November 2012 - Haskell-Cafe

Re: [Haskell-cafe] GHC for mobile devices?
by Andrew Pennebaker 10 Nov '12

10 Nov '12

Awesome! Jeffrey Scofield has ported OCaml to iOS<http://psellos.com/ocaml/compile-to-iossim.html>, so there's also experience there. On Fri, Nov 9, 2012 at 11:46 PM, Kristopher Micinski <krismicinski(a)gmail.com > wrote: > If you have interest in doing this, I have quite a bit of experience > in Android hacking at the system level and above and would be glad to > talk about what might need to happen. (Though I don't know the GHC > internals / toolchain so well.) > > One potential choice is Scala, though from my limited experience > that's a very rough imitation of the uses for Haskell. (Though, > obviously it works mostly out of the box because of the JVM compiler > target..) > > http://www.haskell.org/haskellwiki/Android > > I've been writing up some thoughts on the Android activity lifecycle > already interpreted with respect to FP, apps are quite functional > already for a variety of reasons. > > kris > > On Fri, Nov 9, 2012 at 8:51 PM, Andrew Pennebaker > <andrew.pennebaker(a)gmail.com> wrote: > > I'd love to use Haskell directly for making mobiles apps. How can we make > > this happen, porting GHC to Android, iOS, and Windows Phone? > > > > -- > > Cheers, > > > > Andrew Pennebaker > > www.yellosoft.us > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > -- Cheers, Andrew Pennebaker www.yellosoft.us

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curl package broken in Windows
by Andrew Pennebaker 10 Nov '12

10 Nov '12

When I do "cabal install curl" in Windows, a missing cygwin dll message pops up, and packages that depend on curl, like shpider, fail to install. Specs: * cabal-install 0.14.0 * Cabal 1.14.0 * GHC 7.4.2 * Haskell Platform 2012 * Windows 7 Proffesional x64 -- Cheers, Andrew Pennebaker www.yellosoft.us

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haskell.org committee: Call for nominations
by Jason Dagit 09 Nov '12

09 Nov '12

Dear Haskellers, The second year of the haskell.org committee is drawing to a close and it is therefore time to seek replacements for those members whose term is expiring. This year one member has a term ending, Ganesh Sittampalam. Ganesh has been an excellent contributor and the rest of the committee would like to thank him for his excellent service. To nominate yourself, please send an e-mail to committee at haskell.org by 30 November 2012. As usual, retiring members are eligible to re-nominate themselves. Please feel free to include any information about yourself that you think will help us to make a decision. Being a member of the committee does not require a significant amount of time, but committee members should aim to be responsive during discussions when the committee is called upon to make a decision. More details about the committee's roles and responsibilities are on http://www.haskell.org/haskellwiki/Haskell.org_committee If you have any questions about the process, please feel free to e-mail us at committee at haskell.org or to contact one of us individually. Regards, Jason On behalf of the haskell.org committee

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Waiting for garbage collection can kill parallelism?
by Janek S. 09 Nov '12

09 Nov '12

Today I was reading "Parallel Performance Tuning for Haskell" by Jones, Marlow and Singh and wanted to replicate the results for their first case study. The code goes like this: module Main where import Control.Parallel main :: IO () main = print . parSumFibEuler 38 $ 5300 parSumFibEuler :: Int -> Int -> Int parSumFibEuler a b = f `par` (e `pseq` (e + f)) where f = fib a e = sumEuler b fib :: Int -> Int fib 0 = 0 fib 1 = 1 fib n = fib (n - 1) + fib (n - 2) mkList :: Int -> [Int] mkList n = [1..n-1] relprime :: Int -> Int -> Bool relprime x y = gcd x y == 1 euler :: Int -> Int euler n = length (filter (relprime n) (mkList n)) sumEuler :: Int -> Int sumEuler = sum . (map euler) . mkList sumFibEuler :: Int -> Int -> Int sumFibEuler a b = fib a + sumEuler b This is the version shown on page 3 of the paper, after adding the pseq combinator to enforce correct evaluation order. I compile and run it with: ghc -O2 -rtsopts -threaded -eventlog parallel.hs ./parallel +RTS -s -ls -N2 In the paper authors show that this version does in fact perform computation in parallel and that good speedup is achieved. However, when I run the code what happens is that HEC 1 blocks very quickly requesting GC. HEC 0 (if I am correct the one calculating sumEuler) does not interrupt but instead continues the computations until they are finished. Then the GC is performed and the HEC 1 resumes computation. In this way there is no parallelism, because first HEC 0 does all its computations and after first GC HEC 1 does its computation. My question is why this might be happening? I don't expect the results of the paper to be fully reproducible, because the paper is 3 years old and GHC has developed a lot since then. This however looks like a regression of some sort. I would appreciate if anyone could explain why this. Janek

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Why doesn't GHC optimize recursive functions by converting them into `let`?
by Petr P 09 Nov '12

09 Nov '12

Hi, recently I found out that some recursive functions can be greatly optimized just by rewriting them using `let` so that they're not recursive themselves any more (only the inner let is). To give an example: > fix :: (a -> a) -> a > fix f = f (fix f) isn't optimized, because it's a recursive function and so it isn't inlined or anything. However, defining it as > fix f = let x = f x in x makes it much more efficient, since `fix` is not recursive any more, so it gets inlined and then optimized for a given argument `f`. (See http://stackoverflow.com/questions/12999385/why-does-ghc-make-fix-so-confou… ) Another example is the well-known generic catamorphism function: > newtype Fix f = Fix { unfix :: f (Fix f) } > > catam :: (Functor f) => (f a -> a) -> (Fix f -> a) > catam f = f . fmap (catam f) . unfix is not optimized. When `catam` is used on a data structure such as [] or a tree, at each level `fmap` creates new constructors that are immediately consumed by `f`. But when written as > catam f = let u = f . fmap u . unfix in u this gets inlined and then optimized, and constructor creation/consumption is fused into very effective code. (See http://stackoverflow.com/q/13099203/1333025) As one comment asked, this seems like something GHC could do automatically. Would it be difficult? Is there a reason against it? I suppose in cases where it doesn't help, the additional `let` would get optimized away, so no harm would be done. And in cases like `fix` or `catam` the gain would be substantial. Best regards, Petr

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cabal-install prompt problem
by Magicloud Magiclouds 07 Nov '12

07 Nov '12

Hi, I am using git version of cabal-install (1.17.0). It was fine until today I got this: 920 $ cabal install yesod Resolving dependencies... cabal: dependencies conflict: ghc-7.6.1 requires an installed instance of directory ==1.2.0.1 however: There is nothing more after the ":". What is happening? -- 竹密岂妨流水过山高哪阻野云飞 And for G+, please use magiclouds#gmail.com.

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Haskell Weekly News: Issue 250
by Daniel Santa Cruz 07 Nov '12

07 Nov '12

Welcome to issue 250 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of October 28 to November 3, 2012. Quotes of the Week * Cale: A list is a bit like a loop which hasn't happened yet. This operation is similar to "how do I make the 5th iteration of my loop do something different from the others?" * broombs: Why are there multiple packages for EitherT? broombs: edwardk: which is the better one? edwardk: broombs: either And today, we have our first quote submited via twitter! Thank you @gersei for sending this one in. If you'd like to submit quotes via twitter, add @HWNQuotes to your tweet. * @kmett:We now generate pretty core but have ugly code. Top Reddit Stories * Cartesian Closed Comic #15: Iteratees Domain: ro-che.info, Score: 75, Comments: 2 On Reddit: [1] http://goo.gl/9NpD2 Original: [2] http://goo.gl/SB7HZ * Are there any, or is there interest in starting, an online Haskell course like the Scala course on Coursera? Domain: class.coursera.org, Score: 69, Comments: 16 On Reddit: [3] http://goo.gl/UzBdF Original: [4] http://goo.gl/1Zzct * Why is this simple text processing program so much slower than in dynamic languages? Domain: honza.ca, Score: 65, Comments: 189 On Reddit: [5] http://goo.gl/jW5ow Original: [6] http://goo.gl/pPzIb * Fast Code Nation: The Bright Side of High-Level Languages Domain: bos.github.com, Score: 59, Comments: 11 On Reddit: [7] http://goo.gl/KmBRt Original: [8] http://goo.gl/jm9s7 * Fay slides Domain: fay-lang.org, Score: 56, Comments: 42 On Reddit: [9] http://goo.gl/0UM4T Original: [10] http://goo.gl/h8iZE * Data.Records: extensible records using the latest GHC features Domain: github.com, Score: 55, Comments: 44 On Reddit: [11] http://goo.gl/64wUj Original: [12] http://goo.gl/XVdB2 * Why Cabal Has Problems Domain: softwaresimply.blogspot.com, Score: 50, Comments: 41 On Reddit: [13] http://goo.gl/ZX1Ia Original: [14] http://goo.gl/TgnOG * The cabal/hackage situation, and what you can do about it Domain: alpmestan.com, Score: 49, Comments: 14 On Reddit: [15] http://goo.gl/jEGIj Original: [16] http://goo.gl/EK2DB * Brian O'Sullivan's favorite bug in GHC Domain: twitter.com, Score: 48, Comments: 12 On Reddit: [17] http://goo.gl/nMjeK Original: [18] http://goo.gl/odPH4 * Haskell for all: "Hello, core!" Domain: haskellforall.com, Score: 45, Comments: 6 On Reddit: [19] http://goo.gl/qXRd2 Original: [20] http://goo.gl/9cBeo * Michael Snoyman: Yesod, AngularJS and Fay Domain: yesodweb.com, Score: 43, Comments: 26 On Reddit: [21] http://goo.gl/8yfVd Original: [22] http://goo.gl/0xtPP * Cellular automata as comonads Domain: blog.sigfpe.com, Score: 41, Comments: 10 On Reddit: [23] http://goo.gl/73nr2 Original: [24] http://goo.gl/vH4TK * pipes-2.5: Faster and slimmer Domain: haskellforall.com, Score: 41, Comments: 56 On Reddit: [25] http://goo.gl/wtUZg Original: [26] http://goo.gl/rKR2D Top StackOverflow Questions * Writing cojoin or cobind for n-dimensional grid type votes: 41, answers: 3 Read on SO: [27] http://goo.gl/qbz7z * Why not be dependently typed? votes: 39, answers: 3 Read on SO: [28] http://goo.gl/1TSVh * Concrete example showing that monads are not closed under composition (with proof)? votes: 32, answers: 4 Read on SO: [29] http://goo.gl/sys7F * Monad Transformers vs Passing parameters to functions votes: 24, answers: 1 Read on SO: [30] http://goo.gl/UpU57 * Why is there no IO transformer in Haskell? votes: 16, answers: 2 Read on SO: [31] http://goo.gl/04Xc7 * Is it possible to make GHC optimize (deforest) generic functions such as catamorphisms? votes: 16, answers: 1 Read on SO: [32] http://goo.gl/f4ZlU * Is it possible to use irrefutable tilde (`~`) patterns in lambda expressions? votes: 14, answers: 1 Read on SO: [33] http://goo.gl/BmGY0 * Can Scala's Cake Pattern be implemented in Haskell? votes: 13, answers: 2 Read on SO: [34] http://goo.gl/QiMfg * Confused by the meaning of the 'Alternative' type class and its relationship to other type classes votes: 13, answers: 5 Read on SO: [35] http://goo.gl/RXNuV * How to add fields that only cache something to ADT? votes: 13, answers: 0 Read on SO: [36] http://goo.gl/FjLlX * Type algebra and Knuth's up arrow notation votes: 13, answers: 1 Read on SO: [37] http://goo.gl/kyYIn Until next time, Daniel Santa Cruz References 1. http://ro-che.info/ccc/15.html 2. http://www.reddit.com/r/haskell/comments/11t7ru/cartesian_closed_comic_15_i… 3. https://class.coursera.org/course/progfun 4. http://www.reddit.com/r/haskell/comments/12865b/are_there_any_or_is_there_i… 5. http://honza.ca/2012/10/haskell-strings 6. http://www.reddit.com/r/haskell/comments/120h6i/why_is_this_simple_text_pro… 7. http://bos.github.com/reaktor-dev-day-2012/reaktor-talk-slides.html 8. http://www.reddit.com/r/haskell/comments/11sl47/fast_code_nation_the_bright… 9. http://fay-lang.org/fay-talk/ 10. http://www.reddit.com/r/haskell/comments/11yrpi/fay_slides/ 11. https://github.com/jonsterling/Data.Records 12. http://www.reddit.com/r/haskell/comments/12835b/datarecords_extensible_reco… 13. http://softwaresimply.blogspot.com/2012/11/why-cabal-has-problems.html 14. http://www.reddit.com/r/haskell/comments/12hbzb/why_cabal_has_problems/ 15. http://alpmestan.com/posts/2012-11-02-cabal-hackage-what-you-can-do-about-i… 16. http://www.reddit.com/r/haskell/comments/12ijs7/the_cabalhackage_situation_… 17. https://twitter.com/bos31337/status/116372971509121025 18. http://www.reddit.com/r/haskell/comments/127emo/brian_osullivans_favorite_b… 19. http://www.haskellforall.com/2012/10/hello-core.html 20. http://www.reddit.com/r/haskell/comments/11vuzz/haskell_for_all_hello_core/ 21. http://www.yesodweb.com/blog/2012/10/yesod-fay-js 22. http://www.reddit.com/r/haskell/comments/12ct6p/michael_snoyman_yesod_angul… 23. http://blog.sigfpe.com/2006/12/evaluating-cellular-automata-is.html 24. http://www.reddit.com/r/haskell/comments/11o6nu/cellular_automata_as_comona… 25. http://www.haskellforall.com/2012/10/pipes-25-faster-and-slimmer.html 26. http://www.reddit.com/r/haskell/comments/12gikh/pipes25_faster_and_slimmer/ 27. http://stackoverflow.com/questions/12963733/writing-cojoin-or-cobind-for-n-… 28. http://stackoverflow.com/questions/12961651/why-not-be-dependently-typed 29. http://stackoverflow.com/questions/13034229/concrete-example-showing-that-m… 30. http://stackoverflow.com/questions/12968351/monad-transformers-vs-passing-p… 31. http://stackoverflow.com/questions/13056663/why-is-there-no-io-transformer-… 32. http://stackoverflow.com/questions/13099203/is-it-possible-to-make-ghc-opti… 33. http://stackoverflow.com/questions/13185632/is-it-possible-to-use-irrefutab… 34. http://stackoverflow.com/questions/12947176/can-scalas-cake-pattern-be-impl… 35. http://stackoverflow.com/questions/13080606/confused-by-the-meaning-of-the-… 36. http://stackoverflow.com/questions/13098448/how-to-add-fields-that-only-cac… 37. http://stackoverflow.com/questions/13170803/type-algebra-and-knuths-up-arro…

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Compiling with hsc2hs and -Wextra
by Niklas Hambüchen 07 Nov '12

07 Nov '12

Hi, when I compile my C binding using hsc2hs with the -Wextra enabled in my cabal file: cc-options: -std=c99 -Wall -Wextra -Werror This breaks the compilation started by hsc2hs: MyHsc.hsc: In function ‘main’: MyHsc.hsc:16:15: error: unused parameter ‘argc’ [-Werror=unused-parameter] MyHsc.hsc:16:27: error: unused parameter ‘argv’ [-Werror=unused-parameter] cc1: all warnings being treated as errors compiling dist/build/MyHsc_hsc_make.c failed (exit code 1) I would really like to compile my code with -Wextra -Werror though. Can't we just (void) argc; (void) argv; the arguments in the generated code, like described in [1]? Thanks Niklas [1] http://stackoverflow.com/questions/3599160/unused-parameter-warnings-in-c-c…

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Where is the documentation on exception types kept?
by timothyhobbs＠seznam.cz 07 Nov '12

07 Nov '12

Hello, I am trying to catch an "thread blocked on MVar indefinitely exception." Of course I can use ::SomeException as explained in http://hackage.haskell.org/ packages/archive/base/latest/doc/html/Control-Exception.html#g:3 but there is no explanation as to how to find the more case specific exceptions. http://hackage.haskell.org/packages/archive/base/latest/doc/html/Control- Concurrent-MVar.html#t:MVar does not discuss exceptions at all! Indeed, I don't know of any haddock pages that include descriptions of exception types :( :O Thanks, Timothy

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Cross platform method for getting the /actual/ contents of a directory
by timothyhobbs＠seznam.cz 07 Nov '12

07 Nov '12

Hello, I'm writing a function to find out if a given directory is empty. I came up with the obvious: do{ contents <- getDirectoryContents "/home/timothy/works/current/anonGraph/ empty/" ; let realContents = filter (\file -> case file of "." -> False ; ".." -> False ; _ -> True) contents in return $ case realContents of [] -> True ; _ -> False} And this works fine on linux, but I wonder. On other systems, is "." and ". ." allowed as file names? Couldn't a windows user actually end up with a file name named "." and this method would fail? Thank you Timothy

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