May 2015 - Haskell-Cafe

Increasing parallelism in a frequency counter
by Andreas Pauley 25 May '15

25 May '15

Hi everyone, I've started playing with some parallel programming in Haskell, mostly by applying the first few examples from Simon Marlow's book to my own toy problem: https://github.com/apauley/parallel-frequency I'm quite pleased with the speedup I have so far, the fastest parallel implementation is just over twice as fast as its sequential counterpart. But looking at the threadscope images from the generated event logs I can see that there are still significant portions of my program that runs on only one core. Is anyone interested to see if we can increase the parallelism in this frequency counter? Kind regards, Andreas -- https://keybase.io/apauley http://za.linkedin.com/in/apauley http://www.meetup.com/lambda-luminaries http://www.meetup.com/Bitcoins-Baby-Bitcoins GPG Public Key: https://keybase.io/apauley/key.asc

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Fwd: Abandoning String = [Char]?
by David Feuer 25 May '15

25 May '15

Performance will go utterly out the window if you use pattern synonyms and the like to pretend that Text is [Char]. It would probably work fine (great, even) when matching, but code that assumes String=[Char] would be completely killed on the construction side, because building Text by consing, or repeatedly prepending small chunks, is ridiculously inefficient. These are the very things that are *efficient* with [Char], and that people have assumed to be efficient when writing code that uses it. On May 22, 2015 5:58 PM, "Carter Schonwald" <carter.schonwald(a)gmail.com> wrote: > > one direction that this thread has *COMPLETELY* overlooked is the following: > > could we use pattern synonyms or something along those lines to make the migration to Text or the like more seemless?

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Munich Haskell Meeting, 2015-05-25 @ 19:30
by Christian Neukirchen 23 May '15

23 May '15

Dear all, Next week, our monthly Munich Haskell Meeting will take place again on Monday, May 25 at Cafe Puck at 19h30. For details see here: http://www.haskell-munich.de/dates If you plan to join, please add yourself to this dudle so we can reserve enough seats! It is OK to add yourself to the dudle anonymously or pseudonymously. https://dudle.inf.tu-dresden.de/haskell-munich-may-2015/ Everybody is welcome! cu, -- Christian Neukirchen <chneukirchen(a)gmail.com> http://chneukirchen.org

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open gl type conversions
by briand＠aracnet.com 23 May '15

23 May '15

Following code GLFW.setWindowSizeCallback w (Just (\window w h -> do GL.viewport $= (GL.Position 0 0, GL.Size w h) emo1.hs:42:50: Couldn't match type ‘Int’ with ‘Foreign.C.Types.CInt’ Expected type: GLsizei Actual type: Int In the first argument of ‘Size’, namely ‘w’ In the expression: Size w h I can never seem to figure out if i really need to do a type conversion, or just the make the appropriate type declarations. Sometimes it seems to be a combination of both. Part of the complication here is that w and h are, in fact, `Int` because that's there type in the type signature of WindowSizeCallback (Window -> Int -> Int -> IO () ). GL.Size really needs a CInt so it seems like I have to do some sort of explicit conversion, but I can never seem to figure out exactly how to find the necessary function. Humorously i tried putting Int->CInt in hoogle, and the only thing that looked like it might be useful was unsafeCoerce, but I'm pretty sure that's not the right answer. I was wondering if someone could enlighten me on exactly how you can trace out the solution to this sort of problem because it seeems to come up often (see, for example my confusion over FFTW r). Brian

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Abandoning String = [Char]?
by Andrew Gibiansky 22 May '15

22 May '15

Hey all, In the earlier haskell-cafe discussion of IsString, someone mentioned that it would be nice to abandon [Char] as the blessed string type in Haskell. I've thought about this on and off for a while now, and think that the fact that [Char] is the default string type is a really big issue (for example, it gives beginners the idea that Haskell is incredibly slow, because everything that involves string processing is using linked lists). I am not proposing anything, but am curious as to what already has been discussed: 1. Has the possibility of migrating away from [Char] been investigated before? 2. What gains could we see in ease of use, performance, etc, if [Char] was deprecated? 3. What could replace [Char], while retaining the same ease of use for writing string manipulation functions (pattern matching, etc)? 4. Is there any sort of migration path that would make this change feasible in mainline Haskell in the medium term (2-5 years)? Thanks! I would welcome any references or links that my cursory googling has failed to find. -- Andrew

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Installing ghc-mod on Windows
by Anakreontas 22 May '15

22 May '15

I tried to instal ghc-mod on Windows with cabal. Two dependencies fail to compile because of missing native libraries. ansi-terminal requires kernel32 and lifted-base requires inlinable.h. Have you solved a similar problem? In Linux it would be easy to install the native libraries but in Windows I do not know which programs I should install and where to find them.

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cabal problem with Digits
by Roelof Wobben 22 May '15

22 May '15

Hello, When I want to install digits I see this output: cabal install digits Resolving dependencies.. [1 of 1] Compiling Main ( /tmp/digits-0.2-415/digits-0.2/Setup.lhs, /tmp/digits-0.2-415/digits-0.2/dist/setup/Main.o ) /tmp/digits-0.2-415/digits-0.2/Setup.lhs:3:3: Warning: Module ‘System.Cmd’ is deprecated: Use "System.Process" instead /tmp/digits-0.2-415/digits-0.2/Setup.lhs:5:49: Warning: In the use of ‘runTests’ (imported from Distribution.Simple, but defined in Distribution.Simple.UserHooks): Deprecated: "Please use the new testing interface instead!" Linking /tmp/digits-0.2-415/digits-0.2/dist/setup/setup ... Warning: digits.cabal: A package using 'cabal-version: >=1.2' must use section syntax. See the Cabal user guide for details. Configuring digits-0.2... Building digits-0.2... Preprocessing library digits-0.2... [1 of 1] Compiling Data.Digits ( src/Data/Digits.hs, dist/build/Data/Digits.o ) ****/usr/bin/ld: cannot find -lHSQuickCheck-2.8.1-ghc7.8.3 /usr/bin/ld: cannot find -lHStf-random-0.5-ghc7.8.3 /usr/bin/ld: cannot find -lHSprimitive-0.6-ghc7.8.3 /usr/bin/ld: cannot find -lHSrandom-1.1-ghc7.8.3**** How to soilve this one ? Roelof --- Dit e-mailbericht is gecontroleerd op virussen met Avast antivirussoftware. http://www.avast.com

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Haskell Weekly News: Issue 322
by Semen Trygubenko / Семен Тригубен ко 21 May '15

21 May '15

New Releases 99 Haskell: A web service by Bram Hoskin Solve live Haskell coding problems based on H-99 in the browser to strengthen your understanding of the language. http://www.99haskell.org/ https://github.com/bramgg/99haskell/ Magic Cookies A commercial Android game is released that is written in Haskell using SDL2 for multimedia and the Arrowized Functional Reactive Programming DSL Yampa. The authors had to "escape their functional comfort zones and come up with smarter abstractions that mutable reality and performance demand". The game consists of 2K lines of code, of which 1K is game specific and 400 are Yampa code. The most complex parts were certain Yampa constructs (arrow-based, with lots of tupling/untupling). http://keera.co.uk/blog/2015/03/19/magic-cookies-released-google-play/ https://play.google.com/store/apps/details?id=uk.co.keera.games.magiccookies https://github.com/keera-studios/keera-hails https://github.com/ivanperez-keera/Yampa Discussion Finding a GHC bug by Neil Mitchell A write up of the hunt for bug #10176 http://neilmitchell.blogspot.it/2015/03/finding-ghc-bug.html https://ghc.haskell.org/trac/ghc/ticket/10176 What are your most persuasive examples of using Quickcheck? Quickcheck helped many people in a number of areas — compiler optimisations, date/time validation, regular expressions, encoding/decoding, topological sort, fuzzing HTTP APIs and even exhibiting classical voting paradox! Apart from the obvious benefit of helping to find big juicy bugs as well as valid but potentially harmless tiny bugs in edge cases that no-one cares about, Quickcheck can help attain enlightenment in the sense that spelling out Quickcheck properties in itself can be rewarding because this can reveal assumptions that have been made without realizing it. Roman Cheplyaka reminds us about SmallCheck and that it should be used instead of Quickcheck when one has a good idea about what depth is needed and when exhaustive search at that depth is affordable. https://www.reddit.com/r/haskell/comments/308ps6/what_are_your_most_persuas… http://stackoverflow.com/questions/20092191/how-much-should-one-control-the… What is the difference between free monads and free monoids? A comment by Edward Kmett. https://www.reddit.com/r/haskell/comments/2znhjk/what_is_the_difference_bet… Type-Checked Pseudo-Code Tom Ashworth argues that Haskell is an excellent tool for fleshing out ideas and prototyping solutions, and that it makes one generalize and shake out many conceptual bugs before concepts become code. https://phuu.net/2015/03/24/type-checked-pseudo-code.html Where does GHC spend most of it's time during compilation? Optimisation and codegen, it seems. Also, GHC compilation times went up substantially from version 7.6 to 7.8. https://www.reddit.com/r/haskell/comments/309430/where_does_ghc_spend_most_… Quotes of the Week "Haskell doesn't feel like code. It feels like a language for thinking in: it's expressive, terse and simple, especially as type-checked pseudo-code." (Tom Ashworth) https://phuu.net/2015/03/24/type-checked-pseudo-code.html "Well that's not in the spec. If you want to change the requirements you have to renegotiate the contract =P" (sccrstud92) https://www.reddit.com/r/haskell/comments/2zxekg/while_learning_haskell_i_m… "Unit tests are double entry bookkeeping" (EvanDaniel) https://www.reddit.com/r/haskell/comments/308ps6/what_are_your_most_persuas… augustss> "Well, type checking is worst case exponential…" barsoap> "Yep. That's also the reason why I don't really get the insistence of dependently-typed languages to have to be total at the type level: I don't care whether type-checking takes ten or infinitely many years, both figures are too large." https://www.reddit.com/r/haskell/comments/309430/where_does_ghc_spend_most_… "Our prophet Djistra said that a testing shows the presence, not the absence of bugs … <snip> … Today Djistra would have been stoned under a myriad of inutile unit tests that people perform for a sense of false security, in a sort of superstitious sacrificial ritual, a waste of time to convince himself and others that his software is right." (agocorona) https://www.reddit.com/r/haskell/comments/308ps6/what_are_your_most_persuas… "You might also consider these sorts of infelicities as teachable moments" (ericpashman) https://www.reddit.com/r/haskell/comments/2zxekg/while_learning_haskell_i_m… "Nice gradual progression there. Big O notion, smoothly divided into 3 easy parts, then bam, Coyoneda. =) I approve. [Yes, I realize it is a bag of topics, not a course outline, but it still grabbed me.]" (edwardkmett) https://www.reddit.com/r/haskell/comments/2zqyes/we_released_a_video_librar…

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23% of calls to map fuse away
by Joachim Breitner 21 May '15

21 May '15

Hi, I just compiled all¹ of Stackage LTS 2.9 with GHC 7.8.4 with -ddump-rule-firings, and counted the occurrences of the rules. Here is a fun fact: 23% of all (static) occurrences of "map" fuse away². And 50387 lists are fused away in total³ Full data at https://gist.github.com/nomeata/071c1f87450cf668bbeb Greetings, Joachim ¹ but the SDL2 bindings ² 16925 × rule map 12904 × rule mapList, which fires if no fusion happened. ³ 27846 × rule fold/build 11293 × rule foldr/single 11248 × rule augment/build -- Joachim “nomeata” Breitner mail(a)joachim-breitner.de • http://www.joachim-breitner.de/ Jabber: nomeata(a)joachim-breitner.de • GPG-Key: 0xF0FBF51F Debian Developer: nomeata(a)debian.org

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I'm reading #OutOfTheTarPit does ghc have a relational model built in for state?
by KC 21 May '15

21 May '15

I'm reading #OutOfTheTarPit does ghc have a relational model built in for state? "Out of the Tar Pit" talks about FRP - Functional Relational Programming -- -- Sent from an expensive device which will be obsolete in a few months! :D Casey

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