- Haskell-Cafe - Haskell.org

Cabal extensions tracking
by David Feuer 26 Apr '16

26 Apr '16

It's possible to enable extensions using an "extensions" field in a Cabal file, in which case Cabal will be able to report what language extensions the package requires. Unfortunately, this can be confusing, since the extensions in effect do not appear at the top of each affected module. Is there any way to get Cabal to report required extensions *based on LANGUAGE pragmas*? David

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Re: [Haskell-cafe] (no subject)
by Tom Ellis 25 Apr '16

25 Apr '16

On Sun, Apr 24, 2016 at 08:07:40PM +0000, haskell-cafe-bounces(a)haskell.org wrote: > >From int-e(a)gmx.de Sun Apr 24 21:13:18 2016 > Date: Sun, 24 Apr 2016 21:13:18 +0200 > From: Bertram Felgenhauer <bertram.felgenhauer(a)googlemail.com> > To: haskell-cafe(a)haskell.org > Subject: Re: [Haskell-cafe] Is it possible to make lazy combinators for IO? > Message-ID: <20160424191319.GA32509@24f89f8c-e6a1-4e75-85ee-bb8a3743bb9f> > References: <CAMgWh9t4MbzbbdvnU8zz6Lm-p9Gtf2UO-%2BGaW%3DofPsNLmt4aCA(a)mail.gmail.com> > <CAErCyQ-J%2BiYo2tXVs9S4VtGtuurFaNp%3DzsrY_58%3D7EeL7q0HVQ(a)mail.gmail.com> > <CAMgWh9s3%2BksQOh52oU%3DazKcdLDyuFy2sYKPbz30xaXnTkLK9cw(a)mail.gmail.com> > <CAErCyQ8OVMF0fMDuxLpuvHimCt7RBijjzp1wEBMxRFyyyCXZVg(a)mail.gmail.com> > <CAMgWh9vsxijoCuNrMxeAZmVxML%2BA8iNUwpTy9Es1BF0z_%2BpryQ(a)mail.gmail.com> > <CAMgWh9tV_aq2%3DNkKZ79QZX6Xn6V953RwWoFqTeUMDggF0%3DZZ%2Bg(a)mail.gmail.com> > <CAErCyQ_zUqn53x1%3Dg3RKDjQc3%2BGXfXkj1VvaTxo8ONPeTfym0A(a)mail.gmail.com> > <CAMgWh9s%3D0bAsjGEgGwSr8bPL6vDqNL8yyY%3DG%2BqANoz35G0NKuw(a)mail.gmail.com> > MIME-Version: 1.0 > Content-Type: text/plain; charset=us-ascii > Content-Disposition: inline > In-Reply-To: <CAMgWh9s%3D0bAsjGEgGwSr8bPL6vDqNL8yyY%3DG%2BqANoz35G0NKuw(a)mail.gmail.com> > User-Agent: Mutt/1.5.24 (2015-08-30) > Status: RO > Content-Length: 1283 > Lines: 37 > > David Feuer wrote: > > What I'm looking for is more limited than lazy IO or unsafeInterleaveIO, > > but it seems quite possible that there's no way to get just what I'm > > looking for with the IO type proper using GHC's implementation of IO. Lazy > > IO allows evaluation to drive action. When a thunk is forced, it may > > trigger I/O (spooky action at a distance). What I'm talking about is > > separating what actions are performed from what values are calculated from > > them. Here's a partial concept which won't actually compile because of the > > lazy pattern matches: > > > > data MyIO a = forall b . MyIO (b -> a) (IO b) > > instance Functor MyIO where > > fmap f ~(MyIO t m) = MyIO (f . t) m > > instance Applicative MyIO where > > pure a = MyIO (const a) (pure ()) > > MyIO t1 m1 <*> ~(MyIO t2 m2) = > > MyIO (\(r1, r2) -> t1 r1 (t2 r2)) ((,) <$> m1 <*> m2) > > instance Monad MyIO where > > ??? > > instance MonadFix MyIO where > > ??? > > I believe that using this interface `unsafeInterleaveIO` could be > implemented as follows, making it just as powerful as lazy IO: > > data Box a = Box a > > unsafeInterleaveMyIO :: MyIO a -> MyIO a > unsafeInterleaveMyIO act = do > act' <- Box `fmap` act > return $ case act' of Box !r -> r > > Have I missed anything? Since MyIO and its associate functions don't contain any unsafe primitives it seems highly unlikely you can implement unsafeInterleaveIO with them! In fact I can't see how MyIO is any different to IO. All you can do with the function field is fmap it over the IO action field. Tom

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Munich Haskell Meeting, 2016-04-28 @ 19:30
by Christian Neukirchen 25 Apr '16

25 Apr '16

Dear all, This week, our monthly Munich Haskell Meeting will take place again on Thursday, April 28 at Cafe Puck at 19h30. For details see here: http://chneukirchen.github.io/haskell-munich.de/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-apr-2016/ Everybody is welcome! cu, -- Christian Neukirchen <chneukirchen(a)gmail.com> http://chneukirchen.org

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(no subject)
by None 25 Apr '16

25 Apr '16

>From int-e(a)gmx.de Sun Apr 24 21:13:18 2016 Date: Sun, 24 Apr 2016 21:13:18 +0200 From: Bertram Felgenhauer <bertram.felgenhauer(a)googlemail.com> To: haskell-cafe(a)haskell.org Subject: Re: [Haskell-cafe] Is it possible to make lazy combinators for IO? Message-ID: <20160424191319.GA32509@24f89f8c-e6a1-4e75-85ee-bb8a3743bb9f> References: <CAMgWh9t4MbzbbdvnU8zz6Lm-p9Gtf2UO-%2BGaW%3DofPsNLmt4aCA(a)mail.gmail.com> <CAErCyQ-J%2BiYo2tXVs9S4VtGtuurFaNp%3DzsrY_58%3D7EeL7q0HVQ(a)mail.gmail.com> <CAMgWh9s3%2BksQOh52oU%3DazKcdLDyuFy2sYKPbz30xaXnTkLK9cw(a)mail.gmail.com> <CAErCyQ8OVMF0fMDuxLpuvHimCt7RBijjzp1wEBMxRFyyyCXZVg(a)mail.gmail.com> <CAMgWh9vsxijoCuNrMxeAZmVxML%2BA8iNUwpTy9Es1BF0z_%2BpryQ(a)mail.gmail.com> <CAMgWh9tV_aq2%3DNkKZ79QZX6Xn6V953RwWoFqTeUMDggF0%3DZZ%2Bg(a)mail.gmail.com> <CAErCyQ_zUqn53x1%3Dg3RKDjQc3%2BGXfXkj1VvaTxo8ONPeTfym0A(a)mail.gmail.com> <CAMgWh9s%3D0bAsjGEgGwSr8bPL6vDqNL8yyY%3DG%2BqANoz35G0NKuw(a)mail.gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <CAMgWh9s%3D0bAsjGEgGwSr8bPL6vDqNL8yyY%3DG%2BqANoz35G0NKuw(a)mail.gmail.com> User-Agent: Mutt/1.5.24 (2015-08-30) Status: RO Content-Length: 1283 Lines: 37 David Feuer wrote: > What I'm looking for is more limited than lazy IO or unsafeInterleaveIO, > but it seems quite possible that there's no way to get just what I'm > looking for with the IO type proper using GHC's implementation of IO. Lazy > IO allows evaluation to drive action. When a thunk is forced, it may > trigger I/O (spooky action at a distance). What I'm talking about is > separating what actions are performed from what values are calculated from > them. Here's a partial concept which won't actually compile because of the > lazy pattern matches: > > data MyIO a = forall b . MyIO (b -> a) (IO b) > instance Functor MyIO where > fmap f ~(MyIO t m) = MyIO (f . t) m > instance Applicative MyIO where > pure a = MyIO (const a) (pure ()) > MyIO t1 m1 <*> ~(MyIO t2 m2) = > MyIO (\(r1, r2) -> t1 r1 (t2 r2)) ((,) <$> m1 <*> m2) > instance Monad MyIO where > ??? > instance MonadFix MyIO where > ??? I believe that using this interface `unsafeInterleaveIO` could be implemented as follows, making it just as powerful as lazy IO: data Box a = Box a unsafeInterleaveMyIO :: MyIO a -> MyIO a unsafeInterleaveMyIO act = do act' <- Box `fmap` act return $ case act' of Box !r -> r Have I missed anything? Cheers, Bertram

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Should webassembly be a target for GHC?
by Dimitri DeFigueiredo 25 Apr '16

25 Apr '16

I'm interested in webassembly's recent momentum https://hacks.mozilla.org/2016/03/a-webassembly-milestone/ Is this a way we could get a foot in the web? I'm curious, just putting the thought out there. Cheers, Dimitri

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Automatically Deriving Numeric Type Class Instances
by Jake 24 Apr '16

24 Apr '16

Is it possible to automatically derive instances of Numeric type classes like Num, Fractional, Real, Floating, etc? I currently have two datatypes, Pair and Triple, that are defined like this: data Pair a = Pair a a data Triple a = Triple a a a I wrote these pretty trivial instances for Num and Floating: instance Num a => Num (Pair a) where (+) = liftA2 (+) (*) = liftA2 (*) abs = liftA abs negate = liftA negate signum = liftA signum fromInteger = pure . fromInteger instance Fractional a => Fractional (Pair a) where (/) = liftA2 (/) recip = liftA recip fromRational = pure . fromRational and practically identical instances for Triple as well. Is there anyway to have GHC derive these instances and the other numeric type classes? Thanks, Jake

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Re: [Haskell-cafe] Haskell-Cafe Digest, Vol 152, Issue 27
by Julian Leviston 23 Apr '16

23 Apr '16

There were quite a few responses to you so far, but no one seemed to mention that what you actually asked for are called anamorphisms. https://en.wikipedia.org/wiki/Anamorphism <https://en.wikipedia.org/wiki/Anamorphism> They’re not a *class* per se (as in… they’re not a typeclass), but rather they’re a kind of higher-order function which we call unfold (the kind-of “opposite" action to a fold). Here’s a nice little thing I found which is quite helpful as an explanation: https://gist.github.com/CMCDragonkai/b586ee9ecce297ceeeb3 and Data.List has unfoldr: http://hackage.haskell.org/package/base-4.8.2.0/docs/Data-List.html#v:unfol… <http://hackage.haskell.org/package/base-4.8.2.0/docs/Data-List.html#v:unfol…> If you google around for either "anamorphism Haskell” or “unfold Haskell” you’ll find some nice explanatory resources. Hope that helps! Julian http://www.happylearnjavascripttutorial.com/ <http://www.happylearnjavascripttutorial.com/> - begin programming with JavaScript today using our awesome new method! http://www.happylearnhaskelltutorial.com/ <http://www.happylearnhaskelltutorial.com/> - begin programming with Haskell today using our awesome new method! > On 23 Apr 2016, at 10:00 PM, haskell-cafe-request(a)haskell.org wrote: > > Hi. I'm more in the shallow end of the Haskell pool right now, so > forgive me if this is really basic... In Haskell, what is the class for > things that can provide an infinite sequence? I mean, things that > produce a value as well as function for producing the next value? > Clearly RandomGen does this, but what is the next level of abstraction > above RandomGen (i.e., RandomGen minus the randomness aspect).

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wxhaskell: Play sound not working on OSX?
by Tilmann 23 Apr '16

23 Apr '16

Is it possible that play . sound might not work on OSX or I'm I doing something wrong? This advice from around 2006 is all I could find: "Did you build wxWidgets with configuring --enable-sound build option?" Is this still valid? If so, how do I do that? Tilmann |import Graphics.UI.WX main :: IO () main = start $ do f <- frame [] p <- panel f [] set f [layout := minsize(Size 100 100) $ widget p] play (sound "alarm.wav") | Using OSX 10.9.5, GHC 7.10.2, WX 0.92.2.0 http://stackoverflow.com/questions/36809989/play-sound-with-wxhaskell-not-w…

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[ANN] Aivika: A parallel distributed discrete event simulation library
by David Sorokin 23 Apr '16

23 Apr '16

Hi Cafe! I’m glad to announce the first add-on module [3] of Aivika simulation library that allows creating and running parallel distributed discrete event simulations. It is destined for clusters and supercomputers, although it works on a stand-alone notebook too. My module implements an optimistic strategy that implies transparent rollbacks. It is mainly inspired by ideas of the Time Warp algorithm, which first publication is dated early 80s. Please note that this is the first version. It is mainly experimental, but it already shows good results. For example, when using the reproducible random number generators on different nodes, I always get the same simulation results. Furthermore, for testing purposes, I rewrote one of my examples with discrete processes and the shared resource so that it uses too much messages just to see how it works. The add-on module allows doing everything that the generalized version [2] of Aivika can do in a part related to the discrete event simulation. Moreover, the parallel nodes can communicate to each other sending and receiving asynchronous messages, but these messages have the send and receive times. If the node receives the past message, a transparent rollback occurs so that the node can process the received message. When rolling the computation back, the node sends so called anti-messages to cancel the invalidated messages that came from the node. It allows us to have a predictable distributed simulation satisfying the time constraints and causality. There are different customization parameters that allow tuning the library. For example, by default the library waits 5 seconds to consider the local time of the node defined. Therefore, when running test models, you can see a small delay, which is necessary for finishing the synchronization between the nodes and so called time server. Unfortunately, there is a lack of documentation. But the main idea is as follows. The generalized version [2] is almost the same as the core library [1] which already has a documentation. The represented add-on [3] just allows using the generalized version for creating and running parallel distributed discrete event simulations within the specified computation. Best regards, David Sorokin, Yoshkar-Ola, Russia [1] http://hackage.haskell.org/package/aivika [2] http://hackage.haskell.org/package/aivika-transformers [3] http://hackage.haskell.org/package/aivika-distributed

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Generators? Iterators?
by Christopher Howard 23 Apr '16

23 Apr '16

Hi. I'm more in the shallow end of the Haskell pool right now, so forgive me if this is really basic... In Haskell, what is the class for things that can provide an infinite sequence? I mean, things that produce a value as well as function for producing the next value? Clearly RandomGen does this, but what is the next level of abstraction above RandomGen (i.e., RandomGen minus the randomness aspect). -- http://justonemoremathproblem.com To protect my privacy, please use PGP encryption. It's free and easy to use! My public key ID is 0x340EA95A (pgp.mit.edu)

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