December 2018 - Haskell-Cafe

Jan 2018- Membership of the Group
by William Fearon 29 Dec '18

29 Dec '18

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About understanding FunBind in haskell-src-exts.
by Magicloud Magiclouds 29 Dec '18

29 Dec '18

Hi, Recently I am playing with haskell-src-exts to help understanding Haskell syntax more. Then I got this question. As top level declarations (the type Decl), TypeSig, PatBind are quite straight forward, they mean one item in the source. But `FunBind l [Match l] `, I could not see it maps to "one item" in the source. Apparently, the list of Match can hold a few non-related functions with different names. Then why a FunBind to hold them together? -- 竹密岂妨流水过山高哪阻野云飞

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[GSoC 2019] Call for Ideas
by Jasper Van der Jeugt 28 Dec '18

28 Dec '18

Google Summer of Code will take place again in 2019, for the 15th year of the program [1]! We are very proud that Haskell.Org has been able to take part every year except for the initial year (2015) and 2016-2017, when we had to resort to running our own program. Last year, we were fortunate enough to join again, and we think the results greatly benefited the Haskell community [2]. We are hoping to do the same for 2019. As far as we know, a really important part of our application to GSoC is the list of ideas we provide. For that, I would like to count on all of you. If you are the maintainer or the user of a Haskell project, and you have an improvement in mind which a student could work on during the summer, please submit an idea here: https://summer.haskell.org/ideas.html For context, Google Summer of Code is a program where Google sponsors students to work on open-source projects during the summer. Haskell.org has taken part in this program from 2006 until 2015, and again in 2018. Many important improvements to the ecosystem have been the direct or indirect result of Google Summer of Code projects, and it has also connected new people with the existing community. Projects should benefit as many people as possible – e.g. an improvement to GHC will benefit more people than an update to a specific library or tool, but both are definitely valid. New libraries and applications written in Haskell, rather than improvements to existing ones, are also accepted. Projects should be concrete and small enough in scope such that they can be finished by a student in three months. Warm regards On behalf of the Haskell.Org committee Jasper Van der Jeugt [1]: https://opensource.googleblog.com/2018/11/google-summer-of-code-15-years-st… [2]: https://summer.haskell.org/news/2018-09-01-final-results.html

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hackage.org down?
by Gregory Guthrie 28 Dec '18

28 Dec '18

I get a failure response from hackage.org: 502 Bad Gateway nginx/1.10.3 (Ubuntu)

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Re: [Haskell-cafe] Find out which part of code are causing parallelism?
by Johannes Waldmann 28 Dec '18

28 Dec '18

> however I just found out that without inserting > any parallism manually the program is already parallelised, That's not your program, it's the garbage collector. "By default, all of the capabilities participate in parallel garbage collection." https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/runtime_con… - J.W.

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[Haskell][ANN] vabal - the cabal companion
by Francesco Magliocca 28 Dec '18

28 Dec '18

Hello, I'm please to announce that vabal 1.2.0 has been released on Hackage: http://hackage.haskell.org/package/vabal vabal is a software that helps you determine the ghc version needed to compile a cabal package. It reads the cabal file and extracts the constraints imposed on the base package. Then it uses ghcup ( https://github.com/haskell/ghcup ) to obtain a compiler version compatible with the constraints (possibly downloading it from the official ghc mirror) and can be combined with cabal to build the project using the obtained ghc compiler. In this way you can treat the `base` package dependency as any other package dependency and don't have to manually deal with different ghc versions anymore. vabal is meant to be used in collaboration with cabal. It leverages cabal's ability to work with multiple ghc versions, and tries to be the least intrusive possible, so that you can keep using your usual workflow, but won't have to worry about `base` package dependency issues, anymore. You can get started here: https://github.com/Franciman/vabal#quick-start For details please refer to: https://github.com/Franciman/vabal#vabal---the-cabal-companion Ideas, suggestions, opinions and pull requests are very welcome! A big thank you goes to all the contributors that helped me with code, suggestions and ideas: https://github.com/Franciman/vabal#contributors

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Find out which part of code are causing parallelism?
by ducis 28 Dec '18

28 Dec '18

Hi all, I am working on parallelising some program, however I just found out that without inserting any parallism manually the program is already parallelised, presumably by some of the libraries referenced, as long as I enable -threaded and +RTS -N8. Unfortunately this parallism are actually slowing down my program, probably due to being of too fine granularity. For example, with +RTS -N8 I have 47.03user 38.06system 0:21.68elapsed 392%CPU While with +RTS -N1 I have 16.57user 0.45system 0:17.02elapsed 100%CPU In .prof file I can't find whether a function fired a spark or how much 'system' time a function used. Nor can I find any names of functions/modules browsing through eventlog with ghc-events. Is there a way to find out which functions/modules are causing any parallelism at all? I am also trailing the post with the imports I used in case anyone immediately recognises parallelism by default in the package. import Control.DeepSeq import GHC.Generics (Generic, Generic1) import Text.Parsec import Data.List import Data.Vector.Unboxed(fromList, (!)) import qualified Data.Vector as V import qualified Data.Map as M import qualified Data.Set as S import Data.Char import Control.Applicative((*>),(<*)) import Test.QuickCheck.Monadic import Test.QuickCheck import Control.Monad import Text.Regex import Control.Applicative ((<$>), (<$), (<|>)) import GHC.Read (readLitChar) import Data.Char(isPrint) import Text.ParserCombinators.ReadP import Text.Show.Pretty import Data.Functor import Data.Maybe import System.Environment(getArgs) import Text.Read(readMaybe) import Text.Printf import Data.Dynamic import Debug.Trace import Data.Ratio import qualified Data.DList as D import Control.Monad.Trans.Writer.Strict import Control.Monad.Trans.Class import Data.Typeable(Typeable) import qualified Data.HashMap.Lazy as H Best, ducis

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Re: [Haskell-cafe] trace output statements
by Ian Denhardt 27 Dec '18

27 Dec '18

(Adding the list back to Cc; you forgot to hit reply all) Quoting Damien Mattei (2018-12-25 16:57:04) > i get in trouble understanding what fmap was doing, fmap (for IO) just applies a function to the result of the action, so: fmap f action is equivalent to: do result <- action return (f result) > and why the same thing to do in main and in a function had so > different implementations... I suspect the modified version of the program has some differences that aren't strictly necessary just to factor out the relevant bits into their own definition; this *shouldn't* be major surgery. Hard for me to point out without having the two full examples handy. Quoting Damien Mattei (2018-12-25 16:57:04) > yes, i understand with your python example, it's like read on a web > page, IO are not a "cake" but IO are "a recipe for the cake"... > functions are pure in haskell, so they can not have side effects... > what put me some trouble is an answer 2 weeks ago, someone gave me > hints that lead to this solution for getDB: > getBD :: Connection -> String -> IO Float > getBD conn name = trace "Entering getBD" noBDfp > � where qry_head_BD_Sidonie = "select `N° BD` from Coordonnées where > Nom = ?" :: Query > � � � � � � � bd_rows :: IO [Only Text] > � � � � � � � bd_rows = query conn qry_head_BD_Sidonie (Only > (name::String)) > � � � � � � � noBDtxt :: IO Text > � � � � � � � noBDtxt = trace "assigning noBDtxt" (fmap (fromOnly . > Prelude.head) bd_rows) > � � � � � � � noBDstr :: IO String > � � � � � � � noBDstr = trace "assigning noBDstr" (fmap Text.unpack > noBDtxt) > � � � � � � � noBDfp :: IO Float > � � � � � � � noBDfp = fmap read noBDstr > which was code different from the code in main,i get in trouble > understanding what fmap was doing , and why the same thing to do in > main and in a function had so different implementations... > > On Tue, Dec 25, 2018 at 10:19 PM Ian Denhardt <[1]ian(a)zenhack.net> > wrote: > > The correct type annotation for getDB3 should be: > � � getDB3 :: Connection -> String -> IO Float > Note the IO at the end. Functions in Haskell are just pure > computation; > they can't have side effects -- so a function returning a Float > can't > possibly talk to a database. > Instead, The type `IO a` represents a description of an action to > perform.� It's still just a value -- calling getDB3 doesn't *do* > anything. You can stitch these together using do-notation or > functions > like >>=, and when the program is run the action defined by 'main' > is > performed. > --- > An analogy: you could imagine instead of IO we could give up, and > write > code that computes a program in some other (imperative) programming > language) that we then hand off to an interpreter. For example, we > could > compute a python program that counts from 1 to 99 like so: > � � printNum :: Int -> String > � � printNum n = "print('" ++ show n ++ "')\n" > � � pythonProgram = concatMap printNum [1..99] > So we've defined a variable fullProgram that is a string with the > source > code to a python program like: > � � print('1') > � � print('2') > � � print('3') > � � ... > � � print('99') > ..but we haven't actually run it. To do that we'd have to pass the > string off to the python interpreter. > This is a good way to think about what IO is -- main is like our > pythonProgram above, in that it is a description of actions to > perform, > but *evaluating* it doesn't have any side effects -- it just > computes > the description. When you run a Haskell program, this is like taking > the description defined by Main and passing it off to an > interpreter. > --- > So your definition of getDB3 is a description of actions to perform > to > get a float from the database, but your type declaration says it's a > function that computes a float (without having to perform any > "actions"). > This is a critical distinction that exists in Haskell but not most > other > languages. > Hope this helps, > -Ian > Quoting Damien Mattei (2018-12-25 15:07:35) > >� � yes i use do notation, but for example i have code that works > in main > >� � and not in a function! > >� � i print the code perheaps someone could help me: > >� � first the function, so you have the import list too: > >� � import Database.MySQL.Simple > >� � import Database.MySQL.Simple.QueryResults > >� � import Database.MySQL.Simple.Result > >� � import Database.MySQL.Simple.QueryParams > >� � import Database.MySQL.Simple.Param > >� � import Control.Monad > >� � import Data.Text as Text > >� � --import Data.Int as Int > >� � --import Data.List > >� � import Debug.Trace > >� � import Data.Maybe as Maybe > >� � -- this function will return th N°BD from Sidonie for a > given name > >� � -- note: names have been standardized between Sidonie and WDS > >� � getBD3 :: Connection -> String -> Float > >� � getBD3 conn name = do > >� � � � � � � � � � � � �� let > qry_head_BD_Sidonie = "select `N° BD` from > >� � Coordonnées where Nom = ?" :: Query > >� � � � � � � � � � � � �� (bd_rows :: > [Only Text]) <- query conn > >� � qry_head_BD_Sidonie (Only (name::String)) > >� � � � � � � � � � � � �� let noBDtxt = > fromOnly (Prelude.head bd_rows) :: > >� � Text > >� � � � � � � � � � � � �� let noBDstr = > Text.unpack noBDtxt :: String > >� � � � � � � � � � � � �� let noBDfp = > read $ noBDstr :: Float > >� � � � � � � � � � � � �� return noBDfp > >� � with this function i have this error: > >� � Prelude> :load UpdateSidonie > >� � [1 of 1] Compiling Main� � � � � � � � � > � � �� ( UpdateSidonie.hs, > >� � interpreted ) > >� � UpdateSidonie.hs:54:13: error: > >� � � � �� Couldn't match expected type �Float� > with actual type �IO > >� � Float� > >� � � � �� In a stmt of a 'do' block: > >� � � � � � � � �� (bd_rows :: [Only Text]) <- > query > >� � � � � � � � � � � � � � � � � > � � � � � � � � � � � � � � � � � > � � � � > >� � � �� conn qry_head_BD_Sidonie (Only (name :: String)) > >� � � � � � �� In the expression: > >� � � � � � � � �� do let qry_head_BD_Sidonie = > ... > >� � � � � � � � � � � �� (bd_rows :: [Only > Text]) <- query > >� � � � � � � � � � � � � � � � � > � � � � � � � � � � � � � � � � � > � � � � > >� � � � � � �� conn qry_head_BD_Sidonie (Only (name :: > String)) > >� � � � � � � � � � � �� let noBDtxt = ... > >� � � � � � � � � � � �� let noBDstr = ... > >� � � � � � � � � � � �� .... > >� � � � � � �� In an equation for �getBD3�: > >� � � � � � � � � � �� getBD3 conn name > >� � � � � � � � � � � � �� = do let > qry_head_BD_Sidonie = ... > >� � � � � � � � � � � � � � � � � > �� (bd_rows :: [Only Text]) <- query > >� � � � � � � � � � � � � � � � � > � � � � � � � � � � � � � � � � � > � � � � > >� � � � � � � � � � � � �� conn > qry_head_BD_Sidonie (Only (name :: String)) > >� � � � � � � � � � � � � � � � � > �� let noBDtxt = ... > >� � � � � � � � � � � � � � � � � > �� .... > >� � � �� | > >� � 54 |� � � � � � � � � � � �� > (bd_rows :: [Only Text]) <- query conn > >� � qry_head_BD_Sidonie (Only (name::String)) > >� � � �� |� � � � � � � � � � � � > >� � > ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ > ^^^ > >� � ^^^^^^^^^ > >� � Failed, no modules loaded. > >� � i do not understand the error complaining that i return an IO > >� � float,because i'm sure it's a float in noBDfp > >� � if i put the same lines of code in the main it works !!! : > >� � main :: IO () > >� � main = > >� � �� do > >� � � � �� conn <- connect defaultConnectInfo > >� � � � � � �� { connectHost = "moita", > >� � � � � � � � �� connectUser = "mattei", > >� � � � � � � � �� connectPassword = "sidonie2", > >� � � � � � � � �� connectDatabase = "sidonie" } > >� � � let qry_head_BD_Sidonie = "select `N° BD` from > Coordonnées where > >� � Nom = ?" :: Query > >� � � (bd_rows :: [Only Text]) <- query conn > qry_head_BD_Sidonie (Only > >� � (name::String)) > >� � putStr "bd_rows =" > >� � putStrLn $ show bd_rows > >� � � � �� let noBDtxt = fromOnly (Prelude.head bd_rows) > :: Text > >� � � � �� let noBDstr = Text.unpack noBDtxt :: String > >� � � � �� let noBDfp = read $ noBDstr :: Float > >� � � � �� putStr "noBDfp =" > >� � � � �� (putStrLn (show noBDfp)) > >� � � close conn > >� � it works i have output like this: > >� � *Main> main > >� � bd_rows =[Only {fromOnly = "-04.3982"}] > >� � noBDtxt ="-04.3982" > >� � noBDfp =-4.3982 > >� � noBDfp + 1 = -3.3982 > >� � i'm well getting a float in noBDfp , i even can add 1 to it > :-) ( cool > >� � haskell...) > >� � but i'm just wanting to that in the function getDB3 but it > does not > >� � compile... > >� � ?????? > >� � Damien > > > >� � On Sun, Dec 23, 2018 at 4:54 PM Tom Ellis > >� � <[1][2]tom-lists-haskell-cafe-2017(a)jaguarpaw.co.uk> wrote: > > > >� � � I think forgetting about monads and just using do-notation > will help > >� � � you. > >� � � On Sun, Dec 23, 2018 at 04:44:57PM +0100, Damien Mattei > wrote: > >� � � > i think learning Monads from scratch again will help me > >� � � > > >� � � > On Sun, Dec 23, 2018 at 4:11 PM Tom Ellis < > >� � � > [2][3]tom-lists-haskell-cafe-2017(a)jaguarpaw.co.uk> > wrote: > >� � � > > >� � � > > Yes, exactly! > >� � � > > > >� � � > > On Sun, Dec 23, 2018 at 02:08:57PM +0100, Damien > Mattei wrote: > >� � � > > > lazyness....? > >� � � > > > > >� � � > > > On Sun, Dec 23, 2018 at 8:40 AM Tom Ellis < > >� � � > > > [3][4]tom-lists-haskell-cafe-2017(a)jaguarpaw.co.uk> > wrote: > >� � � > > > > >� � � > > > > On Sat, Dec 22, 2018 at 09:52:18AM +0100, Damien > Mattei > >� � � wrote: > >� � � > > > > > i have inserted trace statement that output > variable > >� � � > > > > > ... i have strange behavior of output: > >� � � > > > > > >� � � > > > > Let's take a simpler example.�� Do you > understand why the > >� � � trace > >� � � > > statments > >� � � > > > > from this small program appear in the order that > they do?� > >� � � (And for > >� � � > > what > >� � � > > > > it's worth I really think you'll be better off > writing > >� � � programs using > >� � � > > do > >� � � > > > > notation). > >� � � > > > > > >� � � > > > > > >� � � > > > > % cat test.hs > >� � � > > > > import Debug.Trace > >� � � > > > > > >� � � > > > > result = > >� � � > > > >�� let a = trace "evaluating a" 2 > >� � � > > > >�� b = trace "evaluating b" 10 > >� � � > > > >�� c = trace "evaluating c" (a + > b) > >� � � > > > >�� in c > >� � � > > > > ~% ghci -e result test.hs > >� � � > > > > evaluating c > >� � � > > > > evaluating b > >� � � > > > > evaluating a > >� � � > > > > 12 > >� � � > > _______________________________________________ > >� � � > > Haskell-Cafe mailing list > >� � � > > To (un)subscribe, modify options or view archives go > to: > >� � � > > > [4][5]http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > >� � � > > Only members subscribed via the mailman list are > allowed to > >� � � post. > >� � � > _______________________________________________ > >� � � > Haskell-Cafe mailing list > >� � � > To (un)subscribe, modify options or view archives go to: > >� � � > > [5][6]http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > >� � � > Only members subscribed via the mailman list are allowed > to post. > >� � � _______________________________________________ > >� � � Haskell-Cafe mailing list > >� � � To (un)subscribe, modify options or view archives go to: > >� � � > [6][7]http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > >� � � Only members subscribed via the mailman list are allowed > to post. > > > > Verweise > > > >� � 1. mailto:[8]tom-lists-haskell-cafe-2017@jaguarpaw.co.uk > >� � 2. mailto:[9]tom-lists-haskell-cafe-2017@jaguarpaw.co.uk > >� � 3. mailto:[10]tom-lists-haskell-cafe-2017@jaguarpaw.co.uk > >� � 4. > [11]http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > >� � 5. > [12]http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > >� � 6. > [13]http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > > Verweise > > 1. mailto:ian@zenhack.net > 2. mailto:tom-lists-haskell-cafe-2017@jaguarpaw.co.uk > 3. mailto:tom-lists-haskell-cafe-2017@jaguarpaw.co.uk > 4. mailto:tom-lists-haskell-cafe-2017@jaguarpaw.co.uk > 5. http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > 6. http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > 7. http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > 8. mailto:tom-lists-haskell-cafe-2017@jaguarpaw.co.uk > 9. mailto:tom-lists-haskell-cafe-2017@jaguarpaw.co.uk > 10. mailto:tom-lists-haskell-cafe-2017@jaguarpaw.co.uk > 11. http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > 12. http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > 13. http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe

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Poor performance when using hsc2hs to cross-compile
by Vanessa McHale 26 Dec '18

26 Dec '18

Hi all, I've been trying to cross-compile X11 for ARM. Unfortunately, the build runs in the preprocessor phase for a very very long time when preprocessing Graphics/X11/Types.hsc Upon passing -v to hsc2hs I saw many steps like checking XK_abrevebelowdot `GreaterOrEqual` Unsigned 17039360 executing: /usr/bin/arm-linux-gnueabihf-gcc -c /home/vanessa/git-builds/junk/X11-1.9/dist-newstyle/build/arm-linux/ghc-8.6.3/X11-1.9/build/Graphics/X11/ExtraTypes/XorgDefault_hsc_test37988.c -o /home/vanessa/git-builds/junk/X11-1.9/dist-newstyle/build/arm-linux/ghc-8.6.3/X11-1.9/build/Graphics/X11/ExtraTypes/XorgDefault_hsc_test37988.o -marm -fno-stack-protector '-fuse-ld=gold' -Wl,-z,noexecstack '-D__GLASGOW_HASKELL__=806' '-Dlinux_BUILD_OS=1' '-Dx86_64_BUILD_ARCH=1' '-Dlinux_HOST_OS=1' '-Darm_HOST_ARCH=1' -Iinclude -I/home/vanessa/git-builds/junk/X11-1.9/dist-newstyle/build/arm-linux/ghc-8.6.3/X11-1.9/build/include -I/home/vanessa/git-builds/junk/X11-1.9/dist-newstyle/build/arm-linux/ghc-8.6.3/X11-1.9/build/autogen -I/home/vanessa/git-builds/junk/X11-1.9/dist-newstyle/build/arm-linux/ghc-8.6.3/X11-1.9/build/global-autogen -include /home/vanessa/git-builds/junk/X11-1.9/dist-newstyle/build/arm-linux/ghc-8.6.3/X11-1.9/build/autogen/cabal_macros.h -I/usr/local/lib/arm-linux-gnueabihf-ghc-8.6.3/base-4.12.0.0/include -I/usr/local/lib/arm-linux-gnueabihf-ghc-8.6.3/integer-gmp-1.0.2.0/include -I/usr/local/lib/arm-linux-gnueabihf-ghc-8.6.3/include result: False which I assume are necessary or at least progressing... Is there any way to speed this up? Is it a bug of some sort in hsc2hs or X11? Is there any hope in reporting upstream to the appropriate project (and what is the appropriate project in this case?)? Cheers, Vanessa

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ZuriHac 2019 registration now open
by Jasper Van der Jeugt 26 Dec '18

26 Dec '18

Hello Friends of Haskell, We are happy to announce that registration for ZuriHac 2019 is now open. Participation is free but limited to 450 attendees. You can register at: https://zfoh.ch/zurihac2019/#registration This year, we've taken dogfooding a step further and rolled our own open source registration system in Haskell, so please let me know if you experience any issues. ZuriHac 2019 will take place from Friday the 14th to Sunday the 16th of June 2019. It will be hosted at the Hochschule Rapperswil on the shores of beautiful Lake Zurich. This will mark the 8th anniversary of ZuriHac since our beginnings in 2010. The Zurich Haskell Hackathon is a free, international, grassroots, and collaborative coding festival. Our goal is to connect Haskellers, expand the community, learn things from each other, and to work on Haskell libraries, tools, and infrastructure. This year, we will enjoy keynotes from: - Simon Peyton Jones - Susan Potter - Richard Eisenberg - Ryan Trinkle More keynote speakers will be announced. The event is open to any experience level, from beginners to gurus. We want to make a special effort to ensure that the event is welcoming and accessible to people completely new to Haskell. That is why we are super excited that, Julie Moronuki, co-author of Haskell Programming from first principles [1] and Joy of Haskell [2], has kindly agreed to teach a beginners course in one of the classrooms we have available. Additionally, there will be mentors that you can approach during the whole event with any Haskell-related questions. This is a great opportunity to meet your fellow Haskellers in real life, find new contributors for your project, improve existing libraries and tools or even start new ones! You can find all of this information and more on our website: https://zfoh.ch/zurihac2019 We would also like to thank our sponsors Digital Asset [3], DFINITY [4] and HSR [5] for their strong commitment to the Haskell community and for supporting this great event! Looking forward to seeing you there, The Zurich Friends of Haskell association. [1]: https://www.goodreads.com/book/show/25587599-haskell-programming [2]: https://joyofhaskell.com/ [3]: https://digitalasset.com/careers.html [4]: https://dfinity.org/jobs [5]: https://www.hsr.ch/

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