February 2019 - Haskell-Cafe

[ANN] Haskell.Org Discourse Instance
by Jasper Van der Jeugt 08 Apr '19

08 Apr '19

Hello all, We would like to announce a new discourse-based forum for the Haskell community, at: https://discourse.haskell.org/ This is inspired by the Rust, Nix and Elm discourse instances, where these forums have had a strong positive effect on the community. We will run this as a "beta" for 6 months and then evaluate its usefulness. It is meant as an extra alternative to Haskell-cafe, reddit and Haskell IRC, but it does not replace any of those. Discourse is completely open source and easily accessible using an email client. We would like to thank the Haskell.Org admin team for provisioning a server and Matthew Pickering for suggesting this and doing the hard work setting up the instance. Warm regards, Jasper Van der Jeugt on behalf of the Haskell.Org Committee

4 3

Feedback on plan for checking code requirements in a GHC plugin
by Chris Smith 03 Apr '19

03 Apr '19

Hey everyone, I'm converging on a design for a new feature of CodeWorld, the Haskell-based educational programming environment that I teach with. I'm wondering if anyone has done something like this before. The goal is that when I give an assignment in a class (e.g., "modify this starting code to generalize the repeated pattern using a function"), I want the user to see a checklist of assignment requirements when they run the code. A prototype implementation is here: https://code.world/haskell#PpwpTF1wv3qIvwhohCfvSrQ There are all sorts of possible requirements, such as: "No lines longer than 80 characters", or "there must be a function called foo", or "all top-level definitions must have type signatures", or "your code must define at least 10 top-level variables, and use at least 3 where clauses", or "your definition of var should be equivalent to this one" (see Joachim Breitner's inspection testing work), or even "the function f must satisfy this QuickCheck property". It's been suggested that the requirements language also include the ability to match patterns in the AST, which I think is a good idea. The current prototype uses a pre-compile step that parses the code using haskell-src-exts, and doesn't implement dynamic requirements (runtime-evaluated) at all. My ultimate plan, though, is to send these requirements to GHC via a plugin, then have it evaluate the static ones at compile time, and generate code to check the dynamic ones. Finally, the plugin would add new code to the beginning of main that will invoke a configurable function with the results of the requirement check (hard-coding the static ones, and evaluating dynamic ones on the fly). (In the CodeWorld environment, this function would display the checklist in the web UI, for example.) Has anyone done anything like this before? Any wisdom to share, or ideas to contribute? Thanks, Chris

3 3

Coercible between data A a b = A a b and (a, b)
by Georgi Lyubenov 07 Mar '19

07 Mar '19

Greetings! Is there any reason behind/what is the reason behind ``` data A a b = A a b ``` and ``` (a, b) ``` not being coercible? And in general all n-ary constructors not being coercible to one another? Thanks in advance! ======= Georgi

4 3

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

2 4

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

3 2

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

3 4

example of monad from http://learnyouahaskell.com not working
by Damien Mattei 27 Feb '19

27 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

4 7

Haskell.Org accepted for GSoC 2019
by Jasper Van der Jeugt 26 Feb '19

26 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

1 0

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

2 3

Where should I report about https://www.haskell.org/ghc/
by Yuji Yamamoto 25 Feb '19

25 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

4 4