- Haskell-Cafe - Haskell.org

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

4 3

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

3 2

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.

4 5

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

2 1

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…

1 0

OpenAL - compiling problem
by Gary Klindt 28 Nov '12

28 Nov '12

Dear Haskell Cafe, I tried installing the haskell openal library via cabal install openal I get the following error: ... [ 7 of 30] Compiling Sound.OpenAL.ALC.QueryUtils ( Sound/OpenAL/ALC/QueryUtils.hs, dist/build/Sound/OpenAL/ALC/QueryUtils.o ) Sound/OpenAL/ALC/QueryUtils.hs:66:1: Unacceptable argument type in foreign declaration: ALCdevice When checking declaration: foreign import ccall unsafe "static alcGetString" alcGetString :: ALCdevice -> ALCenum -> IO (Ptr ALCchar) Sound/OpenAL/ALC/QueryUtils.hs:102:1: Unacceptable argument type in foreign declaration: ALCdevice When checking declaration: foreign import ccall unsafe "static alcGetIntegerv" alcGetIntegerv :: ALCdevice -> ALCenum -> ALCsizei -> Ptr ALCint -> IO () Sound/OpenAL/ALC/QueryUtils.hs:120:1: Unacceptable argument type in foreign declaration: ALCdevice When checking declaration: foreign import ccall unsafe "static alcIsExtensionPresent" alcIsExtensionPresent_ :: ALCdevice -> Ptr ALCchar -> IO ALCboolean Failed to install OpenAL-1.4.0.1 cabal: Error: some packages failed to install: OpenAL-1.4.0.1 failed during the building phase. The exception was: ExitFailure 1 The version of my openal installation is 1.14-1. I also tried to install via darcs, but the error is the same. Any ideas? Greets Gary

2 3

Equality test between types that returns type-level Bool ?
by Takayuki Muranushi 28 Nov '12

28 Nov '12

Is it possible to write type family SameType a b :: Bool which returns True if a and b are the same type, and False otherwise? I encountered this problem when I was practicing promoted lists and tuples in ghc-7.6.1. One of my goal for practice is to write more "modular" version of extensible-dimensional calculations, and to understand whether ghc-7.6.1 is capable of it. http://hackage.haskell.org/packages/archive/dimensional/0.10.2/doc/html/Num… Some of my attempts: https://github.com/nushio3/dimensional-tf/blob/master/attic/list-02.hs This fails because :==: is not an equality test between a and b, but is a equality test within a (promoted) kind. https://github.com/nushio3/dimensional-tf/blob/master/attic/list-03.hs This fails because type instance declarations are not read from top to bottom. (not like function declarations.) https://github.com/nushio3/dimensional-tf/blob/master/attic/map-03.hs I could define a lookup using class constraints, but when I use it, results in overlapping instances. So, will somebody teach me which of the following is correct? * We can write a type family SameType a b :: Bool * We cannot do that because of theoretical reason (that leads to non-termination etc.) * We cannot write SameType, but there are ways to write functions like 'filter' and 'merge' , over type-level lists, without using SameType. Always grateful to your help, -- Takayuki MURANUSHI The Hakubi Center for Advanced Research, Kyoto University http://www.hakubi.kyoto-u.ac.jp/02_mem/h22/muranushi.html

3 4

Conduit and pipelined protocol processing using a threadpool
by Nicolas Trangez 28 Nov '12

28 Nov '12

All, I've written a library to implement servers for some protocol using Conduit (I'll announce more details later). The protocol supports pipelining, i.e. a client can send a 'command' which contains some opaque 'handle' chosen by the client, the server processes this command, then returns some reply which contains this handle. The client is free to send other commands before receiving a reply for any previous request, and the server can process these commands in any order, sequential or concurrently. The library is based on network-conduit's "Application" style [1], as such now I write code like (OTOH) > application :: AppData IO -> IO () > application client = appSource client $= handler $$ appSink client > where > handler = do > negotiateResult <- MyLib.negotiate > liftIO $ validateNegotiateResult negotiateResult > MyLib.sendInformation 123 > loop > > loop = do > command <- MyLib.getCommand > case command of > CommandA handle arg -> do > result <- liftIO $ doComplexProcessingA arg > MyLib.sendReply handle result > loop > Disconnect -> return () This approach handles commands in-order, sequentially. Since command processing can involve quite some IO operations to disk or network, I've been trying to support pipelining on the server-side, but as of now I was unable to get things working. The idea would be to have a pool of worker threads, which receive work items from some channel, then return any result on some other channel, which should then be returned to the client. This means inside "loop" I would have 2 sources: commands coming from the client (using 'MyLib.getCommand :: MonadIO m => Pipe ByteString ByteString o u m Command'), as well as command results coming from the worker threads through the result channel. Whenever the first source produces something, it should be pushed onto the work queue, and whenever the second on yields some result it should be sent to the client using 'MyLib.sendReply :: Monad m => Handle -> Result -> Pipe l i ByteString u m ()' I've been fighting this for a while and haven't managed to get something sensible working. Maybe the design of my library is flawed, or maybe I'm approaching the problem incorrectly, or ... Has this ever been done before, or would anyone have some pointers how to tackle this? Thanks, Nicolas [1] http://hackage.haskell.org/packages/archive/network-conduit/0.6.1.1/doc/htm…

2 4

ANNOUNCE: language-java 0.2.0
by Vincent Hanquez 27 Nov '12

27 Nov '12

Hi Cafe, with the approval of Niklas, the original author and maintainer, i'll be maintaining language-java for now. I've uploaded a new version on hackage [1] with some minor improvements and the repository is now hosted on github [2]. Thanks Niklas for language-java ! [1] http://hackage.haskell.org/package/language-java [2] http://github.com/vincenthz/language-java/ -- Vincent

3 3

Real-time code in Haskell (Was: Can a GC delay TCP connection formation?)
by Mike Meyer 27 Nov '12

27 Nov '12

On Tue, Nov 27, 2012 at 3:45 AM, Gregory Collins <greg(a)gregorycollins.net> wrote: > If you have a hard real-time requirement then a garbage-collected > language may not be appropriate for you. This is a common meme, but frankly, it isn't true. When writing real-time code, you just need to make sure that everything that happens takes a known maximum amount of time. Then, you can sum up the maximums and verify that you do indeed finish in the real-time window of the task. GC is a problem because it's not predictable, and may not have a maximum. However, it's no worse than a modern version of the C function malloc. Some of those even do garbage collection internally before doing an OS call if they're out of memory. The solution is the same in both cases - make sure you don't do GC (or call malloc) in the critical region. Both require knowing implementation details of everything you call, but it isn't impossible, or even particularly difficult. Lazyness, on the other hand ... I haven't thought about. I suspect you need to force the evaluation of everything you're going to need before you start the critical region, but I wonder if that's enough? Has anyone out there investigated this? Thanks, <mike

4 3