrows on top of a Kleisli arrow. Now that Wire is a data family you can have an efficient representation of that, which gets along entirely without the Kleisli constructor wrapping. Now the main bottleneck is the monadic actions performed. This is very satisfactory. Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/

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List fusion of nested lists
by Joachim Breitner 01 Dec '11

01 Dec '11

Dear Cafe, I’m trying to exploit list fusion as provided by GHC (build/foldr). One function that I want to get fusable is this, it splits a list of integeres into maximal monotonous subsequences: streaks :: [Integer] -> [[Integer]] streaks [] = [] streaks (x:xs) = let (this,rest) = oneStreak (x:xs) in this:streaks rest oneStreak :: [Integer] -> ([Integer], [Integer]) oneStreak [x] = ([x],[]) oneStreak l@(x:y:_) = splitWhile2 (\a b -> a `compare` b == x `compare` y) l splitWhile2 :: (Integer -> Integer -> Bool) -> [Integer] -> ([Integer], [Integer]) splitWhile2 p [x] = ([x],[]) splitWhile2 p (x:y:xs) | p x y = let (s,r) = splitWhile2 p (y:xs) in (x:s,r) | otherwise = ([x],y:xs) Now I’d like to implement streaks in terms of build and foldr such that it is subject to list fusion. Especially, when used in concatMap (streaks . func) :: [X] -> [[Integer]] where func :: X -> [Integer] is implemented with buildr, this should ideally remove all intermediate lists. Can this be done with list fusion at all? How would I go about it? If the above example is too complicated, known how it would work for Data.List.group would help me already a lot. Greetings, Joachim -- Joachim "nomeata" Breitner mail(a)joachim-breitner.de | nomeata(a)debian.org | GPG: 0x4743206C xmpp: nomeata(a)joachim-breitner.de | http://www.joachim-breitner.de/

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ANNOUNCE: data-timeout - 64-bit timeouts of nanosecond precision
by Mikhail Vorozhtsov 01 Dec '11

01 Dec '11

Hi. I grew up tired of counting milliseconds, so I wrote a small library[1] that allows me to specify time units for timeouts and convert between them. The library also provides wrapped versions of 'timeout' and 'threadDelay' functions: > threadDelay $ 1 # Minute + 30 # Second Nanosecond precision seems to be enough for RTS and POSIX calls and it also provides a good range for 64-bit representation: > maxBound :: Timeout 30500 w 3 d 23 h 34 m 33 s 709 ms 551 us 615 ns One thing that might disappoint some people is that I chose unsigned underlying type (Word64), which means that "infinite" timeouts cannot be represented. I think "negative" timeouts essentially are performance warts (a way of packing `Maybe Word63` into Word64) that muddle equality and I recommend using `Maybe Timeout` whenever timeout is optional. [1] http://hackage.haskell.org/package/data-timeout

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Munich December Haskell Meeting
by Heinrich Hördegen 01 Dec '11

01 Dec '11

Dear all, on our last meeting, we decided to set up a doodle poll to determine the date of our December's get-together: http://www.doodle.com/x6x4sgnemc35myq3 If you want to join, vote! There are also new dates available for next year's meetings (with rotating week-days): http://www.haskell-munich.de/dates Have a nice December, Heinrich -- -- hoerdegen(a)funktional.info www.funktional.info --

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Using Data,TypeLevel.Num
by Richard O'Keefe 01 Dec '11

01 Dec '11

I'd like to write a small module using Data.TypeLevel.Num. It has type level constants, variables, addition, and maximum, which is pretty much all I need. But I've never used it before, and there's one thing I want to do that I don't understand how to do. val :: Nat t => t -> Int val _ = t as an Int e.g., if x :: D8 then val x = 8. The "value-level reflection functions" in Data.TypeLevel.Num.Ops all seem to be 'undefined'.

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Haskell Weekly News: Issue 209
by Daniel Santa Cruz 01 Dec '11

01 Dec '11

Welcome to issue 209 of the HWN, a newsletter covering developments in the Haskell community. This release covers two weeks, from November 13 to 26, 2011. You can find the HTML version at: http://contemplatecode.blogspot.com/2011/11/haskell-weekly-news-issue-209.h… Announcements By now I'm sure you've heard about Da the lamb. [1] http://goo.gl/WF2o7 Sean Leather annouced that the next Dutch functinal programming day will take place on January 6, 2012, at the university campus "De Uithof" of Utrecht University. [2] http://goo.gl/IfVK8 Ganesh Sittampalam announced that the haskell.org committee for 2011-12 has been formed. [3] http://goo.gl/YFFsa New and Updated Projects * Gtk2Hs (Axel Simon; 0.12.2) Adjustments to Gtk2Hs to work with GHC 7.x.x. Also, looking for a part-time maintainer. [4] http://goo.gl/52M56 * iteratee-compress (Maciej Marcin Piechotka; 0.3.0.0) Provides compressing and decompressing enumerators. [5] http://goo.gl/tRAkg * Netwire (Ertugrul Soeylemez; 2.0.0) Complete rewrite. [6] http://goo.gl/HMCG8 * SBV (Levent Erkok; 0.9.22) [7] http://goo.gl/pSXGb * graphviz (Ivan Lazar Miljenovic; 2999.12.0.4). [8] http://goo.gl/JNVSV * SmallCHeck (Roman Cheplyaka; 0.5) [9] http://goo.gl/8njoh * OpenCL (Luis Cabellos; 1.0.2.2) Better error handling. [10] http://goo.gl/fphJN * clientsession (Michael Snoyman; 0.7.3.4) Major performance enhancement. [11] http://goo.gl/hCqrf Quotes of the Week * dmwit: Let's not make things better, I haven't put enough lipstick on the worse version yet. * mike-burns: Oh good, a semantic argument. Goodnight. * companion_cube: why bother with complicated abstractions like monads, when you can enjoy the taste of the sun on your skin? * applicative: clearly the trouble with haskell is plain: no inbuild syntactic support for quaternions. Wonder if *fortress* has it. * elliott: "with a lot of unicode" is like agda's @faq. "yes, agda can do that with a lot of unicode!" * [shachaf] mauke: EBCDIC? [mauke] shachaf: ah, the data encryption standard invented by IBM? * Tiwazz: It's called "Berp" because that's the sound Haskell makes after it devours a live Python. Top Reddit Stories * Typeclassopedia moved to the Haskell wiki Domain: byorgey.wordpress.com, Score: 72, Comments: 8 On Reddit: [12] http://goo.gl/pbHqr Original: [13] http://goo.gl/tWM0P * New project to support SIMD vector instructions in GHC (x86 SSE & AVX, ARM's NEON etc) Domain: hackage.haskell.org, Score: 62, Comments: 18 On Reddit: [14] http://goo.gl/JRj3o Original: [15] http://goo.gl/OcFCG * GHC 7.4 branched, what's in for Christmas? Domain: haskell.org, Score: 54, Comments: 16 On Reddit: [16] http://goo.gl/ZUN1e Original: [17] http://goo.gl/qWIhq * How to read Haskell like Python Domain: blog.ezyang.com, Score: 52, Comments: 7 On Reddit: [18] http://goo.gl/3cVXY Original: [19] http://goo.gl/nNhDI * xmonad-0.10 released! Domain: hackage.haskell.org, Score: 51, Comments: 15 On Reddit: [20] http://goo.gl/U5rry Original: [21] http://goo.gl/SIFuB * Roy - language with Hindley-Milner type system and monads that compiles to javascript Domain: roy.brianmckenna.org, Score: 43, Comments: 9 On Reddit: [22] http://goo.gl/RLK8S Original: [23] http://goo.gl/dzx8N * The Universe of Discourse : Triples and Closure and Monads Domain: blog.plover.com, Score: 42, Comments: 13 On Reddit: [24] http://goo.gl/7EDJX Original: [25] http://goo.gl/iPLnN * ANN: happstack-lite. A simpler, friendlier version of the Happstack Haskell Web Framework. What do you think? Domain: happstack.blogspot.com, Score: 35, Comments: 12 On Reddit: [26] http://goo.gl/Ii6Ch Original: [27] http://goo.gl/5wSZJ * HN/SPL - a better way to do your boring C++ job (written in Haskell) Domain: code.google.com, Score: 35, Comments: 16 On Reddit: [28] http://goo.gl/VxqIN Original: [29] http://goo.gl/9aAeZ * Fast lookup tables in haskell Domain: tab.snarc.org, Score: 34, Comments: 10 On Reddit: [30] http://goo.gl/mzgyw Original: [31] http://goo.gl/4VnrX * What Yesod Got Right Domain: yesodweb.com, Score: 34, Comments: 12 On Reddit: [32] http://goo.gl/eZhhK Original: [33] http://goo.gl/Li4mY Top StackOverflow Questions * What is a monad in FP, in categorical terms? votes: 30, answers: 7 Read on SO: [34] http://goo.gl/sC7PD * Haskell vs JVM performance [closed] votes: 24, answers: 6 Read on SO: [35] http://goo.gl/olA8g * Are there any Haskell techniques for mixed (part structure, part unmodified text) parsing & rewriting? votes: 24, answers: 1 Read on SO: [36] http://goo.gl/jRydd * STArray documentation for newbies and State/ST related questions votes: 17, answers: 1 Read on SO: [37] http://goo.gl/nw8eH * Should I avoid using Monad fail? votes: 15, answers: 3 Read on SO: [38] http://goo.gl/ruh7s * Effects of monomorphism restriction on type class constraints votes: 15, answers: 2 Read on SO: [39] http://goo.gl/QRbTh * Folding, function composition, monads, and laziness, oh my? votes: 14, answers: 2 Read on SO: [40] http://goo.gl/gped5 * Type error when ascribing a valid forall type to a let-bound variable votes: 13, answers: 2 Read on SO: [41] http://goo.gl/oyx0a * How do I organize my pure functions with my monadic actions idiomatically votes: 13, answers: 3 Read on SO: [42] http://goo.gl/6tByj * Is foldl ever preferable to its strict cousin, foldl'? votes: 13, answers: 2 Read on SO: [43] http://goo.gl/jFIe4 Until next time, Daniel Santa Cruz References 1. http://comments.gmane.org/gmane.comp.lang.haskell.cafe/93887 2. http://comments.gmane.org/gmane.comp.lang.haskell.general/19025 3. http://comments.gmane.org/gmane.comp.lang.haskell.general/19031 4. http://comments.gmane.org/gmane.comp.lang.haskell.general/19019 5. http://comments.gmane.org/gmane.comp.lang.haskell.libraries/16591 6. http://comments.gmane.org/gmane.comp.lang.haskell.cafe/93860 7. http://comments.gmane.org/gmane.comp.lang.haskell.cafe/93866 8. http://comments.gmane.org/gmane.comp.lang.haskell.cafe/93956 9. http://comments.gmane.org/gmane.comp.lang.haskell.cafe/94014 10. http://comments.gmane.org/gmane.comp.lang.haskell.cafe/94103 11. http://comments.gmane.org/gmane.comp.lang.haskell.yesod/941 12. http://byorgey.wordpress.com/2011/11/26/typeclassopedia-v2-moving-my-effort… 13. http://www.reddit.com/r/haskell/comments/mq2nr/typeclassopedia_moved_to_the… 14. http://hackage.haskell.org/trac/ghc/wiki/SIMD 15. http://www.reddit.com/r/haskell/comments/mbpoi/new_project_to_support_simd_… 16. http://www.haskell.org/pipermail/cvs-ghc/2011-November/068352.html 17. http://www.reddit.com/r/haskell/comments/mm68o/ghc_74_branched_whats_in_for… 18. http://blog.ezyang.com/2011/11/how-to-read-haskell/ 19. http://www.reddit.com/r/haskell/comments/mc2mw/how_to_read_haskell_like_pyt… 20. http://hackage.haskell.org/package/xmonad-0.10 21. http://www.reddit.com/r/haskell/comments/mhzu8/xmonad010_released/ 22. http://roy.brianmckenna.org/ 23. http://www.reddit.com/r/haskell/comments/mp245/roy_language_with_hindleymil… 24. http://blog.plover.com/math/monad-closure.html 25. http://www.reddit.com/r/haskell/comments/mfsfz/the_universe_of_discourse_tr… 26. http://happstack.blogspot.com/2011/11/ann-happstack-lite-friendly.html 27. http://www.reddit.com/r/haskell/comments/mcjoz/ann_happstacklite_a_simpler_… 28. http://code.google.com/p/inv/ 29. http://www.reddit.com/r/haskell/comments/mldzq/hnspl_a_better_way_to_do_you… 30. http://tab.snarc.org/posts/haskell/2011-11-15-lookup-tables.html 31. http://www.reddit.com/r/haskell/comments/mh1ox/fast_lookup_tables_in_haskel… 32. http://www.yesodweb.com/blog/2011/11/got-right 33. http://www.reddit.com/r/haskell/comments/mi4r7/what_yesod_got_right/ 34. http://stackoverflow.com/questions/8221395/what-is-a-monad-in-fp-in-categor… 35. http://stackoverflow.com/questions/8185282/haskell-vs-jvm-performance 36. http://stackoverflow.com/questions/8198808/are-there-any-haskell-techniques… 37. http://stackoverflow.com/questions/8197032/starray-documentation-for-newbie… 38. http://stackoverflow.com/questions/8163852/should-i-avoid-using-monad-fail 39. http://stackoverflow.com/questions/8270548/effects-of-monomorphism-restrict… 40. http://stackoverflow.com/questions/8249958/folding-function-composition-mon… 41. http://stackoverflow.com/questions/8190431/type-error-when-ascribing-a-vali… 42. http://stackoverflow.com/questions/8234162/how-do-i-organize-my-pure-functi… 43. http://stackoverflow.com/questions/8235797/is-foldl-ever-preferable-to-its-…

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Haskell Platform 64 bit
by Philippe Sismondi 30 Nov '11

30 Nov '11

I just tried to install the Haskell Platform 64-bit on OS X Snow Leopard 10.6.8. The install fails with "an error". This is all I see in /var/log/install.log: 11-11-25 6:22:01 PM Installer[53992] The Installer encountered an error that caused the installation to fail. Contact the software manufacturer for assistance. Is it known to be broken? I find something in haskell-cafe archives from last spring about a such a problem, but I don't know where to look for anything more recent. I may be blind, but I cannot see anything in the tickets under "known problems". I have been successfully running the 32-bit version for quite a while, but have begun to run into problems with linking haskell stuff against various 64-bit macports libraries. - Phil -

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Re: [Haskell-cafe] containers-deepseq?
by John Lato 29 Nov '11

29 Nov '11

> Message: 22 > Date: Tue, 29 Nov 2011 12:18:41 +0200 > From: Michael Snoyman <michael(a)snoyman.com> > Subject: Re: [Haskell-cafe] containers-deepseq? > > By the way, this is the part of situation that cabal-src is coming to > solve, and it *is* solved... except that the result is a lot of > re-compiling. How is cabal-src different from cabal-dev? John

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minor refactoring problem
by Martin DeMello 29 Nov '11

29 Nov '11

I have the following functions: makePair :: (String, String) -> IO PairBox parseFile :: String -> [(String, String)] importFile :: Editor -> String -> IO () importFile ed path = do s <- readFile path ps <- mapM (\x -> makePair (x, "")) (lines s) es <- return $ V.fromList ps writeIORef ed es loadFile :: Editor -> String -> IO () loadFile ed path = do s <- readFile path ps <- mapM makePair (parseFile s) es <- return $ V.fromList ps writeIORef ed es The problem is that loadFile and importFile are so similar it seems a shame not to combine them somehow, but anything I can think of doing leaves the code looking more rather than less messy. Any nice ideas? martin

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