February 2015 - Haskell-Cafe

Re: [Haskell-cafe] RankNTypes question
by Nikolay Amiantov 26 Feb '15

26 Feb '15

Hm, now that I look at it, I've given the wrong example which badly describes what I want -- sorry. Let me give you a more specific one: {-# LANGUAGE RankNTypes #-} test :: (forall a. (a -> a)) -> (Int -> Int) test = id-- something else should be here i.e., I want to "specialize" a polymorphic value -- in this case, a polymorphic function to the function on `Int`s. On 02/26/2015 09:24 PM, Marcin Mrotek wrote: > Hello, > > id has type a -> a, a function from some type to exactly the same > type. Your function would be from any type to any other (the same type > as given being a special case). It seem to me that > > example _ = undefined > > is the only legal definition. What do you exactly want the function to do? > > Kind regards, > Marcin Mrotek -- Nikolay.

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safe ways how to get head/last of Seq (or Foldable in general)
by Petr Pudlák 26 Feb '15

26 Feb '15

Hi, today I was a bit surprised that apparently there is no easy way how to safely get the head element of `Seq` in a point-free way. Of course there is `viewl`, but it seems the data type has no folding function (something like 'foldViewL :: b -> (a -> Seq a -> b) -> b`. Is there any existing function like `Seq a -> Maybe a` to safely retrieve the head (or last) element? If not, I'd suggest to add headMaybe :: (Foldable t) => t a -> Maybe a headMaybe = getFirst . foldMap (First . Just) and similarly lastMaybe to Data.Foldable. Thanks, Petr

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[ANN] hoodle 0.4
by Ian-Woo Kim 26 Feb '15

26 Feb '15

Hi, Haskellers! I am happy to announce a new version of hoodle that I uploaded to hackage right now. The newest version is 0.4. Hoodle is a pen notetaking program developed in haskell using gtk2hs. You can find relevant information here: http://ianwookim.org/hoodle If all the dependencies are correctly installed (hoodle has rather big dependencies though), the installation of hoodle should be easily doable by cabal install hoodle. I am managing hoodle in nix package repository, so nix users can also easily install hoodle by nix command once after nix expression in nixpkgs is updated. The major change in this update is *tabbed* interface! Like modern web browsers, you can open multiple hoodle files in one hoodle window and navigate them using tabs. Internally, I improved its rendering engine quite much. Now it has a fully asynchronous renderer, which enables a faster change of zoom ratio using cached image and then update rendering later more precisely. This improves display performance from user's point of view a lot. I also made a support for synchronisation and shared documents in association with a web service which I am now preparing for. By default, it is turned off, so all network-related dependencies for that will not be required unless turned on by -fhub flag. In addition, I also made dyre (dynamic reloading as xmonad and yi) opt-in, so you need to use -fdyre if you want to have the feature. Now hoodle is being migrated to gtk3 though this version will not support gtk3 fully yet. I plan to migrate hoodle to gtk3 fully and use gtk3 exclusively in the next version, so this version will be the last gtk2 version. Once this is done, a multi-touch support will be enabled. (and also hoodle will be able to live in wayland, too!) Enjoy hoodle and please bug me if you have any issues. Thank you! best, Ian-Woo Kim

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SBLP 2015: 19th Brazilian Symposium on Programming Languages
by Simon Thompson 26 Feb '15

26 Feb '15

+++++++++++++++++++++++++++++++++++++++++++++++++++ CALL FOR PAPERS - SBLP 2015 19th Brazilian Symposium on Programming Languages 21-26 September 2015 Belo Horizonte, Brazil http://cbsoft.org/sblp2015 +++++++++++++++++++++++++++++++++++++++++++++++++++ IMPORTANT DATES Abstract submission: 20 April, 2015 Paper submission: 27 April, 2015 Author notification: 18 June, 2015 Camera ready deadline: 2 July 2015 INTRODUCTION The Brazilian Symposium on Programming Languages is a well-established symposium which provides a venue for researchers and practitioners interested in the fundamental principles and innovations in the design and implementation of programming languages and systems. SBLP 2015 is part of 6th Brazilian Conference on Software: Theory and Practice, CBSoft 2015, that will be held in Belo Horizonte, Minas Gerais, Brazil, from September 21st to September 26th, 2015. Authors are invited to submit original research on any relevant topic which can be either in the form of regular or short papers. TOPICS Topics of interest include, but are not limited to: * Program generation and transformation, including domain-specific languages and model-driven development in the context of programming languages. * Programming paradigms and styles, including functional, object-oriented, aspect-oriented, scripting languages, real-time, service-oriented, multithreaded, parallel, and distributed programming. * Formal semantics and theoretical foundations, including denotational, operational, algebraic and categorical. * Program analysis and verification, including type systems, static analysis and abstract interpretation. * Programming language design and implementation, including new programming models, programming language environments, compilation and interpretation techniques. INVITED SPEAKERS * Doaitse Swierstra, Utrecht University * TBA SUBMISSION AND PUBLICATION All submissions will be peer-reviewed and judged on the basis of its originality, contribution to the field, technical and presentation quality, and relevance to the symposium. Contributions should be written in Portuguese or English. Papers should fall into one of two different categories: regular papers, which can be up to 15 pages long in LNCS format, or short papers, with up to 5 pages in LNCS format. Short papers can discuss new ideas which are at an early stage of development and which have not yet been thoroughly evaluated. We encourage the submission of short papers reporting partial results of on-going master dissertations or doctoral theses. Accepted papers written in English will be published in a volume of Lecture Notes in Computer Science (LNCS), by Springer. Both regular and short papers must be prepared using the LNCS format, available at http://www.springer.com/computer/lncs?SGWID=0-164-6-793341-0. Papers must be submitted electronically (in PDF format) via the Easychair System: http://www.easychair.org/conferences/?conf=sblp2015. As in previous editions, after the conference, authors of selected papers will be invited to submit an extended version of their work to be considered for publication in a journal special issue. Since 2009, selected papers of each SBPL edition are being published in a special issue of Science of Computer Programming, by Elsevier. PROGRAM CHAIRS Alberto Pardo, Universidad de la República, Uruguay Doaitse Swierstra, Utrecht University, The Netherlands PROGRAM COMMITTEE Alberto Pardo, Universidad de la República (co-chair) Alex Garcia, IME Alvaro Moreira, Federal University of Rio Grande do Sul Andre Rauber Du Bois, Federal University of Pelotas Carlos Camarão, Federal University of Minas Gerais Christiano Braga, Fluminense Federal University Doaitse Swierstra, Utrecht University (co-chair) Fabio Mascarenhas, Federal University of Rio de Janeiro Fernando Pereira, Federal University of Minas Gerais Fernando Castor, Federal University of Pernambuco Francisco Carvalho-Junior, Federal University of Ceara Hans-Wolfgang Loidl, Heriot-Watt University João Saraiva, University of Minho João F. Ferreira, Teesside University Louis-Noel Pouchet, University of California, Los Angeles Lucilia Figueiredo, Federal University of Ouro Preto Luis Barbosa, University of Minho Manuel A. Martins, University of Aveiro Marcelo Maia, Federal University of Uberlândia Marcelo D'Amorim, Federal University of Pernambuco Mariza Bigonha, Federal University of Minas Gerais Martin Musicante, Federal University of Rio Grande do Norte Noemi Rodriguez, PUC-Rio Peter Mosses, Swansea University Rafael Lins, Federal University of Pernambuco Renato Cerqueira, PUC-Rio Roberto Bigonha, Federal University of Minas Gerais Roberto Ierusalimschy PUC-Rio Rodrigo Geraldo, Federal University of Ouro Preto Sandro Rigo, State University of Campinas Sérgio Medeiros, Federal University of Rio Grande do Norte Simon Thompson, University of Kent Varmo Vene, University of Tartu Zongyan Qiu Peking University Simon Thompson | Professor of Logic and Computation School of Computing | University of Kent | Canterbury, CT2 7NF, UK s.j.thompson(a)kent.ac.uk | M +44 7986 085754 | W www.cs.kent.ac.uk/~sjt

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Haskell Weekly News: Issue 318
by Semen Trygubenko / Семен Тригубен ко 25 Feb '15

25 Feb '15

New Releases A very fast BufferBuilder-based JSON encoder for Aeson. Encode JSON 2-5x faster than Aeson. https://hackage.haskell.org/package/buffer-builder-0.2.0.3 https://hackage.haskell.org/package/buffer-builder-aeson-0.2.0.3 http://www.reddit.com/r/haskell/comments/2wwbuj/announcing_bufferbuilder_en… FLTK GUI bindings - Alpha release This library aims to make it easy for users to build native apps that work portably across platforms. http://hackage.haskell.org/package/fltkhs-0.1.0.0/docs/Graphics-UI-FLTK-Low… Calls for Participation Primitive Haskell Originally this is a chapter in Mezzo Haskell, covering primitive operations, evaluation order and mutation. Michael Snoyman wants feedback on style and teaching approach. https://www.fpcomplete.com/blog/2015/02/primitive-haskell https://github.com/fpco/mezzohaskell Intermediate Haskell documentation proposal Michael Snoyman points out that Haskell community lacks middle-weight documentation beyond the levels of RWH and LYaH, or it is redundant, of varying quality and hard to find. He proposes to create conditions that will help Haskell community to write this documentation collectively. Part of Commercial Haskell effort. https://github.com/commercialhaskell/commercialhaskell/blob/master/proposal… https://github.com/commercialhaskell/commercialhaskell https://groups.google.com/forum/#!topic/commercialhaskell/Ou00AvRdDTU Discussion GHC 7.10 will embody Plan FTP In the 11 day window ~800 people participated in the survey and 82% were in favour of Foldable/Traversable Proposal. https://mail.haskell.org/pipermail/libraries/2015-February/025009.html http://www.reddit.com/r/haskell/comments/2wx64g/ghc_710_will_use_plan_ftp/ Web development using Haskell A discussion of (micro)frameworks, active and (seemingly) dormant, and the ultimate minimalism. http://www.reddit.com/r/haskell/comments/2wfap0/web_development_using_haske… Free Monoids in Haskell Lists aren't! http://comonad.com/reader/2015/free-monoids-in-haskell/ https://www.reddit.com/r/haskell/comments/2wqxqv/the_comonadreader_dan_doel… Let's teach GHC idiom brackets! (| pureFunction (fetchWith some params) (fetchMore withOthers) |) vs pureFunction <$> fetchWith some params <*> fetchMore withOthers https://mail.haskell.org/pipermail/haskell-cafe/2015-February/118228.html http://www.reddit.com/r/haskell/comments/2wclln/haskellcafe_lets_teach_ghc_… Is working with numeric formulas on Haskell a pain for anyone else? (/) and quot have different semantics, and this makes it hard to implement formulas. http://www.reddit.com/r/haskell/comments/2ws786/is_working_with_numeric_for… Monads in a non Functional Programming language X Haskell Fluid interfaces and Monads have little in common, although it might look like a lot. http://www.reddit.com/r/haskell/comments/2wefft/monads_in_a_non_functional_… Quotes of the Week "would require parens" in haskell world is like saying "would require taking lives" (chrisdone) "Functional programming beats all GOF Gang of Four patterns." (eniacsparc2xyz) http://www.reddit.com/r/haskell/comments/2wjtq5/haskell_actually_does_chang… "… pure functional programming and HTTP are a match made in heaven (type Application = Request -> Response - it doesn't get any more sensible than that ;)) …" (tdammers) http://www.reddit.com/r/haskell/comments/2wfap0/web_development_using_haske…

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Typed label
by Konstantine Rybnikov 25 Feb '15

25 Feb '15

Hi! I have a problem that seems very basic and fundamental, but at the same time I keep somehow skipping "the right way" to solve it. So I decided to give a try and describe it here. The task itself is very common. You have some data, which has some property inside of it. Often, you want to write a function that works only on an object with specific property being equal to something, but you can't move it out to type-parameter, because then you get all sorts of problems like you can't use regular lists/vectors anymore and other similar things. Let's take a look at real-world example to get a better understanding of a problem: Given a `Vegetable` type with a type that describes it's "type" (saying which concretely veg is it), you need to write a function `embraceOnion`, which would only work on onions (because it doesn't make sense to embrace any other veg). https://gist.github.com/k-bx/32b3f6a770ad81330f51 ``` {-# LANGUAGE DataKinds #-} {-# LANGUAGE KindSignatures #-} module Main where main :: IO () main = putStrLn "Hi!" data Vegetable = Vegetable { vegName :: String , vetType :: VegType } deriving (Show, Eq) data VegType = Potato | Tomato | Cucumber | Onion deriving (Show, Eq) newtype TypedVegetable (t::VegType) = TypedVegetable Vegetable unType :: TypedVegetable (t::VegType) -> Vegetable unType (TypedVegetable v) = v -- | bad embraceOnion :: Vegetable -> IO () embraceOnion veg = putStrLn ("Yay for onion! It's name is: " ++ vegName veg) -- | good, but little ugly because of boilerplate embraceOnion2 :: TypedVegetable Onion -> IO () embraceOnion2 onion = putStrLn ("Yay for onion! It's name is: " ++ vegName (unType onion)) -- | doesn't work, need to specify kind of `t` newtype TypedLabel1 t a = TypedLabel1 a unTyped1 :: TypedLabel1 t a -> a unTyped1 (TypedLabel1 a) = a embraceOnion3 :: TypedLabel1 Onion Vegetable -> IO () embraceOnion3 = undefined ``` Currently, I'm sticking to solution `embraceOnion2` in my code. `embraceOnion3` doesn't work because of: ``` /Users/kb/workspace/typelabels/typelabels.hs The first argument of ‘Main.TypedLabel1’ should have kind ‘*’, but ‘Main.Onion’ has kind ‘Main.VegType’ In the type signature for ‘Main.embraceOnion3’: Main.embraceOnion3 :: Main.TypedLabel1 Main.Onion Main.Vegetable -> GHC.Types.IO () ``` What are the other options and "state of the art"? Thank you!

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PowerPC variants (GHC 7.8.3)
by Jon Schneider 25 Feb '15

25 Feb '15

Good morning, We have a product with an MPC8544E we might want to target. Also known as PowerQUICC and e500v2. The ABI is gnuspe rather than gnueabi. I have built a powerpc---ghc cross compiler but the thing stopping "hello world" is a SIGILL happening in StgCRun.c where the stfd and lfd instructions are used to stash registers. The e500 has different instructions but that cannot be dropped in trivially because of offset encoding Commenting these out fixes "hello world" though this surely only scratches the surface of what would need doing. I notice that whereas ARMv5, v6 and v7 appear to be catered for along with various knobs and whistles there appears to be no such thing for PowerPC. Is anybody else out there working on this area ? Jon

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Trouble with forkProcess
by Rob Leslie 24 Feb '15

24 Feb '15

Hi folks, I’d love to get a sense of the prevailing wisdom with respect to forkProcess from System.Posix.Process. I’m building a multithreaded (STM) application with potentially many threads simultaneously reading and writing to pieces of (an otherwise rather large) shared state. Occasionally I would like to record a whole snapshot of that state, and it seems to me a natural way to accomplish this is to fork a process (eliminating all the other threads) and perform a read-only dump of the state. This dump can be time-consuming, which is why I’d rather not have contention with other threads that may be trying to modify the state while I’m dumping. My experience with forkProcess so far is that sometimes it works brilliantly, and other times it just deadlocks. I’m at a loss to understand what the problem is, but the deadlock seems to occur before the new process gets any control -- certainly before it has started to access any of the shared state. I’m aware of the giant warning in the forkProcess documentation, but I’m not sure if or how it could explain the behavior I’m seeing. Has anyone else used forkProcess successfully? It’s possible too that I’m running into something related to the way I’m setting up a pipe to receive messages from the forked process. Here is what I’m doing: > runInForkedProcess :: ((Text -> IO ()) -> IO ()) -> > Consumer Text IO () -> IO Bool > runInForkedProcess iof consumer = do > process <- newEmptyMVar > result <- newEmptyMVar > > let handler = Catch $ do > status <- getProcessStatus False False =<< readMVar process > case status of > Just (Exited code) -> void $ tryPutMVar result (code == ExitSuccess) > Just Terminated{} -> void $ tryPutMVar result False > _ -> return () > > bracket (installHandler processStatusChanged handler Nothing) > (\former -> installHandler processStatusChanged former Nothing) $ \_ -> do > > (input, output) <- createPipe > > processId <- forkProcess $ do > closeFd input > bracket (fdToHandle output) hClose $ \outHandle -> do > hSetBuffering outHandle NoBuffering > iof (hPutStrLn outHandle) > putMVar process processId > > closeFd output > > bracket (fdToHandle input) hClose $ \inHandle -> do > hSetBuffering inHandle NoBuffering > runEffect $ fromHandle inHandle >-> for cat (yield . T.pack) >-> consumer > > takeMVar result Does anyone have any advice? Many thanks, -- Rob Leslie rob(a)mars.org

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Should foldl' be added to the post-"Plan FTP" Prelude?
by Bob Ippolito 24 Feb '15

24 Feb '15

I would love to have foldl' as part of the Prelude. I use it quite frequently and it seems like a beginner trap to have foldl but not foldl' (which is almost always the better choice). I see that foldMap has been added (in addition to all of Monoid and Traversable), why was foldl' left out? It might be good measure to add foldr' as well, although I use that much less frequently. -bob

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Proposal: Infix expression keyword: -XInfixExpressions
by Christopher Done 24 Feb '15

24 Feb '15

Ahoy, The idiom discussion brought back to mind a general problem (well, for me) in Haskell syntax which is there is no syntactic sugar for interspersing operators to many arguments. Regarding a solution for this, I wrote up a wee proposal here: https://gist.github.com/chrisdone/d9d33e4770a2fef19ad1 If I go ahead and implement this in GHC as -XInfixExpressions or something (better names welcome), would it be likely to be accepted? I could first do an implementation in haskell-src-exts to demonstrate the concept. Ciao!

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