- Haskell-Cafe - Haskell.org

Directed rounding in haskell ?
by Dominik Peteler 15 Apr '14

15 Apr '14

Hello list, I'm going to implement an interval arithmetic library (following the IEEE P1788) in Haskell and for that I need support for directed rounding to the nearest floating point number. I had a quick look at the Decimal package which comes with the `roundTo` function for rounding to a specified number of decimal places but it doesn't provide directed rounding. Is there way to get directed rounding to to the nearest float (maybe even with machine floats) ? I'm also aware of the AERN-* packages but they seem to be unmaintained. At least there is no version which builds with the current GHC. Does anybody know something about that ? Regards dominik

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Cabal flag default based on external program
by Sebastian Wiesner 14 Apr '14

14 Apr '14

Hello, I have a cabal package with some optional flags. The “reasonable” default for these flags depends on the target system, specifically on the result of some external program. Is there a way to dynamically set the default value of a Cabal flag based on the result of an external program in my .cabal file? Greetings, Sebastian Wiesner

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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 14 Apr '14

14 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 14 Apr '14

14 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 14 Apr '14

14 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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Docker image for GHC 7.8.2
by Konstantine Rybnikov 13 Apr '14

13 Apr '14

For people like me who would like to try out 7.8.2, but don't want to make new creative ways of making your system be able to handle multiple ghc/cabal/etc, I created a docker image. Source code, instructions how to build image yourself, or just pull/run it are here: https://bitbucket.org/k_bx/docker-ghc-7.8 Thanks. // I will make a github mirror once hg-git is fixed back by new dulwich lib release soon

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Dependencies (was Re: Patches for (abandoned?) pcre-light)
by Doug Burke 13 Apr '14

13 Apr '14

On Apr 12, 2014 9:10 PM, "Nikolay Amiantov" <nikoamia(a)gmail.com> wrote: > > 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. Is this the information that you want? http://packdeps.haskellers.com/ http://packdeps.haskellers.com/reverse Doug

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Feedback/Ideas for a Master Thesis about creating a Querying Tool for Hackage
by Victor Nithander 13 Apr '14

13 Apr '14

Hi, My name is Victor Nithander and I am currently doing my Master Thesis at the Computer Science and Engineering department at Chalmers in Gothenburg and would like to ask you for some help. My Master Thesis is about creating a querying tool for Hackage written in Haskell and I would like to know what kind of features you would like the querying tool to have. My plan is to design and implement an embedded language for queries to be expressed in and then implement a backend which accordingly takes care of those queries. I then plan to packet the tool as a package using Cabal and upload it to Hackage. Any feedback/ideas on features or other things about the tool would be greatly appreciated as I want to make a tool which benefits the community as much as possible. Examples of queries I'm thinking of to implement is: - Which language features (such as typeclass instances) does a certain package use? - How many packages uses a certain language feature? - Which packages does a certain package depend on? - Which packages depends on a certain package? Of course, this in no way limits what other type of queries could be made. I will also post this at the Haskell Section of reddit at http://www.reddit.com/r/haskell/, sorry for the redundancy. Feel free to respond to this either at this mailing list, at reddit, or as a personal email at nithande(a)student.chalmers.se though I think the best would be to have an open discussion about it. Thanks in advance! / Victor

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Re: [Haskell-cafe] Seems a wrong implementation of Graham Scan in Haskell Wiki
by Yang Leo 12 Apr '14

12 Apr '14

Ah so that’s how a mailing list works.. Sorry for my stupidness... On Apr 12, 2014, at 1:08 AM, Kim-Ee Yeoh <ky3(a)atamo.com> wrote: > > On Sat, Apr 12, 2014 at 12:01 AM, Yang Leo <jeremyrobturtle(a)gmail.com> wrote: > I’ve updated the Haskell wiki now. I haven’t learnt how to do unit tests in Haskell yet, > any tests on my implementations would be welcomed. > > wikipage: http://www.haskell.org/haskellwiki/Graham_Scan_Implementation > > Would you mind forwarding to the haskell-cafe mailing list as well? > > So that everyone else is kept updated? > > -- Kim-Ee

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Haddock markup questions
by Christiaan Baaij 11 Apr '14

11 Apr '14

Hello list, I have two questions regarding proper markup for my haddock documentation. I'm using the latest released haddock, version 2.14.2. The first is about deprecation pragma's and operators usint '<' and '>' I have the following deprecation message: > {-# DEPRECATED Comp "Use 'Applicative' interface and ('<^>') instead" #-} Haddock however generates the following: http://christiaanb.github.io/clash-prelude/CLaSH-Prelude.html#t:Comp Where 'Applicative' gets a proper link to the applicative type class... but '<^>' is translated to a link to '^'... which is a page that doesn't exists. I can get haddock to not generate a link using: > > {-# DEPRECATED Comp "Use 'Applicative' interface and ('\\<^\\>') instead" #-} But that's unwanted, as those backslashes show up in GHC(i) messages. So: how do I get haddock to not parse my '<^>' operator as a link/URL? My second question is again about getting links to operators in general. If you take a look at, e.g., http://hackage.haskell.org/package/base-4.7.0.0/docs/Control-Applicative.ht… You can see that proper documentation links are generated to the '<*>' and such operators. However, if you take a look at the documention I tried to generate for my library: http://christiaanb.github.io/clash-prelude/CLaSH-Signal-Implicit.html#v:-60… You can see that the referenced '^>' operator does not get a proper link. So my question: how do I get haddock to create proper links for my referenced operators. I should note that I create haddock docs using 'cabal haddock' instead of calling haddock directly. Best regards, Christiaan

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[ANN] Yi 0.8
by Mateusz Kowalczyk 11 Apr '14

11 Apr '14

We're pleased to announced Yi 0.8! While this isn't a huge release, there are some nice fixes and breaking changes compared to 0.7.0. Please note that 0.7.1 and 0.7.2 were just maintenance releases so these are lumped under this announcement. What I think is most notable is removal of Yi.Prelude, dropping GHC 7.4 support, making Yi work with GHC 7.8.1 (and soon 7.8.2 when that comes out sometime today), moving from data-accessor to lens and finally dropping the dead front-ends. Please visit us at https://github.com/yi-editor/yi Relevant part of the CHANGELOG: 0.8.0 ----- * This release works with GHC 7.8.1. * And doesn't work with GHC 7.4. * Lens is now used instead of data-accessor. Migration was mostly mechanical, patches for more idiomatic lens usage are welcome. * Yi.Prelude was getting complaints over the years so it's now gone. * Commandline flag for choosing config directory now works. * Vte and Cocoa (issue #481) frontends that were abandoned for years are removed. If you want to revive these or make new frontends (Qt, EFL, SDL, threepenny-gui, etc), patches are welcome! * Test files no longer make case sensitive filesystems mad (issue #458). * Yi no longer eats (20 x filesize) memory when opening a file. * Other bugfixes and usability tweaks here and there. Emacs-specific changes: * Dynamic reconfiguring yi with "M-x reload" now works (issue #515). * Cancel incremental search on cursor movement/return (issue #499). * Remove binding to cabal configure from emacs keymap (issue #522). * Bind M-{ and M-} to jump between paragraphs (issue #106). * Prompt the user for comment style when it's missing (issue #413). Vim2-specific changes: * C-n completion now uses words from all open buffer instead of just current one. * Introduced :buffer, :buffers and :bdelete commands and C-^ and C-6 normal mode bindings. * Meta modifier is now available for use in Vim2 bindings. * :s(ubstitute) now works with delimiters other than '/' (issue #461). * Introduced :cabal command. * More tests. 0.7.2 ----- * This is a maintenance release, upper bound set for QuickCheck. 0.7.1 ----- * This is a maintenance release, yi.cabal got cleaned up a little and dependencies got bumped, some problematic upper bounds were removed. Thanks! -- Mateusz K.

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Hackage upload log
by Michael Snoyman 11 Apr '14

11 Apr '14

There used to be a log of Hackage uploads available at [1], but "it's just not here" anymore. Has that content moved to a different URL? Or is there some new way to get that information? I'd like to know the upload dates of every package. I could definitely screenscrape fro the date on the package listing pages, but I was hoping for something a bit more elegant. [1] http://hackage.haskell.org/packages/archive/log

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Re: [Haskell-cafe] package 'permutation' no longer compiles and the maintainer is uncontactable
by Patrick Perry 11 Apr '14

11 Apr '14

It's fine with me if Sophie takes over maintenance of the "permutation" package. On Friday, April 11, 2014 at 11:03 AM, Patrick Perry wrote: > Hi Sophie, > > It's fine with me if you take over maintenance of the package. > > > Patrick > > On Friday, April 11, 2014 at 10:01 AM, Carter Schonwald wrote: > > > Ccing him now > > Get his ok and I'll give you uploaded acls > > > > On Friday, April 11, 2014, Sophie Taylor <sophie(a)traumapony.org (mailto:sophie@traumapony.org)> wrote: > > > Hi, > > > > > > The permutation package no longer compiles due to the change in comparison primops in 7.8.1. Unfortunately, the maintainer Patrick Perry no longer appears to be contactable (the email address bounces) and so cannot submit a patch. > > > > > > I have a patch I wish to upload and according to https://www.haskell.org/haskellwiki/Taking_over_a_package > > > > > > I have to state my intention to take over the maintainence of the package, so yeah, this is me doing that. > > > > > > Thanks, > > > Sophie Taylor (spacekitteh) > > > > > > > > > > > > > > > > > >

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Re: [Haskell-cafe] Seems a wrong implementation of Graham Scan in Haskell Wiki
by Kim-Ee Yeoh 11 Apr '14

11 Apr '14

On Fri, Apr 11, 2014 at 6:16 PM, Yang Leo <jeremyrobturtle(a)gmail.com> wrote: > I don't have the account on Haskell wiki. How can I get an account on it > so I can contribute? Look here: http://www.haskell.org/haskellwiki/HaskellWiki:Contributing -- Kim-Ee

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package 'permutation' no longer compiles and the maintainer is uncontactable
by Sophie Taylor 11 Apr '14

11 Apr '14

Hi, The permutation package no longer compiles due to the change in comparison primops in 7.8.1. Unfortunately, the maintainer Patrick Perry no longer appears to be contactable (the email address bounces) and so cannot submit a patch. I have a patch I wish to upload and according to https://www.haskell.org/haskellwiki/Taking_over_a_package I have to state my intention to take over the maintainence of the package, so yeah, this is me doing that. Thanks, Sophie Taylor (spacekitteh)

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Seems a wrong implementation of Graham Scan in Haskell Wiki
by Yang Leo 11 Apr '14

11 Apr '14

The implementation in the page: http://www.haskell.org/haskellwiki/Graham_Scan_Implementation seems to be wrong. Given a Point list as follows: pts5 = [Point (0,0), Point (0,4), Point (1,4), Point (2,3), Point (6,5), Point (6,0)] The gscan will output a wrong answer which included a inner point Point (1,4). That is because the implemented algorithm cannot handle the situation where multiple consecutive points need to be removed. Here is my implementation: ``` module GrahamScan where import Data.List (sortBy) import Data.Function (on) -- 9. define data `Direction` data Direction = GoLeft | GoRight | GoStraight deriving (Show, Eq) -- 10. determine direct change via point a->b->c -- -- define 2D point data Pt = Pt (Double, Double) deriving (Show, Eq, Ord) isTurned :: Pt -> Pt -> Pt -> Direction isTurned (Pt (ax, ay)) (Pt (bx, by)) (Pt (cx, cy)) = case sign of EQ -> GoStraight LT -> GoRight GT -> GoLeft where sign = compare ((bx - ax) * (cy - ay)) ((cx - ax) * (by - ay)) -- 12. implement Graham scan algorithm for convex -- -- Helper functions -- -- Find the most button left point buttonLeft :: [Pt] -> Pt buttonLeft [] = Pt (1/0, 1/0) buttonLeft [pt] = pt buttonLeft (pt:pts) = minY pt (buttonLeft pts) where minY (Pt (ax, ay)) (Pt (bx, by)) | ay > by = Pt (bx, by) | ay < by = Pt (ax, ay) | ax < bx = Pt (ax, ay) | otherwise = Pt (bx, by) -- -- Main convex :: [Pt] -> [Pt] convex [] = [] convex [pt] = [pt] convex [pt0, pt1] = [pt0, pt1] convex pts = scan [pt0] spts where -- Find the most buttonleft point pt0 pt0 = buttonLeft pts -- Sort other points `ptx` based on angle <pt0->ptx> spts = tail (sortBy (compare `on` compkey pt0) pts) where compkey (Pt (ax, ay)) (Pt (bx, by)) = (atan2 (by - ay) (bx - ax), {-the secondary key make sure collinear points in order-} abs (bx - ax)) -- Scan the points to find out convex -- -- handle the case that all points are collinear scan [p0] (p1:ps) | isTurned pz p0 p1 == GoStraight = [pz, p0] where pz = last ps scan (x:xs) (y:z:rsts) = case isTurned x y z of GoRight -> scan xs (x:z:rsts) GoStraight -> scan (x:xs) (z:rsts) -- I choose to skip the collinear points GoLeft -> scan (y:x:xs) (z:rsts) scan xs [z] = z : xs ``` The source file is at https://github.com/robturtle/Haskell/blob/master/ch03/GrahamScan.hs Maybe not elegant for I just finished the first 3 chapters in Real World Haskell. However it at least correctly computes the example `pts5`

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How to find consumer packages on Hackage?
by Han Joosten 11 Apr '14

11 Apr '14

Hi, I wonder if there is a (simple?) way to find out which packages use some package foo? The reason I would like this is that I am interested in using configFile in a project, and I would like to look at some example usage of it. So in my case I am interested in knowing what packages use configFile. I presume there is a generic way to find out.... Cheers Han Joosten

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Confused about type seen in the wild
by Alexander Solla 10 Apr '14

10 Apr '14

I'm using the yesod-markdown package as a "template" for making my own yesod-mathjax package. I'm pretty much done, and it does compile now, but I'm just really confused by a type that looks like bad syntax. In fact, I ended up trying to "fix" it, which lead to errors: mathJaxField :: Monad m => RenderMessage (HandlerSite m) FormMessage => Field m MathJax Notice that there are two type arrows (=>). Interestingly, mathJaxField :: ( Monad m , RenderMessage (HandlerSite m) FormMessage ) => Field m MathJax compiles too. So it looks like currying/uncurrying, at the type level. But when did this start? I'm using GHC 7.6.3

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ANN.: TreeViz v0.0.6 release.
by David Banas 10 Apr '14

10 Apr '14

I’ve just posted version `v0.0.6’ of the TreeViz package to both Hackage and GitHub. This release fixes the “mixed decimation style failure” bug, in which a DIF stage followed by a DIT stage, followed by anything was giving incorrect numerical results for the FFT. Cheers, -db

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ANN.: TreeViz v0.0.6 release.
by David Banas 10 Apr '14

10 Apr '14

I’ve just posted version `v0.0.6’ of the TreeViz package to both Hackage and GitHub. This release fixes the “mixed decimation style failure” bug, in which a DIF stage followed by a DIT stage, followed by anything was giving incorrect numerical results for the FFT. Cheers, -db

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Stackage with GHC 7.8: build results
by Michael Snoyman 10 Apr '14

10 Apr '14

I'm happy to report that after about six months, enough upper bounds have been corrected on Hackage that a Stackage build with GHC 7.8 can now proceed. I've run a build with 7.8.0rc2, and have uploaded the build.log[1] and the complete cabal output for each package[2]. The good news is that most packages are building correctly, though there is still at least one package deep in the dependency tree which does not build (pcre-light). Here's a list of packages which failed to build, ignoring those which couldn't be built due to broken dependencies: * fpco-api (yes, I'm working on it) * gitlib * groundhog * monad-peel * parsestar * pcre-light * recursion-schemes * statistics * syb-extras * thyme * uniqueid [1] http://download.fpcomplete.com/ghc-7.8-builds/2014-04-09/build.log [2] http://download.fpcomplete.com/ghc-7.8-builds/2014-04-09/logs.zip

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Re: [Haskell-cafe] [Haskell] ANNOUNCE: GHC version 7.8.1
by Ketil Malde 10 Apr '14

10 Apr '14

Austin Seipp <austin(a)well-typed.com> writes: > The (Interactive) Glasgow Haskell Compiler -- version 7.8.1 Congrats! > * Massive scalability improvements to the I/O manager Just a brief note from the front lines - I recompiled a heavily threaded program with (a recent pre-release) of 7.8.1 and got a 10% performance increase over 7.6.3. Nothing like free lunches - so thanks again for the great work! -k -- If I haven't seen further, it is by standing in the footprints of giants

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Haskell Weekly News: Issue 290
by Daniel Santa Cruz 10 Apr '14

10 Apr '14

Welcome to issue 289 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers from April 1 to 5, 2014 Quotes of the Week * edwardk: Zombies are expensive. Top Reddit Stories * foldl is broken! Domain: well-typed.com, Score: 148, Comments: 63 On Reddit: [1] http://goo.gl/LXmJTV Original: [2] http://goo.gl/viOG5O * A small Haskell extension Domain: augustss.blogspot.nl, Score: 76, Comments: 23 On Reddit: [3] http://goo.gl/buI46t Original: [4] http://goo.gl/LnAV9y * New GHC Features for 7.10.1 and Beyond Domain: haskell.org, Score: 74, Comments: 26 On Reddit: [5] http://goo.gl/DdXTfO Original: [6] http://goo.gl/g0FPBS * Libc considered harmful Domain: sylvain-henry.info, Score: 58, Comments: 28 On Reddit: [7] http://goo.gl/aq39Mf Original: [8] http://goo.gl/qdQDXm * Category Theory by Tom LaGatta Domain: youtube.com, Score: 58, Comments: 53 On Reddit: [9] http://goo.gl/oIzG1e Original: [10] http://goo.gl/DeLu5G * Idris 0.9.12 released with major laziness, codata, and documentation updates plus metavars in types Domain: idris-lang.org, Score: 55, Comments: 25 On Reddit: [11] http://goo.gl/4MtXJS Original: [12] http://goo.gl/3tXbqC * A cool paper I found on "Functional Programming and 3D Games". It discusses everything from collision detection to physics and texturing. [PDF] Domain: cse.unsw.edu.au, Score: 52, Comments: 3 On Reddit: [13] http://goo.gl/6PYB2e Original: [14] http://goo.gl/Tf6fa2 * Error reporting with locations Domain: augustss.blogspot.fr, Score: 47, Comments: 28 On Reddit: [15] http://goo.gl/ikaENm Original: [16] http://goo.gl/hntlJT * [ANN] Haste now has a website and a mailing list Domain: haste-lang.org, Score: 42, Comments: 7 On Reddit: [17] http://goo.gl/Ife2FA Original: [18] http://goo.gl/lRWmhv * Haskell in the Newsroom Domain: infoq.com, Score: 41, Comments: 5 On Reddit: [19] http://goo.gl/M4tehD Original: [20] http://goo.gl/bpvgLA * Haskell application architecture best practices Domain: self.haskell, Score: 32, Comments: 23 On Reddit: [21] http://goo.gl/XjHfNJ Original: [22] http://goo.gl/XjHfNJ Top StackOverflow Questions * Haskell - Is it possible to make pointfree functions more readable using different combinators than (.)? votes: 11, answers: 1 Read on SO: [23] http://goo.gl/5b036l * How to detect if a program has been compiled using -threaded? votes: 10, answers: 2 Read on SO: [24] http://goo.gl/Cr7pMA * What does the star mean in this haskell code? votes: 9, answers: 2 Read on SO: [25] http://goo.gl/qnolS7 * What about arrows? votes: 9, answers: 2 Read on SO: [26] http://goo.gl/b1wwp4 * Triggering a documentation update for a Hackage package votes: 8, answers: 1 Read on SO: [27] http://goo.gl/ErTVzf * Combining Data.Dynamic and type classes votes: 7, answers: 0 Read on SO: [28] http://goo.gl/n8qMo9 * Systematically applying a function to all fields of a haskell record votes: 6, answers: 3 Read on SO: [29] http://goo.gl/tNkLGu * Transforming a List of 2-Tuples in Haskell votes: 6, answers: 3 Read on SO: [30] http://goo.gl/dDe8QP * Error in ghci which I cannot reproduce in written haskell file votes: 6, answers: 1 Read on SO: [31] http://goo.gl/bg8DCw * Why can I call a monadic function without supplying a monad? votes: 6, answers: 1 Read on SO: [32] http://goo.gl/xZZ5iO * Deriving default instances using GHC.Generics votes: 6, answers: 1 Read on SO: [33] http://goo.gl/Autu6r Until next time, [34]+Daniel Santa Cruz References 1. http://www.well-typed.com/blog/90/ 2. http://www.reddit.com/r/haskell/comments/21wvk7/foldl_is_broken/ 3. http://augustss.blogspot.nl/2014/04/a-small-haskell-extension.html 4. http://www.reddit.com/r/haskell/comments/2239wh/a_small_haskell_extension/ 5. http://www.haskell.org/pipermail/haskell-cafe/2014-April/113373.html 6. http://www.reddit.com/r/haskell/comments/21wbme/new_ghc_features_for_7101_a… 7. http://sylvain-henry.info/blog/posts/2014-04-01-libc-considered-harmful.html 8. http://www.reddit.com/r/haskell/comments/220ae4/libc_considered_harmful/ 9. https://www.youtube.com/watch?v=o6L6XeNdd_k 10. http://www.reddit.com/r/haskell/comments/229scs/category_theory_by_tom_laga… 11. http://www.idris-lang.org/idris-0-9-12-released/ 12. http://www.reddit.com/r/haskell/comments/2284s5/idris_0912_released_with_ma… 13. http://www.cse.unsw.edu.au/~pls/thesis/munc-thesis.pdf 14. http://www.reddit.com/r/haskell/comments/21zush/a_cool_paper_i_found_on_fun… 15. http://augustss.blogspot.fr/2014/04/haskell-error-reporting-with-locations.… 16. http://www.reddit.com/r/haskell/comments/226otd/error_reporting_with_locati… 17. http://haste-lang.org/ 18. http://www.reddit.com/r/haskell/comments/21x4wm/ann_haste_now_has_a_website… 19. http://www.infoq.com/presentations/haskell-newsroom-nyt 20. http://www.reddit.com/r/haskell/comments/222i82/haskell_in_the_newsroom/ 21. http://www.reddit.com/r/haskell/comments/223i80/haskell_application_archite… 22. http://www.reddit.com/r/haskell/comments/223i80/haskell_application_archite… 23. http://stackoverflow.com/questions/22872098/haskell-is-it-possible-to-make-… 24. http://stackoverflow.com/questions/22839824/how-to-detect-if-a-program-has-… 25. http://stackoverflow.com/questions/22807728/what-does-the-star-mean-in-this… 26. http://stackoverflow.com/questions/22871648/what-about-arrows 27. http://stackoverflow.com/questions/22783637/triggering-a-documentation-upda… 28. http://stackoverflow.com/questions/22876370/combining-data-dynamic-and-type… 29. http://stackoverflow.com/questions/22807619/systematically-applying-a-funct… 30. http://stackoverflow.com/questions/22828259/transforming-a-list-of-2-tuples… 31. http://stackoverflow.com/questions/22839784/error-in-ghci-which-i-cannot-re… 32. http://stackoverflow.com/questions/22841899/why-can-i-call-a-monadic-functi… 33. http://stackoverflow.com/questions/22850983/deriving-default-instances-usin… 34. https://plus.google.com/105107667630152149014/about

1 0

www.galois.com' response to Heartbleed .... .
by Vasili I. Galchin 10 Apr '14

10 Apr '14

http://corp.galois.com/blog/2014/4/9/heartbleed-a-great-time-to-think-about… BTW I am always very careful when I use memcpy .. .hate that function ... read "buffer overrun" big time .....

1 1

Automatically infer Newtype instance
by Dmitry Bogatov 09 Apr '14

09 Apr '14

Hello! I think, that Newtype instances are transitive, and there is only one sane definiton is (pack . pack). But in example below I have to use UndecidableInstances and they seems to loop somewhere (Context reduction stack overflow; size = 132). To my understanding, Ghc tries to prove, that exists only one type b, and somewhy fail at it(It is unclear, why), but is it any way to say to it that I take responsibility, that ANY b would be nice? I know about TH, but interested in more elegant solution. {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE FunctionalDependencies #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE UndecidableInstances #-} import Control.Newtype newtype A1 = A1 Int deriving (Eq, Show) newtype A2 = A2 A1 deriving (Eq, Show) instance Newtype A1 Int where pack = A1 unpack (A1 a) = a instance Newtype A2 A1 where pack = A2 unpack (A2 a) = a -- Here comes UndecidableInstances instance (Newtype a b, Newtype b c) => Newtype a c where pack = pack . pack unpack = unpack . unpack main = let foo :: A2 foo = pack "46" in print foo -- Best regards, Dmitry Bogatov <KAction(a)gnu.org>, Free Software supporter, esperantisto and netiquette guardian. git://kaction.name/rc-files.git GPG: 54B7F00D

4 6

Looking for maintainer of pwstore-fast
by Michael Snoyman 09 Apr '14

09 Apr '14

I just tried emailing Peter Scott, the maintainer of pwstore-fast[1], but the listed email address is no longer working. He also hasn't been responding to Github pings. Does anyone know how to get in touch with him? [1] http://hackage.haskell.org/package/pwstore-fast

2 2

Heart Bleed bug in OpenSSL
by Vasili I. Galchin 09 Apr '14

09 Apr '14

http://heartbleed.com/ Ok .. I just scanned this .. but is this problem a "logic" bug in the OpenSSL C/C++ code or is a type correctness issue? Thanks, Vasili

7 6

Re: [Haskell-cafe] ANNOUNCE: GHC version 7.8.1
by Felipe Lessa 09 Apr '14

09 Apr '14

Em 09-04-2014 11:23, Austin Seipp escreveu: > An updated SHA256SUMS.sig is attached. Thanks for Edsko de Vries for > pointing it out! If anybody is, like myself, trying to find a link for the SHA256SUMS file on the download page, here it is: https://www.haskell.org/ghc/dist/7.8.1/SHA256SUMS For some reason it seems that it's not linked anywhere on that page. Cheers, and thanks for the release! =) -- Felipe.

1 0

Library for creating interactive diagrams?
by Kai Zhang 09 Apr '14

09 Apr '14

Hi cafe, Is there any active project about creating interactive diagrams, like D3js http://d3js.org/ ? The Diagrams library has already done a good job of making static images. Would it be possible to extend this library to support declaring interactive diagrams and then use JS and HTML as backend to draw them? Example: circle 1 $ with & hover .~ (change style) PS: I don't want a D3js binding.

4 6

GHC Optimization seriously hurting performance by unsharing.
by Michael Karcher 09 Apr '14

09 Apr '14

Hello Haskell folks, Can you give me a hint whether the GHC optimizer behaviour demonstrated below should be considered a bug? If it is no bug, what should I keep in mind to make sure not to hit that trap again? This posting is (obviously) formatted as literal Haskell file, so feel free to try. This is a case where -O2 completely kills performance, because it unshares an object that should be shared. I stumbled across this behaviour of GHC 7.6.3 when trying to solve RMQ from hackerrank. Lets start with a typical header, the reason for the IORef module is discussed later. > module Main where > import Data.Set (member, fromAscList) > import Control.Monad (replicateM_) > import Data.IORef Now lets get to the code. > main = do The original problem statement requested to read a set of numbers from the standard input. To have this testcase self-contained, we use a "virtual file" instead, which is an (IORef String). The key point is that we need to get the string with the space-sperated test element from an IO operation, whereas it does not matter whether it is "getLine" or "readIORef" > ior <- newIORef $ unwords $ map show $ [1..20000] Now read the space-separated test input from an IO action. > testdata <- fmap (map read.words) $ readIORef ior and build a set from the number array we obtained to speed up membership queries. You don't want to search 20.000 numbers on each iteration. > let bigset = fromAscList testdata finally query the membership of the number 5 several times. This happens to be reasonably fast if compiled without optimization, but goes down to dog slow if compiled with optimization, as the set building process is hoisted into the loop and thus performed 10 times. > replicateM_ 10 . print . member 5 $ bigset this is the solution (or the workaround) to the problem. If you use this line, things get fast, because the loop gets created after evaluting the set to WHNF, thus forcing sharing replicateM_ 10 . print . member 5 $! bigset Enabling both of the replicateM_ lines also speeds up the first one, as the use of bigset in both lines prevents hoisting set construction into the first loop. Regards, Michael Karcher

2 1

ANNOUNCE slot-lambda
by ducis 08 Apr '14

08 Apr '14

http://hackage.haskell.org/package/slot-lambda It lets your write lambdas with 'slots' without inventing names for the parameters. [s| ı + ı |] = \x y -> x+y The unicode character ı(305) representing a 'slot' can be input in vim with the digraph 'i.' . Use _ı to refer to the immediate left ı, and _0, _1, _2, ... to refer to the 1st, 2nd, 3rd, ... arguments respectively. e.g. [s| ı : ı : _ı : ı : _ı : _ı : _0 : [] |] 'a' 'b' 'c' = (\x y z -> x:y:y:z:z:z:x:[]) 'a' 'b' 'c' = "abbccca" I originally intended using '_' to represent the 'slots' but that doesn't parse as legal haskell. Actually I wonder how to achieve that without rolling my own haskell parser or forking haskell-src-exts.

9 17

Towards a better time library (announcing tz)
by Mihaly Barasz 08 Apr '14

08 Apr '14

I would like to propose reforming the 'time' [1] library. Initially, I was just planning to announce my new 'tz' [2] library, but realized that I have a more important agenda. Namely: Haskell needs a better time library! Let me summarize what are ‒ in my view ‒ the biggest deficiencies of 'time': 1. Inefficient data structures and implementations. 2. Ad-hoc API which is hard to remember and frustrating to work with. 3. Conceptually wrong representations and/or missing concepts. The wonderful thyme [3] package (by Liyang HU) improves a lot on #1 by choosing better data structures and careful implementations and on #2 by lensifying the API. But, it was the #3 that caused me the most frustration lately; most importantly the time zone handling. There is a TimeZone data type in 'time', but it is a misnomer, it basically represents a fixed time difference (with a label and a DST flag). 'time' basically adapts the broken approach from libc: you can work with one time zone at a time, which is defined globally for your program (via the TZ environment variable). So, the transformation between UTCTime and LocalTime which should have been a pure function can only be done in IO now. Like this: do tz <- getTimeZone ut return $ utcToLocalTime tz ut Oh, and just to hammer down on the point #1 from the list above. This code runs in about 6100 ns on my machine. The drop-in replacement from tz: utcToLocalTimeTZ [4] (which is actually pure) runs in 2300 ns. While this is a significant improvement, it's easy to miss the point where the bulk of the inefficiency comes from the data structures. In my main project we represent times as Int64 (raw nanoseconds since UNIX epoch; and similar representation for zoned times). And to convert those to and from different time zones we need 40 ns. That's right, a 150 _times_ improvement! (There are many other interesting benchmark results that I could mention. An exciting bottom line: we can actually beat the libc in many use-cases!) The 'tz' package is still very much in flux. I will try to solidify the API soon, but until then it should be considered more of a proof of concept. There is some missing functionality, for example. On the other hand, there are the 'timezone-series' [5] and 'timezone-olson' [6] packages that together provide about the same functionality as 'tz' (minus the efficiency), and I'd like to explore if we could remove some of the overlap. But, all kind of suggestions and requests are welcome! More importantly, I'd like to hear the opinions of the community about the general issue of a better time library! Do we need one? How should we proceed about it? I think, Haskell could potentially have one of the best time libraries, but the current de-facto standard is mediocre at best. Unfortunately, designing a good time library is very far from trivial, as many existing examples demonstrate. And I definitely don't know enough for it. (I understand time zone info files, now that I wrote tz, but that's just a tiny fraction of what's needed.) So, if you think you can contribute to the design (have important use-cases in mind, know good examples of API, have some experience working with dates and time, etc. etc.) ‒ speak up! Mihaly Footnotes: [1] http://hackage.haskell.org/package/time [2] http://hackage.haskell.org/package/tz [3] http://hackage.haskell.org/package/thyme [4] http://hackage.haskell.org/package/tz-0.0.0.1/docs/Data-Time-Zones.html#v:u… [5] http://hackage.haskell.org/package/timezone-series [6] http://hackage.haskell.org/package/timezone-olson

4 9

Haskell + emacs + Ghc-mod 4.0.1
by Arnaud Bailly 08 Apr '14

08 Apr '14

Hello, I upgrade my emacs configuration with latest ghc-mod and all of sudden nothing works (again). When I try to load a .hs file from a project I got the first line highlighted with the error "cannot satisfy -package spec" and I cannot got type, info or doc from that file. Also I do not have anymore syntax errors highlighting I used to see. Any help greatly appreciated. I would also appreciate any pointer explaining how to setup and maintain a proper Haskell dev environment. Haskell is not the language I use at day work but I have been using it for years and every so often my emacs configuration is broken and I have a hard time getting it back to sane state. Thanks in advance Arnaud Here is my emacs configuration: ;; haskell coding (require 'haskell-mode) (require 'haskell-cabal) (autoload 'ghc-init "ghc" nil t) (add-hook 'haskell-mode-hook (lambda () (ghc-init))) (eval-after-load "haskell-mode" '(progn (define-key haskell-mode-map (kbd "C-,") 'haskell-move-nested-left) (define-key haskell-mode-map (kbd "C-.") 'haskell-move-nested-right) (define-key haskell-mode-map (kbd "C-c v c") 'haskell-cabal-visit-file) (define-key haskell-mode-map (kbd "C-c C-c") 'haskell-compile) (define-key haskell-mode-map (kbd "C-x C-d") nil) (setq haskell-font-lock-symbols t))) (add-hook 'haskell-mode-hook 'turn-on-haskell-decl-scan) (add-hook 'haskell-mode-hook 'turn-on-haskell-indentation) (eval-after-load "which-func" '(add-to-list 'which-func-modes 'haskell-mode)) (eval-after-load "haskell-cabal" '(define-key haskell-cabal-mode-map (kbd "C-c C-c") 'haskell-compile)) Here is the output of ls -l ~/Library/Haskell/bin: lrwxr-xr-x 1 arnaud staff 45 Dec 27 22:36 HsColour -> ../ghc-7.6.3/lib/hscolour-1.20.3/bin/HsColour lrwxr-xr-x 1 arnaud staff 50 Dec 31 07:11 aeson-pretty -> ../ghc-7.6.3/lib/aeson-pretty-0.7/bin/aeson-pretty lrwxr-xr-x 1 arnaud staff 52 Dec 31 07:11 biblio2yaml -> ../ghc-7.6.3/lib/pandoc-citeproc-0.2/bin/biblio2yaml lrwxr-xr-x 1 arnaud staff 49 Jan 10 16:25 cabal -> ../ghc-7.6.3/lib/cabal-install-1.18.0.2/bin/cabal lrwxr-xr-x 1 arnaud staff 46 Jan 15 13:51 cabal-dev -> ../ghc-7.6.3/lib/cabal-dev-0.9.2/bin/cabal-dev lrwxr-xr-x 1 arnaud staff 39 Dec 27 22:36 cpphs -> ../ghc-7.6.3/lib/cpphs-1.17.1/bin/cpphs lrwxr-xr-x 1 arnaud staff 43 Dec 28 14:17 doctest -> ../ghc-7.6.3/lib/doctest-0.9.10/bin/doctest lrwxr-xr-x 1 arnaud staff 55 Jan 15 13:51 fake-ghc-cabal-dev -> ../ghc-7.6.3/lib/cabal-dev-0.9.2/bin/fake-ghc-cabal-dev lrwxr-xr-x 1 arnaud staff 62 Jan 26 11:15 ghc-events -> ../../../projects/data-scientist/.cabal-sandbox/bin/ghc-events lrwxr-xr-x 1 arnaud staff 42 Apr 3 15:04 ghc-mod -> ../ghc-7.6.3/lib/ghc-mod-4.0.1/bin/ghc-mod lrwxr-xr-x 1 arnaud staff 43 Apr 3 13:35 ghc-modi -> ../ghc-7.6.3/lib/ghc-mod-4.0.1/bin/ghc-modi lrwxr-xr-x 1 arnaud staff 55 Jan 15 13:51 ghc-pkg-6_8-compat -> ../ghc-7.6.3/lib/cabal-dev-0.9.2/bin/ghc-pkg-6_8-compat lrwxr-xr-x 1 arnaud staff 47 Dec 31 07:11 hakyll-init -> ../ghc-7.6.3/lib/hakyll-4.4.2.0/bin/hakyll-init lrwxr-xr-x 1 arnaud staff 45 Mar 31 13:57 hasktags -> ../ghc-7.6.3/lib/hasktags-0.68.7/bin/hasktags lrwxr-xr-x 1 arnaud staff 36 Mar 17 06:09 hell -> ../ghc-7.6.3/lib/hell-0.0.1/bin/hell lrwxr-xr-x 1 arnaud staff 39 Dec 27 22:36 hlint -> ../ghc-7.6.3/lib/hlint-1.8.55/bin/hlint lrwxr-xr-x 1 arnaud staff 47 Apr 3 15:07 hspec-discover -> ../ghc-7.6.3/lib/hspec-1.9.1/bin/hspec-discover lrwxr-xr-x 1 arnaud staff 58 Dec 31 07:11 make-pandoc-man-pages -> ../ghc-7.6.3/lib/pandoc-1.12.2.1/bin/make-pandoc-man-pages lrwxr-xr-x 1 arnaud staff 62 Jan 15 04:25 operational-TicTacToe -> ../ghc-7.6.3/lib/operational-0.2.2.1/bin/operational-TicTacToe lrwxr-xr-x 1 arnaud staff 43 Jan 15 05:14 pandoc -> ../ghc-7.6.3/lib/pandoc-1.12.2.1/bin/pandoc lrwxr-xr-x 1 arnaud staff 56 Dec 31 07:11 pandoc-citeproc -> ../ghc-7.6.3/lib/pandoc-citeproc-0.2/bin/pandoc-citeproc lrwxr-xr-x 1 arnaud staff 44 Dec 27 19:34 pdf2line -> ../ghc-7.6.3/lib/pdf2line-0.0.1/bin/pdf2line lrwxr-xr-x 1 arnaud staff 43 Dec 27 19:34 pdfdump -> ../ghc-7.6.3/lib/pdf2line-0.0.1/bin/pdfdump lrwxr-xr-x 1 arnaud staff 42 Jan 15 05:26 word2vec -> ../ghc-7.6.3/lib/word2vec-0.1/bin/word2vec lrwxr-xr-x 1 arnaud staff 43 Dec 31 07:11 yaml2json -> ../ghc-7.6.3/lib/yaml-0.8.5.2/bin/yaml2json lrwxr-xr-x 1 arnaud staff 34 Mar 13 08:58 yate -> ../ghc-7.6.3/lib/yate-0.1/bin/yate -- Arnaud Bailly FoldLabs Associate: http://foldlabs.com

3 5

Question about typeclass & constraints
by jean-christophe mincke 08 Apr '14

08 Apr '14

Hello Consider the following data, class and instance: class PP m where create :: a -> m a data A a = A a instance PP A where create a = A a And then: class CB a where fb :: a -> Int data B m a = B (m a) If I try to define an instance of PP for B with the following implementation: instance (PP m) => PP (B m) where create a = let _ = fb a in B (create a) GHC issues the (expected) error: Could not deduce (CB a) arising from a use of 'fb' from the context (PP m). So the workaround I am using is to define a new class PP' where the type a is made more explicit. That allows the constraint (CB a) to be stated in an instance declaration. class PP' m a where create' :: a -> m a instance (PP m) => PP' m a where create' = create instance (PP m, CB a) => PP' (B m) a where create' a = let _ = fb a in B (create a) So, my question is: is there a way to achieve the same results without having to create the new class PP'? Thank you for your help J-C

3 5

ANN: Melbourne Haskell Users Group
by Lyndon Maydwell 08 Apr '14

08 Apr '14

Hi Haskellers, A few people have been meeting up in the Melbourne CBD to talk about Haskell for the past couple of months. This month I've asked Bernie Pope (who organised the Melbourne Functional Programming Union) to give a talk about two libraries he has written in Haskell that compile Python code to Haskell, and Python Byte-Code. I thought I'd take the opportunity to announce the this meetup and the group in general to the wider Haskell community in case there is anyone in Melbourne who would be interested in coming along. The talk will be on Thursday, April 24. Details can be found at: http://www.meetup.com/Melbourne-Haskell-Users-Group/events/174401572/ Hope to see you there! - Lyndon

1 0

Can I write this function without doing the action on a different line?
by Matt Joiner 08 Apr '14

08 Apr '14

1. packageBase <- do 2. curDir <- getCurrentDirectory 3. return . last . splitDirectories . normalise . joinPath $ curDir:maybeToList package

3 2

Fwd: c2hs: E.g. #include <iostream> OK in *.cpp's, but not in *.h's
by Nick Rudnick 07 Apr '14

07 Apr '14

---------- Forwarded message ---------- From: Nick Rudnick <nick.rudnick(a)gmail.com> Date: 2014-04-07 15:28 GMT+02:00 Subject: Re: [Haskell-cafe] c2hs: E.g. #include <iostream> OK in *.cpp's, but not in *.h's To: Ian Ross <ian(a)skybluetrades.net> Hi Ian, thanks for clearing up :-) So you mean the trick of hsqml primarily is (A) dummy typedefs to char, e.g.: typedef char HsQMLStringHandle; (B) 'extern' sigantures, e.g.: extern void hsqml_init_string(HsQMLStringHandle*); THIS WORKS... Occasionally, one might use two headers, e.g. a *.hpp as actual C++ header neither listed in the *.cabal nor the *.chs, where instead it would be represented by a *.h, as described above. No more...?? Thanks and cheers, Nick 2014-04-07 14:04 GMT+02:00 Ian Ross <ian(a)skybluetrades.net>: Hi Nick, > > As far as I can tell from a quick look at hsqml, the stuff in Setup.hs > isn't related to C2HS. What's more relevant is that hsqml defines a C > library wrapper around the C++ library it uses -- C2HS is only used on that > C wrapper, not on the C++ library itself. In general, C2HS doesn't support > C++ at all, because it needs to be able to parse the header files that it > includes and we use the C parser from the languagge-c package to do that. > Any C++ constructs will just cause breakage. And your "old school" FFI > definition doesn't look in any header files, which is why it has no trouble. > > Basically, if you want to do something like what hsqml does, just take a > look in the cbits directory there, in particular at hsqml.h which is the > header that defines the C wrapper around the C++ library. This hsqml.h > header is the one that's included in the C2HS files. If you want to wrap a > C++ library directly, C2HS probably isn't the tool for the job. I know > there was some work during the last GSOC on a C++ FFI wrapper, but I don't > know if anything concrete came of it. > > Cheers, > > Ian. > > > > On 6 April 2014 20:22, Nick Rudnick <nick.rudnick(a)gmail.com> wrote: > >> Dear all, >> >> maybe this is a piece of c2hs behaviour I missed to understand, but it >> appears happening with C++ style includes deep in the header call hierarchy >> – in case of a such backend C++ library, if one wishes to refer to by *.h's >> in c2hs (e.g. for access to types of it), one might have a problem. >> >> - 8< cSample.h --------------------------------------------- >> #include <iostream> ///////// here, with c2hs, leading to build abort >> #ifdef __cplusplus >> extern "C" { >> #endif >> void sampleFunction(); >> #ifdef __cplusplus >> } >> #endif >> - 8< cSample.cpp --------------------------------------------- >> #include "cSample.h" >> #include <iostream> ///////// here no problem >> >> void sampleFunction(){ >> std::cout << "OK" << std::endl; >> } >> - 8< ---------------------------------------------------------------- >> >> While 'old school' FFI (foreign import ccall) handles this effortlessly, >> c2hs uses to abort with an exception like >> C/cSample.h:3:20: fatal error: iostream: No such file or directory >> compilation terminated. >> >> I see that hsqml gets around this, but apparently by tweaking Setup.hs in >> a way not quite trivial – would this be necessary in any case, or are there >> simpler alternatives? >> >> Please excuse if I oversaw something, but I am afraid I didn't see this >> interesting issue covered elsewhere. >> >> Thank you a lot in advance & cheers, Nick >> >> >> >> >> >> >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >> > > > -- > Ian Ross Tel: +43(0)6804451378 ian(a)skybluetrades.net > www.skybluetrades.net >

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Domain Events in haskell
by Rouan van Dalen 07 Apr '14

07 Apr '14

Hi Cafe, I am trying to write a very simple implementation of an event publisher pattern but I am stuck and do not know how to do this in Haskell. I have the following code: ======================== {-# LANGUAGE RankNTypes, NamedFieldPuns #-} module Domain.DomainEventPublisher where import Control.Monad (forM_) import HsFu.Data.DateTime import Domain.Client data DomainEvent = ClientChangeAgeDomainEvent data DomainEventContext = DomainEventContext { domainEventContext_event :: DomainEvent , domainEventContext_occurredOn :: DateTime } deriving (Show) data DomainEventPublisher = DomainEventPublisher { domainEventPublisher_subscribers :: [DomainEventContext -> IO ()] } mkEventPublisher :: DomainEventPublisher mkEventPublisher = DomainEventPublisher [] subscribe :: DomainEventPublisher -> (DomainEventContext -> IO ()) -> DomainEventPublisher subscribe publisher eventHandler = DomainEventPublisher { domainEventPublisher_subscribers = eventHandler : (domainEventPublisher_subscribers publisher) } publish :: DomainEventPublisher -> DomainEventContext -> IO () publish DomainEventPublisher{ domainEventPublisher_subscribers } event = forM_ domainEventPublisher_subscribers ($ event) ======================== My problem is that the publish method returns IO (), which means that events can only be published from the IO monad, but I would like events to be 'publish-able' from pure code. I can live with event handlers (passed into the subscribe function) being in the IO monad. Is there a better way to implement this pattern in Haskell? I have been racking my brain on this for a while now and cannot seem to come up with a good implementation. Regards --Rouan.

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c2hs: E.g. #include <iostream> OK in *.cpp's, but not in *.h's
by Nick Rudnick 07 Apr '14

07 Apr '14

Dear all, maybe this is a piece of c2hs behaviour I missed to understand, but it appears happening with C++ style includes deep in the header call hierarchy – in case of a such backend C++ library, if one wishes to refer to by *.h's in c2hs (e.g. for access to types of it), one might have a problem. - 8< cSample.h --------------------------------------------- #include <iostream> ///////// here, with c2hs, leading to build abort #ifdef __cplusplus extern "C" { #endif void sampleFunction(); #ifdef __cplusplus } #endif - 8< cSample.cpp --------------------------------------------- #include "cSample.h" #include <iostream> ///////// here no problem void sampleFunction(){ std::cout << "OK" << std::endl; } - 8< ---------------------------------------------------------------- While 'old school' FFI (foreign import ccall) handles this effortlessly, c2hs uses to abort with an exception like C/cSample.h:3:20: fatal error: iostream: No such file or directory compilation terminated. I see that hsqml gets around this, but apparently by tweaking Setup.hs in a way not quite trivial – would this be necessary in any case, or are there simpler alternatives? Please excuse if I oversaw something, but I am afraid I didn't see this interesting issue covered elsewhere. Thank you a lot in advance & cheers, Nick

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Functional Software Engineering Posts
by Khushil Dep (Sphonic) 07 Apr '14

07 Apr '14

Symphonic Solutions Ltd (www.sphonic.com) is a disruptive technology company that leverages functional paradigms to enable engineered innovation. In real terms, we build high performance, low latency platforms that integrate to a plethora of third party API’s and apply intelligent manipulation and analysis to the data via a DSL. We help turn ‘Big Data’ into ‘Useful Information’. We also don’t use Hadoop - which is nice. We’re looking for software engineers who understand the functional domain extremely well. Ideally you will be a polyglot software engineer with skills and at least twelve months (commercial) experience in Scala (with with Akka) || Clojure || Haskell || NodeJS. We run all of these in production so it’s important you understand the difference between experimentation and production quality code. You should also be able to at least read one of either Python, Bash or C. We don’t allow Ruby anywhere on our platform so you should be comfortable about that. We’re looking for people who want to live the functional dream, not people who think it’s a ‘nice to have’ on their C.V please. You should understand the sympathetic orchestration of human, software, operating system and hardware. You should also be able to translate complex functional requirements into simple, well composed programs which work in concert to produce the desired result. Experience in working within a micro-services architecture will be looked on very favourably. We are flexible on working locations, including home working, with a London office based near Edgware Road station. During the early phases of engineering design some time in the office will be a necessity however and you should be comfortable about that. You should be able to work independently and within a wider team as well as manage your time such that your responsibilities in each sprint are delivered to a high quality and in a timely manner. You’ll be expected and empowered to make software design choices specific to your responsibilities in concert with the platform engineering team. You should be happy to work within an agile project management framework which supports Kanban. You should be familiar with working with JIRA, Confluence and Git. You will be expected to produce documentation around your sprint responsibilities which deliver a high level of knowledge retention and transfer. Your programs will be run on SmartOS platforms and so you should be able to build technology agnostic programs. In simple terms you should be happy not to rely on any operating system specific functionality in order to write your programs. You will be trained in technologies such as DTrace to better aid your diagnostic efforts. You will be provided with local build environments based off Vagrant which you should be happy to manage yourself though training and support from engineering will be available at all times. If this interests you, in the first instance, please answer the following questions : Question 1 Explain the relationship between the human and the machine. Describe the relationship in as much detail as you feel comfortable with. Question 2 Explain tail recursion to a 5 year old. Question 3 Explain how you would manage state in a functional manner to a 5 year old. We expect that you have public GitHub/Bitbucket repo’s sharing your work and/or research with functional languages and programs so please also send your GitHub/Bitbucket handle with your answers.Please send your answers and whatever you have of a C.V. to jobs(a)sphonic.com. --- Khushil Dep Head Of Engineering Symphonic Solutions Ltd khushildep(a)sphonic.com 07799908654 07415372737

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ANN: Budapest Hackathon 2014
by Csaba Hruska 07 Apr '14

07 Apr '14

Dear Haskellers, We're organizing the first ever Haskell Hackathon in Budapest, Hungary. When: Saturday 31 May 2014 - Sunday 1 June 2014 This will be a great opportunity to meet up with fellow Haskellers, and show off your cool hobby projects in one of the most vibrant cities in Europe. There's also room for working on existing tools, for example you can team up with Luite Stegeman at the event to hack on GHC-JS. Details about the when, where and how: http://www.haskell.org/haskellwiki/BudapestHackathon2014 See you all there!

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Second CFP: Higher-order, Typed, Inferred, Strict: ML Family Workshop
by oleg＠okmij.org 07 Apr '14

07 Apr '14

Higher-order, Typed, Inferred, Strict: ACM SIGPLAN ML Family Workshop Thursday September 4, 2014, Gothenburg, Sweden (immediately following ICFP and preceding OCaml Users and Developers Workshop) Call For Papers http://okmij.org/ftp/ML/ML14.html News: Post-proceedings will be published in EPTCS ML is a very large family of programming languages that includes Standard ML, OCaml, F#, SML#, Manticore, MetaOCaml, JoCaml, Alice ML, Dependent ML, Flow Caml, and many others. All ML languages, beside the great deal of syntax, share several fundamental traits. They are all higher-order, strict, mostly pure, and typed, with algebraic and other data types. Their type systems inherit from Hindley-Milner. The development of these languages has inspired a significant amount of computer science research and influenced a number of programming languages, including Haskell, Scala and Clojure, as well as Rust, ATS and many others. ML workshops have been held in affiliation with ICFP continuously since 2005. This workshop specifically aims to recognize the entire extended ML family and to provide the forum to present and discuss common issues, both practical (compilation techniques, implementations of concurrency and parallelism, programming for the Web) and theoretical (fancy types, module systems, metaprogramming). The scope of the workshop includes all aspects of the design, semantics, theory, application, implementation, and teaching of the members of the ML family. We also encourage presentations from related languages (such as Scala, Rust, Nemerle, ATS, etc.), to exchange experience of further developing ML ideas. The ML family workshop will be held in close coordination with the OCaml Users and Developers Workshop. Format Since 2010, the ML workshop has adopted an informal model. Presentations are selected from submitted abstracts. There are no published proceedings, so any contributions may be submitted for publication elsewhere. We hope that this format encourages the presentation of exciting (if unpolished) research and deliver a lively workshop atmosphere. Each presentation should take 20-25 minutes, except demos, which should take 10-15 minutes. The exact time will be decided based on the number of accepted submissions. The presentations will likely be recorded. Post-conference proceedings The post-proceedings of selected papers from the ML Family and the OCaml Users and Developers workshops will be published in the Electronic Proceedings in Theoretical Computer Science (EPTCS). The Program Committee shall invite interested authors of selected presentations to expand their abstract for inclusion in the proceedings. The submissions are to be reviewed according to the EPTCS standards. Coordination with the OCaml Users and Developers Workshop The OCaml workshop is seen as more practical and is dedicated in significant part to the OCaml community building and the evolution of the OCaml system. In contrast, the ML family workshop is not focused on any language in particular, is more research oriented, and deals with general issues of the ML-style programming and type systems. Yet there is an overlap, which we are keen to explore in various ways. The authors who feel their submission fits both workshops are encouraged to mention it at submission time or contact the Program Chairs. Scope We acknowledge the whole breadth of the ML family and aim to include languages that are closely related (although not by blood), such as Rust, ATS, Scala, Typed Clojure. Those languages have implemented and investigated run-time and type system choices that may be worth considering for OCaml, F# and other ML languages. We also hope that the exposure to the state of the art ML might favorably influence those related languages. Specifically, we seek research presentations on topics including but not limited to * Design: concurrency, distribution and mobility, programming for the web and embedded systems, handling semi-structured data, facilitating interactive programming, higher forms of polymorphism, generic programming, objects * Implementation: compilation techniques, interpreters, type checkers, partial evaluators, runtime systems, garbage collectors, etc. * Type systems: fancy types, inference, effects, overloading, modules, contracts, specifications and assertions, dynamic typing, error reporting, etc. * Applications: case studies, experience reports, pearls, etc. * Environments: libraries, tools, editors, debuggers, cross-language interoperability, functional data structures, etc. * Education: ML and ML-like languages in college or high-school, in general or computer science curriculum. Four kinds of submissions will be accepted: Informed Positions, Research Presentations, Experience Reports and Demos. * Informed Positions: A justified argument for or against a language feature. The argument must be substantiated, either theoretically (e.g., by a demonstration of (un)soundness, an inference algorithm, a complexity analysis), empirically or by a substantial experience. Personal experience is accepted as justification so long as it is extensive and illustrated with concrete examples. * Research Presentations: Research presentations should describe new ideas, experimental results, or significant advances in ML-related projects. We especially encourage presentations that describe work in progress, that outline a future research agenda, or that encourage lively discussion. These presentations should be structured in a way which can be, at least in part, of interest to (advanced) users. * Experience Reports: Users are invited to submit Experience Reports about their use of ML and related languages. These presentations do not need to contain original research but they should tell an interesting story to researchers or other advanced users, such as an innovative or unexpected use of advanced features or a description of the challenges they are facing or attempting to solve. * Demos: Live demonstrations or short tutorials should show new developments, interesting prototypes, or work in progress, in the form of tools, libraries, or applications built on or related to ML. (You will need to provide all the hardware and software required for your demo; the workshop organizers are only able to provide a projector.) Important dates Monday May 19 (any time zone): Abstract submission Monday June 30: Author notification Thursday September 4, 2014: ML Family Workshop Submission Submissions should be at most two pages, in PDF format, and printable on US Letter or A4 sized paper. A submission should have a synopsis (2-3 lines) and a body between 1 and 2 pages, in one- or two-column layout. The synopsis should be suitable for inclusion in the workshop program. Submissions must be uploaded to the workshop submission website before the submission deadline (Monday May 19, 2014). For any question concerning the scope of the workshop or the submission process, please contact the program chair. Program Committee Kenichi Asai Ochanomizu University, Japan Matthew Fluet Rochester Institute of Technology, USA Jacques Garrigue Nagoya University, Japan Dave Herman Mozilla, USA Stefan Holdermans Vector Fabrics, Netherlands Oleg Kiselyov (Chair) Monterey, CA, USA Keiko Nakata Tallinn University of Technology, Estonia Didier Remy INRIA Paris-Rocquencourt, France Zhong Shao Yale University, USA Hongwei Xi Boston University, USA

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New Functional Programming Job Opportunities
by Functional Jobs 07 Apr '14

07 Apr '14

Here are some functional programming job opportunities that were posted recently: Server Game Developer at Quark Games http://functionaljobs.com/jobs/8701-server-game-developer-at-quark-games Infrastructure Engineer at Zeta Project Germany GmbH http://functionaljobs.com/jobs/8699-infrastructure-engineer-at-zeta-project… Cheers, Sean Murphy FunctionalJobs.com

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