- Haskell-Cafe - Haskell.org

show in monad
by Damien Mattei 01 Mar '19

01 Mar '19

just for tracing the monad i have this : import Control.Monad import Data.Ratio import Data.List (all) import Debug.Trace newtype Prob a = Prob { getProb :: [(a,Rational)] } deriving Show instance Functor Prob where fmap f (Prob xs) = trace " Functor Prob " Prob $ map (\(x,p) -> (f x,p)) xs t flatten :: Prob (Prob a) -> Prob a flatten (Prob xs) = trace (" flatten " ++ show xs) Prob $ concat $ map multAll xs where multAll (Prob innerxs,p) = trace " multAll " map (\(x,r) -> (x,p*r)) innerxs instance Applicative Prob where pure = trace " Applicative Prob return " return (<*>) = trace " Applicative Prob ap " ap instance Monad Prob where return x = trace " Monad Prob return " Prob [(x,1%1)] m >>= f = trace " Monad Prob >>= " flatten (fmap f m) fail _ = trace " Monad Prob fail " Prob [] {- instance Applicative Prob where pure a = Prob [(a,1%1)] Prob fs <*> Prob as = Prob [(f a,x*y) | (f,x) <- fs, (a,y) <- as] instance Monad Prob where Prob as >>= f = Prob [(b,x*y) | (a,x) <- as, let Prob bs = f a, (b,y) <- bs] -} in this : flatten :: Prob (Prob a) -> Prob a flatten (Prob xs) = trace (" flatten " ++ show xs) Prob $ concat $ map multAll xs where multAll (Prob innerxs,p) = trace " multAll " map (\(x,r) -> (x,p*r)) innerxs i have this error: [1 of 1] Compiling Main ( monade.hs, interpreted ) monade.hs:22:43: error: • No instance for (Show a) arising from a use of ‘show’ Possible fix: add (Show a) to the context of the type signature for: flatten :: forall a. Prob (Prob a) -> Prob a • In the second argument of ‘(++)’, namely ‘show xs’ In the first argument of ‘trace’, namely ‘(" flatten " ++ show xs)’ In the expression: trace (" flatten " ++ show xs) Prob | 22 | flatten (Prob xs) = trace (" flatten " ++ show xs) | ^^^^^^^ Failed, no modules loaded. how can i implement a show for xs ? regards, damien

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qtHaskell
by David Harley 28 Feb '19

28 Feb '19

I've made a new version of qtHaskell - version 1.1.10.1 available at the following site: http://www.isptech.co.uk/qtHaskell/download.html It comprises bindings to about 50 Qt modules and includes a version of Asteroids which uses functional reactive programming and some code running in a non-IO based monad, plus a Qt based version of Threadscope (qts). The installation of a code set of this sort can be tricky, it builds using a combination of cabal, qmake and make controled from perl. I wish it were simpler, but in most cases: ./build lite -jx will build all modules required for the demos and examples. It has currently only been tested on Linux hence, the build script which used to work on Windows will probably not work without some modification. I'll get around to that if there's any interest. It has also only been tested against regular builds of Qt 5.11/5.12 build from the qt-everywhere-src-5.11/12.x codesets. Good luck with any building and testing of qtHaskell-1.1.10.1, and I'd be delighted to get any feedback David Harley

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rounding and undefined behavior
by Matt Peddie 28 Feb '19

28 Feb '19

Hi cafe, It's been my general impression that when neither Haskell nor C defines behavior in a particular situation, the behavior nonetheless matches. I was surprised to observe Prelude Data.Int> round (4294967295 :: Double) :: Int16 -1 when #include <stdio.h> #include <inttypes.h> int main(void) { double d = 4294967295; int16_t r = (int16_t) d; printf("%"PRId16"\n", r); return 0; } yields 0 when compiled and run. As far as I can tell, neither language defines what this result should be, so neither is doing anything wrong here. But I was surprised that they differ; does anyone know why Haskell's rounding operation behaves the way it does (maybe there is some historical reason)? Or can someone perhaps point me to a standards document I missed that states how the language must round out-of-bounds inputs? Regards Matt Peddie

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example of monad from http://learnyouahaskell.com not working
by Damien Mattei 28 Feb '19

28 Feb '19

i'm trying this example (see code below) from : http://learnyouahaskell.com/for-a-few-monads-more#making-monads when trying to compile this: import Data.Ratio newtype Prob a = Prob { getProb :: [(a,Rational)] } deriving Show instance Functor Prob where fmap f (Prob xs) = Prob $ map (\(x,p) -> (f x,p)) xs thisSituation :: Prob (Prob Char) thisSituation = Prob [( Prob [('a',1%2),('b',1%2)] , 1%4 ) ,( Prob [('c',1%2),('d',1%2)] , 3%4) ] flatten :: Prob (Prob a) -> Prob a flatten (Prob xs) = Prob $ concat $ map multAll xs where multAll (Prob innerxs,p) = map (\(x,r) -> (x,p*r)) innerxs instance Monad Prob where return x = Prob [(x,1%1)] m >>= f = flatten (fmap f m) fail _ = Prob [] l1 = Prob [('a',2%3),('b',1%3)] multAllExt :: (Prob a, Rational) -> [(a, Rational)] multAllExt (Prob innerxs,p) = map (\(x,r) -> (x,p*r)) innerxs --Main> :type multAllExt --multAllExt :: (Prob a, Rational) -> [(a, Rational)] --Main> multAllExt (l1,1 % 4) --[('a',1 % 6),('b',1 % 12)] i get this error: GHCi, version 8.4.3: http://www.haskell.org/ghc/ :? for help Prelude> :load monade.hs [1 of 1] Compiling Main ( monade.hs, interpreted ) monade.hs:21:10: error: • No instance for (Applicative Prob) arising from the superclasses of an instance declaration • In the instance declaration for ‘Monad Prob’ | 21 | instance Monad Prob where | ^^^^^^^^^^ Failed, no modules loaded. it fails when i add the last part of the example: instance Monad Prob where return x = Prob [(x,1%1)] m >>= f = flatten (fmap f m) fail _ = Prob [] seems the Monad needs an instance of the Applicative to be instanciated... what is wrong? regards, Damien

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Haskell.Org accepted for GSoC 2019
by Jasper Van der Jeugt 27 Feb '19

27 Feb '19

Hi all, We're very excited to announce that Haskell.Org has been accepted [1] into the Google Summer of Code 2019 program [2]. We hope that, like last year, it will lead to a whole range of improvements to the Haskell ecosystem, and to new faces joining our community! We would like to thank everyone who submitted ideas -- this is a key part of being accepted into GSoC. Now, here's the near term timeline: - Today - March 25: Potential student participants discuss application ideas with mentors - March 25 - April 9: Students can submit applications - May 6: Accepted student proposals announced At this point, we're looking for both students and extra mentors. We would like to assign at least two mentors to each project if possible, so the students get the support they deserve. Additional ideas for projects are still welcome! There's a lot of information on our Summer of Haskell page [3]. If there are any students who are not sure where to begin, feel free to reach out to us directly [4]! [1]: https://summerofcode.withgoogle.com/organizations/5556388114202624/ [2]: https://summerofcode.withgoogle.com/ [3]: https://summer.haskell.org/ [4]: https://summer.haskell.org/contact.html Warm regards Jasper Van der Jeugt on behalf of the Haskell.Org Committee

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generics-sop equivalent of everywhere/mkT?
by Scott Michel 26 Feb '19

26 Feb '19

Before I cut down my code to a test case, are there any examples of a generics-sop equivalent of syb's everywhere/mkT? What's the way to operate on a product, replace the "I" argument with a compatible argument and walk back through the isomorphism, i.e. "to $ <something> $ from". I'm hacking on a Z80 system emulator (TRS-80 Model I system, more specifically). There are a couple of places where it'd be smoother in the disassembler to transform the disassembled instruction sequence (converting addresses to labels) before output. Consequently, cutting down code to an example is a bit painful -- examples would help. -scooter

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Where should I report about https://www.haskell.org/ghc/
by Yuji Yamamoto 26 Feb '19

26 Feb '19

Hello Haskellers, I found a typo [in the download page of GHC][1]: the "Current Stable Release" section must be "8.6.3" instead of "8.6.2"! So I want to file an issue for the repository or sending a patch via pull request or merge request. But as long as I looking for https://gitlab.haskell.org/ghc/ghc , I can't get sure of where the download page is managed. Can you tell me where is the right place to report or how to send a patch? Thanks in advance! [1] https://www.haskell.org/ghc/download.html -- 山本悠滋 twitter: https://twitter.com/igrep GitHub: https://github.com/igrep GitLab: https://gitlab.com/igrep Facebook: http://www.facebook.com/igrep

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Munich Haskell Meeting, 2019-02-26 @ 19:30
by Leah Neukirchen 25 Feb '19

25 Feb '19

Dear all, Next week, our monthly Munich Haskell Meeting will take place again on Tuesday, February 26 at Rumpler at 19h30. For details see here: http://muenchen.haskell.bayern/dates.html If you plan to join, please add yourself to this dudle so we can reserve enough seats! It is OK to add yourself to the dudle anonymously or pseudonymously. https://dudle.inf.tu-dresden.de/haskell-munich-feb-2019/ Everybody is welcome! cu, -- Leah Neukirchen <leah(a)vuxu.org> http://leah.zone

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Feedback on new warning for MINIMAL pragmas
by Levent Erkok 24 Feb '19

24 Feb '19

Dear Haskell enthusiasts: I recently came across a scenario where GHC's output confused me regarding MINIMAL pragmas, and filed a "feature request" for GHC: https://github.com/ghc-proposals/ghc-proposals/pull/210 Simon PJ asked me to turn it into a proposal to gather feedback: https://github.com/ghc-proposals/ghc-proposals/pull/210 Alas, it gathered no feedback so far, aside from Simon PJ and myself. It's a very simple change and hopefully one that is also very easy to implement. If you could take a moment to comment on the proposal (or better yet, volunteer to implement it!), it would be much appreciated. I'm always very pleasantly surprised on the thoughtfulness and the technical depth of the comments this community provides, and it would be nice to make sure there are no gotcha scenarios. To keep the traffic on the haskell-cafe low, please comment directly on the proposal itself. Thanks, -Levent.

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segfault depending on code position in source file
by Damien Mattei 22 Feb '19

22 Feb '19

my code is structured like this , in summary: -- this function will return th N°BD from Sidonie for a given name -- note: names have been standardized between Sidonie and WDS getAlphaDelta :: Connection -> String -> IO (Double,Double) getAlphaDelta conn name = do let qry_head_AlphaDelta_AngularDistance = "select alpha,delta from AngularDistance where Nom = ?" :: Query (alpha_delta_rows :: [(Double,Double)]) <- query conn qry_head_AlphaDelta_AngularDistance (Only (name::String)) -- putStrLn $ show alpha_delta_rows return (head alpha_delta_rows) main = do conn <- connect defaultConnectInfo { connectHost = "moita", connectUser = "mattei", connectPassword = "sidonie2", connectDatabase = "sidonie" } -- check getAlphaDelta works: let name3 = "A 7" putStrLn $ show name3 ad3 <- getAlphaDelta conn name3 putStr "ad3 =" putStrLn $ show ad3 this works i got result such as: "A 7" ad3 =(11297.0,-619.0) if i move the 5 lines of code at the end of file it segfault between the begin and end i have some DB connection and queriung , filtering, mapping... any idea? should i really upgrade the version of GHCI? Damien

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