April 2016 - Haskell-Cafe

Hackage package index download failure
by Sean Leather 28 Apr '16

28 Apr '16

I'm unable to download the package index file used by cabal: $ cabal update Downloading the latest package list from hackage.haskell.org cabal: Failed to download http://hackage.haskell.org/packages/archive/00-index.tar.gz : ErrorMisc "Error HTTP code: 502" A few hours ago the error was a 503. Anybody know what's going on? Regards, Sean

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Interactive ASCII graphics
by Jeffrey Brown 27 Apr '16

27 Apr '16

Gary Fixler recently gave a talk, to the Santa Monica Haskell Users Group at Brainium, in which he creates an interactive explorer for (among other images) the Mandelbrot set, using nothing but text. No complex libraries for graphics or reactivity are involved -- the only foreign import statements are to Data.Monoid, Data.Function, Data.Complex, and System.IO. The code is here: https://github.com/gfixler/continou This gif demonstrates it: http://www.garyfixler.com/2016-04-18-012037_3840x1024_scrot.gif The video is here: https://vimeo.com/163287967/99e0595942 -- Jeffrey Benjamin Brown

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When using Cereal, what is the right way to write `get` for multi-constructor type?
by Magicloud Magiclouds 27 Apr '16

27 Apr '16

Hi, Say I have code already like: data STH = A | B | C instance Serialize STH where put A = putWord8 1 put B = putWord8 66 put C = putWord8 111 Then what is the better way to do `get`? Is following the only one? get = do i <- getWord8 case i of 1 -> return A 66 -> return B 111 -> return C Thanks.

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Dropping support for NHC98 and old GHC from containers
by David Feuer 26 Apr '16

26 Apr '16

The containers package currently includes CPP and Cabal directives intended to support NHC98 and versions of GHC before 7.0. However, our TravisCI script tests only GHC versions 7.0 and above. It is therefore likely that the compatibility code is bit-rotted to some extent. Wren Romano and I are strongly inclined to immediately remove all code intended to support NHC98 or pre-7.0 GHC. If anyone uses any such implementations with the current version of containers, please speak now or forever hold your peace. Thanks, David Feuer

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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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