April 2014 - Haskell-Cafe

Best practice to target different monad transformer libraries?
by Ismael Figueroa 14 Apr '14

14 Apr '14

I am developing the effective-aspects package, which currently depends on the mtl library. I want to develop a new version based on the "Monads, Zippers and Views" approach, developed by Schrijvers and Oliveira in ICFP'11. The question is: do I need to publish a new package, say, effective-aspects-mzv, which will be an almost duplicate of the original package? or is there another more elegant way to deal with this situations? Thanks -- Ismael

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Patches for (abandoned?) pcre-light
by Nikolay Amiantov 14 Apr '14

14 Apr '14

Hello, I've tried the new ghc-7.8.1 and have had an issue with building pcre-light as one of dependencies for my project. I've made a patch which fixes this, but I can't reach maintainer by e-mail -- what can I do in this situation? Also, the second (unrelated) question would be: how can I find a package which depends on it in my project? I have found no way to dump a dependency tree in cabal. Thanks in advance, Nikolay.

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Using GHC.Generics to print data type structure
by J. Stutterheim 14 Apr '14

14 Apr '14

Hi all, I'm trying to use GHC.Generics to print a data type's structure. I have a test type TestRecord: data TestRecord = TestRecord { trId :: Int , someStr :: String } deriving Generic And a PrintDT class: class PrintDT a where printDT :: a -> String default printDT :: (Generic a, GPrintDT (Rep a)) => a -> String printDT = gprintDT . from With a corresponding instance for TestRecord. The GPrintDT class and instances are defined as follows: class GPrintDT f where gprintDT :: f a -> String instance (GPrintDT a, GPrintDT b) => GPrintDT (a :*: b) where gprintDT (x :*: y) = " { " ++ gprintDT x ++ ", " ++ gprintDT y ++ " }" instance (Datatype d, GPrintDT f) => GPrintDT (D1 d f) where gprintDT d = "data " ++ datatypeName d ++ " = " ++ (gprintDT $ unM1 d) instance (Constructor c, GPrintDT f) => GPrintDT (C1 c f) where gprintDT con | conIsRecord con = conName con ++ (gprintDT $ unM1 con) | otherwise = "No record" instance (Selector s, GPrintDT a) => GPrintDT (S1 s a) where gprintDT m = selName m instance (PrintDT a) => GPrintDT (K1 i a) where gprintDT _ = "" instance PrintDT Int where printDT n = show n instance PrintDT String where printDT xs = xs In a first attempt, I apply printDT to a TestRecord value: test1 :: String test1 = printDT (TestRecord 1 "foo") This prints the expected result: "data TestRecord = TestRecord { trId, someStr }" Now ideally, I wouldn't have to specify some value of TestRecord to get this output, since so far, I'm only printing the structure of the TestRecord type, not the values. I would want the following to give me the same result string: test2 :: String test2 = printDT (undefined :: TestRecord) With the current implementation, test2 gives me the following output: "data TestRecord = TestRecord*** Exception: Prelude.undefined Is there a way to implement GPrintDT such that test2 gives me the same output as test1? Thanks! - Jurriën

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Trouble splitting up source into multiple files, when using data abstraction.
by David Banas 14 Apr '14

14 Apr '14

Hi all, I’ve defined a typeclass, LogTree, and would like to put each instance definition in its own source file, in order that LogTree.hs not grow to a ridiculous length. I’m attempting to use data abstraction, in order to future-proof the user interface to this class. So, for instance, I don’t make all of the data constructors defined in LogTree accessible, via the module export list, but rather force the user to use certain “helper” functions, instead. However, the individual instance definitions do need access to these data constructors, but they’re in a different source file. Is this possible? That is, is it possible to provide different export lists to “friendly” vs. “unknown” client code? Thanks, -db

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ANN: haskell-src-exts-1.15.0
by Roman Cheplyaka 13 Apr '14

13 Apr '14

I'm pleased to announce the release of haskell-src-exts-1.15.0! Hackage: https://hackage.haskell.org/package/haskell-src-exts-1.15.0 GitHub: https://github.com/haskell-suite/haskell-src-exts Changes from the last 1.14.0.1 release: * Add support for extensions: - MultiWayIf - LambdaCase - DataKinds * Remove support for old (deprecated) Generics * Derive GHC's Generic instances for datatypes * Derive some missing Data and Typeable instances * Many bug fixes * Add missing Functor and Applicative instances for monads * Remove support for GHCs older than 7.4 This release is brought to you by: Adam Vogt Andrés Sicard-Ramírez Ben Gamari Daniel Oom Danylo Hlynskyi Michael Sloan Neil Mitchell Niklas Broberg Niklas Hambüchen Philipp Schuster phischu Roman Cheplyaka Stefan Wehr Stijn van Drongelen

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Handling Windows file name conventions without unix (Posix) package?
by Dominick Samperi 13 Apr '14

13 Apr '14

Hello, The unix package (System.Posix) is not supported under Windows and I wonder if there is an alternative solution that deals with the odd file naming convention under windows (backward vs unix forward slash), spaces in file names, etc. It looks like System.Environment is a good substitute for System.Posix.Env, but it is not clear how to deal with file naming conventions. Any pointers or tips would be much appreciated. Thanks, Dominick

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Video chair at ICFP
by Johan Jeuring 13 Apr '14

13 Apr '14

Dear people, We are busy organising ICFP 2014, which will happen together with the Haskell Symposium and many other events in Göteborg in Sweden from August 31 until September 6 2014. I am looking for a Video chair to organise the recording of the events on video. Tasks - prepare the video recording of the events: equipment (camera's, software, microphones?), required post-processing, ... - instruct student-volunteers in assisting the recording process - oversee the post-processing of the recordings, and the uploads Budget - we have some budget available for buying or renting equipment for the video recordings, or we can pay you for you bringing your own equipment - we will defray your costs for travelling to Göteborg, staying in Göteborg, and attending ICFP 2014 and affiliated events Please contact me if you are interested in this position/task, With kind regards, Johan Jeuring General chair for ICFP 2014

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warp vs scotty + some benchmark numbers
by Miro Karpis 13 Apr '14

13 Apr '14

Hi, I'm trying to make a small benchmarking for warp and scotty (later with json/no-json text performance test). My client is a Qt c++ application. I made a minimum code in both Haskell and C++. The problem is the numbers I'm getting. (Benchmark::pong-warp) - total requests: 10000 , send/received in msec: 3848 (Benchmark::pong-scotty) - total requests: 10000 , send/received in msec: 3814 Only sending 10k requests from my c++ client takes around 1 sec. What I don't understand is: a) how is it possible that scotty is so close to warp (in this run it even wins)? b) 10k requests I'm getting in approximately 4 seconds. Here<http://www.yesodweb.com/blog/2011/03/preliminary-warp-cross-language-benchm…>is one 2 years old benchmark with 80k/second. Why is there so big difference? I'm running on macbook-pro 16g, 2.3 GHz Scotty and warp is compiled with: -O2 -threaded --scotty import Web.Scotty import Data.Monoid (mconcat) main = scotty 3005 $ do get "/" $ do text "pong" --warp import Network.Wai(responseLBS, Application) import Network.HTTP.Types(status200) import Network.Wai.Handler.Warp (run) app :: Application app _ = return $ responseLBS status200 [("Content-Type", "text/plain")] "pong" main :: IO () main = do let port = 3000 putStrLn $ "Listening on port " ++ show port run port app --c++ client sample (Qt) simStart = QDateTime().currentMSecsSinceEpoch(); for(int i=0; i<requests; i++){ QNetworkReply *reply = manager->get(QNetworkRequest(url)); QEventLoop eventLoop; QObject::connect(manager, SIGNAL(finished(QNetworkReply *)), &eventLoop, SLOT(quit())); eventLoop.exec(); QObject::disconnect(manager, SIGNAL(finished(QNetworkReply *)), &eventLoop, SLOT(quit())); } qDebug() << "(Benchmark::pong) - total requests: " << requests << ", send in msec: " << (QDateTime().currentMSecsSinceEpoch()-simStart); cheers, miro

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Simple OverloadedLists question
by Konstantine Rybnikov 13 Apr '14

13 Apr '14

Hi! I wanted to check out great new OverloadedLists [0] feature, but got an error as in [1]. What am I doing wrong? Thanks. [0]: https://www.haskell.org/ghc/docs/7.8.1/html/users_guide/type-class-extensio… [1]: https://gist.github.com/k-bx/10574806

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OverloadedLists pattern-matching
by Konstantine Rybnikov 13 Apr '14

13 Apr '14

Continuing playing with OverloadedLists and GHC 7.8.2. For this code: ``` {-# LANGUAGE OverloadedLists #-} {-# LANGUAGE OverloadedStrings #-} {-# LANGUAGE TypeFamilies #-} import Data.Map import Data.Text import GHC.Exts instance (Ord k) => IsList (Map k v) where type Item (Map k v) = (k,v) fromList = Data.Map.fromList toList = Data.Map.toList main :: IO () main = do let m = [("foo", 1), ("bar", 2)] :: Map Text Int putStrLn "My map looks like this:" case m of [] -> putStrLn "impossible!" [x,_] -> putStrLn $ "ok, some random elem is: " ++ show x print m ``` I have this output from compiler: ``` root@b575c8a9c84b:~/overloaded_lists# cabal build --ghc-options="-fforce-recomp" Building overloaded-lists-0.1.0.0... Preprocessing executable 'overloaded-lists' for overloaded-lists-0.1.0.0... [1 of 1] Compiling Main ( src/Main.hs, dist/build/overloaded-lists/overloaded-lists-tmp/Main.o ) src/Main.hs:19:5: Warning: Pattern match(es) are non-exhaustive In a case alternative: Patterns not matched: _ src/Main.hs:9:10: Warning: Orphan instance: instance Ord k => IsList (Map k v) [1 of 1] Compiling Main ( src/Main.hs, dist/build/overloaded-lists/overloaded-lists-tmp/Main.o ) src/Main.hs:19:5: Warning: Pattern match(es) are non-exhaustive In a case alternative: Patterns not matched: _ src/Main.hs:9:10: Warning: Orphan instance: instance Ord k => IsList (Map k v) Linking dist/build/overloaded-lists/overloaded-lists ... ``` Couple things: 1. It shows same warnings two times. Is this a bug? 2. It doesn't seem to be that warning that pattern "_" wasn't matched should be there. Should I also create a bug report? Thank you!

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