November 2012 - Haskell-Cafe

computation over containers, greatly simplified notation.
by Takayuki Muranushi 01 Dec '12

01 Dec '12

Dear all, I came up with an idea to greatly simplify some kinds of array computations. It should work well with many kinds of arrays. Is this new? https://gist.github.com/4162375 These few days, I've been trying to rewrite a hydrodynamic simulation code that used Data.Vector (~250 lines), to Repa [1] . It seemed promising, but soon I realized that I needed to use Repa.map and Repa.zipWith almost everywhere. I need careful thinking to transform every lines (that used vector's indexing) to Repa's point-free stile. Is there any better ways? Then I realized that I was the author of Paraiso [2], a DSL for stencil computation. One of its feature is to write down array computation just as if it were scalar computation. Basically what I need is ZipList-like Applicative instances for vectors and Repa arrays. Why not they support ZipVector? Because 'pure' of zipList was an infinite list and you can't do infinite vectors. Then I came up with this idea. https://gist.github.com/4162375 the wrapper W does several things: it represents the 'pure,' homogeneous array in a space-efficient manner, and also serves as a newtype-wrapper of Num (and possibly Fractional, Floating...) instances. Questions are: is this technology new? or promising? doomed? It seems to me like a free-Applicative, like the free-Monad theory. Are they related? The function 'backend' helps to mix in the non-zip-like computations. How can we remove the 'undefined' in the 'backend?' Some of Repa computations are Monads. W needs to be a monad transformer to incooperate this. Also I'm grateful to past cafe discussion on existing Zippable implementations [3][4] . [1] hackage.haskell.org/package/repa [2] http://hackage.haskell.org/package/Paraiso [3] http://www.haskell.org/pipermail/haskell-cafe/2009-July/064403.html [4] http://hackage.haskell.org/packages/archive/category-extras/latest/doc/html… -- Takayuki MURANUSHI The Hakubi Center for Advanced Research, Kyoto University http://www.hakubi.kyoto-u.ac.jp/02_mem/h22/muranushi.html

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Why Kleisli composition is not in the Monad signature?
by damodar kulkarni 30 Nov '12

30 Nov '12

Hi, The Monad class makes us define bind (>>=) and unit (return) for our monads. Why the Kleisli composition (>=>) or (<=<) is not made a part of Monad class instead of bind (>>=)? Is there any historical reason behind this? The bind (>>=) is not as elegant as (>=>), at least as I find it. Am I missing something? - Damodar

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Can a GC delay TCP connection formation?
by Jeff Shaw 30 Nov '12

30 Nov '12

Hello, I've run into an issue that makes me think that when the GHC GC runs while a Snap or Warp HTTP server is serving connections, the GC prevents or delays TCP connections from forming. My application requires that TCP connections form within a few tens of milliseconds. I'm wondering if anyone else has run into this issue, and if there are some GC flags that could help. I've tried a few, such as -H and -c, and haven't found anything to help. I'm using GHC 7.4.1. Thanks, Jeff

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ANN: zeromq3-conduit, Conduit bindings for zeromq3-haskell
by Nicolas Trangez 30 Nov '12

30 Nov '12

Hi, I published zeromq3-conduit, a small library which facilitates using zeromq3-haskell, a binding for ZeroMQ 3.x, in a Conduit-based application. The 'examples' folder in the source repository contains ports of the zeromq3-haskell examples. The library also contains a module which might ease using plain zeromq3-haskell: it lifts some of the IO actions into MonadIO and provides a ReaderT which carries a Context around, so sockets can be created without passing this around explicitly. Package is on Hackage [1] and Github [2]. Comments/feedback/pull requests/... welcome! Nicolas [1] http://hackage.haskell.org/package/zeromq3-conduit [2] https://github.com/NicolasT/zeromq3-conduit/

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ANN: base-compat
by Simon Hengel 30 Nov '12

30 Nov '12

Hi, I just released the first version of a package that "provide new additions to base for older versions of base" [2]. So far the following is covered: readMaybe readEither lookupEnv getExecutablePath Source is on GitHub [2]; patches welcome ;) Cheers, Simon [1] http://hackage.haskell.org/package/base-compat [2] https://github.com/sol/base-compat

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Category Theory and Haskell
by Gytis Žilinskas 30 Nov '12

30 Nov '12

Greetings, I'm only taking my very first steps learning Haskell, but I believe that this mailing list might be appropriate for my question. How difficult would it be to study category theory and simultaneously come up with Haskell examples of various results that it presents? I believe some background is in order. I am a second year maths student (well actually I'm in-between 2nd and 3rd, but let's not delve into details). I have also done some 2nd year CS courses so I have some background in that too. My general interests seem to lie somewhere between maths and compsci (I guess theoretical cs is the word for it), but it doesn't look like there is a good way to study the subject at an undergrad level. Most CompSci undergrad courses seem to concentrate too much on the trade of being a programmer and only present the basics of the theoretical results. While maths departments seem to be reluctant to make room in the undergrad curriculum for a huge subject that, after all, is maths. Recently, whenever I try reading up on something that looks like an exciting topic to me, I hit my head against category theory. That seems to suggest, that I should check it out, as it might be a fun subject to study, or at least provide valuable insights elsewhere. I've tried looking for some resources online. Most of them are outright scary and clearly have prerequisites that I lack. Some a little bit more accessible. Overall, various similar questions on different stackexchanges and even some old discussions on this mailing list, imply that to study category theory one has to know many examples from other advanced fields, in order to see the patterns that category theory describes. Now I obviously lack that. Some basic group theory, linear algebra, real/complex analysis and metric spaces is pretty much all I've got. Moreover, most sources say that without the knowledge of examples - category theory is an extremely dry subject to study. Therefore, my preliminary plan is the following: Read: * Barry Mazur - When is one thing equal to some other thing? * Conceptual mathematics : a first introduction to categories / F. William Lawvere, Stephen H. Schanuel. (People say that it is rather lightweight, but at this point my goal is to see the general idea of category theory and get some taste of it) Then, in the 2nd semester, I have an option of taking a self-study module, and if I don't get bored by cats by that time - it would be great to invest some more time in it (or rather, I'll do anything to avoid applied maths *shudder*). However, I'll need to present my case before the department staff and this, basically, is why I'm here. Right now, it looks like the main argument against me, would be that I can't appreciate the subject since I lack the fundamentals in other areas of maths. So I was hoping, I could convince them that everything I lack example wise - I'll find in Haskell. Okay, "everything" might be too strong of a statement, but I don't really know what I'm talking about so it's a bit difficult to calibrate. Could you comment whether Haskell would serve me well for this purpose? I've also seen these lecture notes: http://www.haskell.org/haskellwiki/User:Michiexile/MATH198. But I'm not sure if they won't be deemed too comp sci targeted. Therefore, I'm putting more hope into "The Joy of Cats" which, at least according to the first few pages, doesn't assume any prior knowledge that I don't have. So that's that, I would appreciate any advice you could give. Gytis

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Recursive timezone-loading function
by David Thomas 29 Nov '12

29 Nov '12

https://github.com/dlthomas/tzcache A small bit of code, but seems likely to be useful enough that I figured I should share. I've a few notes/questions: 1) Does this already exist somewhere I missed? 2) It seems silly to make this its own library - any suggestions where it could be added? 3) Is the traverse-a-directory-and-populate-a-map pattern one worth abstracting? If so, where should that go? 4) Presently, it's a static cache entirely pre-loaded. This seems fine, as it's not a terribly huge amount of data, but it's worth noting. 5) Any comments on the code generally? Improvements? Complaints? Thanks, - David

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delete http://www.haskell.org/haskellwiki/Haskell_IDE
by Roman Beslik 29 Nov '12

29 Nov '12

Hi. There is more verbose page http://www.haskell.org/haskellwiki/IDEs . I registered on http://www.haskell.org/haskellwiki/ , but have not found the "Delete Page" command, wiki software help pages, or feedback channel, so I'm writing here.

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SOX - play simple
by Gary Klindt 29 Nov '12

29 Nov '12

Dear Cafe, after installing the Sox library (cabal install sox) I wanted to let run a minimal example from http://hackage.haskell.org/packages/archive/sox/0.2.2.2/doc/html/Sound-Sox-… module Main where import Sound.Sox.Play import Sound.Sox.Signal.List --import Sound.Sox.Option.Format import Data.Int main = do simple Sound.Sox.Signal.List.put Option.none 11025 (iterate (1000+) (0::Data.Int.Int16)) in this version, I get the error: Not in scope: `Option.none' So, I imported Sound.Sox.Option.Format, because there is a none with the right type. I also changed Option.none to none. Then the program compiles and I get the runtime error: fd:4: hClose: resource vanished (Broken pipe) What is wrong here? I appreciate your help! Best regards, Gary

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Haskell Weekly News: Issue 252
by Daniel Santa Cruz 29 Nov '12

29 Nov '12

Welcome to issue 252 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of November 18 to 24, 2012. Quotes of the Week * rwbarton: edwardk now has Lens under Control * atriq: My son looks a bit like me, he can put away the plates after dinner now thanks to edwardk! * mgsloan: Lens got 99 operators, but a (|>) ain't one. * hiptobecubic: I feel like this program is just a bunch of glue and duct tape holding a bottle of nitroglycerine off of the ground. * xplat: Some software is worth trying; most is just plain trying. * edwardk: Control.Lens is my secret ploy to get everyone to give up on Haskell and just move on to Agda as they find the Haskell types too complicated by comparison. * rwbarton: alpha reduction is what happens when you let edwardk maintain your module * edwardk: er that was a tongue sticking out, not an operator suggestion Top Reddit Stories * [Haskell] Leaving Microsoft Domain: haskell.org, Score: 139, Comments: 24 On Reddit: [1] http://goo.gl/DnzZF Original: [2] http://goo.gl/eVcXi * Help Nikki (Game written in Haskell) jump on Steam! Domain: joyridelabs.de, Score: 77, Comments: 8 On Reddit: [3] http://goo.gl/EzLh8 Original: [4] http://goo.gl/H84Nl * Gaussian distributions are monoids Domain: izbicki.me, Score: 46, Comments: 24 On Reddit: [5] http://goo.gl/i4keo Original: [6] http://goo.gl/C0gnN * Getting Started with Lenses Domain: newartisans.com, Score: 42, Comments: 32 On Reddit: [7] http://goo.gl/nv4ab Original: [8] http://goo.gl/h2zCa * Designing the Haskell IDE Domain: fpcomplete.com, Score: 41, Comments: 112 On Reddit: [9] http://goo.gl/yz1Rf Original: [10] http://goo.gl/gpJd7 * Putting haskell down Domain: b7j0c.org, Score: 33, Comments: 279 On Reddit: [11] http://goo.gl/8JWmr Original: [12] http://goo.gl/tZcbj * London Haskell Video (24-Oct-2012) – Why Do Monads Matter? Domain: youtube.com, Score: 33, Comments: 5 On Reddit: [13] http://goo.gl/uWH6w Original: [14] http://goo.gl/KqDji * Combinatorial species definition (Brent Yorgey) Domain: byorgey.wordpress.com, Score: 31, Comments: On Reddit: [15] http://goo.gl/0llrA Original: [16] http://goo.gl/PWIep * PhD Positions in Functional Programming at Chalmers (appl. deadline 2012-12-12) Domain: chalmers.se, Score: 28, Comments: On Reddit: [17] http://goo.gl/WAmYP Original: [18] http://goo.gl/JenYO * Video of the presentations at the first NYC Haskell User's Group meetup. Domain: vimeo.com, Score: 25, Comments: 10 On Reddit: [19] http://goo.gl/rxN75 Original: [20] http://goo.gl/uV4wc Top StackOverflow Questions * Where to start with dependent type programming? votes: 16, answers: 1 Read on SO: [21] http://goo.gl/HXdOu * What other ways can state be handled in a pure functional language besides with Monads? votes: 13, answers: 3 Read on SO: [22] http://goo.gl/D86hV * Why are some Prelude functions defined in terms of foldl? votes: 8, answers: 3 Read on SO: [23] http://goo.gl/wewrb * cabal FFI dependency votes: 8, answers: 1 Read on SO: [24] http://goo.gl/JFtJM * How to write Ctrl-C handler in Haskell? votes: 7, answers: 1 Read on SO: [25] http://goo.gl/pS651 * Why does the 2-tuple Functor instance only apply the function to the second element? votes: 5, answers: 2 Read on SO: [26] http://goo.gl/qGgSv * Type Families with GHC.Generics or Data.Data votes: 5, answers: 0 Read on SO: [27] http://goo.gl/mERdP * Haskell Programmatically/Dynamically Define Functions votes: 5, answers: 6 Read on SO: [28] http://goo.gl/22Wf0 * is this implementation of merge sort good? votes: 5, answers: 1 Read on SO: [29] http://goo.gl/Skznl * Railroad diagrams for Haskell? votes: 5, answers: 1 Read on SO: [30] http://goo.gl/ZeJa9 * Is FRP a proper way to implement most “event-driven” things? votes: 5, answers: 1 Read on SO: [31] http://goo.gl/QglPp * Stack overflow in very simple code votes: 5, answers: 1 Read on SO: [32] http://goo.gl/wB0kg * Mapping over Either's Left votes: 5, answers: 3 Read on SO: [33] http://goo.gl/oJX4v * Haskell - strict vs non-strict with foldl votes: 5, answers: 2 Read on SO: [34] http://goo.gl/3ElzK Until next time, Daniel Santa Cruz References 1. http://www.haskell.org/pipermail/haskell/2012-November/023566.html 2. http://www.reddit.com/r/haskell/comments/13m8eu/haskell_leaving_microsoft/ 3. http://joyridelabs.de/blog/?p=1548 4. http://www.reddit.com/r/haskell/comments/13muex/help_nikki_game_written_in_… 5. http://izbicki.me/blog/gausian-distributions-are-monoids 6. http://www.reddit.com/r/haskell/comments/13qyg2/gaussian_distributions_are_… 7. http://newartisans.com/2012/11/getting-started-with-lenses/ 8. http://www.reddit.com/r/haskell/comments/13l9vj/getting_started_with_lenses/ 9. http://fpcomplete.com/designing-the-haskell-ide/ 10. http://www.reddit.com/r/haskell/comments/13gn8k/designing_the_haskell_ide/ 11. https://www.b7j0c.org/blog/putting_haskell_down.html 12. http://www.reddit.com/r/haskell/comments/13g0ul/putting_haskell_down/ 13. http://www.youtube.com/watch?v=3q8xYFDYLeI 14. http://www.reddit.com/r/haskell/comments/13nwxu/london_haskell_video_24oct2… 15. http://byorgey.wordpress.com/2012/11/20/combinatorial-species-definition/ 16. http://www.reddit.com/r/haskell/comments/13iz3p/combinatorial_species_defin… 17. http://www.chalmers.se/en/about-chalmers/vacancies/Pages/default.aspx?rmpag… 18. http://www.reddit.com/r/haskell/comments/13nxtx/phd_positions_in_functional… 19. http://vimeo.com/53906049 20. http://www.reddit.com/r/haskell/comments/13hyjt/video_of_the_presentations_… 21. http://stackoverflow.com/questions/13497865/where-to-start-with-dependent-t… 22. http://stackoverflow.com/questions/13536761/what-other-ways-can-state-be-ha… 23. http://stackoverflow.com/questions/13474932/why-are-some-prelude-functions-… 24. http://stackoverflow.com/questions/13544642/cabal-ffi-dependency 25. http://stackoverflow.com/questions/13441676/how-to-write-ctrl-c-handler-in-… 26. http://stackoverflow.com/questions/13442544/why-does-the-2-tuple-functor-in… 27. http://stackoverflow.com/questions/13448361/type-families-with-ghc-generics… 28. http://stackoverflow.com/questions/13452907/haskell-programmatically-dynami… 29. http://stackoverflow.com/questions/13455849/is-this-implementation-of-merge… 30. http://stackoverflow.com/questions/13485374/railroad-diagrams-for-haskell 31. http://stackoverflow.com/questions/13486293/is-frp-a-proper-way-to-implemen… 32. http://stackoverflow.com/questions/13489676/stack-overflow-in-very-simple-c… 33. http://stackoverflow.com/questions/13503965/mapping-over-eithers-left 34. http://stackoverflow.com/questions/13533586/haskell-strict-vs-non-strict-wi…

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