- Haskell-Cafe - Haskell.org

Problems with type family in a class
by Leza Morais Lutonda 22 Jul '14

22 Jul '14

Hi, Consider the following type classes: class (Signal sx) => Complexable sx where type ComplexSignalType .... class (Complexable sx) => FourierTransformable sx where fft :: (cx ~ ComplexSignalType sx) => cx -> cx .... and an instance: instance (RealFloat e) => Complexable [e] where type ComplexSignalType = [Complex e] instance (RealFloat e) => FourierTransformable [e] where fft = ... Why this example will give errors: *DSP.Signals.Core Prelude Data.Complex> :t ax ax :: [Complex Double] *DSP.Signals.Core Prelude Data.Complex> fft ax <interactive>:90:1: No instance for (FourierTransformable s0 [Complex Double]) arising from a use of ‘fft’ The type variable ‘s0’ is ambiguous Note: there is a potential instance available: instance [overlap ok] (RealFloat e, Math.FFT.Base.FFTWReal e) => FourierTransformable [e] [Complex e] -- Defined at src/DSP/Signals/Instances/List.hs:40:10 In the expression: fft ax In an equation for ‘it’: it = fft ax Thanks! -- Lemol-C 50 Aniversario de la Cujae. Inaugurada por Fidel el 2 de diciembre de 1964 http://cujae.edu.cu

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Extensible Effects & Multiple Operations
by Justin Bailey 21 Jul '14

21 Jul '14

I am playing around with the extensible-effects framework and cannot figure out how to solve this problem. I am trying to define a set of operations that only work after a file has been opened. For example, reading a character from a file or detecting EOF. My problem is I cannot figure out how to define one "runner" for my Effect that can deal with operations with different result types. For example, I would like my `readFrom` and `atEOF` operations to have the following types: data OpenFile r v = ... -- my "OpenFile" effect -- read a character from the file readFrom :: Eff r Char -- detect EOF atEOF :: Eff r Bool And I would like to use this operations as follows (where I read a byte from the file, check EOF, and return the concatenation of the two values): main = do c <- runOpenFile "Setup.hs" (do a <- readFrom -- `a` has `Char` type eof <- atEOF -- `eof` has `Bool` type return $ [a] ++ show eof) putStrLn c However, I cannot define a `runOpenFile` that compiles. For example, this definition gives me "Overlapping Instance" errors: runOpenFile :: FilePath -> Eff (OpenFile v :> r) result -> IO result The best I've come up with is to define a `Result` type that holds all possible results: data Result = AtEOF Bool | ReadChar Char The `Result` type then lets me put a single type on the `OpenFile` effect in `runOpenFile`: runOpenFile :: FilePath -> Eff (OpenFile Result :> r) result -> IO result But that makes my `main` program pretty ugly, because I have to deconstruct those result types each time: main = do c <- runOpenFile "Setup.hs" (do (ReadChar a) <- readFrom (AtEOF eof) <- atEOF return $ [a] ++ show eof) putStrLn c Does anyone have suggestions? Justin p.s. The complete code (in a sort-of similar form) can be seen at https://github.com/m4dc4p/ext-eff-sample/blob/master/src/OpenFile.hs

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Cabal question
by Jurriaan Hage 21 Jul '14

21 Jul '14

Hi there, I am busy making the Helium compiler available as a Cabal install. What I still need is a way to run some kind of postprocessing (like running a Makefile) after installation (as it happens this is for compiling the libraries that come with Helium). Does anyone know here whether that is supported and how, without having to resort to build-types like Make that for I actually want to avoid? best, Jur

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Another Cabal question
by Jurriaan Hage 21 Jul '14

21 Jul '14

Dear all, I have another issue: I have a Cabal package (call it A) that runtime depends on another (say B), where B provides (only) an executable that is compiled with the Make build-type. Now, I would like to list B under the build-depends of A (since there is no way to specify having just a run-time dependency), but it seems that when I cabal install B the package is not registered (although the executable does in fact become available to use within the standard directory.) So when I then compile A with Cabal it will complain the package is not registered. Does anyone know whether the behaviour for B is normal, or whether this is a bug of some kind? If the former, what is an easy and clean way of dealing with this situation. best, Jur

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applicative question
by grant 21 Jul '14

21 Jul '14

Hi guys, I am trying to simplify some code. A user provides a combination of boolean expressions and for each expression an async task is run until that expression becomes true. The boolean expressions have type (Event e => e -> Bool) eg (PC.availableFlag .&&. "asmith" .== PC.userId) Unfortunately the way I have coded this, the user needs to include the \r -> r ? for each expression, with a unique number indicating the position in the sequence (so I can keep track of the asyncs when restarting after a failure). The 'r' function just wraps the expression in a Concurrently constructor. r :: (forall e.Event e => (e -> Bool) -> Concurrently e) -> Concurrently a Here is a sample expression that someone might provide: expr1 = runit (\r -> (,) <$> r 1 (PC.availableFlag .&&. "asmith" .== PC.userId) <*> r 2 (22 .== SC.signalNumber)) So, is there a way to rewrite or simplify this to avoid having to pass the r + unique Int to each subexpression? Or, is there a different format that I can write this in and then transform to the Concurrently applicative? Here is the runit method: runit expr = runConcurrently (expr (Concurrently . queryToIOAction)) And some more sample queries: expr1 = runit (\r -> r (PC.availableFlag .&&. "asmith" .== PC.userId)) expr2 = runit (\r -> (,) <$> r 1 (PC.availableFlag .&&. "asmith" .== PC.userId) <*> r 2 (22 .== SC.signalNumber)) expr3 = runit (\r -> (\(a b -> Left (a,b)) <$> r 1 (PC.availableFlag .&&. "asmith" .== PC.userId) -- (Event e => e -> Bool) <*> r 2 (2 .== SC.signalNumber)) <|> Right <$> r 3 (1 .== SC.signalNumber)) Thanks for any help! Grant

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Q about last&init
by Закиров Марат 21 Jul '14

21 Jul '14

I am teaching myself haskell. The first impression is very good. But phrase "haskell is polynomially reducible" is making me sad :(. Anyway I am trying to backport my algorithm written in C. The key to performance is to have ability to remove element from the end of a list in O(1). But the original haskell functions last and init are O(n). My questions are: 1) Is last function is something like "black box" written in C++ which perform O(1)? So I shouldn't even try to imagine some haskell O(1) equivalent. 2) Or will optimizer (llvm?) reduce init&last complexity to 1? 3) Some people suggest to use sequences package, but still how do they implement O(1) init&last sequences equivalent in haskell? -- Regards, Marat.

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Memory consumption issues under heavy network throughput/concurrency loads
by Ben Bangert 21 Jul '14

21 Jul '14

I have been testing solutions in several languages for running a network daemon that accepts hundreds of thousands of websocket connections, and moves messages between them. Originally I used this websocket lib, http://jaspervdj.be/websockets/, but upon discovering a rather severe memory leak issue even when sending just a basic ping, switched to Michael Snoyman's first stab at a websocket impl for Yesod here: https://github.com/yesodweb/yesod/commit/66437453f57e6a2747ff7c2199aa7ad25d…. When under mild load (5k connections, each pinging once every 10+ seconds), memory usage remained stable and acceptable around 205 MB. Switching to higher load (pinging every second or less), memory usage spiked to 560 MB, and continued to 'leak' slowly. When I quit the server with the profile diagnostics, it indicate that hundreds of MB were "lost due to fragmentation". In addition, merely opening the connections and dropping them, repeatedly, made the base memory usage go up. Somewhere, memory is not being fully reclaimed when the connections are dropped. For a few thousand connections doing little, this wouldn't matter much. However, I'm trying to gauge whether its feasible to use Haskell to handle 150-200k connections that regularly come/go and are held open for long periods of time. Such massive memory use with what looks like leaks (or fragmentation issues) is problematic. I have created a very simple TCP based echo client/server here: https://github.com/bbangert/echo The server can be run after compiling with no additional options, and will listen on port 8080. The client can be run like so: ./dist/build/echoclient/echoclient localhost 8080 2000 0.5 The last number is the frequency to ping (every half second), the second to last is how many clients to connect. Under my local tests, when pinging every 5+ seconds, 2k clients will take about 50-75 MB of ram. Pinging every 0.5 seconds jumps to ~ 180 MB of ram, and this is for a mere 2k clients. Starting/stopping the echoclient repeatedly also causes the servers overall memory usage to continue to climb higher and higher. Releasing the connections never quite gets the memory usage down to where it started. The issue seems to occur under both GHC 7.6.3 and 7.8.2. I know there's a variety of ghc options that might be tuned, am I missing some critical option to keep memory usage under control? Is there a better way to build high TCP throughput/concurrent servers in Haskell that I'm missing? Thanks, Ben

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Creating a Point
by Rafael Almeida 20 Jul '14

20 Jul '14

Hello, I was talking to friends about how could you make a Point type in haskell. It's intuitive to make a 2D point such as: type Point2D = (Double, Double) Let's define one operation on this new type: add2D (x1, y1) (x2, y2) = (x1+x2, y1+y2) Let's say now we want a 3D point. Then we'd be tempted to do: type Point3D = (Double, Double, Double) add3D (x1, y1, z1) (x2, y2, z2) = (x1+x2, y1+y2, z1+z2) Although those types work great and you could make a type class so you don't have to suffix each function with 2D and 3D. It feels like we are just repeating code when defining the add function. If we want to go 4D, 5D, etc it would be more repeated code. Using a list would be more general: type Point = [Double] now we have a nice, general add function add = zipWith (+) It's not so fun that we are able to do something like: add [2,3] [5,7,11] We have no type-safety that we can only operate on points with the same dimension. How could we address this? Can we make a general function, yet retain the type safety? I suppose maybe there's something that could be done with TH so that we automatically generate those Point2D, Point3D, etc code. I'm not sure that would be a nice path to follow, though. []'s Rafael

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Obtaining kind of a HsType
by Alejandro Serrano Mena 20 Jul '14

20 Jul '14

Dear Haskell-café, I'm trying to get some information of the kinds of types in a program. In particular, I want to obtain the kinds of arguments to type family instances, that is, given: type family F a b :: Nat type instance F (List k) b = b I would like to obtain the kinds of "k" and "b" in the left-hand side of the type instance. In particular, I'm obtaining type family instance information via LTyFamInstEqn [ http://www.haskell.org/ghc/docs/latest/html/libraries/ghc/HsDecls.html#t:LT…] Inside, I get a list of binders of type HsWithBndrs [LHsType name]. I'm assuming each of the LHsTypes inside is a pattern in the type family instance. Thus, I'm looking at those elements and try to find their kind there. Thus, the specific question is: given a HsType, can I obtain its kind somehow? Thanks in advance

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ANN: loop - fast for loops
by Niklas Hambüchen 20 Jul '14

20 Jul '14

Announcing the first version of the loop package: http://hackage.haskell.org/package/loop This is for iterating over numeric or otherwise iteratable things in a fast fashion. Looks like C, runs like C. It's trivial, but very robust performance-wise and more independent of GHC smartness or non-smartness in regards to optimising alternatives like `forM_ [1..n]`. It comes with a lot of benchmarks. Contributions are very welcome.

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ANN: hackage-diff - Compare the public API of different versions of a Hackage library
by Tim C. Schroeder 20 Jul '14

20 Jul '14

Just a first shot at the problem, hope it’s useful to somebody! https://github.com/blitzcode/hackage-diff Cheers, Tim

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Is there a name for this property: x + x - x == x
by Omari Norman 20 Jul '14

20 Jul '14

It's not quite idempotence, because more than one function is involved. It's a common property and I figure I can write a higher order function to build QuickCheck tests for it. I was just wondering if it has a name.

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Conduit & state
by Павел Гаврилов 20 Jul '14

20 Jul '14

Hi! I have: runUnixServer (serverSettings socket) $ \appData -> appSource appData $$ CL.map handleBS =$ appSink appData I want to store some data in priority queue in Monad.State and be able to access it inside the handleBS (now: handleBS :: ByteString -> ByteString). Can you send me on the right path? Thanx, Pavel.

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No `base-4.7.0.1` in hackage
by Alois Cochard 19 Jul '14

19 Jul '14

Hi, GHC 7.8.3 ships with base 4.7.0.1, but this module is not published in hackage. Is there any reason why? That is an issue in my tool `codex` because he used hackage to download the sources and tags them [1]. [1]: https://github.com/aloiscochard/codex/issues/15 -- *A\ois* http://twitter.com/aloiscochard http://github.com/aloiscochard

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Unable to build vector on OSX (ghc 7.4, gcc 4.8)
by Edsko de Vries 18 Jul '14

18 Jul '14

Hi guys, I am currently unable to build vector on OSX: Loading package base ... <command line>: can't load .so/.DLL for: /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.9.sdk/usr/lib/libiconv.dylib (dlopen(/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.9.sdk/usr/lib/libiconv.dylib, 9): no suitable image found. Did find: /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.9.sdk/usr/lib/libiconv.dylib: mach-o, but wrong filetype) cabal: Error: some packages failed to install: vector-0.10.11.0 failed during the building phase. The exception was: This has been reported before as some kind of clang issue, but I am using gcc 4.8 (ghc-7.4.2/settings has "gcc-4.8" for "C compiler command"). Any suggestions? Thanks! Edsko

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Functional Programing-Oriented Machine Learning Startup Seeks DevOps Guru
by charles＠charlesweitzer.com 18 Jul '14

18 Jul '14

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Re: [Haskell-cafe] Conduit & state
by Toby Goodwin 18 Jul '14

18 Jul '14

Hi Pavel, >runUnixServer (serverSettings socket) $ \appData -> > ... >I want to store some data in priority queue in Monad.State and be able to >access it inside the handleBS (now: handleBS :: ByteString -> ByteString). Funnily enough, I've just been looking at an analogous problem (with runTCPClient and Persistent). It seems that in the recent move from conduit-network to conduit-extra, the run*Client and run*Server functions were specialized to the IO monad. I don't know what the motivation for this change was (simpler error messages perhaps?) but it does seem unhelpful. The workaround I deployed last night was to roll my own runTCPClient' which merges the code from the (deprecated) conduit-network package and the (current) streaming-commons, but this is hardly satisfactory. I'm sure I must be missing something. (I don't know if there's a better list to discuss conduit.) Toby.

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Haskell Weekly News: Issue 299
by Daniel Santa Cruz 17 Jul '14

17 Jul '14

Welcome to issue 299 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers from June 29 to July 12, 2014 Quotes of the Week * donri: maybe good thing Applicative isn't called StrongLaxSemimonoidalEndofunctor * ninja_code: haskell is used to debug thinking * johnw: the first rule of category theory club is to state your identity Top Reddit Stories * The new haskell.org design Domain: new-www.haskell.org, Score: 130, Comments: 95 Original: [1] http://new-www.haskell.org/ On Reddit: [2] http://goo.gl/qG7683 * GHC-7.8.3 is out! Domain: haskell.org, Score: 115, Comments: 40 Original: [3] http://goo.gl/fyHQZd On Reddit: [4] http://goo.gl/7bsDXj * Announcing rest - A Haskell REST framework Domain: engineering.silk.co, Score: 97, Comments: 41 Original: [5] http://goo.gl/F3W628 On Reddit: [6] http://goo.gl/VqWdi9 * GHC plans for 7.10.1 Domain: ghc.haskell.org, Score: 77, Comments: 54 Original: [7] http://goo.gl/YH6516 On Reddit: [8] http://goo.gl/g16PE3 * unagi-chan: Fast and scalable concurrent queues for x86, with a Chan-like API Domain: hackage.haskell.org, Score: 61, Comments: 17 Original: [9] http://goo.gl/7dE0e4 On Reddit: [10] http://goo.gl/zaMC6o * Come practice Haskell *and* make a difference in the world by helping us build a new Free/Libre/Open patronage economy Domain: snowdrift.coop, Score: 59, Comments: 19 Original: [11] http://goo.gl/vU2cx On Reddit: [12] http://goo.gl/PRkG3B * What's wrong with String Domain: haskell.org, Score: 55, Comments: 123 Original: [13] http://goo.gl/8etnBS On Reddit: [14] http://goo.gl/k2FZ9u * Why is package management so awful? Domain: self.haskell, Score: 55, Comments: 100 Original: [15] http://goo.gl/7h7cWn On Reddit: [16] http://goo.gl/7h7cWn * Reﬂection without Remorse: Revealing a hidden sequence to speed up monadic reﬂection [pdf] Domain: homepages.cwi.nl, Score: 49, Comments: 36 Original: [17] http://goo.gl/QgiwtW On Reddit: [18] http://goo.gl/BpYmcS * Mutable Algorithms in Immutable Languages, Part 1 Domain: tel.github.io, Score: 49, Comments: 27 Original: [19] http://goo.gl/hn6218 On Reddit: [20] http://goo.gl/apKO9f * Where is Haskell going in industry? Domain: self.haskell, Score: 47, Comments: 114 Original: [21] http://goo.gl/Io2hvy On Reddit: [22] http://goo.gl/Io2hvy * Why Dependently Typed Programming Will (One Day) Rock Your World Domain: ejenk.com, Score: 45, Comments: 154 Original: [23] http://goo.gl/d3zkiG On Reddit: [24] http://goo.gl/z3ZhDf * Using Phantom Types for Extra Safety Domain: blog.jakubarnold.cz, Score: 41, Comments: 19 Original: [25] http://goo.gl/WnZH0Z On Reddit: [26] http://goo.gl/EJTpxl * Designing Dependently Typed Languages - OPLSS '14 Domain: self.haskell, Score: 37, Comments: 11 Original: [27] http://goo.gl/P2mV1A On Reddit: [28] http://goo.gl/P2mV1A * Hackage update, part 3 Domain: lambda.xyz, Score: 37, Comments: 13 Original: [29] http://goo.gl/sLHVfX On Reddit: [30] http://goo.gl/G7dHLc * Announce: The most complete prelude formed from only the "base" package Domain: hackage.haskell.org, Score: 37, Comments: 55 Original: [31] http://goo.gl/lSoKVh On Reddit: [32] http://goo.gl/UeNUYL * New generic programming library (with paper accepted in WGP 2014) Domain: andres-loeh.de, Score: 34, Comments: 11 Original: [33] http://goo.gl/PgV7yV On Reddit: [34] http://goo.gl/qFOlLB * Ideal programming language for a new modern OS built from scratch? Domain: self.haskell, Score: 32, Comments: 63 Original: [35] http://goo.gl/eTkOFd On Reddit: [36] http://goo.gl/eTkOFd * Neil Mitchell's Haskell Blog: Optimisation with Continuations Domain: neilmitchell.blogspot.com.es, Score: 30, Comments: 15 Original: [37] http://goo.gl/lvo8Oa On Reddit: [38] http://goo.gl/haJyX4 * can we do better than cabal sandbox? Domain: self.haskell, Score: 30, Comments: 38 Original: [39] http://goo.gl/RS3j0s On Reddit: [40] http://goo.gl/RS3j0s Top StackOverflow Questions * Is there a monad that doesn't have a corresponding monad transformer (except IO)? votes: 37, answers: 1 Read on SO: [41] http://goo.gl/wBH70C * Arrows are exactly equivalent to applicative functors? votes: 21, answers: 3 Read on SO: [42] http://goo.gl/WRjQx0 * Why does GHC typecheck before desugaring? votes: 19, answers: 1 Read on SO: [43] http://goo.gl/EzaZVr * List of Functors votes: 15, answers: 3 Read on SO: [44] http://goo.gl/RmJvDC * Or-patterns in Haskell votes: 13, answers: 2 Read on SO: [45] http://goo.gl/ukvJm1 * What does this list permutations implementation in Haskell exactly do? votes: 12, answers: 2 Read on SO: [46] http://goo.gl/CXOCch * Should Latitude, Longitude and Elevation have their own type in Haskell? votes: 12, answers: 2 Read on SO: [47] http://goo.gl/fkElae * Haskell List Comprehension Speed Inconsistencies votes: 12, answers: 1 Read on SO: [48] http://goo.gl/0o4rwI * What are some examples of type-level programming? [closed] votes: 11, answers: 3 Read on SO: [49] http://goo.gl/fowZ6b * QuickCheck Gen is not a monad votes: 11, answers: 1 Read on SO: [50] http://goo.gl/hiJ0q8 Until next time, [51]+Daniel Santa Cruz References 1. http://new-www.haskell.org/ 2. http://www.reddit.com/r/haskell/comments/2a77f2/the_new_haskellorg_design/ 3. http://www.haskell.org/ghc/docs/7.8.3/html/users_guide/release-7-8-3.html 4. http://www.reddit.com/r/haskell/comments/2af4jf/ghc783_is_out/ 5. http://engineering.silk.co/post/90354057868/announcing-rest-a-haskell-rest-… 6. http://www.reddit.com/r/haskell/comments/29h32i/announcing_rest_a_haskell_r… 7. https://ghc.haskell.org/trac/ghc/wiki/Status/GHC-7.10.1 8. http://www.reddit.com/r/haskell/comments/29v4lj/ghc_plans_for_7101/ 9. http://hackage.haskell.org/package/unagi-chan-0.1.0.1 10. http://www.reddit.com/r/haskell/comments/2adh2f/unagichan_fast_and_scalable… 11. https://snowdrift.coop/ 12. http://www.reddit.com/r/haskell/comments/29zw5j/come_practice_haskell_and_m… 13. http://www.haskell.org/pipermail/haskell-cafe/2014-June/114745.html 14. http://www.reddit.com/r/haskell/comments/29jw0s/whats_wrong_with_string/ 15. http://www.reddit.com/r/haskell/comments/2a9n0t/why_is_package_management_s… 16. http://www.reddit.com/r/haskell/comments/2a9n0t/why_is_package_management_s… 17. http://homepages.cwi.nl/~ploeg/papers/zseq.pdf 18. http://www.reddit.com/r/haskell/comments/29i94o/re%EF%ACection_without_remo… 19. http://tel.github.io/2014/07/12/mutable_algorithms_in_immutable_languges_pa… 20. http://www.reddit.com/r/haskell/comments/2ahorr/mutable_algorithms_in_immut… 21. http://www.reddit.com/r/haskell/comments/2a310v/where_is_haskell_going_in_i… 22. http://www.reddit.com/r/haskell/comments/2a310v/where_is_haskell_going_in_i… 23. http://ejenk.com/blog/why-dependently-typed-programming-will-one-day-rock-y… 24. http://www.reddit.com/r/haskell/comments/29kv2y/why_dependently_typed_progr… 25. http://blog.jakubarnold.cz/2014/07/08/using-phantom-types-for-extra-safety.… 26. http://www.reddit.com/r/haskell/comments/2a5duv/using_phantom_types_for_ext… 27. http://www.reddit.com/r/haskell/comments/2a115h/designing_dependently_typed… 28. http://www.reddit.com/r/haskell/comments/2a115h/designing_dependently_typed… 29. http://lambda.xyz/blog/hackage-part-3/ 30. http://www.reddit.com/r/haskell/comments/2abi06/hackage_update_part_3/ 31. http://hackage.haskell.org/package/base-prelude 32. http://www.reddit.com/r/haskell/comments/2ahq11/announce_the_most_complete_… 33. http://www.andres-loeh.de/TrueSumsOfProducts/ 34. http://www.reddit.com/r/haskell/comments/2a535x/new_generic_programming_lib… 35. http://www.reddit.com/r/haskell/comments/29tgjd/ideal_programming_language_… 36. http://www.reddit.com/r/haskell/comments/29tgjd/ideal_programming_language_… 37. http://neilmitchell.blogspot.com.es/2014/06/optimisation-with-continuations… 38. http://www.reddit.com/r/haskell/comments/29fjg4/neil_mitchells_haskell_blog… 39. http://www.reddit.com/r/haskell/comments/2a2ec7/can_we_do_better_than_cabal… 40. http://www.reddit.com/r/haskell/comments/2a2ec7/can_we_do_better_than_cabal… 41. http://stackoverflow.com/questions/24515876/is-there-a-monad-that-doesnt-ha… 42. http://stackoverflow.com/questions/24668313/arrows-are-exactly-equivalent-t… 43. http://stackoverflow.com/questions/24639279/why-does-ghc-typecheck-before-d… 44. http://stackoverflow.com/questions/24589995/list-of-functors 45. http://stackoverflow.com/questions/24700762/or-patterns-in-haskell 46. http://stackoverflow.com/questions/24484348/what-does-this-list-permutation… 47. http://stackoverflow.com/questions/24521976/should-latitude-longitude-and-e… 48. http://stackoverflow.com/questions/24690406/haskell-list-comprehension-spee… 49. http://stackoverflow.com/questions/24481113/what-are-some-examples-of-type-… 50. http://stackoverflow.com/questions/24481648/quickcheck-gen-is-not-a-monad 51. https://plus.google.com/105107667630152149014/about

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help with multiparam type class
by gbwey9 17 Jul '14

17 Jul '14

Hi, Given any Event constructor, the 'conv' method passes in the field name to 'val' for each field. There is a Z instance for each Event arity. The call "conv E1::E1" works but requires that I explicitly specify the type. Is there any way to avoid that having to do that? For some reason 'conv' does not have enough information to know the type. Is there a way to do this with functional dependencies or type classes or something else? Here is the paste http://lpaste.net/107632 and here is the error: *Main> conv E1 <interactive>:3:1: No instance for (Z e0 (String -> E1)) arising from a use of `conv' The type variable `e0' is ambiguous Possible fix: add a type signature that fixes these type variable(s) Note: there are several potential instances: instance (Event e, Field a) => Z e (a -> e) -- Defined at testfamily.hs:28:10 instance Event e => Z e e -- Defined at testfamily.hs:20:10 Possible fix: add an instance declaration for (Z e0 (String -> E1)) In the expression: conv E1 In an equation for `it': it = conv E1 Thanks for any help, Grant

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Feature Proposal: GHC Flag for implicit external Prelude
by Andrew Gibiansky 16 Jul '14

16 Jul '14

Hey all, I would like to propose a very minor flag to add to GHC. I would like GHC to have a --with-prelude flag, which would specify an alternate Prelude to use instead of the default Haskell prelude. This would have an effect similar to -XNoImplicitPrelude and an additional import MyNewPrelude in the source file. It might be a *little* different semantically, as a qualified import would disable the original implicit import, just like it does with the default Haskell prelude. The benefit this would have is that this would give alternate preludes a more first-class status. Instead of having to import an alternative prelude everywhere, you could just have a ghc-options: --with-prelude=... flag in your *.cabal file, and have a different prelude be used. This is important for my own work, as I highly prefer other preludes for my non-library development; I think this is a feature which will be very useful as Haskell develops and we try to figure out how to get rid of the warts in the current Prelude. In order to specify the actula module, I propose that you just pass the module name as the --with-prelude argument. You could thus invoke GHC like this: ghc --with-prelude=MyNewPrelude Main.hs Then, the same rules apply for figuring out which MyNewPrelude module you are referring to as if you had written a `import MyNewPrelude` statement in the file. I am willing to implement this for GHC if people agree that this is a useful flag (or at least agree that isn't not a *bad* idea). What are your thoughts on this proposal? -- Andrew

15 16

Looking for architecture/infrastructure advice on coping with thousands of concurrent clients
by Isak Hansen 16 Jul '14

16 Jul '14

Let's just imagine there aren't enough poker sites out there already and that it makes sense to build another one. Basically, there will be players connecting to one or more tables, most likely talking some kind of JSON based protocol with the server(s). #1 - What's a good way to set up and maintain compressed and encrypted connections from the clients to my cluster? I've been wanting to try out websockets, think that'll do? #2 - I'm new to Haskell and would appreciate thoughts on how to store and process game state. Imagine a plain application managing 1000 tables of poker, responding to player input (that magically appears) and timer events (e.g. folding players that fail to take action). What data structures and libraries should I be looking at here? Thoughts on concurrency and how I organize program flow? #3 - I'm thinking of ZMQ to wire components together. Would I be better off basing my cluster on Cloud Haskell or some other library for distributed work? Would love some quick pointers and feedback here, thanks in advance, Isak

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[ANN](and feedback request) unagi-chan: Fast and scalable concurrent queues for x86, with a Chan-like API
by Brandon Simmons 16 Jul '14

16 Jul '14

I'm happy to finally release unagi-chan, an implementation of high-performance concurrent FIFO queues that have an API very similar to Control.Concurrent.Chan. You can see benchmarks and documentation here: http://hackage.haskell.org/package/unagi-chan If you have a moment (especially if you're on a non-x86 architecture) please take 10 minutes and run the tests with: $ cabal configure --enable-tests $ cabal build $ time ./dist/build/test/test Thanks to Ryan Newton for helping answer some low-level questions about his atomic-primops package which provides the CAS and fetch-and-add implementations that are the core of unagi-chan. REQUEST FOR FEEDBACK: would anyone be interested in any functionality like the following: - concurrent builder for Text, something like: new :: IO (InTextChan , Lazy.Text) write :: String -> InTextChan -> IO () - something similar for ByteString (is there a class for types convertable to ByteString?) - concurrent Text and/or ByteString readers, something like: reader :: Text -> IO TextReader takeText :: Int -> TextReader -> IO Text That's very sketchy, but I ask because I'm very close to that already with Unagi.Unboxed, and should be able to implement something like any of the above without too much trouble if it would be useful. Brandon http://brandon.si

3 4

extensible-transformers
by Ben Foppa 15 Jul '14

15 Jul '14

Hi cafe, I whipped up extensible-transformers ( https://github.com/RobotGymnast/extensible-transformers) this afternoon. The idea is to make Monad transformer code more like extensible-effects code (http://hackage.haskell.org/package/extensible-effects) Here's a sample: {-# LANGUAGE FlexibleContexts #-} module Main(main) where import Control.Monad.Trans.Flexible import Control.Monad.Trans.List import Control.Monad.Trans.State.Strict -- A flexible transformer stack built from existing transformers using `liftT`. bar :: (In (StateT Int) t, In ListT t) => t () bar = do n <- liftT get liftT $ ListT $ return $ replicate n () -- A flexible transformer stack built from existing transformers using `liftT`. baz :: In (StateT Int) t => t () baz = do liftT $ state $ \i -> ((), i + (1 :: Int)) -- A flexible transformer monad stack composed of two other flexible -- transformer monad stacks. foo :: (In (StateT Int) t, In ListT t) => t () foo = do bar baz main :: IO () main = do evalStateT (runListT foo) (1 :: Int) >>= putStrLn . show runListT (evalStateT foo (2 :: Int)) >>= putStrLn . show Any feedback on this? Does such a package already exist? Thanks, Ben

2 4

Doctest, quickcheck and typeclass
by Gautier DI FOLCO 14 Jul '14

14 Jul '14

Hi all, Is there a way, with doctest and quickcheck, to declare test at typeclass level (such as laws) and play them automatically at instance level? For example: -- | Convert from/to a List to/from an arbitrary data structure -- -- >>> prop > fromList . toList == id class Listable d where fromList :: [a] -> d a toList :: d a -> [a] And this test will be played for each instance. Thanks in advance for your help.

2 2

how to get ghci to load compiled modules in 7.8?
by Evan Laforge 14 Jul '14

14 Jul '14

I recently upgraded to 7.8.2 and I have a silly question. How do you get ghci to load compiled modules? When I try this: % cat >T.hs module T where x :: Int x = 42 % ghc -c -dynamic-too T.hs % s T.dyn_hi T.dyn_o T.hi T.hs T.o % ghci GHCi, version 7.8.2: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package filepath-1.3.0.2 ... linking ... done. Prelude> :l T [1 of 1] Compiling T ( T.hs, interpreted ) Ok, modules loaded: T. *T> It still loads the file interpreted, even though there is a .dyn_o present. What am I doing wrong? This is on x86-64 OS X.

1 1

How to walk under binders "for free" in 'bound'?
by Christiaan Baaij 14 Jul '14

14 Jul '14

Dear list, I'm trying to implement a small dependently typed language (based on [1]) using the generalised De Bruijn indices from the 'bound' library [2]. My main two datatypes are: > data Binder n a > = Lam {binder :: Name n a} > | Pi {binder :: Name n a} Representing my two types of binders, and: > data Term n a > = Var !a > | Universe Integer > | App !(Term n a) !(Term n a) > | Bind !(Binder n (Term n a)) !(Scope (Name n ()) (Term n) a) Representing terms. My problem lies with implementing type-inference for lambda-binders. Originally I started with: > inferType1 :: Eq a > => (a -> Term n a) -- ^ Context > -> Term n a -- ^ Term > -> Term n a -- ^ Inferred type > inferType1 ctx (Var a) = ctx a > inferType1 ctx (Bind (Lam b) s) = Bind (Pi b) s' > where > s' = toScope . inferType1 ctx' . fromScope $ s > ctx' = unvar bCtx fCtx > bCtx _ = fmap F $ extract b > fCtx = fmap F . ctx Where `toScope . inferType1 ctx' . fromScope` entails doing three subsequent traversals: - `fromScope` traverses the term in `s`, which quotients placements of 'F' distributing them to the leaves - `inferType ctx'` then traverses the result of `fromScope` to infer the type - `toScope` transforms the inferred type to a generalised De Bruijn form, requiring a full tree traversal. Also, the new context, `ctx'`, is defined in term of a traversal (fmap). Also note that, for every Scope I walk under, `ctx'` gets an additional traversal. Doing so many traversals seems/is expensive, so after some thinking I improved `inferType2` to: > inferType2 ctx (Bind (Lam b) s) = Bind (Pi b) s' > where > s' = Scope . inferType2 ctx' . unscope $ s > ctx' = unvar bCtx fCtx > bCtx _ = fmap (F . Var) . extract $ b > fCtx = fmap (F . Var) . inferType2 ctx Where the only traversal needed to get `s'` is done by `inferType ctx'`. However, the new context, `ctx'`, remains defined in terms of traversals (fmap). Again, note that, for every Scope I walk under, `ctx'` gets an additional traversal. I could get rid of the remaining traversals in `ctx'` by implementing `inferType3` as: > inferType3 ctx (Bind (Lam b) s) = Bind (Pi b) s' > where > s' = Scope . inferType3 ctx' . unscope $ s > ctx' = unvar bCtx fCtx > bCtx _ = Var . F . extract $ b > fCtx = Var . F . inferType3 ctx But if I do that I start running into problems elsewhere, because the context is now returning lifted expressions. These problems arise for example in `inferPi`, which checks if the type of a term is a pi-binder: > inferPi :: Eq a > => (Term n a -> Term n a) -- ^ Type inference function > -> Term n a > -> (n,Term n a,Scope (Name n ()) (Term n) a) > inferPi inferTy tm = case inferTy tm of > Bind (Pi b) s -> (name b, extract b, s) > t -> error "Function expected" Let's assume that I'm doing type inference for the term: > \(a:*).\(b:*).\(f:(pi(_:a).b).\(x:a).f x Where `*` is `Universe 0`. In the application `f x`, I need to determine if `f` has a pi-type. When using either `inferType` or `inferType2`, I will see that `f` has the shape: > Bind (Pi {binder = Name "_" (Var (F (Var (F (Var (F (Var (B (Name "a" ())))))))))}) > (Scope (Var (F (Var (F (Var (F (Var (B (Name "b" ())))))))))) And so, `inferPi` will have no problems seeing that `f` has a pi-type. However, when using `inferType3`, where the context, `ctx`, returns lifted expressions , `f` has the shape: > Var (F (Var (F (Bind (Pi {binder = Name "_" (Var (F (Var (B (Name "a" ())))))}) > (Scope (Var (F (Var (B (Name "b" ())))))))))) Now `inferPi` will have problems seeing that `f` has a pi-type, it will need to quotient out and distribute the (Var . F) first. The (non-)solution is to quotient and distribute on demand, by defining `inferType4`: > inferType4 :: Eq a > => (a -> Term n a) -- ^ Context > -> (Term n a -> Term n a) -- Quotient and distribute (Var . F) > -> Term n a -- ^ Term > -> Term n a -- ^ Inferred type > inferType4 ctx dist (Var a) = dist (ctx a) > inferType4 ctx dist (Bind (Lam b) s) = Bind (Pi b) s' > where > s' = Scope . inferType4 ctx' dist' . unscope $ s > ctx' = unvar bCtx fCtx > bCtx _ = Var . F . extract $ b > fCtx = Var . F . inferType4 ctx dist > > dist' (Var (F a)) = fmap (F . Var) . dist $ a > dist' e = e However, `inferType4` simply moves the traversal from one place to the other, giving us no improvement over `inferType2`. What we do however see is that, after having descended two Scopes, we would get a `dist'` function that does: > fmap (F . Var) . fmap (F . Var) . dist This gives me hope that instead of having to do two traversals using `fmap`, we could get one traversal of the form: > fmap (F . Var . F . Var) . dist According to the Functor law. I have absolutely no idea how to achieve this single traversal solution though. This leads me to have several, quite general, questions: - Is there a way to get a `dist` function, as defined in `inferType4`, that does a single traversal? - Would it be possible to even get rid of the traversal for the context `ctx`? Perhaps by approaching the `inferType` function completely differently? - Or is there simply no way to implement `inferType` for free (without traversals) using generalised De Bruijn indices? The complete code can be found at: https://gist.github.com/christiaanb/4e0fd1777aee927999aa Cheers, Christiaan Baaij [1] http://math.andrej.com/2012/11/08/how-to-implement-dependent-type-theory-i/ [2] http://hackage.haskell.org/package/bound-1.0.3

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ANN: HacBerlin - Haskell Hackathon in Berlin, 26-28 Sep 2014
by Stefan Wehr 14 Jul '14

14 Jul '14

Dear Haskellers, another Haskell Hackathon is waiting for you! Where: Berlin, Germany When: Fri 26 - Sun 28 September 2014 Meet in Berlin, discuss, hack together and improve the Haskell infrastructure. We welcome all programmers interested in Haskell, beginners and experts! For all details, visit our wiki page (http://www.haskell.org/haskellwiki/HacBerlin2014) and make sure to register early. By the way: we are currently looking for one or two keynote talks. If you feel qualified, please let us know. Happy hacking, Stefan

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book "Haskell Data Analysis Cookbook" by Nishant Shukla
by Vasili I. Galchin 14 Jul '14

14 Jul '14

I believe there is a non-thread safe code fragment in Chapter 1(page 15): import System.Directory (doesFileExist) ..... exist <- doesFileExist filename -- these two lines are non-thread safe .. yes?? to be thread-safe the above line and the following line would have to be together atomic .... input <- if exists then readFile filename else return "" ........... Vasili

3 4

What is the state of the art in testing code generation?
by Tom Ellis 14 Jul '14

14 Jul '14

I am implementing an EDSL that compiles to SQL and I am wondering what is the state of the art in testing code generation. All the Haskell libraries I could find that deal with SQL generation are tested by implementing multiple one-off adhoc queries and checking that when either compiled to SQL or run against a database they give the expected, prespecified result. * https://github.com/prowdsponsor/esqueleto/blob/master/test/Test.hs * https://github.com/m4dc4p/haskelldb/blob/master/test/TestCases.hs * https://github.com/yesodweb/persistent/blob/master/persistent-test/SumTypeT… I couldn't find any tests for groundhog. * https://github.com/lykahb/groundhog I also had a look at Javascript generators. They take a similar adhoc, one-off approach. * https://github.com/valderman/haste-compiler/tree/master/Tests * https://github.com/faylang/fay/tree/master/tests Is this the best we can do in Haskell? Certainly it seems hard to use a QuickCheck/SmallCheck approach for this purpose. Is there any way this kind of testing can be automated or made more robust? Thanks, Tom

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Diony Rosa - 7/14/2014 2:14:04 AM
by Diony Rosa 14 Jul '14

14 Jul '14

http://butikpastalar.net/rhad/cfttllmcrlpibdgdyufrzbhvsd.aucahicfstvtogdrs

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[ANN] cabal-bounds 0.7: update bounds by haskell platform release
by Daniel Trstenjak 13 Jul '14

13 Jul '14

Hi cafe, cabal-bounds[1] is a command line program for managing the bounds/versions of the dependencies in a cabal file. cabal-bounds 0.7 adds the feature to set the bounds of dependencies to the library versions used by a haskell platform[2] release. For further details please consult the README[3]. Greetings, Daniel [1] https://github.com/dan-t/cabal-bounds [2] https://www.haskell.org/platform/ [3] https://github.com/dan-t/cabal-bounds/blob/master/README.md

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Re: [Haskell-cafe] book "Haskell Data Analysis Cookbook" by Nishant Shukla
by Vasili I. Galchin 13 Jul '14

13 Jul '14

well .. if my memory is correct .. a previous example was based on using an exemption to make the code safe (.. found it on pg. 14) ..... i was just being lazy(my total bad) .. not being stupid .. ... in any case, aren't books refereed ....not impugning the author .. hey I make mistakes .. e.g. working too fast .. too much booze the night before ... :-)

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[ANNOUNCE] New release of SBV (v3.1)
by Levent Erkok 12 Jul '14

12 Jul '14

I'm pleased to announce v3.1 release of SBV, a library for integrating SMT solvers into Haskell. This release coincides with GHC 7.8.3: A a prior bug in the 7.8 series caused SBV to crash under heavy load. GHC 7.8.3 fixes this bug; so if you're an SBV user, please upgrade to both GHC 7.8.3 and your version of SBV. Also new in this release are two oft-requested features: - Parallel solving capabilities: Using multiple SMT solvers at the same time to get the fastest result (speed), or get all results (to make sure they all behave the same way, safety). - A variant of symbolic if-then-else (called sBranch) that can call the external solver during simulation before it symbolically simulates "then" and "else" branches. This is useful for programming with recursive functions where termination depends on symbolic values. Full release notes: https://github.com/LeventErkok/sbv/blob/master/CHANGES.md SBV web page: http://leventerkok.github.io/sbv/ As usual, bug reports and feedback are most welcome! -Levent.

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help with some code
by gbwey9 12 Jul '14

12 Jul '14

Hi Cafe, I've got some code where a user can provide a filter to subscribe to a particular type of event. The events come in as xml and are parsed using the toEvent method. The code works fine but is repetitive since each event record potentially has the information needed to parse the xml using the field names and types. So is there a way to get rid of the repetition and streamline the code? Should I be using TH or lenses or something else entirely? Here is the paste. http://lpaste.net/107338 Thanks for any pointers! Grant

2 2

regular-xmlpickler
by gbwey9 12 Jul '14

12 Jul '14

Hi, I am using a very cool package called regular-xmlpickler. Does anyone know of a way to skip one level of nesting for nested complex fields? So I would like to elide that outer/inner Header. Here is the full code: http://lpaste.net/107362 data User = User { name :: String , admin :: Bool , dt :: UTCTime , header :: Header } data Header = Header { header1 :: String , header2 :: String } so instead of generating <header> twice <header> <header> <header1>abb</header1> <header2>abb</header2> </header> </header> I would like to have it generate this. <header> <header1>abb</header1> <header2>abb</header2> </header> Thanks, Grant

2 2

Re: [Haskell-cafe] [Haskell] ANNOUNCE: GHC version 7.8.3
by Henk-Jan van Tuyl 12 Jul '14

12 Jul '14

On Fri, 11 Jul 2014 15:40:54 +0200, Austin Seipp <austin(a)well-typed.com> wrote: > Included in this email is a signed copy of the SHA256 hashes for the > tarballs, using my GPG key (keyid 0x3B58D86F). Where can I find your public key? Regards, Henk-Jan van Tuyl -- Folding@home What if you could share your unused computer power to help find a cure? In just 5 minutes you can join the world's biggest networked computer and get us closer sooner. Watch the video. http://folding.stanford.edu/ http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --

2 1

Haskell Platform release schedule?
by Dominick Samperi 11 Jul '14

11 Jul '14

The Haskell Platform wiki page says the platform follows a 6-month release schedule, yet the last release occurred more than one year ago, and I couldn't find information about future plans. Are there plans to update the platform with ghc 7.8.2? Thanks, Dominick

6 9

Embedding version info in executables
by Roman Cheplyaka 11 Jul '14

11 Jul '14

What are existing solutions for embedding version info (git revision, build date/time, versions of dependencies) in Haskell programs? Roman

6 6

OCL 2014: Submission Deadline Extended by One Week
by Achim D. Brucker 11 Jul '14

11 Jul '14

(Apologies for duplicates) ************************************************************** ** Submission Deadline Extended to July 18th, 2014 ** ************************************************************** CALL FOR PAPERS 14th International Workshop on OCL and Textual Modeling Applications and Case Studies (OCL 2014) Co-located with ACM/IEEE 17th International Conference on Model Driven Engineering Languages and Systems (MODELS 2014) September 30, 2014, VALENCIA, SPAIN http://www.software.imdea.org/OCL2014/ Modeling started out with UML and its precursors as a graphical notation. Such visual representations enable direct intuitive capturing of reality, but some of their features are difficult to formalize and lack the level of precision required to create complete and unambiguous specifications. Limitations of the graphical notations encouraged the development of text-based modeling languages that either integrate with or replace graphical notations for modeling. Typical examples of such languages are OCL, textual MOF, Epsilon, and Alloy. Textual modeling languages have their roots in formal language paradigms like logic, programming and databases. The goal of this workshop is create a forum where researchers and practitioners interested in building models using OCL or other kinds of textual languages can directly interact, report advances, share results, identify tools for language development, and discuss appropriate standards. In particular, the workshop will encourage discussions for achieving synergy from different modeling language concepts and modeling language use. The close interaction will enable researchers and practitioners to identify common interests and options for potential cooperation. Topics of interest include (but are not limited to) =================================================== - Mappings between textual modeling languages and other languages/formalisms - Algorithms, evaluation strategies and optimizations in the context of textual modeling languages for -- validation, verification, and testing, -- model transformation and code generation, -- metamodeling and DSLs, and -- query and constraint specifications - Alternative graphical/textual notations for textual modeling languages - Evolution, transformation and simplification of textual modeling expressions - Libraries, templates and patterns for textual modeling languages - Complexity results for textual modeling languages - Quality models and benchmarks for comparing and evaluating textual modeling tools and algorithms - Successful applications of textual modeling languages - Case studies on industrial applications of textual modeling languages - Experience reports -- usage of textual modeling languages and tools in complex domains, -- usability of textual modeling languages and tools for end-users - Empirical studies about the benefits and drawbacks of textual modeling languages - Innovative textual modeling tools - Comparison, evaluation and integration of modeling languages - Correlation between modeling languages and modeling tasks This year, we particularly encourage submissions describing applications and case studies of textual modeling as well as test suites and benchmark collections for evaluating textual modeling tools. Venue ===== The workshop will be organized as a part of MODELS 2014 Conference in Valencia, Spain. It continues the series of OCL workshops held at UML/MODELS conferences: York (2000), Toronto (2001), San Francisco (2003), Lisbon (2004), Montego Bay (2005), Genova (2006), Nashville (2007), Toulouse (2008), Denver (2009), Oslo (2010), Zurich (2011, at the TOOLs conference), 2012 in Innsbruck, and 2013 in Miami. Similar to its predecessors, the workshop addresses both people from academia and industry. The aim is to provide a forum for addressing integration of OCL and other textual modeling languages, as well as tools for textual modeling, and for disseminating good practice and discussing the new requirements for textual modeling. Workshop Format =============== The workshop will include short (about 15 min) presentations, parallel sessions of working groups, and sum-up discussions. Submissions =========== Two types of papers will be considered: * short papers (6 pages) and * full papers (10 pages) in LNCS format. Submissions should be uploaded to EasyChair (https://www.easychair.org/conferences/?conf=ocl2014) The program committee will review the submissions (minimum 2 reviews per paper, usually 3 reviews) and select papers according to their relevance and interest for discussions that will take place at the workshop. Accepted papers will be published online in a pre-conference edition of CEUR (http://www.ceur-ws.org) Authors of selected papers will be invited to submit an extended version of their workshop paper to a special issue of the Electronic Communications of the EASST (http://journal.ub.tu-berlin.de/eceasst) Important Dates =============== Submission of papers: July 18, 2014 (extended) Notification: August 8, 2014 Workshop date: September 30, 2014 Organizers ========== Achim D. Brucker, SAP AG, Germany Carolina Dania, IMDEA Software Institute, Madrid, Spain Geri Georg, Colorado State University, Fort Collins, Colorado, USA Martin Gogolla, University of Bremen, Germany Programme Committee =================== Michael Altenhofen, SAP AG, Germany Thomas Baar, University of Applied Sciences Berlin, Germany Mira Balaban, Ben-Gurion University of the Negev, Israel Tricia Balfe, Nomos Software, Ireland Fabian Buettner, Ecole des Mines de Nantes, France Achim D. Brucker, SAP AG, Germany Jordi Cabot, INRIA-Ecole des Mines de Nantes, France Yoonsik Cheon, University of Texas, USA Dan Chiorean, Babes-Bolyai University, Romania Robert Clariso, Universitat Oberta de Catalunya, Spain Tony Clark, Middlesex University, UK Manuel Clavel, IMDEA Software Institute, Madrid, Spain Carolina Dania, IMDEA Software Institute, Madrid, Spain Birgit Demuth, Technische Universitat Dresden, Germany Marina Egea, Atos Research, Madrid, Spain Geri Georg, Colorado State University, Fort Collins, Colorado, USA Martin Gogolla, University of Bremen, Germany Pieter Van Gorp, Eindhoven University of Technology, The Netherlands Heinrich Hussmann, LMU Munchen, Germany Tihamer Levendovszky, Vanderbilt University, USA Shahar Maoz, Tel Aviv University, Israel Istvan Rath, Budapest University of Technology and Economics, Hungary Bernhard Rumpe, RWTH Aachen, Germany Shane Sendall, Snowie Research SA, Switzerland Michael Wahler, ABB Switzerland Ltd Corporate Research, Switzerland Claas Wilke, Technische Universitat Dresden, Germany Edward Willink, Willink Transformations Ltd., UK Burkhart Wolff, Univ Paris-Sud, France Steffen Zschaler, King’s College, London, UK -- Dr. Achim D. Brucker, SAP SE, Vincenz-Priessnitz-Str. 1, D-76131 Karlsruhe Phone: +49 6227 7-52595, http://www.brucker.ch/

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ANN: FFI bindings to cuBLAS and cuSPARSE
by Ben Sherman 11 Jul '14

11 Jul '14

I have written FFI bindings to the cuBLAS and cuSPARSE libraries, which are CUDA libraries for executing linear algebra computations on the GPU. It's a relatively straightforward translation of the C API. It's slightly novel in that I use language-c and Template Haskell to parse the C headers and create the FFI declarations, avoiding the boilerplate that may otherwise be necessary, even using a preprocessor such as c2hs. http://hackage.haskell.org/package/cublas-0.2.0.0 I've done a similar thing with a subset of the MAGMA GPU library. It's less polished, and the installation process is more unforgiving, so I haven't put it up on Hackage. https://github.com/bmsherman/magma-gpu Finally, I've written a library which abstracts the immutable API of hmatrix and provides a pure, hmatrix-like interface for cuBLAS/MAGMA, enabling simultaneous development of linear algebra programs using either hmatrix or the above GPU bindings as backends. Additionally, I have written "medium-level" mutable and immutable interfaces to cuBLAS/MAGMA (in LinAlg-magma). I use a type system trick reminiscent of the ST monad to allow mutable operations to read from either immutable or mutable datatypes, so it is easy to mix pure and destructive computations in a safe manner. I really made these libraries only complete enough to suit my purposes; I imagine many improvements can be made. https://github.com/bmsherman/LinAlg https://github.com/bmsherman/LinAlg-magma https://github.com/bmsherman/LinAlg-hmatrix Any feedback, bug reports, or pull requests are most welcome!

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Meetup - Calling all haskellers from Croatia
by Deni Bertović 10 Jul '14

10 Jul '14

We've opened a meetup group for all haskellers from Croatia and someone suggested we should post info about it here as it might reach more interested people that way. Here's the link to the meetup group (please join): http://www.meetup.com/haskellzg/ Sorry for the spam if this doesn't apply to you. -Deni

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Re: [Haskell-cafe] Taking over setlocale
by Clint Adams 10 Jul '14

10 Jul '14

On Tue, Jul 08, 2014 at 03:41:00PM +0200, Sven Bartscher wrote: > To solve this problem, I rewrote the setlocale binding with the same API > under the BSD3-clause. Have you considered submitting this as a patch to the unix package, since setlocale() is POSIX?

2 1

security update practice?
by Mark Wotton 10 Jul '14

10 Jul '14

Hi all, there was a security update to the underlying library to one of my bindings last night (lz4) and it got me thinking - how do we handle security updates as a community? I typically find out from IRC or twitter now, which isn't particularly reliable. Might it be possible to mark an update on Hackage as a security update rather than feature update? cheers Mark -- A UNIX signature isn't a return address, it's the ASCII equivalent of a black velvet clown painting. It's a rectangle of carets surrounding a quote from a literary giant of weeniedom like Heinlein or Dr. Who. -- Chris Maeda

5 4

Webdriver: Couldn't match type `Element' with `WD Element'
by Cody Goodman 09 Jul '14

09 Jul '14

I received some help/guidance in #haskell, but I still can't get this to work. I'm basically trying to find all the text elemnts in a webpage with the webdriver package. Here is my code and errors: {-# LANGUAGE OverloadedStrings #-} import Control.Monad import Control.Monad.IO.Class import Test.WebDriver import Test.WebDriver.Classes (WebDriver (..)) import Test.WebDriver.Commands import Test.WebDriver.Commands.Wait main = do runSession defaultSession capabilities $ do openPage "http://www.appnitro.com/demo/view.php?id=1" inputs <- findElems $ ByTag "input" textElems <- filterM (liftM $ ((==) "text" . (`attr` "type"))) inputs -- wait 20 seconds waitUntil 20 (getText <=< findElem $ ByCSS ".doesnotexist") `onTimeout` return "" liftIO $ putStrLn "done" where capabilities = allCaps { browser=firefox } -- [1 of 1] Compiling Main ( src/Main.hs, interpreted ) -- src/Main.hs:168:70: -- Couldn't match type `Element' with `WD Element' -- Expected type: [WD Element] -- Actual type: [Element] -- In the second argument of `filterM', namely `inputs' -- In a stmt of a 'do' block: -- textElems <- filterM -- (liftM $ ((==) "text" . (`attr` "type"))) inputs -- In the second argument of `($)', namely -- `do { openPage "http://www.appnitro.com/demo/view.php?id=1"; -- inputs <- findElems $ ByTag "input"; -- textElems <- filterM -- (liftM $ ((==) "text" . (`attr` "type"))) inputs; -- waitUntil 20 (getText <=< findElem $ ByCSS ".doesnotexist") -- `onTimeout` return "" }' -- Failed, modules loaded: none.

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Designing widgets in Reactive style
by JP Moresmau 09 Jul '14

09 Jul '14

Hello, I'm a bit confused on how to design graphical widgets to take advantage of FRP design. I'm trying to do things in threepenny-gui but I suppose the same question applies to any framework. Suppose I want to create a "file selection widget". Currently I have a function that takes a (FilePath -> UI()) function as a parameter: what to do when a file is selected; my function returns the "widget", so that the calling code can incorporate it in its UI. This works fine, but is not FRP, right? Should my function take no arguments and return a (Widget,Event FilePath) tuple, and let the calling code decide what to do with that event? Should it be a Behavior (the currently selected file of the widget)? I *think* I understand the principles of FRP, but I'm not getting what it means in terms of design. What are the advantages of returning an Event and letting the calling code do "on myEvent myWidget $ \file -> ..." instead of just passing the handler function as a parameter? I don't know if I make much sense. In any case, any pointer to non trivial FRP GUI code would be good! Thanks a million all, -- JP Moresmau http://jpmoresmau.blogspot.com/

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Turning a message parser into a protocol handler
by Magnus Therning 09 Jul '14

09 Jul '14

The last few days I've been playing around with the combination of attoparsec/blaze/conduit to build a parser and a generator of messages in a small proprietary protocol used at work. The goal is to easily build small clients of the protocol, and possibly, if time permits, build a server in Haskell which mimics the C implementation of the server. I've come so far that the message parsing and generating is working. I've also put together a couple of tiny clients to verify that my Haskell code can communicate with the production implementation of the server. I've done this by manually tying the toy example behaviour into my conduit process. However, I recognise that as the required behaviour becomes more and more complex my thinking about the behaviour more and more resembles a finite automaton (or FSM). This led me to look around for an FSM generator tool or library for Haskell, but found none. Did I miss something in my search? Or maybe Haskell already IS the FSM generator tool I need. /M -- Magnus Therning OpenPGP: 0xAB4DFBA4 email: magnus(a)therning.org jabber: magnus(a)therning.org twitter: magthe http://therning.org/magnus I invented the term Object-Oriented, and I can tell you I did not have C++ in mind. -- Alan Kay

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ANNOUNCE: sai-shape-syb Generic mapping to homogeneous types (etc.)
by Andrew Seniuk 09 Jul '14

09 Jul '14

This package provides some support for dealing with polytypic data. It lets you escape from the generics world and work with a homogeneous rose tree, which can sometimes be convenient. I realise a more experienced programmer would probably do this differently, but anyhow it was useful to me and I've uploaded it. http://hackage.haskell.org/package/sai-shape-syb There are some examples at http://fremissant.net/shape-syb In particular, it supports GHC staged traversals, so for those trying to work with the GHC AST this might be a helpful tool for filtering, debugging, and suchlike. Kind Regards, Andrew Seniuk rasfar on #haskell

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Travis CI
by Kazu Yamamoto 09 Jul '14

09 Jul '14

Hi, This is just FYI. As you may know, Travis CI itself provides multiple versions of GHC now: http://docs.travis-ci.com/user/languages/haskell/ But I was suffering from test failures indicated by: ghc_find: command not found I heard that this problem is fixed today. https://github.com/travis-ci/travis-ci/issues/2364 --Kazu

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How to have a managed window unfocusable?
by Magicloud Magiclouds 08 Jul '14

08 Jul '14

Hi, I'd like to have a transparent clock floating on my desktop. If I made it ignored (unmanaged), it would not be above and would be blocked by other windows. If I made it managed and float, when I floated up and down, it would be focused and I needed to press hotkey again to move focus to correct window. Anyway I could manage it, but not focusing on it? -- 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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Nottingham Research Fellowships
by Graham Hutton 08 Jul '14

08 Jul '14

Dear all, The University of Nottingham is seeking applications for a number of Nottingham Research Fellowships: http://tinyurl.com/notts-fellows These are highly prestigious three-year fellowships, which are normally expected to lead into permenant academic positions. Candidates should have no more than 8 years postdoctoral experience, and fellowships normally start in October 2015. The deadline for submission of initial Expressions of Interest is ** 20th October 2014 **. Applicants in the area of the Functional Programming (FP) lab would be most welcome. The FP lab is keen to receive applications from outstanding candidates with an excellent publication record, experience in combining theory with practice, and the ability to secure external funding to support their research. Further information about the FP lab is available from http://fp.cs.nott.ac.uk As an approximate guideline, candidates in the area of the FP lab would normally be expected to have at least 3 years postdoc experience, and a number of strong publications in leading international venues such as ICFP, POPL, LICS, JFP, Haskell Symposium, etc. Best wishes, Graham Hutton -- Prof Graham Hutton Functional Programming Lab School of Computer Science University of Nottingham, UK http://www.cs.nott.ac.uk/~gmh This message and any attachment are intended solely for the addressee and may contain confidential information. If you have received this message in error, please send it back to me, and immediately delete it. Please do not use, copy or disclose the information contained in this message or in any attachment. Any views or opinions expressed by the author of this email do not necessarily reflect the views of the University of Nottingham. This message has been checked for viruses but the contents of an attachment may still contain software viruses which could damage your computer system, you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

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