March 2015 - Haskell-Cafe

ANN: Lambdabot 5.0.1
by Bertram Felgenhauer 22 Mar '15

22 Mar '15

Dear Haskell users, I'm pleased to announce a minor release of Lambdabot. Lambdabot is an IRC bot that can also be used offline; it provides tools for querying information about and even for producing Haskell code. To install it, use the following command: cabal install lambdabot --constraint 'transformers installed' (the constraint helps cabal-install to find an install plan) * What's new - lambdabot is now smarter about locating its data files. This means that commands like @src now work out of the box, without having to copy files into ~/.lambdabot/State - lambdabot-5.0.1 uses (and requires) monad-control 1.0 * Development Development of lambdabot happens on github, where you can also find the bugtracker: https://github.com/lambdabot/lambdabot Have fun with lambdabot, Bertram

1 0

time and TimeLocale compatibility
by Tom Ellis 22 Mar '15

22 Mar '15

Hello all, The `formatTime` function from time-1.4 uses the TimeLocale type from old-locale, whereas time-1.5 provides its own definition of the TimeLocale type. I want to provide `formatTime` with the argument `defaultTimeLocale`. 1.4 requires the version in old-locale whereas 1.5 requires the version it defines itself. Is it possible to use this function in a way that is compatible with both 1.4 and 1.5? If time-1.5 had merely reexported the old-locale version, and then (the future) time-1.6 replaced it completely this would have offered a smoother upgrade path, but as far as I can tell nothing like this was done. What are my options? Tom

3 4

Munich Haskell Meeting, 2015-03-25 @ 19:30
by Christian Neukirchen 22 Mar '15

22 Mar '15

Dear all, Next week, our monthly Munich Haskell Meeting will take place again on Wednesday, March 25 at Cafe Puck at 19h30. For details see here: http://www.haskell-munich.de/dates 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-mar-2015/ Everybody is welcome! cu, -- Christian Neukirchen <chneukirchen(a)gmail.com> http://chneukirchen.org

1 0

GSoC Proposal: Refactor program with HLint suggestions
by Matthew Pickering 22 Mar '15

22 Mar '15

Dear Café, I would appreciate any comments on the proposal I have written for this years GSoC. The proposal focuses on an oft-requested feature, the ability for HLint to perform the refactorings which it suggests. I believe this is now possible with the work that Alan Zimmerman has recently completed on ghc-exactprint. More details can be found here - http://mpickering.github.io/gsoc2015.html (There is also a pdf version available - http://mpickering.github.io/gsoc2015.pdf) Matt

1 0

netwire example error message
by briand＠aracnet.com 22 Mar '15

22 Mar '15

The error message: time.hs:13:8: No instance for (Show e0) arising from a use of ‘testWire’ The type variable ‘e0’ is ambiguous Note: there are several potential instances: instance Show Double -- Defined in ‘GHC.Float’ instance Show Float -- Defined in ‘GHC.Float’ instance (Integral a, Show a) => Show (GHC.Real.Ratio a) -- Defined in ‘GHC.Real’ ...plus 45 others In the expression: testWire clockSession_ pos In an equation for ‘main’: main = testWire clockSession_ pos Failed, modules loaded: none. So here's my type debugging. (netwire sure is convoluted) pos => Wire s e m a Float integral => a -> Wire s e m a a clockSession_ => Session m (Timed NominalDiffTime ()) looks to me like the problem is probably rooted in the interaction of pos with (Timed NominalDiffTime) as it does not seem to have a float instance, but that doesn't seem like it should result in an error about ambiguity. Any help appreciated. Thanks Brian ---- import Prelude hiding ((.)) -- To use (.) in the scope of Categories instead import Control.Wire import FRP.Netwire speed :: Float speed = 0.5 pos :: (HasTime t s, Monad m) => Wire s e m a Float pos = integral 0 . pure speed -- The main function main :: IO () main = testWire clockSession_ pos

2 2

Re: [Haskell-cafe] [Haskell-beginners] Multi-parameter type classes and ambiguous type variables...
by Sumit Sahrawat, Maths & Computing, IIT (BHU) 21 Mar '15

21 Mar '15

This might be better answered at the haskell-cafe. Sending to cafe. On 21 March 2015 at 03:09, Stuart Hungerford <stuart.hungerford(a)gmail.com> wrote: > Hi, > > As a learning exercise I'm modelling some algebraic structures as > Haskell typeclasses. Some of these are multi-parameter type classes. > Here's a very simplified version of the type class relationships: > > class MM a where > one :: a > > class AM a where > zero :: a > > class (AM a, MM a) => SR a > > class (AM a) => AG a where > inv :: a -> a > > class (SR a) => R a where > neg :: a -> a > > class (R r, AG g) => M r g where > sca :: r -> g -> g > > check :: (Eq g, M r g) => g -> Bool > check x = sca one x == x > > The problem I have is that GHC is finding the "r" type variable in the > "check" function ambiguous. Given my still limited Haskell knowledge > I'm not surprised this is happening. What I would like to know is how > experienced Haskellers handle this situation in practice: is there an > idiomatic way of disambiguating "r" or is it a sign of poor type class > design? > > Thanks, > > Stu > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners > -- Regards Sumit Sahrawat

3 3

Continuation vs Codensity
by silvio 21 Mar '15

21 Mar '15

I just saw this http://stackoverflow.com/questions/25827227/why-cant-there-be-an-instance-o… Personally, I think the suggested type for callCC newtype ContT m a = ContT { runContT :: forall r. (a -> m r) -> m r } class MonadCont m where callCC :: (forall b. (a -> m b) -> m b) -> m a is much more intuitive and resembles the actual type of ContT much more closely then the current callCC and it can be Fixed. So I was wondering if there was an other reason than convention for not using that type. Silvio

1 0

What's the story of this weird type?
by David Feuer 21 Mar '15

21 Mar '15

I was reading an answer by Luis Casillas on Stack Overflow http://stackoverflow.com/a/23418746/1477667 and he used a somewhat peculiar type, with apparently not-so-useful constraints: newtype f :-> g = Natural { eta :: forall x. (Functor f, Functor g) => f x -> g x } There are a couple things about it that I don't understand. 1. I can construct a :-> from a function that doesn't satisfy the constraints, and I can pattern match to get access to it, but I can't use it. Why am I allowed to construct it at all? data Mb a = N | J a -- Not a Functor instance -- This is accepted (why?) pot = Natural (\case [] -> N (a:_) -> J a) -- And I can even do this: mb :: Functor Mb => Mb Int mb = case pot of Natural f -> f [3] But then of course I can't satisfy the constraint to use it. 2. I would have thought that such a weird/broken thing as this would only be allowed with -XDatatypeContexts, but in fact it is allowed without it. Why? David

2 3

apt-get install haskell-platform fails on ubuntu 14.04
by David Gladstein 20 Mar '15

20 Mar '15

http://askubuntu.com/questions/597278/haskell-platform-unmet-dependencies I'm installing on a newly-built system. I looked at the haskell.org download instructions, and I couldn't find anything that made sense. apt-get install haskell-platform worked about a month ago. Does anyone know what happened?

2 3

Strongly typed data / API specification librares
by Ignas Vyšniauskas 20 Mar '15

20 Mar '15

Hi cafe, Does anyone know of any libraries that allow one to describe data / interfaces (think IDLs), with emphasis on good support for modules, strong typing and extensibility? I am thinking in the lines of Thrift[1], Piqi[2], Swagger[3], but with the core data specification language as decoupled as possible from any kind of implementation related things (such as actual representation of data, or RPC / RESTful bindings). haskell-servant[4] and silk-rest[5] are kind of relevant, but they are very much oriented towards RESTful APIs. Sorry if this is slightly off-topic, but I couldn't think anywhere else to inquire about this. Thanks, Ignas [1]: https://thrift.apache.org/ [2]: http://piqi.org/ [3]: http://swagger.io/ [4]: http://haskell-servant.github.io/ [5]: http://silkapp.github.io/rest/

2 1