October 2014 - Haskell-Cafe

Timing execution of individual functions in ghci?
by Magnus Therning 24 Oct '14

24 Oct '14

When playing around with various implementations of a single function it would be nice to time the execution. Is there some way to do that easily from within GHCi? Or am I forced to look to criterion for that? /M -- Magnus Therning OpenPGP: 0xAB4DFBA4 email: magnus(a)therning.org jabber: magnus(a)therning.org twitter: magthe http://therning.org/magnus Heuristic is an algorithm in a clown suit. It’s less predictable, it’s more fun, and it comes without a 30-day, money-back guarantee. -- Steve McConnell, Code Complete

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pattern matches are overlapped
by briand＠aracnet.com 23 Oct '14

23 Oct '14

This works fine keyboardAct a d p (SpecialKey KeyLeft) Down Modifiers {shift=Down, ctrl=Up, alt=Up} = do (x,y) <- get p p $= (x-0.1, y) This keyboardAct a d p (SpecialKey KeyLeft) Down shiftDown = do (x,y) <- get p p $= (x-0.1, y) where, shiftDown = Modifiers {shift=Down, ctrl=Up, alt=Up} gives me : KeyBindings.hs:32:1: Warning: Pattern match(es) are overlapped In an equation for `keyboardAct': keyboardAct angle delta p (SpecialKey KeyLeft) Down Modifiers {shift = Up, ctrl = Up, alt = Up} = ... keyboardAct is used several times via pattern matching, e.g. keyboardAct angle delta p (SpecialKey KeyLeft) Down Modifiers {shift=Up, ctrl=Up, alt=Up} = do (x,y) <- get p a <- get angle d <- get delta angle $= a+d and there are several more. So clearly, the pattern match is overlapped, just like the warning says. The question is why I don't get the same warning before I make use of shiftDown. Thanks Brian

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acid-state problem.
by Magicloud Magiclouds 23 Oct '14

23 Oct '14

Hi, I have following code. In init, I called update, createCheckpoint, and close. So every time the code runs, if the DB did not exist, it would ask some configurations and create one. But actually it asks for init every time. How should I use it? state <- openLocalStateFrom dbFolder $ State ConfNone conf <- query state QueryState case conf of ConfNone -> init state m c -> process state m c -- 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 310
by Daniel Santa Cruz 23 Oct '14

23 Oct '14

Welcome to issue 310 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers from October 5 to 18, 2014 Quotes of the Week * spopejoy: That [Spineless Tagless G Machine] will always sound like the villian in an upcoming Ghostbusters sequel to me :) * pigworker: When you can make data out of potatoes, why would you want to encode them as functions? * fishcorn: When someone mentions lens on freenode... [1]zeus-lens! * neelk: So constructively we know this style can be used to meet hard performance deadlines in domains where the penalty for failure is literally radioactive flaming death. * edwardkmett: At one point we had a type in lens where one of its arguments started taking parameters of the form i m a s t a b u. Upon reflection, we let it win the argument and decided not to implement the function. * Ivan Appel: ... while the popular opinion says that the main reason to pick Haskell is "to have fun hacking," in fact it's not so fun to hack in Haskell because there's not such much hacking to do at the first place. For instance, you don't need to spend lots of effort tracing and fixing obscure bugs, so if your definition of "hacking" includes heroic bugfixing, you'd be better off with JavaScript or Perl. Top Reddit Stories * From 60 Frames per Second to 500 in Haskell Domain: keera.co.uk, Score: 116, Comments: 82 Original: [2] http://goo.gl/tiA202 On Reddit: [3] http://goo.gl/sl8TTl * Note: Erik Meijer's MOOC - "Introduction to Functional Programming" starts next Wednesday Domain: edx.org, Score: 109, Comments: 34 Original: [4] http://goo.gl/QNXb5M On Reddit: [5] http://goo.gl/sVfmE4 * The New Haskell.org Domain: blog.haskell.org, Score: 98, Comments: 72 Original: [6] http://goo.gl/TbBGrW On Reddit: [7] http://goo.gl/kdaLFq * New Haskell Book: Thinking Functionally with Haskell Domain: amazon.com, Score: 95, Comments: 39 Original: [8] http://goo.gl/oj35Ox On Reddit: [9] http://goo.gl/rYKq6c * Some notes on reimplementing a NodeJS service in Haskell Domain: gist.github.com, Score: 67, Comments: 5 Original: [10] http://goo.gl/ZoSy2a On Reddit: [11] http://goo.gl/kQ6mOs * Release of threepenny-gui 0.5.0.0: easy UI in Haskell! Domain: apfelmus.nfshost.com, Score: 65, Comments: 1 Original: [12] http://goo.gl/NClc0o On Reddit: [13] http://goo.gl/qDos1R * Bake: a Continuous Integration System Domain: neilmitchell.blogspot.fr, Score: 64, Comments: 34 Original: [14] http://goo.gl/u5bznA On Reddit: [15] http://goo.gl/7oC0N8 * On concerns about Haskell's Prelude favoring Foldable/Traversable Domain: yesodweb.com, Score: 62, Comments: 101 Original: [16] http://goo.gl/yJ7yMJ On Reddit: [17] http://goo.gl/M1M6we * Making GHCi awesomer? Domain: haskell.org, Score: 57, Comments: 3 Original: [18] http://goo.gl/KUkSQ7 On Reddit: [19] http://goo.gl/GRxzb6 * lens over tea, part 1: lenses 101, traversals 101, and some implementation details Domain: artyom.me, Score: 56, Comments: 25 Original: [20] http://goo.gl/rH0tdJ On Reddit: [21] http://goo.gl/jgsBeN * A neat trick for GHCi Domain: mega-nerd.com, Score: 54, Comments: 12 Original: [22] http://goo.gl/o9yrqJ On Reddit: [23] http://goo.gl/oB8Nhb * Shaking up GHC Domain: blogs.ncl.ac.uk, Score: 53, Comments: 21 Original: [24] http://goo.gl/jkRnPn On Reddit: [25] http://goo.gl/byWnxV * FRP Zoo: the TodoMVC of FRP libraries Domain: github.com, Score: 53, Comments: 12 Original: [26] http://goo.gl/JyKRtL On Reddit: [27] http://goo.gl/wJV4Vt * [Video] Simon Peyton Jones - Zero-Cost Coercions in Haskell at Haskell eXchange Domain: skillsmatter.com, Score: 50, Comments: 46 Original: [28] http://goo.gl/yXp43E On Reddit: [29] http://goo.gl/l0mo5O * Bryan O'Sullivan - Performance Measurement and Optimization in Haskell [video] Domain: skillsmatter.com, Score: 49, Comments: 23 Original: [30] http://goo.gl/HO4Sjy On Reddit: [31] http://goo.gl/4sqGgX * How to Rewrite the Prelude Domain: neilmitchell.blogspot.it, Score: 42, Comments: 81 Original: [32] http://goo.gl/PnnoO2 On Reddit: [33] http://goo.gl/EP9CaL * Generalizing function composition Domain: jaspervdj.be, Score: 42, Comments: 31 Original: [34] http://goo.gl/vCfmqT On Reddit: [35] http://goo.gl/GXb4gt * C structures in Haskell FFI Domain: ghc.haskell.org, Score: 40, Comments: 19 Original: [36] http://goo.gl/OT7UFH On Reddit: [37] http://goo.gl/uKJWCl * Yi 0.10, Anniversary Edition Domain: haskell.org, Score: 38, Comments: 16 Original: [38] http://goo.gl/T58nN6 On Reddit: [39] http://goo.gl/Fz6NQo * Curry-Howard, the Ontological Ultimate Domain: psnively.github.io, Score: 38, Comments: 28 Original: [40] http://goo.gl/HpPhXz On Reddit: [41] http://goo.gl/LlYUiM Top StackOverflow Questions * Is it possible to directly invoke Haskell code from Bash and output to stdout? votes: 17, answers: 3 Read on SO: [42] http://goo.gl/jkG0vv * Why does foldr use a helper function? votes: 16, answers: 4 Read on SO: [43] http://goo.gl/hsmIsF * Why do hGetBuf, hPutBuf, etc. allocate memory? votes: 14, answers: 2 Read on SO: [44] http://goo.gl/G40NfW * Is it possible to get the Kind of a Type Constructor in Haskell? votes: 10, answers: 1 Read on SO: [45] http://goo.gl/Pajd4R * Printing out Haskell's evaluation (rewriting) steps for educational/learning purposes. Is it possible? votes: 9, answers: 3 Read on SO: [46] http://goo.gl/Wlfeol Until next time, [47]+Daniel Santa Cruz References 1. https://s3.amazonaws.com/fishcorn/zeus-lens.gif 2. http://keera.co.uk/blog/2014/10/15/from-60-fps-to-500/ 3. http://www.reddit.com/r/haskell/comments/2jbl78/from_60_frames_per_second_t… 4. https://www.edx.org/course/delftx/delftx-fp101x-introduction-functional-212… 5. http://www.reddit.com/r/haskell/comments/2ixpcf/note_erik_meijers_mooc_intr… 6. https://blog.haskell.org/post/the_new_haskell_org/ 7. http://www.reddit.com/r/haskell/comments/2j4sim/the_new_haskellorg/ 8. http://www.amazon.com/Thinking-Functionally-Haskell-Richard-Bird/dp/1107087… 9. http://www.reddit.com/r/haskell/comments/2jele8/new_haskell_book_thinking_f… 10. https://gist.github.com/paf31/9c4d402d400d61a49656 11. http://www.reddit.com/r/haskell/comments/2j8lhd/some_notes_on_reimplementin… 12. http://apfelmus.nfshost.com/blog/2014/10/04-threepenny-gui-0-5.html 13. http://www.reddit.com/r/haskell/comments/2idep2/release_of_threepennygui_05… 14. http://neilmitchell.blogspot.fr/2014/10/bake-continuous-integration-system.… 15. http://www.reddit.com/r/haskell/comments/2in244/bake_a_continuous_integrati… 16. http://www.yesodweb.com/blog/2014/10/classy-base-prelude 17. http://www.reddit.com/r/haskell/comments/2if0fu/on_concerns_about_haskells_… 18. http://www.haskell.org/pipermail/ghc-devs/2014-October/006771.html 19. http://www.reddit.com/r/haskell/comments/2jm4nk/making_ghci_awesomer/ 20. http://artyom.me/lens-over-tea-1 21. http://www.reddit.com/r/haskell/comments/2igp1v/lens_over_tea_part_1_lenses… 22. http://www.mega-nerd.com/erikd/Blog/CodeHacking/Haskell/ghci-trick.html 23. http://www.reddit.com/r/haskell/comments/2jketr/a_neat_trick_for_ghci/ 24. https://blogs.ncl.ac.uk/andreymokhov/shaking-up-ghc/ 25. http://www.reddit.com/r/haskell/comments/2j07m0/shaking_up_ghc/ 26. https://github.com/gelisam/frp-zoo#readme 27. http://www.reddit.com/r/haskell/comments/2j0u4h/frp_zoo_the_todomvc_of_frp_… 28. https://skillsmatter.com/skillscasts/5296-safe-zero-cost-coercions-in-haske… 29. http://www.reddit.com/r/haskell/comments/2iqrr5/video_simon_peyton_jones_ze… 30. https://skillsmatter.com/skillscasts/5466-bryan-o-sullivan 31. http://www.reddit.com/r/haskell/comments/2is5am/bryan_osullivan_performance… 32. http://neilmitchell.blogspot.it/2014/10/how-to-rewrite-prelude.html 33. http://www.reddit.com/r/haskell/comments/2icjmf/how_to_rewrite_the_prelude/ 34. http://jaspervdj.be/posts/2014-10-17-generalizing-function-composition.html 35. http://www.reddit.com/r/haskell/comments/2jivxa/generalizing_function_compo… 36. https://ghc.haskell.org/trac/ghc/wiki/CStructures 37. http://www.reddit.com/r/haskell/comments/2jefui/c_structures_in_haskell_ffi/ 38. http://www.haskell.org/pipermail/haskell-cafe/2014-October/116463.html 39. http://www.reddit.com/r/haskell/comments/2j8pfv/yi_010_anniversary_edition/ 40. http://psnively.github.io/blog/2014/10/14/Curry-Howard-the-Ontological-Ulti… 41. http://www.reddit.com/r/haskell/comments/2jb1n7/curryhoward_the_ontological… 42. http://stackoverflow.com/questions/26417136/is-it-possible-to-directly-invo… 43. http://stackoverflow.com/questions/26243014/why-does-foldr-use-a-helper-fun… 44. http://stackoverflow.com/questions/26333815/why-do-hgetbuf-hputbuf-etc-allo… 45. http://stackoverflow.com/questions/26291740/is-it-possible-to-get-the-kind-… 46. http://stackoverflow.com/questions/26214586/printing-out-haskells-evaluatio… 47. https://plus.google.com/105107667630152149014/about

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Yosemite
by Kazu Yamamoto 23 Oct '14

23 Oct '14

Hi, Has anybody tried GHC on Yosemite? I cannot afford to do it by myself this time. So, I would like to know. --Kazu

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Precise timing
by Jeffrey Brown 23 Oct '14

23 Oct '14

Hi, An earlier version of this question is cross-posted at StackOverflow <http://stackoverflow.com/questions/26437770/timing-inaccuracy-in-haskell-th…> . What should I use for precise (and eventually, concurrency-compatible) timing in Haskell? I want to make rhythms. I tried the following to produce a repeating rhythm in which one note is twice as long as the other two. (That rhythm is encoded by the list [1,1,2].) import Control.Concurrent import Text.Printf import Control.Monad main = mapM_ note (cycle [1,1,2]) beat = round (10^6 / 4) -- measured in microseconds note :: Int -> IO () note n = do threadDelay $ beat * n printf "\BEL\n" When I run it the long note is three times as long as the others, not twice. If I speed it up, by changing the number 4 to a 10, the rhythm is destroyed completely: the notes all have the same length. threadDelay is guaranteed <http://hackage.haskell.org/package/base-4.7.0.1/docs/Control-Concurrent.htm…> to wait at least as long, but potentially longer than, what the caller specifies. Another potential problem could be buffering in printf. (Eventually I intend to replace the printf statement with OSC output to a sound generator.) Thanks, Jeff

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a textured quad/triangle
by Vlad Lopatin 22 Oct '14

22 Oct '14

Hello, I am looking for a working example of texture binding, using VBOs and OpenGL4+ standard. A textured quad/triangle would be great. Does anybody have one? Regards, Vlad

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How to send a string somewhere via UDP
by Jeffrey Brown 22 Oct '14

22 Oct '14

Chapter 27 of Real World Haskell <http://book.realworldhaskell.org/read/sockets-and-syslog.html> explains a way to send a message over UDP. From a folder containing the two files "syslogclient.hs" and "SyslogTypes.hs" (which are attached to this email, and also available here <http://book.realworldhaskell.org/read/sockets-and-syslog.html>), supposedly one can run these three commands from GHCI, to send the message "This is my message" to localhost, port number 514: :load syslogclient.hs h <- openlog "localhost" "514" "testprog" syslog h USER INFO "This is my message" I have a Max/MSP patch that accepts UDP on localhost port number 9000. I substitute 514 -> 9000 and run that, and I get nothing. I know the Max/MSP "udpreceive" object is working, because from Python, using the aiosc module, I can run aiosc.send(('127.0.0.1', 9000), "This is a message from Python") ) and "This is a message from Python" reaches Max. There is a UDP library <http://hackage.haskell.org/package/network-conduit-0.6.1.1/docs/Data-Condui…> for Haskell, which I presume could serve as an alternative to the RWH code, but I don't understand it. I run OS X 10.9 on an early 2011 MBP. Many thanks, Jeff

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Limit elements of a list to a certain value
by Vlatko Basic 21 Oct '14

21 Oct '14

Hello Cafe, I have these datatypes data Event = E1 {...} | E2 {...} | E3 {...} data Parsed = PEvent Event | PLine Text | PHeader Text I'd like to split the list of Parsed values into separate lists with a guaranty that each of those lists contains only the elements of the same type, as in [PEvent E1] [PEvent E2] [PLine Text] ... How can that be achieved on type level (without creating newtypes for each case)? I tried with type families, but failed. Also, an explanation of why the Event above (or "data Q = Q1 | Q2 String") is not promotable to kind level would be welcome. best regards, vlatko

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Recommended way to install, use and maintain haskell tools on OS X
by Nicola Gigante 20 Oct '14

20 Oct '14

Hi I know this question might be a little old, but I happen to have a from scratch installation of OS X (yosemite) on my machine and I want to install the haskell tools in the currently best way. So I summarize the options that I understand are available: 1) A non-option first: the Homebrew formula for haskell-platform is no longer available, but… 2) There are formulas for GHC (7.8.x) and cabal, so I can install those and install the other packages with the homebrew cabal. 3) I can install the binary distribution of the Haskell Platform for OS X directly from the website, and then install any needed package with cabal. 4) I’ve heard of Nix, what is this all about? 5) Installing manually the binary distributions of ghc and cabal. No thanks. That said, what is the recommended way to manage packages once I have cabal and ghc? 1) Installing everything locally into ~/Library/Haskell, which afaik is the default behavior of cabal. That also means having ~/Library/Haskell/bin in the PATH. But do I have to also have ~/.cabal/bin in the PATH? Why having two different local bin directories? What happens to the packages installed by haskell platform system-wide? They are left there, shadowed by the local ones, and never again touched nor updated? 2) Installing everything globally, changing the cabal configuration to make it the default. Afaik bad idea, but then I don’t have obsoleted packages installed system-wide. 3) Installing nothing, and relying on sandboxes to install exactly what is needed for any single package I need. This option is still a bit mysterious to me. 3a) If I need to install a package I need for development, I do cabal sandbox init in my source tree and then if I install something from inside it, cabal installs it in the sandbox. Am I correct? 3a) If I need to install a package I need to _use_, instead, what to do? Sorry for the confusion and thanks for the clarification. Bye, Nicola

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