- Haskell-Cafe - Haskell.org

Why is "Cont" out of scope?
by Daryoush Mehrtash 03 Apr '13

03 Apr '13

I am trying to use the Cont in Control.Monad.Cont but it seems to be missing Prelude> import Control.Monad.Cont Prelude Control.Monad.Cont> :t Cont <interactive>:1:1: Not in scope: data constructor `Cont' Perhaps you meant `ContT' (imported from Control.Monad.Cont) Prelude Control.Monad.Cont> :t runCont runCont :: Cont r a -> (a -> r) -> r Prelude Control.Monad.Cont> :t ContT ContT :: ((a -> m r) -> m r) -> ContT r m a Prelude Control.Monad.Cont> Any ideas? Thanks -- Daryoush Weblog: http://onfp.blogspot.com/

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closed world instances, closed type families
by Henning Thielemann 03 Apr '13

03 Apr '13

Recently I needed to define a class with a restricted set of instances. After some failed attempts I looked into the DataKinds extension and in "Giving Haskell a Promotion" I found the example of a new kind Nat for type level peano numbers. However the interesting part of a complete case analysis on type level peano numbers was only sketched in section "8.4 Closed type families". Thus I tried again and finally found a solution that works with existing GHC extensions: data Zero data Succ n class Nat n where switch :: f Zero -> (forall m. Nat m => f (Succ m)) -> f n instance Nat Zero where switch x _ = x instance Nat n => Nat (Succ n) where switch _ x = x That's all. I do not need more methods in Nat, since I can express everything by the type case analysis provided by "switch". I can implement any method on Nat types using a newtype around the method which instantiates the f. E.g. newtype Append m a n = Append {runAppend :: Vec n a -> Vec m a -> Vec (Add n m) a} type family Add n m :: * type instance Add Zero m = m type instance Add (Succ n) m = Succ (Add n m) append :: Nat n => Vec n a -> Vec m a -> Vec (Add n m) a append = runAppend $ switch (Append $ \_empty x -> x) (Append $ \x y -> case decons x of (a,as) -> cons a (append as y)) decons :: Vec (Succ n) a -> (a, Vec n a) cons :: a -> Vec n a -> Vec (Succ n) a The technique reminds me on GADTless programming. Has it already a name?

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ANNOUNCE: Ajhc 0.8.0.3 Release
by Kiwamu Okabe 02 Apr '13

02 Apr '13

We are happy to announce Ajhc 0.8.0.3. Major change on this release is supporting Windows MinGW32 platform. And you can cabal install ajhc. http://hackage.haskell.org/package/ajhc Perhaps changes on the announce will be merged to jhc. Ajhc's project web site is found at http://ajhc.masterq.net/. You can get the source code from https://github.com/ajhc/ajhc/tags. ## Changes * Support Windows MinGW32 platform * You can cabal install Ajhc. * Use DrIFT-cabalized command also to generate DrIFT deriving * Dump Haskell heap with gdb script (utils/gdb_jhc.py) * Use haskeline package to replace readline or editline * No more depend on haskell98 package - - - Metasepi team

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APLAS 2013 call for papers
by Chung-chieh Shan 02 Apr '13

02 Apr '13

=============================================================== APLAS 2013 11th Asian Symposium on Programming Languages and Systems 9-11 December 2013 Melbourne, Australia (colocated with CPP 2013) CALL FOR PAPERS =============================================================== ========== BACKGROUND ========== APLAS aims to stimulate programming language research by providing a forum for the presentation of latest results and the exchange of ideas in programming languages and systems. APLAS is based in Asia, but is an international forum that serves the worldwide programming language community. APLAS is sponsored by the Asian Association for Foundation of Software (AAFS) founded by Asian researchers in cooperation with many researchers from Europe and the USA. Past APLAS symposiums were successfully held in Kyoto ('12), Kenting ('11), Shanghai ('10), Seoul ('09), Bangalore ('08), Singapore ('07), Sydney ('06), Tsukuba ('05), Taipei ('04) and Beijing ('03) after three informal workshops. Proceedings of the past symposiums were published in Springer's LNCS. ====== TOPICS ====== The symposium is devoted to foundational and practical issues in programming languages and systems. Papers are solicited on topics such as * semantics, logics, foundational theory; * design of languages, type systems and foundational calculi; * domain-specific languages; * compilers, interpreters, abstract machines; * program derivation, synthesis and transformation; * program analysis, verification, model-checking; * logic, constraint, probabilistic and quantum programming; * software security; * concurrency and parallelism; * tools and environments for programming and implementation. Topics are not limited to those discussed in previous symposiums. Papers identifying future directions of programming and those addressing the rapid changes of the underlying computing platforms are especially welcome. Demonstration of systems and tools in the scope of APLAS are welcome to the System and Tool presentations category. Authors concerned about the appropriateness of a topic are welcome to consult with program chair prior to submission. ========== SUBMISSION ========== We solicit submissions in two categories: *Regular research papers* describing original scientific research results, including tool development and case studies. Regular research papers should not exceed 16 pages in the Springer LNCS format, including bibliography and figures. They should clearly identify what has been accomplished and why it is significant. Submissions will be judged on the basis of significance, relevance, correctness, originality, and clarity. In case of lack of space, proofs, experimental results, or any information supporting the technical results of the paper could be provided as an appendix or a link to a web page, but reviewers are not obliged to read them. *System and Tool presentations* describing systems or tools that support theory, program construction, reasoning, or program execution in the scope of APLAS. System and Tool presentations are expected to be centered around a demonstration. The paper and the demonstration should identify the novelties of the tools and use motivating examples. System and Tool papers should not exceed 8 pages in the Springer LNCS format, including bibliography and figures. Submissions will be judged based on both the papers and the described systems or tools. It is highly desirable that the tools are available on the web. Papers should be submitted electronically via the submission web page: https://www.easychair.org/conferences/?conf=aplas2013 Acceptable formats are PostScript or PDF. Submitted papers must be unpublished and not submitted for publication elsewhere. Papers must be written in English. The proceedings will be published as a volume in Springer's LNCS series. Accepted papers must be presented at the conference. (While the general chair and the program chair cannot submit papers, other members of the program committee can.) ===== DATES ===== Abstract due: 10 June 2013 (Monday), 23:59 UTC Submission due: 14 June 2013 (Friday), 23:59 UTC Notification: 26 August 2013 (Monday) Final paper due: 19 September 2013 (Thursday) Conference: 9-11 December 2013 (Monday-Wednesday) ========== ORGANIZERS ========== General chair: Peter Schachte (University of Melbourne) Program chair: Chung-chieh Shan (Indiana University) Program committee: Filippo Bonchi (CNRS, ENS-Lyon, France) Yu-Fang Chen (Academia Sinica, Taiwan) Shigeru Chiba (The University of Tokyo, Japan) Jacques Garrigue (Nagoya University, Japan) Robert Glück (University of Copenhagen, Denmark) R. Govindarajan (Indian Institute of Science, India) Kazuhiro Inaba (Google, Inc., Japan) Jie-Hong Roland Jiang (National Taiwan University, Taiwan) Shin-ya Katsumata (Kyoto University, Japan) Gabriele Keller (University of New South Wales, Australia) Ana Milanova (Rensselaer Polytechnic Institute, USA) Keisuke Nakano (The University of Electro-Communications, Japan) Hakjoo Oh (Seoul National University, Korea) Bruno C. d. S. Oliveira (National University of Singapore, Singapore) Kaushik Rajan (Microsoft Research, India) Max Schäfer (Nanyang Technological University, Singapore) Ulrich Schöpp (Ludwig-Maximilians-Universität München, Germany) Paula Severi (University of Leicester, UK) Gang Tan (Lehigh University, USA) Hiroshi Unno (University of Tsukuba, Japan) Meng Wang (Chalmers University of Technology, Sweden) Jingling Xue (University of New South Wales, Australia) Mingsheng Ying (University of Technology, Sydney, Australia) Kenny Q. Zhu (Shanghai Jiao Tong University, China) ======= CONTACT ======= http://aplas2013.soic.indiana.edu/ aplas2013(a)easychair.org

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mueval, interpreter options
by Johannes Waldmann 02 Apr '13

02 Apr '13

What am I missing? I want to use mueval to read a file that contains "instance Serial m a => Serial m (Tree a)" (as in http://hackage.haskell.org/packages/archive/smallcheck/1.0.2/doc/html/Test-…) so I put in my call of mueval: let Right opts = M.interpreterOpts [] keepCurrentDir $ do System.Directory.setCurrentDirectory d I.runInterpreter $ Mueval.Interpreter.interpreter $ opts { M.timeLimit = 10 -- seconds? , M.modules = Just [ "Prelude" , "Test.SmallCheck.Drivers", "Test.SmallCheck.Series" , "GHC.Generics" ] , M.namedExtensions = [ "MultiParamTypeClasses" , "FlexibleInstances", "DeriveGeneric" ] but it gives me "MultiParamTypeClasses is not enabled" on inputs like {-# language MultiParamTypeClasses #-} import Test.SmallCheck.Series import GHC.Generics data Tree a = Null | Fork (Tree a) a (Tree a) deriving Generic instance Serial m a => Serial m (Tree a) where I don't understand mueval's design anyway here: do the interpreter options mean that these are automatically on, or just that the source text will be allowed to switch then on? (I'd prefer the latter.) Thanks - J.W.

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Optimizing Fold Expressions
by J. J. W. 02 Apr '13

02 Apr '13

Dear all, I was wondering whether it was possible to write fold expressions more elegantly. Suppose I have the following datastructure: data Expr = Add Expr Expr | Sub Expr Expr | Mul Expr Expr | Eq Expr Expr | B Bool | I Int deriving Show type ExprAlgebra r = (r -> r -> r, -- Add r -> r -> r, -- Sub r -> r -> r, -- Mul r -> r -> r, -- Eq Bool -> r, -- Bool Int -> r -- Int ) foldAlgebra :: ExprAlgebra r -> Expr -> r foldAlgebra alg@(a, b, c ,d, e, f) (Add x y) = a (foldAlgebra alg x) (foldAlgebra alg y) foldAlgebra alg@(a, b, c ,d, e, f) (Sub x y) = b (foldAlgebra alg x) (foldAlgebra alg y) foldAlgebra alg@(a, b, c ,d, e, f) (Mul x y) = c (foldAlgebra alg x) (foldAlgebra alg y) foldAlgebra alg@(a, b, c ,d, e, f) (Eq x y) = d (foldAlgebra alg x) (foldAlgebra alg y) foldAlgebra alg@(a, b, c ,d, e, f) (B b') = e b' foldAlgebra alg@(a, b, c ,d, e, f) (I i) = f i If I am correct, this works, however if we for example would like to replace all Int's by booleans (note: this is to illustrate my problem): replaceIntByBool = foldAlgebra (Add, Sub, Mul, Eq, B, \x -> if x == 0 then B False else B True) As you can see, a lot of "useless" identity code. Can I somehow optimize this? Can someone give me some pointers how I can write this more clearly (or with less code?) So I constantly don't have to write Add, Sub, Mul, for those things that I just want an "identity function"? Thanks in advance! Jun Jie

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Announcement - HGamer3D - 0.2.1 - why netwire
by Peter Althainz 02 Apr '13

02 Apr '13

Peter Althainz wrote: > Dear All, > > I'm happy to announce release 0.2.1 of HGamer3D, the game engine with > Haskell API, featuring FRP based API and FRP based GUI. The new FRP API > is based on the netwire package. Currently only available on Windows: > http://www.hgamer3d.org. Nice work! Of course, I have to ask: what influenced your choice of FRP library in favor of netwire instead of reactive-banana ? Best regards, Heinrich Apfelmus -- http://apfelmus.nfshost.com Hi Heinrich good question, actually I need to thank you for your excellent tutorials on FRP and GUI on the WEB. I tried the version of reactive-banana without switches as the first FRP framework to have contact with and I liked its simplicity and the cool introduction around Excel cells you gave on the Web. HGamer3D is my personal way to get more insight into FP and Haskell especially and from the beginning I wanted to have a FRP API to try it with game examples. So your intro on FRP and the examples were very helpful with that. After reading a lot on the web it became clear, that currently reactive-banana and netwire are good candidates to start with. So why in the end I decided to use netwire for the binding? It's some personal things and I do not claim to have done a proper evaluation or comparison. I also cannot judge on performance or other relevant topics. Having said that, I can give you some points why I choosed netwire: - The cool simplicity of reactive-banana API seems to have suffered a little bit after the introduction of the switch functionality. - After getting around Monads and Applicative by great help of "Learning a Haskell for great good" I was shocked to see, there is even more to learn, when I detected Arrows. So I started to look at it and discovered some nice tutorials for Arrows. - What struck me was introduction of netwire author Ertugrul Söylemez on Arrows and the explanations of local state, which can be kept into an arrow. Since I was also curious on OOP and FP and game state handling, actually this raised some interest. So I think this "Arrows keep local state" argument was the killer feature. But also behaviours keep local state and maybe I got misguided here. - I then did some trials with netwire and I felt it's a quite comprehensive and nice API, so I got started with that. regards Peter

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Re: [Haskell-cafe] Cabal install pandoc
by Roger Mason 02 Apr '13

02 Apr '13

Hello Albert, On 04/01/2013 11:41 PM, Albert Y. C. Lai wrote: > On 13-04-01 06:26 AM, Roger Mason wrote: >> It turned out that there was a stale version of 'array' lurking in the >> ghc package db. In spite of reinstalling ghc it did not go away until I >> unregistered it. I think it was persisting because re-installing ghc >> simply unpacked over the old directory leaving that pre-existing file >> intact. > > See my http://www.vex.net/~trebla/haskell/sicp.xhtml for how does GHC > know or not know what libs you have. In particular, it has very little > to do with files, and clearing GHC is only half the story. > > And how to have the same kind of problems recur in the future. Thank you. I have read and filed away the article for future reference. I guess the best (least error prone) method of installing ghc and packages is to obtain a binary ghc (outside one's package manager), build haskell platform and then maintain ghc and packages outside the distro package manager. Comments welcome. Roger This electronic communication is governed by the terms and conditions at http://www.mun.ca/cc/policies/electronic_communications_disclaimer_2012.php

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N-ANN: HBlog-0.1
by Tom Murphy 02 Apr '13

02 Apr '13

Hi All, Not particularly happy to announce the non-release of my latest library, HBlog 0.1. I've been working on it steadily for a long time now, but it's still not ready. The library -- when it's finished -- will be a 100% Haskell representation of blogs, from authors and posts up to the most abstract notion of an entire blog as a Haskell data type. I'd like to finish the library sometime, but I've been so busy! Any spare time I've had, I've just ended up watching TV. I've been so caught up with the final season of The Office, I didn't realize how much time had passed since I originally had the idea! (2 years.) Didn't it seem like Jim and Pam were going to break up a couple weeks ago?! Anyway, here's the code I've definitely settled on so far: data Author = Author { name :: String } deriving (Show, Eq) data Post = Post { title :: String , body :: String , authors :: [Author] } deriving (Show) data Blog = Blog [Post] hasSingleAuthor :: Post -> Bool hasSingleAuthor post = 1 == (length $ authors post) Feel free to use portions of this code in your own projects! Just be sure to put at the top of all your source files: "Portions of this code (c) 2013 Tom Murphy. All rights reserved." ROADMAP: -- Lenses sound pretty cool, so I've started writing lenses for the Author type, as well as finding out what a lens is. Maybe one day there'll be lenses for the Post type, too -- keep your fingers crossed! -- One time when I was walking home from work, I had the idea for blogs being modeled as monads. Wouldn't that be awesome?: "The Blog Monad." I haven't done it yet, though, because I haven't figured out what bind would do. If you figure out how to model blogs as monads, just be sure to include this in all of your sourcefiles: "Idea for blog monad (c) 2013 Tom Murphy. All rights reserved." Happy hacking! Tom

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cabal install pandoc
by Roger Mason 02 Apr '13

02 Apr '13

Hello, I installed ghc (7.6.2) on an Arch Linux machine. I'm trying to install pandoc via cabal but it fails: ... Configuring text-0.11.2.3... Warning: This package indirectly depends on multiple versions of the same package. This is highly likely to cause a compile failure. package deepseq-1.3.0.1 requires array-0.4.0.1 package text-0.11.2.3 requires array-0.4.0.1 Building text-0.11.2.3... Preprocessing library text-0.11.2.3... <command line>: cannot satisfy -package-id array-0.4.0.1-db49bb8b0087ae85b5875d4c0cc12874 (use -v for more information) Failed to install text-0.11.2.3 ... There are then errors for other packages that depend on 'text' or 'array'. I will be grateful for any help. Thanks, Roger This electronic communication is governed by the terms and conditions at http://www.mun.ca/cc/policies/electronic_communications_disclaimer_2012.php

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