- Haskell-Cafe - Haskell.org

ANNOUNCE: fgl-arbitrary
by Ivan Lazar Miljenovic 22 Jun '15

22 Jun '15

I've just released the first version of fgl-arbitrary [1], which provides Arbitrary instances for fgl graphs for use with QuickCheck. Also provided are some wrapper newtypes to produce specific types of graphs, and also the ability to generate just valid node and edge lists for use with other graph-like data structures. [1]: http://hackage.haskell.org/package/fgl-arbitrary -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com http://IvanMiljenovic.wordpress.com

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"SameConstraints?" type constraints
by Leza Morais Lutonda 22 Jun '15

22 Jun '15

Hi All, I have the following data type: data S e where SC :: d -> e -> S e And I want to declare a Show instance for it in the following way: instance Show e => Show (S e) where show (SC x y) = show x ++ show y But, of course it don't typechecks because: could not deduce `Show d` arising from a use of `show`. Is there a way to constraint the `d` type in the `SC` constructor definition to have the same constraints of the `e` type? Something like: SC :: SameConstraints d e => d -> e -> S e ??? Thanks! -- Leza Morais Lutonda, Lemol-C http://lemol.github.io 50 Aniversario de la Cujae. Inaugurada por Fidel el 2 de diciembre de 1964 http://cujae.edu.cu

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stack mailing list
by Michael Snoyman 22 Jun '15

22 Jun '15

We recently started a mailing list for discussions of stack[1], a new Haskell build tool (see initial release announcement[2]). This list is open for questions, requests, discussions, and internal development (though if that last becomes too chatty, we can always move it to a separate list). You can sign up for the mailing list at: https://groups.google.com/d/forum/haskell-stack [1] https://github.com/commercialhaskell/stack#readme [2] https://www.fpcomplete.com/blog/2015/06/announcing-first-public-beta-stack

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Do people use visitor pattern in Haskell...
by Nicholls, Mark 22 Jun '15

22 Jun '15

My initial guess would be no...its built into the language But.... I find myself tempted to use it (in order to match on a type that's the instance of a typeclass)...which makes me think I've missed something. So... > data Foo = Foo > data Bar = Bar Ooo...this looks like an OO design pattern...surely wrong? > class Wibble a where > visit :: (Foo -> b) -> (Bar -> b) -> a -> b > instance Wibble Foo where > visit f _ x = f x > instance Wibble Bar where > visit _ f x = f x I want a list of Wibbles.... hmmm... (Wibble a) => [a] is clearly wrong... I want [(Wibble a) =>a] so I package it up? > data WibblePackage where > WibblePackage :: (Wibble a) => a -> WibblePackage lets try this now...so pointless function across Wibbles in a list. > fizzBuzz :: [WibblePackage] -> Integer > fizzBuzz [] = 0 > fizzBuzz ((WibblePackage x) : xs) = (visit (\_ -> 1) (\_ -> 2) x) + (fizzBuzz xs) > help :: Integer > help = fizzBuzz [WibblePackage Foo,WibblePackage Bar] That works! OO nerds will get uptight about "closing" the Wibble typeclass....but I've managed to create a list of different types and have a mechanism for recovering the type. How would a Haskell nerd do this? (I have looked, but they look more complicated....maybe to my OO eye). CONFIDENTIALITY NOTICE This e-mail (and any attached files) is confidential and protected by copyright (and other intellectual property rights). If you are not the intended recipient please e-mail the sender and then delete the email and any attached files immediately. Any further use or dissemination is prohibited. While MTV Networks Europe has taken steps to ensure that this email and any attachments are virus free, it is your responsibility to ensure that this message and any attachments are virus free and do not affect your systems / data. Communicating by email is not 100% secure and carries risks such as delay, data corruption, non-delivery, wrongful interception and unauthorised amendment. If you communicate with us by e-mail, you acknowledge and assume these risks, and you agree to take appropriate measures to minimise these risks when e-mailing us. MTV Networks International, MTV Networks UK & Ireland, Greenhouse, Nickelodeon Viacom Consumer Products, VBSi, Viacom Brand Solutions International, Be Viacom, Viacom International Media Networks and VIMN and Comedy Central are all trading names of MTV Networks Europe. MTV Networks Europe is a partnership between MTV Networks Europe Inc. and Viacom Networks Europe Inc. Address for service in Great Britain is 17-29 Hawley Crescent, London, NW1 8TT.

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"Universal" constraints?
by Michael Walker 21 Jun '15

21 Jun '15

Hi all, I was wondering about constraints in class declarations. Suppose we have something like this: class Monad m => Store m where type Var m :: * -> * new :: a -> m (Var m a) read :: Var m a -> m a write :: Var m a -> a -> m () Then some reasonable implementations would be IO and ST, using IORefs and STRefs, instance Store IO where type Var IO = IORef new = newIORef read = readIORef write = writeIORef instance Store (ST s) where type Var (ST s) = STRef s new = newSTRef read = readSTRef write = writeSTRef Now, both IORefs and STRefs have equality, which doesn't require equality of the contained type, instance Eq (IORef a) where ... instance Eq (STRef s a) where ... This corresponds to pointer equality and, because reading the values is a monadic action, is as good an Eq instance as we can get. Given this, it would be nice to be able to specify that all instances of our Store class have this property. Something like, class (Monad m, Eq (Var m a)) => Store m where ... But we can't write this! The `a` isn't in scope! What I really want to do is to be able to write class constraints like this, class (forall a. C (T a)) => D x where ... Or why not get really crazy: class (forall a. C a => D (T a)) => E x where ... Is there some extension or combination of extensions that would make this work without completely sacrificing type safety? FlexibleContexts allows it (the simpler form, at least) for function constraints, but not for class constraints. If not, has anyone thought about this sort of thing before? -- Michael Walker (http://www.barrucadu.co.uk)

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Troubles with HStringTemplate (compilation/runtime errors)
by Ilya Seleznev 21 Jun '15

21 Jun '15

Hello Haskellers, I hope I chose proper mailing list, if not I apologize. And thanks for reading this! I run into a problem with HStringTemplate ( http://hackage.haskell.org/package/HStringTemplate) that I have never seen before. Code, similar to {-# LANGUAGE OverloadedStrings, DeriveDataTypeable, NoImplicitPrelude #-} module Main where import Data.Data import qualified Data.Text as T import qualified Text.StringTemplate as Template renderTextTemplate :: (Template.ToSElem a) => T.Text -> a -> IO T.Text renderTextTemplate name input = do templatesGroup <- Template.directoryGroup "templates" case Template.getStringTemplate (T.unpack name) templatesGroup of Nothing -> return $ "Internal Error: template '" `mappend` name `mappend` "' can not be rendered" Just template -> return $ Template.render (Template.setAttribute "it" input (template:: Template.StringTemplate T.Text)) data Foo = Foo {value:: T.Text} deriving (Data, Typeable) main :: IO () main = do content <- renderTextTemplate "test" $ Foo {value = "oh hi there!"} putStrLn content with a simple template like this: Just a test $it.value$! started to cause complaints from GHC (7.8.2 and 7.8.4 on Linux x86_64): No instance for (Template.ToSElem Foo) Problem is - it didn't do that before. What even more confusing, older project[0] uses same approach, compiles (same machine, same compiler) and works just fine! I've exhausted all means to fix the problem that I could think off: - Adding 'instance Template.ToSElem Foo' allows code to compile but it doesn't work and prints message in console like this: Main.hs:30:10-29: No instance nor default method for class operation Text.StringTemplate.Classes.toSElem - Tried to use same version of HStringTemplate deps (syb was different) - All kinds of desperate messing around (exporting records from modules, changing field names...) - Updated ghc from 7.8.2 to 7.8.4 - Googled error messages to no avail. I hope almighty All would give me some ideas/directions because I have none left. [0] Function: https://github.com/itsuart/fdc_archivist/blob/master/src/HttpUtils.hs#L59 Data: https://github.com/itsuart/fdc_archivist/blob/master/src/ViewModels/HomeVie… Usage: https://github.com/itsuart/fdc_archivist/blob/master/src/HomeViewFeature.hs… -- With best regards, Ilya Seleznev

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shakespeare >= 2.0.2 fails to install in OS X 10.6.8, Haskell-platform 2014.2.0.0; cabal-install 1.22.4.0
by Miguel A. Santos 20 Jun '15

20 Jun '15

(I already sent a comment on the Shakespeare site but couldn't get too far. So, if this isn't the right place for this issue, I'd appreciate you point me to the right one.) Hi, I can't install yesod. Doing a cabal install yesod-bin or cabal install yesod fails because it fails to build shakespeare. I tried with all versions of shapeare >= 2.0, as required by yesod-bin. Below are all outputs. My environment: OS X 10.6.8, Xcode 3.2 Haskell-platform 2014.2.0.0 ghc-7.8.3 cabal-install 1.22.4.0 using version 1.22.3.0 of the Cabal library. This corresponds to a fresh install of the HP, ghc, and cabal I didn't before trying to install yesod. The developer of Shakespeare suspects that be a bug of HP and suggested I try a clean install of GHC as explained in http://www.stackage.org/install#mac-os-x However, the bindist available there doesn't work with my OS 10.6.8 I have a large screen output from those trials that you can check on the Shakespeare github page corresponding to the thread I started there https://github.com/yesodweb/shakespeare/issues/156 If you prefer, I can paste it here as well. Thanks in advance. MA -- Public key ID: E8FE60D7 Public key server: see, e.g., hkp://keys.gnupg.net

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Re: [Haskell-cafe] [Haskell-beginners] Haskell -> JavaScript -> nodejs
by Sumit Sahrawat, Maths & Computing, IIT (BHU) 20 Jun '15

20 Jun '15

Better try on the cafe. cc-ing. On 20 June 2015 at 15:26, Martin Vlk <martin(a)vlkk.cz> wrote: > I made some progress - in order for Haskell modules to work with ghcjs > you need to use cabal with the --ghcjs flag to install them. > > Martin > > Martin Vlk: > > Hi, > > I can successfully compile a basic Hello World into JavaScript with > > ghcjs and run it with nodejs. > > But what I ultimately need to do is write a nodejs module in Haskell and > > compile down to JavaScript with ghcjs. I need to be able to require > > other nodejs modules as well. > > > > Could somebody point me in the right direction? E.g. what approach and > > libraries to use... > > > > I have tried a simple thing with jsaddle/lens, but when I try to compile > > with ghcjs it is not finding the imported modules, even though when > > compiling with ghc it does find them (they are installed using cabal). > > > > Many Thanks > > Martin > > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners > -- Regards Sumit Sahrawat

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Haskell and Spark(Hadoop)
by Vasili I. Galchin 20 Jun '15

20 Jun '15

> Hello, > > There was a posting from a Polish guy(Wojcieh .,,,, been trying to > contact him) on implementing a library/wrapper for Spark. Does anybody know > the status? > > Vasili >

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Question about terminateProcess
by Niklas Hambüchen 20 Jun '15

20 Jun '15

Hi, http://hackage.haskell.org/package/process-1.2.0.0/docs/System-Process.html… says: " Note: on Windows, if the process was a shell command created by createProcess with shell, or created by runCommand or runInteractiveCommand, then terminateProcess will only terminate the shell, not the command itself. On Unix systems, both processes are in a process group and will be terminated together. " I don't see that happening, at least not in ghci. When I do import System.Process (_,_,_,p) <- createProcess $ (shell "sleep 10") -- wait a second or so terminateProcess p in ghci, then in `ps aux | grep "sleep 10"`, the sleep is still running; same when I'm using `(shell "sleep 10"){ create_group = True }` (`shell` defaults to `create_group = False`). What am I missing?

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