- Haskell-Cafe - Haskell.org

Destructive updates to plain ADTs
by Milan Straka 11 Sep '12

11 Sep '12

Hi all, is there any way to perform a destructive update on a plain ADT? Imagine I have a simple data Tree a = Nil | Node a (Tree a) (Tree a) I would like to be able to modify right subtree of an existing tree. I can do that for example when using IORefs by changing the datatype to data Tree a = Nil | Node a (IORef (Tree a)) (IORef (Tree a)) and use unsafePerformIO + writeIORef. But the IORefs cause additional complexity when working with the data type. At the moment I am interested in any GHC solution, be it non-portable or version specific. I would like just to run some benchmarks and see the results. Cheers, Milan

5 7

ANNOUNCE: hsqml-0.1.1 (and hsqml-morris-0.1.0)
by Robin KAY 11 Sep '12

11 Sep '12

Dear All, I would like to announce a new package called 'hsqml' [1]. HsQML provides a Haskell binding to the Qt Quick framework. It allows you to write graphical applications where the front-end is written in Qt Quick's QML language (incorporating JavaScript) and the back-end is written in Haskell. The two layers are coupled together via a facility to define custom JavaScript objects through which QML code can call into Haskell. HsQML requires an installation of Qt 4.7 (including QtDeclarative) or later on your path. It has been tested with GHC 7.4 on both Linux and Windows. There is also a simple example program available using HsQML on Hackage called 'hsqml-morris'. It implements a human v computer game of Nine Men's Morris with game logic and AI written in Haskell and the presentation layer in QML. The library is currently limited by a lack of support for Qt signals, so calls can only be made from JavaScript to Haskell and not the other way. I plan to add support for these in a future release. Migration over to Qt 5.x is also likely once it's released. [1] http://hackage.haskell.org/package/hsqml-0.1.1 [2] http://hackage.haskell.org/package/hsqml-morris-0.1.0 Regards, -- Robin KAY

1 0

The Architecture of the Mighttpd High-Speed Web Server
by Kazu Yamamoto 10 Sep '12

10 Sep '12

Hello from Copenhagen, In last may, I wrote an article about Mighttpd in Japanese because a PR staff of IIJ asked me to do so. Since this article got popular in Japan, the PR staff decided to translate it into English. The English version is now open to the public: http://www.iij.ad.jp/en/company/development/tech/mighttpd/ # Mighttpd is based on Yesod's Warp but I did touch it to make the # article simple. I guess that we can use this article to explain why Haskell is suitable for network programming to other programming language communities. FYI, the original Japanese article can be found: http://www.iij.ad.jp/company/development/tech/activities/mighttpd/ Regards, --Kazu

1 0

type variable in class instance
by Corentin Dupont 10 Sep '12

10 Sep '12

Hello everybody! I'm soliciting once again your help! It's been several days I'm blocked by this problem: *{-# LANGUAGE DeriveDataTypeable #-} import Data.Typeable class (Typeable e) => Event e data Player = Player Int deriving (Typeable) data Message m = Message String deriving (Typeable) instance Event Player instance (Typeable m) => Event (Message m) viewEvent :: (Event e) => e -> IO () viewEvent event = do case cast event of Just (Player a) -> putStrLn $ show a Nothing -> return () case cast event of Just (Message s) -> putStrLn $ show s Nothing -> return () * gives me a: * Ambiguous type variable `t0' in the constraint: (Typeable t0) arising from a use of `cast' Probable fix: add a type signature that fixes these type variable(s) In the expression: cast event In the expression: case cast event of { Just (Message s) -> putStrLn $ show s Nothing -> return () }* This is because *Message* has a type variable, while *Player* has not... How to get this to work? I tried everything, existential types, scoped type variables etc. without success... Thanks!! Corentin

3 4

extensible-exceptions no longer a part of GHC 7.6.1?
by Peter Simons 10 Sep '12

10 Sep '12

Hi, 'extensible-exceptions' used to be a part of GHC, but it appears that the package has been dropped from 7.6.1. Yet, the release notes on haskell.org don't say anything about this subject (other than TODO). Was that change intentional? Take care, Peter

2 1

ANNOUNCE: grid-1.1
by Amy de Buitléir 10 Sep '12

10 Sep '12

I'm happy to announce a new package called grid: http://hackage.haskell.org/package/grid https://github.com/mhwombat/grid/wiki (wiki) Grid provides tools for working with regular arrangements of tiles, such as might be used in a board game or self-organising map (SOM). Grid currently supports triangular, square, and hexagonal tiles, with various 2D and toroidal layouts. If you need a tile shape or layout that isn't currently provided, please let me know. See Math.Geometry.Grid for an example of how to use the package. Suggestions for improvement are welcome.

6 8

ANNOUCE: Haddock 2.10.0 and 2.11.0
by David Waern 09 Sep '12

09 Sep '12

-------------------------------------------- -- Haddock 2.10.0 and 2.11.0 -------------------------------------------- Two new versions of Haddock have been uploaded to Hackage: version 2.10.0 which comes with GHC 7.4.2 and 2.11.0 which comes with the new GHC 7.6.1! -------------------------------------------- -- Changes in version 2.12.0 -------------------------------------------- * Labeled URLs (e.g <http://example.net/ some label>) * Improved memory usage (new dependency: deepseq) -------------------------------------------- -- Changes in version 2.11.0 -------------------------------------------- * Show deprecation messages for identifiers * List identifiers declared on the same line (with a common type) separately * Don't crash on unicode strings in doc comments * Fix reporting of modules safe haskell mode * Fix a case where we were generating invalid xhtml * Improved --qual option (no crashes, proper error messages) * A new --qual option "aliased" which qualifies identifers by the module alias used in the source code * The Haddock API restores GHC's static flags after invocation * Access to unexported identifiers through the Haddock API again -------------------------------------------- -- Links -------------------------------------------- Homepage: http://www.haskell.org/haddock Hackage page: http://hackage.haskell.org/package/haddock-2.12.0 Bugtracker and wiki: http://trac.haskell.org/haddock Mailing list: haddock(a)projects.haskell.org Code repository: http://darcs.haskell.org/haddock.git -------------------------------------------- -- Contributors -------------------------------------------- The following people contributed patches to this release: Paolo Capriotti Simon Hengel Ian Lynagh Simon Peyton Jones Iavor S. Diatchki David Terei Henning Thielemann David Waern -------------------------------------------- -- Get Involved -------------------------------------------- We would be very happy to get more contributors. To get involved, start by grabbing the code: http://darcs.haskell.org/haddock.git Then take a look at the bug and feature tracker for things to work on: http://trac.haskell.org/haddock

4 4

package categories
by Sergey Mironov 08 Sep '12

08 Sep '12

Hello list. Do we have a guideline for selecting correct category for a package being developed? I'd like to choose between Web.MyLib and Network.MyLib. Or probably just MyLib if I feel ambitious? Sergey.

2 1

ANNOUNCE vkhs-0.0.3
by Sergey Mironov 08 Sep '12

08 Sep '12

I'm happy to announce a new package called VKHS: http://hackage.haskell.org/package/VKHS https://github.com/ierton/vkhs VKHS provides access to Vkontakte [1] (popular Russian social network) API methods. Library can be used to login into the network as a standalone application (OAuth implicit flow as they call it). Interaction with user is not required. For now, vkhs offers limited error detection and no captcha support. Here is an example code: import Web.VKHS.Login import Web.VKHS.Api main = do let client_id = "111111" -- application id, register first let user_of_interest = "222222" let e = env client_id "user(a)example.com" "password" [Photos,Audio,Groups] (Right at) <- login e (Right ans) <- api e at "users.get" [ ("uids",user_of_interest) , ("fields","first_name,last_name,nickname,screen_name") , ("name_case","nom") ] putStrLn ans Internally, library uses small curl-based HTTP automata and tagsoup for jumping over relocations and submitting various 'Yes I agree' forms. Sergey. [1] - http://vk.com

1 0

Haskell with all the safeties off
by David Feuer 08 Sep '12

08 Sep '12

I have no plans to do such a thing anytime soon, but is there a way to tell GHC to allow nasal demons to fly if the program forces bottom? This mode of operation would seem to be a useful optimization when compiling a program produced by Coq or similar, enabling various transformations that can turn bottom into non-bottom, eliminating runtime checks in incomplete patterns, etc.

4 5