September 2014 - Haskell-Cafe

Debugging with ghci
by Dominick Samperi 27 Sep '14

27 Sep '14

For a long time I've worked under the assumption that a breakpoint can only be used once in a ghci session, because if you run the function (main say) again, it runs to completion, ignoring the breakpoint. Restarting ghci and resetting the break point resolves the problem, but does not explain why a breakpoint cannot be used more than once. How can this behavior be explained? Also, when I stop at a breakpoint :step works as expected, but :trace does not? It simply runs the rest of the function, and using :back subsequently results in the message "not stopped at a breakpoint." Any comments on this behavior? Thanks, Dominick

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Sum and Product do not respect the Monoid laws
by Jason Choy 27 Sep '14

27 Sep '14

The Monoid instances for Sum and Product do not respect the Monoid laws. The instances are: Num a => instance Monoid (Sum a) Num a => instance Monoid (Product a) Float and Double are instances of the Num typeclass, however, floating point addition and multiplication are not associative. Here's an example: > (Sum 1234.567 `mappend` Sum 45.67834) `mappend` Sum 0.0004 Sum {getSum = 1280.2457399999998} > Sum 1234.567 `mappend` (Sum 45.67834 `mappend` Sum 0.0004) Sum {getSum = 1280.24574} Shouldn't these instances be constrained to Integral types? Jason

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Munich Haskell Meeting
by Heinrich Hördegen 27 Sep '14

27 Sep '14

Dear all, our next Haskell meeting in Munich takes place at Cafe Puck at 19h30 on Mon, 29th of September. Please go to http://www.haskell-munich.de/dates and click the button if you plan to join. Everyone is welcome! Heinrich

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foreign ccall export / ghc 7.6.3
by Clint Adams 27 Sep '14

27 Sep '14

Are there examples of "foreign ccall export" that work with GHC 7.6.3?

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SIGVTALRM and Unbound threads
by Lane Seppala 26 Sep '14

26 Sep '14

Howdy, I've banged into this issue of FFI errors stemming from a C library improperly handling system call interrupts caused by the SIGVTALRM/SIGALRMs emitted by the Haskell runtime. Usually the 'proper' solution is to fix the C library—easier said than done for most C quagmires. The most popular workaround, described by Bryan O'Sullivan[1] and several others, is to block the culprit signals from reaching the FFI call. I found another workaround using unbound threads, and it seems to work generally for this problem. However, I'm not satisfied with my understanding about *why* it works given what I know about FFI, bound vs unbound threads, and sys calls. As the simplest example, compiled with the threaded runtime (and neglecting imports): main = void $ sleep 10 will be interrupted, whereas main = void $ runInUnboundThread $ sleep 10 will complete its 10 second sleep. How does running the FFI call on an unbound thread block SIGVTALRM or otherwise avoid the interrupt? My understanding is that these signals *should* reach the FFI process, since they're on the same OS thread the timers were set. Is there any danger to consider before saying that this is a viable workaround? Kindly, Lane [1]: http://www.serpentine.com/blog/2010/09/04/dealing-with-fragile-c-libraries-…

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Re: [Haskell-cafe] What happened to hugs?
by Doug McIlroy 26 Sep '14

26 Sep '14

> We know where the old contents of the hugs documentation and source are, > and are going to get them back online sometime soon. Welcome news! For my purposes, there is no substitute. GHCi's disregard for "default" is a fatal flaw. Doug McIlroy

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Hackage bad gateway?
by Dominick Samperi 26 Sep '14

26 Sep '14

Today (9/25/2014) Hackage is not reachable due to "bad gateway." Is there a status page? Thanks, Dominick

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api-tools questions
by Karsten Gebbert 25 Sep '14

25 Sep '14

Hi All, I have a question concerning the api-tools package. Its not clear from the tests, sources or the tutorial how I can use other date/time formats that the default `utc` type can handle. I'm trying to wrap a JSON API that is not under my control, so I have to adhere to whatever I get back from it. Could anybody with experience with the package point me to some example, relevant bits in the sources or a tip how to do it? I'm planning to create some more documentation around the package to contribute back once I figured out a few more details, because I think its quite a useful abstraction when dealing with (foreign) APIs IMO. Thanks already for any hints, karsten

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Replacement for Ptr () in FFI declaration?
by Howard B. Golden 25 Sep '14

25 Sep '14

Hi, Apparently, before GHC 7.8, it was possible to specify FFI declarations using Ptr (). However, it now gets an error message: This is from com-1.2.3.1 when I try to compile it: ------ message follows ----- [12 of 21] Compiling System.Win32.Com.Automation.TypeLib ( System\Win32\Com\Automation\TypeLib.hs, dist\build\System\Win 32\Com\Automation\TypeLib.o ) System\Win32\Com\Automation\TypeLib.hs:1379:1: Unacceptable argument type in foreign declaration: Ptr () When checking declaration: foreign import stdcall safe "dynamic" prim_System_Win32_Com_Automation_TypeLib_setGuid :: Ptr () -> Ptr () -> Ptr GUID -> IO Int32 ------ end of message ----- What is the proper declaration to use instead of Ptr () ? Thanks. Howard B. Golden

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Haskell Weekly News: Issue 307
by Daniel Santa Cruz 24 Sep '14

24 Sep '14

Welcome to issue 307 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers from September 14 to 20, 2014 Quotes of the Week * Qfwfq: I seq what you mean. * trap_exit_: if haskell makes programmers 10x more productive, then why are there not better haskell ides? :-) Fuuzetsu: trap_exit_: because you don't need one when you're 10x more productive already ;P * joshcough: im just surprised when im looking for something and i dont find a package by ed kmett. Top Reddit Stories * Wayward Tide is Being Written in Haskell Domain: blog.chucklefish.org, Score: 131, Comments: 30 Original: [1] http://goo.gl/00xr22 On Reddit: [2] http://goo.gl/kJBqTe * Hython: The simplest possible language Domain: callcc.io, Score: 82, Comments: 16 Original: [3] http://goo.gl/T95xQ3 On Reddit: [4] http://goo.gl/LBKTks * Hey /r/haskell, we're hiring! Domain: blog.chucklefish.org, Score: 79, Comments: 49 Original: [5] http://goo.gl/93gW3N On Reddit: [6] http://goo.gl/zULSci * hindent: A Haskell indenter Domain: chrisdone.com, Score: 58, Comments: 18 Original: [7] http://goo.gl/4XlJpL On Reddit: [8] http://goo.gl/SCXxOS * Reflecting strictness in Haskell types Domain: h2.jaguarpaw.co.uk, Score: 51, Comments: 27 Original: [9] http://goo.gl/lfPkYD On Reddit: [10] http://goo.gl/hx6XtM * Towards Shake 1.0 Domain: neilmitchell.blogspot.com, Score: 49, Comments: 7 Original: [11] http://goo.gl/RPlddo On Reddit: [12] http://goo.gl/ckKD2m * Formatting in Haskell (Chris Done) Domain: chrisdone.com, Score: 49, Comments: 11 Original: [13] http://goo.gl/eq7hAF On Reddit: [14] http://goo.gl/f1Amec * Haskell Dice of Doom - Part 1 Domain: derekmcloughlin.github.io, Score: 39, Comments: 6 Original: [15] http://goo.gl/oF3ot3 On Reddit: [16] http://goo.gl/oLz2Fv * Beginner error messages in C++ vs Haskell Domain: izbicki.me, Score: 39, Comments: 65 Original: [17] http://goo.gl/j05AXH On Reddit: [18] http://goo.gl/ZOLvZW * Blog: GHC Weekly News - 2014/09/15 Domain: ghc.haskell.org, Score: 28, Comments: 0 Original: [19] http://goo.gl/8Dt0e8 On Reddit: [20] http://goo.gl/P4uz3y * Let's Build a Browser Engine in Haskell: part 3 Domain: hrothen.github.io, Score: 27, Comments: 8 Original: [21] http://goo.gl/0ckGRH On Reddit: [22] http://goo.gl/vZXZOy * ANN: best-haskell - haskell download ranking Domain: best-haskell.herokuapp.com, Score: 24, Comments: 11 Original: [23] http://goo.gl/9RtgrI On Reddit: [24] http://goo.gl/3zO7SN * ICFP "The Ghost of Church" - writeup by Olle Fredriksson who was the first to solve it! Domain: reddit.com, Score: 23, Comments: 3 Original: [25] http://goo.gl/9j7hG3 On Reddit: [26] http://goo.gl/nDxc4T * What is wrong with the Monoid instance for Maybe? Domain: self.haskell, Score: 21, Comments: 21 Original: [27] http://goo.gl/kgVlFf On Reddit: [28] http://goo.gl/kgVlFf * Inferring Algebraic Effects Domain: lmcs-online.org, Score: 20, Comments: 0 Original: [29] http://goo.gl/Wjklhq On Reddit: [30] http://goo.gl/PBJY7N * Are typeclasses essential? (includes Luke Palmer's view on it) Domain: stackoverflow.com, Score: 19, Comments: 30 Original: [31] http://goo.gl/5rh2dq On Reddit: [32] http://goo.gl/RuX5i7 * Built on Haskell: Findmelike - visual search for art Domain: findmelike.com, Score: 19, Comments: 1 Original: [33] http://goo.gl/MgM69t On Reddit: [34] http://goo.gl/Vzrc6R * Let's Build a Browser Engine in Haskell: setting up tests Domain: hrothen.github.io, Score: 17, Comments: 9 Original: [35] http://goo.gl/qwLXRq On Reddit: [36] http://goo.gl/mUqX6w Top StackOverflow Questions * Are typeclasses essential? votes: 20, answers: 4 Read on SO: [37] http://goo.gl/5rh2dq * Ambigous instance resolution in Haskell votes: 14, answers: 2 Read on SO: [38] http://goo.gl/KzV5jC * Understanding the casts involved in patterns matching a datatype that is indexed over a user defined kind votes: 11, answers: 1 Read on SO: [39] http://goo.gl/TppaAg * What does fixIO do? votes: 10, answers: 2 Read on SO: [40] http://goo.gl/yECrrk Until next time, [41]+Daniel Santa Cruz References 1. http://blog.chucklefish.org/?p=154 2. http://www.reddit.com/r/haskell/comments/2glk10/wayward_tide_is_being_writt… 3. http://callcc.io/hython-the-simplest-possible-language/ 4. http://www.reddit.com/r/haskell/comments/2go4v6/hython_the_simplest_possibl… 5. http://blog.chucklefish.org/?p=322 6. http://www.reddit.com/r/haskell/comments/2gyn1w/hey_rhaskell_were_hiring/ 7. http://chrisdone.com/posts/hindent 8. http://www.reddit.com/r/haskell/comments/2gyaw7/hindent_a_haskell_indenter/ 9. http://h2.jaguarpaw.co.uk/posts/strictness-in-types/ 10. http://www.reddit.com/r/haskell/comments/2gk51p/reflecting_strictness_in_ha… 11. http://neilmitchell.blogspot.com/2014/09/towards-shake-10.html 12. http://www.reddit.com/r/haskell/comments/2gnat7/towards_shake_10/ 13. http://chrisdone.com/posts/formatting 14. http://www.reddit.com/r/haskell/comments/2gyrvp/formatting_in_haskell_chris… 15. http://derekmcloughlin.github.io/2014/09/13/Haskell-Dice-Of-Doom-Part-1/ 16. http://www.reddit.com/r/haskell/comments/2gg3um/haskell_dice_of_doom_part_1/ 17. https://izbicki.me/blog/error-messages-in-ghc-vs-g++.html 18. http://www.reddit.com/r/haskell/comments/2go92u/beginner_error_messages_in_… 19. https://ghc.haskell.org/trac/ghc/blog/weekly20140915 20. http://www.reddit.com/r/haskell/comments/2gia13/blog_ghc_weekly_news_201409… 21. http://hrothen.github.io/2014/09/19/lets-build-a-browser-engine-in-haskell-… 22. http://www.reddit.com/r/haskell/comments/2gw2bw/lets_build_a_browser_engine… 23. http://best-haskell.herokuapp.com/ 24. http://www.reddit.com/r/haskell/comments/2gsjk0/ann_besthaskell_haskell_dow… 25. http://www.reddit.com/r/theghostofchurch/comments/2fccjw/i_found_this_trink… 26. http://www.reddit.com/r/haskell/comments/2gkjij/icfp_the_ghost_of_church_wr… 27. http://www.reddit.com/r/haskell/comments/2guo44/what_is_wrong_with_the_mono… 28. http://www.reddit.com/r/haskell/comments/2guo44/what_is_wrong_with_the_mono… 29. http://www.lmcs-online.org/ojs/viewarticle.php?id=1469 30. http://www.reddit.com/r/haskell/comments/2gdl31/inferring_algebraic_effects/ 31. http://stackoverflow.com/questions/25855507/are-typeclasses-essential 32. http://www.reddit.com/r/haskell/comments/2gjqop/are_typeclasses_essential_i… 33. http://findmelike.com/ 34. http://www.reddit.com/r/haskell/comments/2gn81y/built_on_haskell_findmelike… 35. http://hrothen.github.io/2014/09/14/lets-build-a-browser-engine-in-haskell-… 36. http://www.reddit.com/r/haskell/comments/2ge7om/lets_build_a_browser_engine… 37. http://stackoverflow.com/questions/25855507/are-typeclasses-essential 38. http://stackoverflow.com/questions/25854072/ambigous-instance-resolution-in… 39. http://stackoverflow.com/questions/25934209/understanding-the-casts-involve… 40. http://stackoverflow.com/questions/25876042/what-does-fixio-do 41. https://plus.google.com/105107667630152149014/about

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