November 2014 - Haskell-Cafe

ISO 8601 Date time format without old-locale?
by Bram Neijt 18 Nov '14

18 Nov '14

I want to do ISO8601 formatting of a date with a numeric zone offset and I can't seem to find a way of doing it without requiring the old-locale package[1]. What is the current method of obtaining and formatting time representation if old-locale is old? Greetings, Bram [1] A sketch of the code using old-locale: -- 2014-11-18T06:37:04+00:00 iso8601DateTimeFormat :: String iso8601DateTimeFormat = "%FT%T%z" main = do now <- getCurrentTime putStrLn $ formatTime defaultTimeLocale iso8601DateTimeFormat now Which requires "defaultTimeLocale" from the old-locale package.

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The next 7000... abstractions to learn!
by Gautier DI FOLCO 18 Nov '14

18 Nov '14

Hi all, I'm using Haskell since nearly three years now and I think I have well understood that types are the most important part of our programs. I have looked all around to learn as many datatypes/typeclasses as possible. But I suffer for a lack of direction to pursue my learning. I have well understood Prelude's one and some random ones (include someones for Category Theory), but I can't handle Kmett's code. Are there some intermediates abstractions I can learn? (For example in some librairies). Thanks by advance for your help. Regards.

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What could possibly be the reason of new GC-related problems after upgrading from ghc-7.6 to ghc-7.8.3 ?
by ducis 18 Nov '14

18 Nov '14

Recently I revived one of my projects set aside a year ago. As I'm an archlinux user, the ghc version has now inevitably become 7.8.3. Unfortunatly, the program starts to somehow have very bad GC performance (less than 50% productivity) . While I don't have the statistics of the older versions I'm pretty sure the current version has become visually less responsive. And if the user performs a series of actions in quick succession, the heap is not garbage collected in time and overflows. If the user performs the same actions with longer pauses then everything is fine. The GC seems to have become more loose in 7.8. There is quite a large underdocumented pile of code so I don't think posting it here would help. But can anyone give me a hint about what could be the reason?

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How to Kill Haskell
by Eric Crockett 18 Nov '14

18 Nov '14

For the purposes of profiling a partially evaluated program, I'm interested in knowing the best way to terminate a GHC program. This is useful for profiling programs that take a long time to run, possibly as long as forever. It appears that *if* the program can be terminated with a single ^c, then profiling information is written to output. Sometimes, a single ^c doesn't kill the program, at not before I get impatient and hit ^c again. Usually two ^c will kill the program, but then no profiling data is written to output. As a simple example, Main.hs fib n = fib (n - 1) + fib (n - 2) main = print $ fib 100 Compiling with -prof and running with +RTS -p, I can kill this program with a *single* ^c in the first approximately 10 seconds of execution, but after that only two ^c will do the job. Looking at my resources, this change appears to coincide with the program using all of my physical memory and moving to swap space, however that could also be coincidental. Why does ^c work sometimes, but not other times for the same program? What is the easiest way to ensure that profiling data will get printed when the program does not terminate on its own?

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OS X 10.10
by Hans Aberg 18 Nov '14

18 Nov '14

The URL [1] says the platform “Works even on 10.10 beta!”, however, OS X 10.10.1 has just been released. Is there anyone out there who does not run the beta? :-) 1. https://www.haskell.org/platform/mac.html

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[ANN] elevator: getting to a top of a monad transformer stack
by Fumiaki Kinoshita 17 Nov '14

17 Nov '14

Hello everybody, I created an elevator to navigate a monad transformer stack, liberating us from creating a Monad* class and instances. The key function, elevate, can be whatever you need: elevate :: IO a -> IO a elevate :: IO a -> StateT s IO a elevate :: State s a -> StateT s IO a elevate :: IO a -> MaybeT (StateT s IO) a If you want to elevate actions above your monad, just write instance Tower YourMonad to declare that YourMonad is one-storied. Hackage: http://hackage.haskell.org/package/elevator-0.1/docs/Control-Elevator.html Cheers Fumiaki Kinoshita

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[Haskell] ANNOUNCE: Haskell Communities and Activities Report (27th ed., November 2014)
by Mihai Maruseac 17 Nov '14

17 Nov '14

On behalf of all the contributors, we are pleased to announce that the Haskell Communities and Activities Report (27th edition, November 2014) is now available, in PDF and HTML formats: http://haskell.org/communities/11-2014/report.pdf http://haskell.org/communities/11-2014/html/report.html Many thanks go to all the people that contributed to this report, both directly, by sending in descriptions, and indirectly, by doing all the interesting things that are reported. We hope you will find it as interesting a read as we did. If you have not encountered the Haskell Communities and Activities Reports before, you may like to know that the first of these reports was published in November 2001. Their goal is to improve the communication between the increasingly diverse groups, projects, and individuals working on, with, or inspired by Haskell. The idea behind these reports is simple: Every six months, a call goes out to all of you enjoying Haskell to contribute brief summaries of your own area of work. Many of you respond (eagerly, unprompted, and sometimes in time for the actual deadline) to the call. The editors collect all the contributions into a single report and feed that back to the community. When we try for the next update, six months from now, you might want to report on your own work, project, research area or group as well. So, please put the following into your diaries now: ======================================== End of April 2015: target deadline for contributions to the May 2015 edition of the HC&A Report ======================================== Unfortunately, many Haskellers working on interesting projects are so busy with their work that they seem to have lost the time to follow the Haskell related mailing lists and newsgroups, and have trouble even finding time to report on their work. If you are a member, user or friend of a project so burdened, please find someone willing to make time to report and ask them to "register" with the editors for a simple e-mail reminder in October (you could point us to them as well, and we can then politely ask if they want to contribute, but it might work better if you do the initial asking). Of course, they will still have to find the ten to fifteen minutes to draw up their report, but maybe we can increase our coverage of all that is going on in the community. Feel free to circulate this announcement further in order to reach people who might otherwise not see it. Enjoy! Mihai Maruseac and Alejandro Serrano Mena <hcar at haskell.org> -- Mihai Maruseac (MM) "If you don't know, the thing to do is not to get scared, but to learn." -- Atlas Shrugged.

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Using wreq Session handling in a monad transformer stack
by Cody Goodman 16 Nov '14

16 Nov '14

Could I somehow add a Wreq.Session.Session to my monad transformer stack? Should I use something other than Wreq? How would I create a default Session, Wreq doesn't seem to export the constructor. Basically I want to achieve Sessions/cookie handling. lpaste of my code: http://lpaste.net/114405 my full code (for those who want to view in email): {-# LANGUAGE OverloadedStrings #-} module Network.Scraper.State where import Control.Lens ((^.)) import Control.Monad import Control.Monad.IO.Class (liftIO) import qualified Control.Monad.Trans.State.Strict as ST import qualified Data.ByteString.Lazy as LBS import Data.Maybe (fromJust, fromMaybe) import Data.Monoid import qualified Data.Text as T import qualified Data.Text.IO as TIO import Network.Wreq (FormParam (..)) import qualified Network.Wreq as Wreq import Network.Wreq.Session (Session (..), withSession) import qualified Network.Wreq.Session as Sesh import Network.Wreq.Types import Safe import Text.HTML.DOM (parseLBS) import Text.XML.Cursor import qualified Text.XML.Cursor.Generic as CG data ScraperState = PS { currentOptions :: Wreq.Options , currentHtml :: LBS.ByteString , currentCursor :: Maybe Cursor , currentSession :: Session } deriving (Show) type Scraper = ST.StateT ScraperState IO toCursor = fromDocument . parseLBS initialSt = PS { currentOptions = Wreq.defaults , currentHtml = ("" :: LBS.ByteString) , currentCursor = Nothing -- , currentSession = ... how do I get a Session? Wreq doesn't seem to export this type } setCurrentOptions :: Wreq.Options -> Scraper () setCurrentOptions o = do scraper <- ST.get ST.put $ scraper { currentOptions = o } -- getCurrentPage :: Shpider Page getCurrentCursor :: Scraper (Maybe Cursor) getCurrentCursor = do scraper <- ST.get return $ currentCursor scraper getCurrentSession :: Scraper (Session) getCurrentSession = do scraper <- ST.get return $ currentSession scraper setCurrentSession :: Session -> Scraper () setCurrentSession s = do scraper <- ST.get ST.put $ scraper { currentSession = s} setCurrentCursor :: Cursor -> Scraper ( ) setCurrentCursor c = do scraper <- ST.get ST.put $ scraper { currentCursor = Just c } setCurrentHtml :: LBS.ByteString -> Scraper () setCurrentHtml html = do scraper <- ST.get ST.put $ scraper { currentHtml = html } runScraper :: Scraper a -> IO a runScraper k = evalScraperWith k initialSt evalScraperWith :: Scraper a -> ScraperState -> IO a evalScraperWith k s = withSession $ \sesh -> do -- set the current session to the mutable session variable return $ setCurrentSession sesh ST.evalStateT k s formShortInfo' f = formInfo' where go Nothing = "N/A" go (Just x) = x formInfo = (headMay . attribute "name" $ f, headMay . attribute "action" $ f) formInfo' = (\(x,y) -> (go x, go y)) formInfo ppTuple :: (T.Text, T.Text) -> T.Text ppTuple = \(x,y) -> "[" <> x <> "]" <> ": " <> y -- move to ../Spider.hs printFormNames :: Scraper () printFormNames = do c <- getCurrentCursor let c' = fromMaybe (error "No cursor set") c forms = c' $// element "form" formInfo = map (ppTuple . formShortInfo') forms liftIO $ mapM_ (TIO.putStrLn) formInfo getFormByName :: T.Text -> Scraper [Cursor] getFormByName name = do c <- getCurrentCursor let c' = fromMaybe (error "No cursor set") c return $ c' $// element "form" >=> attributeIs "name" name get :: String -> Scraper (LBS.ByteString) get url = do r <- liftIO $ Wreq.get url let html = r ^. Wreq.responseBody setCurrentHtml html setCurrentCursor (toCursor html) return html post :: Postable a => String -> a -> Scraper (LBS.ByteString) post url params = do r <- liftIO $ Wreq.post url params let html = r ^. Wreq.responseBody setCurrentHtml html setCurrentCursor (toCursor html) return html test :: Scraper () test = do get "https://www.google.com" >> printFormNames

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Re: [Haskell-cafe] data analysis question
by Dominic Steinitz 16 Nov '14

16 Nov '14

I’d much prefer that. I really dislike the google group experience. I’ll drop Austin a note. Dominic Steinitz dominic(a)steinitz.org http://idontgetoutmuch.wordpress.com On 15 Nov 2014, at 15:42, Ben Gamari <ben(a)smart-cactus.org> wrote: > On November 15, 2014 1:54:38 AM EST, Dominic Steinitz <dominic(a)steinitz.org> wrote: >> Mark Fredrickson <mark.m.fredrickson <at> gmail.com> writes: >> >>> >>> Is there a mailing list for statistics/analytics/simulation/numerical >>> analysis/etc. using Haskell? If not, I purpose we start one. (Not to >>> take away from general discussion, but to provide a forum to hash out >>> these issues among the primary user base). >> >> Sadly not but I think there are sufficient numbers of people >> interested in this subject that it is probably worth setting one up. I >> really don't like the google group experience but maybe that is the >> best place to start? >> > I agree that this would be a worthwhile forum to have. Why not just stay with Haskell.org infrastructure? I'm sure Austin would set up a mailing list for the cause. > > Cheers, > > - Ben > > >

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Call for Presentations: Compose Conference [New York, Jan 30-Feb 1]
by Gershom B 16 Nov '14

16 Nov '14

Compose is a new conference for typed functional programmers, focused specifically on Haskell, OCaml, F#, and related technologies. It will be held in New York from Jan 30-Feb 1, and registration is opening shortly. http://www.composeconference.org/ Below is our call for presentations. We recognize the deadline is tight, so feel free to submit proposals and ideas on the less-polished side. Call for Presentations and Speakers. http://www.composeconference.org/call/index.html --- The audience for Compose is Haskell, OCaml, or F# developers who are looking to increase their skills or learn new technologies and libraries. Presentations should be aimed at teaching or introducing new ideas or tools. We are also interested in presentations aiming at taking complex concepts, such as program derivation, and putting them into productive use. However proposals on anything that you suspect our audience may find interesting are welcome. The following are some of the types of talks we would welcome: Library/Tool Talks — Exploring the uses of a powerful toolkit or library, be it for parsing, testing, data access and analysis, or anything else. Production Systems — Experience reports on deploying functional techniques in real systems; insights revealed, mistakes made, lessons learned. Theory made Practical — Just because it’s locked away in papers doesn’t mean it’s hard! Accessible lectures on classic results and why they matter to us today. Such talks can include simply introducing the principles of a field of research so as to help the audience read up on it in the future; from abstract machines to program derivation to branch-and-bound algorithms, the sky’s the limit. We also welcome proposals for more formal tutorials for the Sunday unconference. Such tutorials should be aimed at a smaller audience of beginner-to-novice understanding, and ideally include hands-on exercises. The due date for submissions is November 30, 2014. We will send out notice of acceptance by 10 December. We prefer that submissions be via the EasyChair website (https://easychair.org/conferences/?conf=compose2015) Please suggest a title, and describe the topic you intend to speak on. Additional information may be included on both your expertise and the interesting elements of your topic, going on what might be included in a public abstract. Furthermore, if your abstract doesn't feel "final"—don't worry! We'll work with you to polish it up. If you want to discuss your proposal(s) before submitting, or to further nail down what you intend to speak on, please feel free to contact us at info(a)composeconference.org. We're happy to work with you, even if you are a new or inexperienced speaker, to help your talk be great. —Gershom

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