- Haskell-Cafe - Haskell.org

Free monoids in Haskell
by Dr. Olaf Klinke 27 Feb '15

27 Feb '15

Dear Dan, you posted an entry on the Comonad.Reader blog about free monoids in Haskell. I commented about FM being a monad, and meanwhile verified that indeed every monoid is an FM-algebra. The proofs are analogous to proving that the continuation (_ -> r) -> r for fixed r is a monad and that r is an algebra for it, respectively. Moreover, the FM-algebra stucture is a monoid homomorphism w.r.t. the monoid instance you gave for (FM a). I'd like to ask what happens when one omits the Monoid constraint. That is, what are the elements of newtype Y a = Y { runY :: forall b. (a -> b) -> b } (Y a) is like (Control.ForFree.Yoneda Identity a), that is, elements of (Y a) should be natural transformations from the reader functor ((->) a) to the identity functor. If that is true, then the Yoneda lemma tells us that (Y a) is isomorphic to (Identity a), but at least operationally that seems not to be the case: u = runY (return undefined) has different semantics than u' = runY (Y undefined) as can be seen by applying both to const (). So return does not map ⊥ to (Y ⊥). I wonder whether one can exhibit other elements not equal to any return(x). Of course (Identity a) is actually the lifted a, since ⊥≠I dentiyt ⊥. Yet this does not serve as an explanation for u vs. u', as both u and u' are of the form (Y _), but one function evaluates its argument while the other does not. Olaf

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Kind synonyms
by Tom Ellis 27 Feb '15

27 Feb '15

Is there an extension that will allow me to do type T = (*, *) so I can replace type family F (k :: (*, *)) with type family F (k :: T) ? Tom

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Idiom Brackets for GHC (first full proposal)
by Oliver Charles 27 Feb '15

27 Feb '15

Hi all, A few days ago I made a post here to gauge interest in adding idiom brackets to GHC. Response was a bit more mixed than I was hoping, but no one seemed to drastically against the idea, so I've moved forward with a more detailed proposal. You can find the full proposal here: https://ocharles.org.uk/IdiomBrackets.html A particular difference in my proposal from existing solutions comes from my desire to lift almost *all* expressions - with the original syntax - into idiom brackets. This means normal function application and tuples, but also case expressions, let bindings, record construction, record update, infix notation, and so on. At first I was skeptical about this, but I am finding uses for this more and more. I really like how it lets me use the interesting data (that is, whatever is "under" the applicative functor) where it's most relevant - rather than having to build a function and thread that value back through. Examples of this can be seen in my proposal. To prove its use, I've been working with this Template Haskell expression: https://ocharles.org.uk/IdiomExp.hs It *almost* does exactly what I want, the only problem is I can't get Template Haskell to give me a difference between $(i [| const True False |]) and $(i [| (const True) False |]) which I was planning to be significant. The proposal has mostly grown into my own personal notebook, so I'm happy to clarify anything that is vague/contradictory/confusing. Look forward to hearing your thoughts! -- Ollie

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RankNTypes question
by Nikolay Amiantov 26 Feb '15

26 Feb '15

Hi Cafe, I can't find how can I do this: {-# LANGUAGE RankNTypes #-} example :: forall a. (forall b. b) -> a example = undefined The first `forall` is for cosmetic reasons. If I understand correctly this is a safe and legit function (please correct me if I'm wrong!). I thought that I can just define `example = id` and GHC would do the rest, but alas, that didn't happen. Thanks! -- Nikolay.

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

26 Feb '15

Ah, thanks! For some reason I haven't even thought of it -- I knew that `id` was wrong because of the type, but had no idea what else to try. Feeling stupid now. On 02/26/2015 09:46 PM, Marcin Mrotek wrote: > Well still, id is (a -> a). So given some function (forall b. b -> b) > it's result is a function (forall b. b -> b). This works: > > test :: (forall a. a -> a) -> Int -> Int > test f a = f a > > Kind regards, > Marcin Mrotek -- Nikolay.

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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 26 Feb '15

26 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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