- Haskell-Cafe - Haskell.org

ANN: libxkbcommon bindings
by Auke Booij 13 Aug '13

13 Aug '13

There's a lot of work nowadays towards replacing the X11 infrastructure. Notably, wayland intends to replace some of the functionality that X.Org now provides, and it uses libxkbcommon to replace the X11 XKB.h keyboard event parsing code (actually it is not a dependency, but it is the de facto standard). I think it would be great if haskell supports these new libraries as soon as they are being used, which is why I wrote haskell FFI bindings for libxkbcommon. Partly because I am not exactly an experienced haskell developer, and partly because I am unsure how users will use libxkbcommon, at this point haskell-xkbcommon is rather thin. I intend to add a layer of code that makes this look more haskelly based on your feedback. Hence, feedback is MOST welcome in any area, be it code style, documentation, or API preferences. I did not upload haskell-xkbcommon to hackage yet - should I at this stage? https://github.com/tulcod/haskell-xkbcommon PS: If this turns out to be a success, I'd like to develop wayland bindings as well, so if you have anything to say about that feel free to contact me.

1 0

Default query for AcidState?
by Kyle Hanson 12 Aug '13

12 Aug '13

I am looking for something with a type signature like this: grabAcidState :: AcidState as -> IO as basically I just want to return my data from database abstractly or based on a class. The reason for this is because I want to have a class instance of state and then I can implement my library like so: data MyCoolData :: MyCoolData T.Text class HasCoolData a where getCoolDataList :: a -> MyCoolData grabStoredList :: HasCoolData as => AcidState as -> IO [MyCoolData] grabStoredList as = do cs <- grabAcidState as return $ getCoolDataList cs and users could implement it like so: data AppState = AppState { someCoolData :: [MyCoolData] } instance HasCoolData AppState { getCoolDataList = someCoolData } However trying to implement this simple behavior has been proving quite difficult. Maybe I am making this too difficult and I should just have a seperate AcidState datatype for my library. Any help would be appreciated.

1 0

ANN: restricted-workers-0.1
by Daniel F 12 Aug '13

12 Aug '13

Introducing: restricted-workers library, version 0.1.0. This library provides an abstract interface for running various kinds of workers under resource restrictions. It is being developed as part of the interactive-diagrams project and you can read more about the origins of the library in my GSoC report: <http://parenz.wordpress.com/2013/07/15/interactive-diagrams-gsoc-progress-r…> The library provides a convenient way of running worker processes, saving data obtained by the workers at start-up, a simple pool abstraction and a configurable security and resource limitations. Right now there are several kinds of security restrictions that could be applied to the worker process: - RLimits - chroot jail - custom process euid - cgroups - process niceness - SELinux security context You can read more about the library on the wiki: <https://github.com/co-dan/interactive-diagrams/wiki/Restricted-Workers> The library has been uploaded to hackage and you can install it using cabal-install. -- Thanks -- Daniil Frumin

1 0

Value-weak hash tables in Haskell ?
by Aleksey Uymanov 11 Aug '13

11 Aug '13

Hello, haskellers. Is there any package implementing magic hash tables weak in value? I mean when the value is garbage collected, then this key+value authomatically removes from the hash table. -- Aleksey Uymanov <s9gf4ult(a)gmail.com>

4 3

Rank N Kinds
by Wvv 11 Aug '13

11 Aug '13

It was discussed a bit here: http://ghc.haskell.org/trac/ghc/ticket/8090 Rank N Kinds: Main Idea is: If we assume an infinite hierarchy of classifications, we have True :: Bool :: * :: ** :: *** :: **** :: ... Bool = False, True, ... * = Bool, Sting, Maybe Int, ... ** = *, *->Bool, *->(*->*), ... *** = **, **->*, (**->**)->*, ... ... RankNKinds is also a part of lambda-cube. PlyKinds is just type of ** (Rank2Kinds) class Foo (a :: k) where <<==>> class Foo (a :: **) where *** is significant to work with ** data and classes; more general: Rank(N)Kinds is significant to work with Rank(N-1)Kinds First useful use is in Typeable. In GHC 7.8 class Typeable (a::k) where ... <<==>> class Typeable (a ::**) where ... But we can't write data Foo (a::k)->(a::k)->* ... deriving Typeable If we redeclare class Typeable (a ::***) where ... or even class Typeable (a ::******) where ... it becomes far enough for many years I'm asking to find other useful examples -- View this message in context: http://haskell.1045720.n5.nabble.com/Rank-N-Kinds-tp5733482.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

5 10

Installling Leksah on ghc7.6.3 with haskell-platform 2013?
by Carlo Hamalainen 10 Aug '13

10 Aug '13

Hi, I'm trying to install Leksah from hackage on my Debian testing laptop, which has ghc 7.6.3 and the Haskell Platform 2013. I fixed a bunch of errors about "catch" not being exported from the Prelude, but now I'm stuck on these two errors in leksah-server-0.8.0.5: [10 of 13] Compiling IDE.Utils.GHCUtils ( src/IDE/Utils/GHCUtils.hs, dist/build/IDE/Utils/GHCUtils.o ) src/IDE/Utils/GHCUtils.hs:94:40-62: Not in scope: data constructor `Opt_ReadUserPackageConf' src/IDE/Utils/GHCUtils.hs:161:44-50: Not in scope: data constructor `Opt_Cpp' Perhaps you meant one of these: `Opt_CSE' (imported from GHC), `Opt_Pp' (imported from GHC) Where have Opt_ReadUserPackageConf and Opt_Cpp gone in ghc 7.6.3? Cheers, -- Carlo

2 3

Importing more modules by default
by aditya bhargava 10 Aug '13

10 Aug '13

Hi there, It seems like every Haskell program I write imports the following modules: Control.Monad Control.Applicative Data.Maybe Data.List Is there a good reason why these modules aren't imported by default? When I write a simple script usually a 1/4th of the script is just imports, and my code just looks uglier. Adit

3 3

Re: [Haskell-cafe] Some philosophy (Was: Alternative name for return)
by man 09 Aug '13

09 Aug '13

On Fri, Aug 9, 2013 at 5:44 PM, Jerzy Karczmarczuk < jerzy.karczmarczuk(a)unicaen.fr> wrote: > I have the impression that a nice part of our dispute comes from the fact > that despite our ambitions, and a decent /technical/ level of understanding > of what we are talking about, most of us are (and I am one of the worst...) > -- > > -- pitiful philosophers... > I just leave this here, Analytic Philosophy: An Anthology http://www.amazon.com/gp/product/0631216472 -- happy hacking... man

1 0

translate imperative pseudo code into haskell
by Joerg Fritsch 09 Aug '13

09 Aug '13

I would need some help to get to a reasonable function involving the DB read, addition and multiplication. for 0 <= i < row dimension of A for 0 <= j < column dimension of B for 0 <= k < column dimension of A = row dimension of B sum += (read A (i,k))* (read B(k,j)) I started like this but then somehow lost the compass: main = do map my.read (map (\(x,y) -> "matrixA:" ++ show row ++ ":" ++ show column) [ (i, j, k) | i <- [1..50], j <- [1..20], k <- [1..30] ]) Can you pls help? --Joerg

2 2

ANN: hi-0.0.2, a Haskell project generator
by Daisuke Fujimura 09 Aug '13

09 Aug '13

Hello cafe, I'm very happy to announce the release of hi, a Haskell project generator. http://hackage.haskell.org/package/hi This application will generate a scaffold for new Haskell project with given command line argument(or value in configuration file), using git repository as a template. If you frequently create new project which has test suite in hspec, generating new project by hi will be a good alternative to cabal-init. It comes with test suite in hspec and already buildable. A project made by the default configuration is here. https://github.com/fujimura/hi-example More details is in the README: https://github.com/fujimura/hi Hope you enjoy it :-) -- Fujimura Daisuke http://fujimuradaisuke.com/

1 0

Re: [Haskell-cafe] Why are field selectors functions?
by David Banas 09 Aug '13

09 Aug '13

Wouldn't the implementation hiding feature of the *newtype *idiom be broken, if field selectors were not first class functions? For instance, the following code (taken shamelessly from Ch. 10 of *Real World Haskell*): module Parse ( runParser ) where data ParseState = ParseState { string :: String } deriving (Show) newtype Parser a = Parser { runParser :: ParseState -> Either String (a, ParseState) } has the attractive feature of hiding the internal implementation of the *ParseState *and *Parser *types from the user, preventing him from, for instance, pattern matching on either and thus writing code, which may break when we change the implementation. I believe this is only possible, because the *runParser *accessor is exportable as a first class function.

1 0

ANN: hi2 -- a better indentation mode for Emacs' haskell-mode
by Gergely Risko 09 Aug '13

09 Aug '13

Hi, In the last 2-3 weeks I've been working on Haskell indentation inside Emacs. I had some annoyances for a long time and fixed some of them. The new mode is called hi2, it's heavily based on the current haskell-indentation (part of haskell-mode). The changes are mainly to the UI, although, I plan to have a look on the parser too. The code can be found on github: https://github.com/errge/hi2. Feel free to send me feedback, bug reports, pull requests, etc. It's also hosted on MELPA. The most notable changes so far are the following: - DEL and C-d is not mapped: if you want to indent backwards, you can use S-TAB. This means no random jumping on backspace anymore. - TAB steps to the right as before, but when the end is reached, it starts going to the left instead of wrapping around. - TAB stays inside the code, instead of going to the beginning of the line. As in pyhton-mode and perl-mode. - Region indentation common case is supported: TAB and S-TAB is simply moving the whole region to the left/right by 1-column. Can be pressed repeatedly. - The current indentations are shown as underscores in the current line. So you have some visual indication on what's gonna happen. Also useful while hacking on the parser and want to see the results. Can be turned off by setting hi2-show-indentations to nil in your init file or calling hi2-disable-show-indentations from the buffer. If there are collisions with other overlay hacking modes (e.g. fill-column-indicator), try to turn off hi2-show-indentations-after-eol. - The buffer is not changed when indentation is not changed (so there are no undo points created and no dirty flag in the buffer if pressing TAB had no effect). - The code for all this is somewhat commented and cleaned. I'm not trying to fork haskell-mode, it's maintained very actively on github and the maintainers were always friendly through code reviews. But the current haskell-indentation-mode required so many fixes that it became unpleasant to contribute patch-by-patch, I wanted to move fast. When (and if) hi2 gets stable and better in every respect than haskell-indentation-mode, I'm happy to merge and contribute it back. Gergely

2 1

starting GHC development -- two questions
by Ömer Sinan Ağacan 09 Aug '13

09 Aug '13

Hi all, I want to start working on GHC, I cloned the repo, made some trivial changes(change some strings etc.) and re-built and observed the results. Now I'll continue reading and understanding the source. While doing this, I think one feature would greatly help me finding my way through GHC source, which is huge: I want to see definition of some symbols. Normally what I would do for this is to load source into GHCi and run :info command. But in the case of GHC, even if it's possible to load GHC into GHCi, I don't think it will be faster than running "ack --haskell someSymbol" and searching through results manually. First idea came to my mind was to generate tags files and then navigate from within vim(my editor of choice). tags file can be added to Makefile as a goal and then tags can be regenerated after each build. Did anyone else try this before? My second question is do we have any low-hanging fruits in trac, to help new people start contributing to GHC? I know several open source projects using that approach and it's really helpful for beginners. I just skimmed over trac and most issues look way too advanced for a starter. Thanks in advance, --- Ömer Sinan Ağacan http://osa1.net

3 3

Errors with Template Haskell
by Jose A. Lopes 09 Aug '13

09 Aug '13

Hi, In Template Haskell, what is the proper way of signalling an error ? For example, you are generating code and you detect that a given parameter does not fulfill a precondition (e.g., String is empty), and you want to abort compilation with a descriptive error message. Thanks, Jose -- Jose Antonio Lopes Ganeti Engineering Google Germany GmbH Dienerstr. 12, 80331, München Registergericht und -nummer: Hamburg, HRB 86891 Sitz der Gesellschaft: Hamburg Geschäftsführer: Graham Law, Christine Elizabeth Flores Steuernummer: 48/725/00206 Umsatzsteueridentifikationsnummer: DE813741370

2 2

Re: [Haskell-cafe] Haskell Platform and Leksah on Windows
by Carter Schonwald 09 Aug '13

09 Aug '13

Hey Dorin, I don't understand your claims. 1) haskell has worked perfectly well on windows for quite some time. I used HUGs nearly a decade ago, and in more recent time (2-3 years ago) I helped teach an introductory first computer science class using GHC where many students were doing great work using notepad++ and ghci. I don't understand your focus on emacs and make files. 2) if you want an "IDE" experience, Sublime Text with the right plugins, or perhaps EclipseFP are worth checking out. 3) likewise, if you're finding tooling on windows unsatisfactory, help fix it! Bug reports, patches, or new tools and libraries are always welcome. Haskell is a relatively small community, and thusly limited manpower (we're all volunteers), so way to fix any problem is help out! cheers On Thu, Aug 8, 2013 at 3:30 AM, Dorin Lazar <dorin.lazar(a)gmail.com> wrote: > Hello, > I am the original author of the post, and I finally received the > emails from the mailman (probably there was an issue with the > automated requests). > My answers are inlined. > > > 1) Leksah should not be considered an "official haskell ide", but merely > one of > > many community supported editing tools. And frankly one of the less > widely > > used ones at that! Leksah is not used much at all by anyone, though > theres > > probably a handful of folks who do use it. > > Many folks use editors like Sublime Tex (2/3), Emacs, Vi(m), textmate, > and > > many more. Its worth noting that the sublime-haskell plugin for sublime > > text, and analogous packages for many other editors, provide haskell > > IDE-like powers, or at least a nice subset thereof. > Unfortunately, I think the problem with this is that we have a > different vision on how development should be done. I have extensive > experience of working from console, with a simple text editor and > hand-made Makefiles or anything similar. However, an IDE should be a > productivity tool, that can help you improve your understanding of the > language, and can assist you in following the proper syntax for a new > language. While learning by doing 'write, save, compile, examine error > message' is ok with me, it is slow, and it limits the time I can > dedicate to learning the language itself. A better cycle is the > current 'write, examine error message' of most IDEs, since it's faster > and requires no context switch. Sure, editors can help there. IDEs do > this by default. > So it's normal of me to search for an IDE to better learn the > language, I'll leave the emacs + console version for when I am > productive in the language. > > > 2) There are people working on building better easily portable native gui > > toolkits, but in many respects, a nice haskelly gui toolkit is still > > something people are experimetning with how to do well. theres lots of > great > > tools out as of the past year or two, many more in progress on various > time > > scales, and gtk2hs is great for linux (and thats fine). > Unfortunately, this is not what's advertised. In fact, on the leksah > site, the recommended method is to have the IDE installed via cabal. > In another mail Mihai calls me unreasonable, but I think it's > reasonable to think that the recommended method should be the one that > works. > But the easy to tell truth is that the Haskell Platform for Windows > is not mature enough yet. That is something I can understand, and I > can recommend other beginners to install a Linux VM for Haskell. That > is perfectly fine, zero cost, 100% gain. However, the mistakes from > the Haskell Platform as it is now on Windows should be pointed out, > and although I've been called a mystical animal that wants only free > support, I think what I had in that blog post was actually a bug > report for the people that can actually add 1+1 to make 2 when it > comes to the Haskell Platform for Windows. Surely, I was harsh. But > that's the first experience of a beginner with Haskell, and I chose to > contribute my experience to people more knowledgeable instead of > shutting up and hiding the dust under the rug. > > Many thanks, > Dorin >

5 4

deriving Data.HashTable - stack overflow
by Lyle Kopnicky 09 Aug '13

09 Aug '13

Hi folks, In GHC 7.6.3, the base Data.HashTable is deprecated, so I installed the hashtables package. In order to work on your datatype, you need an instance of Data.Hashable.Hashable. So I went to the Data.Hashable page and looked up examples on how to derive a Hashable instance for my datatype: http://hackage.haskell.org/packages/archive/hashable/latest/doc/html/Data-H… The problem occurs even when using the sample code on the page: {-# LANGUAGE DeriveGeneric #-} import GHC.Generics (Generic) import Data.Hashable data Colour = Red | Green | Blue deriving Generic instance Hashable Colour If I then type `hash Red` I get a stack overflow. I am using the Haskell Platform, so I have hashable-1.1.2.5, but I notice the docs are for hashable-1.2.0.10. If I install 1.2.0.10 though, other code in my project breaks - seems like one part doesn't recognize the instances from another part. So I'll stick with the platform version. I am running the 32-bit Windows GHC 7.6.3. Haskell Platform 2013.2.0.0. Any thoughts? Thanks, Lyle

4 6

Haskell Platform and Leksah on Windows
by Mihai Maruseac 09 Aug '13

09 Aug '13

Hello, A friend of mine tried to install Haskell Platform and Leksah on Windows and was troubled by the amount of problems he encountered as a beginner in this. I've told him to ask over IRC and mailing list but it seems he has some problems with registration. Anyway, he blogged about his problems at http://dorinlazar.ro/haskell-platform-windows-crippled/ and I'm sure that we can work on fixing some of them. -- MM "All we have to decide is what we do with the time that is given to us"

7 8

Contracts
by Julian Arni 09 Aug '13

09 Aug '13

I've come across interesting papers - and old, sometimes inaccessible, repositories - related contracts in Haskell (Dana Xu and SPJ's papers; haskellcontracts and the Programatica Project). And then some newer, apparently not quite mature projects (Dan Rosen's repositories, for instance). Is there anything in between - mature, but not abandoned? Thanks, Julian

3 4

ANN: FunGEn 0.4.2, an easy cross-platform OpenGL/GLUT game engine
by Simon Michael 08 Aug '13

08 Aug '13

I'm pleased to announce the hackage release of FunGEn 0.4! (Actually 0.4.2 as my 0.4 announcement did not reach the mail lists.) FunGEn (Functional Game Engine) is a BSD-licensed, cross-platform, OpenGL/GLUT-based, imperative game engine/framework. With very few dependencies and two example games, it's one of the easiest ways to get started with game development in Haskell. FunGEn was probably the first Haskell game framework, created by Andre Furtado in 2002 (!). Here's his original feature list: * Initialization, updating, removing, rendering and grouping routines for game objects; * Definition of a game background (or map), including texture-based maps and tile maps; * Reading and intepretation of the player's keyboard input; * Collision detection; * Time-based functions and pre-defined game actions; * Loading and displaying of 24-bit bitmap files; * Some debugging and game performance evaluation facilities; * Sound support (actually for windows platforms only... :-[ ) What's new in 0.4.x: * a new hakyll-based website, incorporating the old site content * new haddock documentation * tested with GHC 7.6 * fixed buggy input when holding down keys on windows * input handlers now receive mouse position and modifier state (inspired by Pradeep Kumar; see fungentest.hs for examples) * added q as quit key in examples Home: http://joyful.com/fungen Hackage: http://hackage.haskell.org/package/FunGEn Code: http://hub.darcs.net/simon/fungen Install from hackage: $ cabal update $ cabal install FunGEn Install source and run examples: $ darcs get http://hub.darcs.net/simon/fungen $ cd fungen $ cabal install $ (cd examples/pong; ghc pong; ./pong) $ (cd examples/worms; ghc worms; ./worms) Contribute patches: - log in to hub.darcs.net and fork http://hub.darcs.net/simon/fungen - push changes to your branch - give me a "pull request" on #haskell-game I have maintained FunGEn very sporadically. If you'd like to take it and run with it, or co-maintain, let's chat! I'm sm on the #haskell-game IRC channel. -Simon

1 0

Installing wxHaskel on Snow Leopard
by Eduardo Sato 08 Aug '13

08 Aug '13

Hello, guys. Has anybody tried to install wxhaskell on Snow Leopard? I followed these instructions: http://www.haskell.org/haskellwiki/WxHaskell/Mac , but got an error: src/haskell/Graphics/UI/WXCore/WxcClassesAL.hs:13085:1: Unacceptable argument type in foreign declaration: CInt When checking declaration: foreign import ccall safe "static wxLogWindow_Create" wxLogWindow_Create :: Ptr (TWindow a) -> CWString -> CBool -> CBool -> IO (Ptr (TLogWindow ())) src/haskell/Graphics/UI/WXCore/WxcClassesAL.hs:13085:1: Unacceptable argument type in foreign declaration: CInt When checking declaration: foreign import ccall safe "static wxLogWindow_Create" wxLogWindow_Create :: Ptr (TWindow a) -> CWString -> CBool -> CBool -> IO (Ptr (TLogWindow ())) Failed to install wxcore-0.90.0.3 cabal: Error: some packages failed to install: wx-0.90.0.1 depends on wxcore-0.90.0.3 which failed to install. wxcore-0.90.0.3 failed during the building phase. The exception was: ExitFailure 1 http://lpaste.net/91634 I'm using ghc 7.6.3. I've heard the above problems are related to GHC 7.6.3 being more pedantic about FFI declarations. Is there any way around it? Thanks -- Eduardo Sato

4 7

Call for Participation: Workshop on Functional Art, Music, Modeling and Design
by Brent Yorgey 08 Aug '13

08 Aug '13

====================================================================== FARM 2013: Call for Participation ACM SIGPLAN Workshop on Functional Art, Music, Modeling and Design Boston, Massachusetts, USA 28th September, 2013 (directly after ICFP) http://www.cis.upenn.edu/~byorgey/farm13/ ====================================================================== The functional programming community is largely interested in writing beautiful programs. This workshop is intended to gather researchers and practitioners interested in writing beautiful programs that generate beautiful artifacts. Such artifacts may include visual art, music, 3D sculptures, animations, GUIs, video games, physical models, architectural models, choreographies for dance, poetry, and even physical objects such as VLSI layouts, GPU configurations, or mechanical engineering designs. The goal of FARM is to gather together researchers, practitioners, and educators in this interdisciplinary field, as well as anyone else with even a casual interest in the area. We wish to share ideas, look for common ground, and encourage more activity. We also hope to legitimize work in the field and facilitate potential collaboration among the participants. Registration ------------ Deadline for early registration: 22 August 2013 https://regmaster3.com/2013conf/ICFP13/register.php This is the registration site for ICFP 2013 and all the affiliated workshops including FARM 2013. Accepted papers and demonstrations ---------------------------------- - Samuel Aaron and Alan F. Blackwell. From Sonic Pi to Overtone: Creative Musical Experiences with Domain-Speciﬁc and Functional Languages - Henrik Bäärnhielm, Mikael Vejdemo-Johansson and Daniel Sundström. Using Haskell as DSL for controlling immersive media experiences (Demo) - Guillaume Baudart, Louis Mandel and Marc Pouzet. Programming Mixed Music in ReactiveML - Jean Bresson, Raphael Foulon and Marco Stroppa. Reduction as a Transition Controller for Sound Synthesis Events - Kelsey D'Souza. PySTEMM - A STEM Learning Tool for Exploring and Building Executable Concept Models (Demo) - Andy Gill and Brent A. Yorgey. Functional active animation (Demo) - Jason Hemann and Eric Holk. Visualizing the Turing Tarpit - Paul Hudak. Euterpea: From Signals to Symphonies (Demo) - David Janin, Florent Berthaut, Myriam Desainte-Catherine, Yann Orlarey and Sylvain Salvati. The T-Calculus : towards a structured programing of (musical) time and space - David Janin and Florent Berthaut. LiveTuiles for tiled composition of audio patterns (Demo) - Thomas Jordan. Spontaneous Musical Explorations of Visible Symmetric Structures (Demo) - Hendrik Vincent Koops, José Pedro Magalhães and W. Bas de Haas. A Functional Approach To Automatic Melody Harmonisation - José Pedro Magalhães, Bas De Haas, Gijs Bekenkamp, Dion ten Heggeler and Tijmen Ruizendaal. Chordify: Chord Transcription for the Masses (Demo) - Donya Quick and Paul Hudak. Grammar-Based Automated Music Composition in Haskell - Chung‐chieh Shan and Dylan Thurston. Braiding in circles (Demo) Workshop Organization --------------------- Workshop Chair: Paul Hudak Program Chair: Conal Elliott Publicity Chair: Brent Yorgey Program Committee ----------------- - Daniel Cukier (AgileAndArt) - Conal Elliott (Tabula) (chair) - Kathleen Fisher (Tufts University) - Richard Gabriel (IBM Research) - George Giorgidze (University of Tübingen) - Paul Hudak (Yale University) - José Pedro Magalhães (University of Oxford) - Alex McLean (University of Leeds) - John Peterson (Western State Colorado University) - Michael Sperber (Active Group) - Henning Thielemann (Martin-Luther-Universität Halle-Wittenberg) - Brent Yorgey (University of Pennsylvania)

1 0

Proposal for adding finallyError to MonadError
by David Sorokin 08 Aug '13

08 Aug '13

Hi, Café! I have a proposal. I offer to add a new function to the MonadError type class: finallyError :: MonadError e m => m a -> m b -> m a This is a generalization of the standard finally function that has the following signature: finally: IO a -> IO b -> IO a Like function finally, the finallyError function runs the first computation and then runs the second computation regardless of whether an error might arise in the first computation. The first computation defines the result. Introducing semantics of a possible cancellation of the m computation, the finallyError function accepts an additional meaning. The second computation must be performed even in case of cancelling the computation, while function catchError must be ignored. It literally means that the finallyError function cannot be expressed in terms of the catchError function. This idea is essentially inspired by the F# async workflow that does allow cancelling the Async computation. I implemented a similar monad Simulation.Aivika.Cont.Cont for my simulation library Aivika [1]. My monad Cont is an extension of the standard monad with the same name. It uses continuations (namely, three different continuations within one computation). It supports the exception handling and it allows cancelling the computation (the liftIO function should also call finallyError to catch the possible cancellation event if my Cont was a generic monad transformer). In the simulation the cancellation of the computation (actually, of the discontinuous process) is a typical use case. Therefore it should be executed as fast as possible. If I raised a specialized IOException for the cancellation event, then it would be a more costly operation. This is a practical point why the finallyError function cannot be expressed in terms of catchError. Another reason is that the catchError function can eat the cancellation exception, which would be absolutely unacceptable. This is a theoretical reason for introducing finallyError. The finallyError function is very useful for guaranteed releasing the external resources (such as the modeled entities in Aivika) in case of raising an exception and even in case of cancelling the active computation, which is important for simulation, for example. Regarding the standard instances of MonadError, the finallyError function seems to be simple for implementing. It is difficult mainly for the Cont-based monads. So, in Aivika it is more difficult than catchError for my Cont monad. The actual name of the function is not important but here it looks consistent with other functions of the MonadError type class. Finally, the finallyError function can be actually defined in an additional type class, which would extend MonadError like that how the ArrowChoice type class extends Arrow. Kind regards, David Sorokin [1] https://github.com/dsorokin/aivika

1 0

Why are field selectors functions? [was: ghc-users A possible alternative to dot notation for record access]
by AntC 08 Aug '13

08 Aug '13

No! This isn't more bikeshedding about notation. It's a bit of Haskell archaeology. > On Sun, Jun 30, 2013 at 2:59 AM, Judah Jacobson wrote: [This isn't exactly what Judah wrote.] > ... > > Instead of `x f` (to access field x of record f), > maybe we could write `f{x}` as the record selection. > The more I thought about that ... We use { ... } to declare records, build them, update them. We use { ... } in pattern matching to access named fields. Why don't we use { ... } to access named fields in terms? The syntax `e{ foo }` is unused in H98. (Or at least it was in 1998.) Can someone who was there at the time (1994?, TRex?) remember if that was ever considered? In the declaration, the build/update, and pattern match: data R = MkR { foo :: Int } r = MkR { foo = 7 } \ (MkFoo { foo }) -> ... -- using NamedFieldPuns `foo` is clearly bound to an Int. So why is there this `foo` thing that isn't an Int? AntC

2 1

ANN: Monad.Reader Issue 22
by Edward Z. Yang 08 Aug '13

08 Aug '13

I am pleased to announce that Issue 22 of the Monad Reader is now available. http://themonadreader.files.wordpress.com/2013/08/issue22.pdf Issue 22 consists of the following two articles: * "Generalized Algebraic Data Types in Haskell" by Anton Dergunov * "Error Reporting Parsers: a Monad Transformer Approach" by Matt Fenwick and Jay Vyas * "Two Monoids for Approximating NP-Complete Problems" by Mike Izbicki Feel free to browse the source files. You can check out the entire repository using Git: git clone https://github.com/ezyang/tmr-issue22.git If you’d like to write something for Issue 23, please get in touch! Cheers, Edward

3 2

Is there a Galois correspondence between a Haskell class hierarchy and a given instance hierarchy?
by Patrick Browne 08 Aug '13

08 Aug '13

1 0

ANNOUNCE: tasty, a new testing framework
by Roman Cheplyaka 08 Aug '13

08 Aug '13

I am pleased to announce the first release of tasty, a new testing framework for Haskell. It is meant to be a successor to test-framework (which is unmaintained). Tasty supports HUnit, SmallCheck, QuickCheck, and golden tests out of the box (through the standard packages), but it is very extensible, so you can write your own test providers. Please see the home page for more information: http://documentup.com/feuerbach/tasty Roman

8 18

Module names from a function name
by Jong-won Choi 08 Aug '13

08 Aug '13

Hi, I asked this question to beginner mailing list and no luck so far, so I'm trying here. How can I get all the module names which (re-)export a given function name? I'd like to fix Emacs Haskell mode - interactive documentation browser and I need the list of such module names. Thanks! - Jong-won

2 1

Re: [Haskell-cafe] Module names from a function name
by Jong-won Choi 08 Aug '13

08 Aug '13

Thanks for the suggestion, but I am looking for a programmatic way. Is module in Haskell first class object? Does it maintain function names which are exported? On 08/08/13 12:41, Patrick Mylund Nielsen wrote: > > hoogle <function name> after running cabal install hoogle and hoogle data > > On Aug 7, 2013 10:35 PM, "Jong-won Choi" <oz.jongwon.choi(a)gmail.com > <mailto:oz.jongwon.choi@gmail.com>> wrote: > > Hi, > > I asked this question to beginner mailing list and no luck so far, > so I'm trying here. > > How can I get all the module names which (re-)export a given > function name? > > I'd like to fix Emacs Haskell mode - interactive documentation > browser and I need the list of such module names. > > Thanks! > > - Jong-won > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org <mailto:Haskell-Cafe@haskell.org> > http://www.haskell.org/mailman/listinfo/haskell-cafe >

1 0

Haskell Weekly News: Issue 275
by Daniel Santa Cruz 08 Aug '13

08 Aug '13

Welcome to issue 275 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of July 28 to August 3, 2013. Quotes of the Week * littlecalculist: combinatorics. n. math's way of punishing you for failing to use abstraction. * dmwit: ?let instance Num Bool where (+) = (/=); (*) = (&&); negate = id; abs = id; signum = id; fromInteger = odd * edwardk: clearly it should be \x -> x <$ do f . g $ x. do is a magical () killing machine * dmwit: Type families are not injective. Your first three inscrutable error messages with type families in them will be because you don't understand this rule. * elliott: halting problem, n. the property which makes stating anything about programs impossible, according to at least one person in any room * carmack: Everything that is syntactically legal that the compiler will accept will eventually wind up in your codebase. Top Reddit Stories * John Carmack talking at length about Haskell in his QuakeCon Keynote speech (live) Domain: twitch.tv, Score: 105, Comments: 47 On Reddit: [1] http://goo.gl/1g1zqD Original: [2] http://goo.gl/C17ci8 * SPJ: GHC's Glorious future Domain: haskell.org, Score: 79, Comments: 16 On Reddit: [3] http://goo.gl/97DGuc Original: [4] http://goo.gl/YAY6wC * There's a massive gap between the "average" and "expert" Haskeller. How do I bridge that gap? Domain: self.haskell, Score: 73, Comments: 53 On Reddit: [5] http://goo.gl/Y3jEoa Original: [6] http://goo.gl/Y3jEoa * Getting Started With GHC Hacking Domain: dagit.github.io, Score: 64, Comments: 2 On Reddit: [7] http://goo.gl/ZwHVpm Original: [8] http://goo.gl/ljhyff * Extensible Effects: An Alternative to Monad Transformers Domain: cs.indiana.edu, Score: 63, Comments: 47 On Reddit: [9] http://goo.gl/2JqImi Original: [10] http://goo.gl/zq5klw * The Intel Labs Haskell Research Compiler [pdf] Domain: leafpetersen.com, Score: 55, Comments: 19 On Reddit: [11] http://goo.gl/wRQs7E Original: [12] http://goo.gl/2eZhEX * Goto in Haskell Domain: self.haskell, Score: 36, Comments: 34 On Reddit: [13] http://goo.gl/pelEJo Original: [14] http://goo.gl/pelEJo * The Algebra of Algebraic Data Type Domain: chris-taylor.github.io, Score: 35, Comments: 3 On Reddit: [15] http://goo.gl/PPi0M1 Original: [16] http://goo.gl/xE78Ko * Functors and monads for analyzing data Domain: izbicki.me, Score: 32, Comments: 16 On Reddit: [17] http://goo.gl/42ux1T Original: [18] http://goo.gl/LAJ7ti * Why use _ <-? Domain: self.haskell, Score: 31, Comments: 24 On Reddit: [19] http://goo.gl/QpGPP1 Original: [20] http://goo.gl/QpGPP1 * CIS 194: Introduction to Haskell (Spring 2013) Domain: cis.upenn.edu, Score: 30, Comments: 3 On Reddit: [21] http://goo.gl/9zHuXb Original: [22] http://goo.gl/cNyEOT * The Commutative monad Domain: gelisam.blogspot.ca, Score: 30, Comments: 41 On Reddit: [23] http://goo.gl/kDVTrl Original: [24] http://goo.gl/mTCHL0 * FRP GUI - reactive-banana + threepenny-gui = awesome Domain: apfelmus.nfshost.com, Score: 30, Comments: 11 On Reddit: [25] http://goo.gl/xu91UA Original: [26] http://goo.gl/9Vu6ng Top StackOverflow Questions * Monad Stack Penetration Classes with Free/Operational Monad Transformers? votes: 16, answers: 1 Read on SO: [27] http://goo.gl/t4F6kd * Haskell reexport everything votes: 12, answers: 1 Read on SO: [28] http://goo.gl/7HQCVC * What are the main differences between Scala and Frege (in programming paradigms)? votes: 11, answers: 2 Read on SO: [29] http://goo.gl/TRhjDB * If a thunk results in an exception, is the exception kept as the result of the thunk? votes: 10, answers: 1 Read on SO: [30] http://goo.gl/rPSd6J * Check if import is redundant votes: 9, answers: 1 Read on SO: [31] http://goo.gl/jEqBnt * Haskell Parser Combinators votes: 9, answers: 1 Read on SO: [32] http://goo.gl/YCrsX3 * A library implementation of a recursion scheme votes: 8, answers: 1 Read on SO: [33] http://goo.gl/DdYPtw Until next time, [34]+Daniel Santa Cruz References 1. http://www.twitch.tv/bethesda?utm_campaign=live_embed_click&utm_source=www.… 2. http://www.reddit.com/r/haskell/comments/1jj40p/john_carmack_talking_at_len… 3. http://www.haskell.org/pipermail/ghc-devs/2013-August/001818.html 4. http://www.reddit.com/r/haskell/comments/1jky60/spj_ghcs_glorious_future/ 5. http://www.reddit.com/r/haskell/comments/1jeo0p/theres_a_massive_gap_betwee… 6. http://www.reddit.com/r/haskell/comments/1jeo0p/theres_a_massive_gap_betwee… 7. http://dagit.github.io/posts/2013-08-03-getting-started-with-ghc-hacking.ht… 8. http://www.reddit.com/r/haskell/comments/1jnaz5/getting_started_with_ghc_ha… 9. http://www.cs.indiana.edu/~sabry/papers/exteff.pdf 10. http://www.reddit.com/r/haskell/comments/1j9n5y/extensible_effects_an_alter… 11. http://www.leafpetersen.com/leaf/publications/hs2013/hrc-paper.pdf 12. http://www.reddit.com/r/haskell/comments/1jh4iv/the_intel_labs_haskell_rese… 13. http://www.reddit.com/r/haskell/comments/1jk06q/goto_in_haskell/ 14. http://www.reddit.com/r/haskell/comments/1jk06q/goto_in_haskell/ 15. http://chris-taylor.github.io/blog/2013/02/13/the-algebra-of-algebraic-data… 16. http://www.reddit.com/r/haskell/comments/1jhq7k/the_algebra_of_algebraic_da… 17. http://izbicki.me/blog/functors-and-monads-for-analyzing-data 18. http://www.reddit.com/r/haskell/comments/1ja3js/functors_and_monads_for_ana… 19. http://www.reddit.com/r/haskell/comments/1jcls5/why_use/ 20. http://www.reddit.com/r/haskell/comments/1jcls5/why_use/ 21. http://www.cis.upenn.edu/~cis194/ 22. http://www.reddit.com/r/haskell/comments/1j9m5e/cis_194_introduction_to_has… 23. http://gelisam.blogspot.ca/2013/07/the-commutative-monad.html 24. http://www.reddit.com/r/haskell/comments/1jegxb/the_commutative_monad/ 25. http://apfelmus.nfshost.com/blog/2013/08/02-reactive-banana-threepenny.html 26. http://www.reddit.com/r/haskell/comments/1jk16t/frp_gui_reactivebanana_thre… 27. http://stackoverflow.com/questions/17936900/monad-stack-penetration-classes… 28. http://stackoverflow.com/questions/18035458/haskell-reexport-everything 29. http://stackoverflow.com/questions/17905322/what-are-the-main-differences-b… 30. http://stackoverflow.com/questions/18018254/if-a-thunk-results-in-an-except… 31. http://stackoverflow.com/questions/17904265/check-if-import-is-redundant 32. http://stackoverflow.com/questions/18028220/haskell-parser-combinators 33. http://stackoverflow.com/questions/18013547/a-library-implementation-of-a-r… 34. https://plus.google.com/105107667630152149014/about

1 0

Hackage upload problem
by Thomas Hallgren 07 Aug '13

07 Aug '13

Hi, I get the following error when I try to upload gf-3.5.tar.gz [1] to Hackage. 400 Error in upload could not extract gf-3.5 directory from gf-3.5.tar.gz I get the same error when I try to Check the previous version, gf-3.4.tar.gz [2], which was uploaded without problems 6 months ago [3], so it seems that something has changed on the server. Does anyone know what it could be that is going wrong? Is there a way to get a more informative error message? Thomas H [1] http://www.grammaticalframework.org/download/gf-3.5.tar.gz [2] http://www.grammaticalframework.org/download/gf-3.4.tar.gz [3] http://hackage.haskell.org/package/gf-3.4

2 2

ScopedTypeVariables
by Evan Laforge 06 Aug '13

06 Aug '13

Occasionally I have to explicitly add a type annotation, either for clarity or to help choose a typeclass instance. Usually top-level type annotations take care of this, but sometimes it's convenient to only annotate a certain value, e.g. one argument of a lambda. I've noticed that while vanilla haskell is happy to allow me to put type annotations on variables where they are used (e.g. '\x -> f (x :: T)'), if I put it on the variable where it is bound (e.g. '\(x :: T) -> f x'), it wants me to turn on ScopedTypeVariables. I think ScopedTypeVariables is a nice extension and I'm sure it comes from a perfectly respectable family and all, but it feels like annotations on arguments comes in as a side-effect. Would it make sense to split it into a separate extension like TypesOnArguments so I can more accurately express my deviation from haskell2010 orthodoxy? Or is there some deeper tie between scoped type variables and annotations on arguments? Now that I think of it, it seems inconsistent that 'x :: A -> B; x a = ...' is valid, but 'x = \(a :: A) -> (...) :: B' is not. Doesn't the former desugar to the latter? And what about getting ScopedTypeVariables into haskell prime? As far as I know everyone loves it, or at least no one actually hates it :)

3 2

Haddock GSOC project progress
by Mateusz Kowalczyk 05 Aug '13

05 Aug '13

Greetings cafe, As some of you might know, I'm hacking on Haddock as part of Google Summer of Code. I was recently advised to create a blog and document some of what I have been doing recently. You can find the blog at [1] if you're interested. The first post goes over the work from the last month or so. Future posts should be shorter and on more specific topics. There's an overview of what has happened/changed/will change at the bottom of the post if you're short on time. Thanks. [1] - http://fuuzetsu.co.uk/blog -- Mateusz K.

6 7

Call For Papers, FLOPS 2014
by unknown＠example.com 04 Aug '13

04 Aug '13

Dear Colleagues, Please find below the CFP for FLOPS 2014. Apologies for possible cross posting. Program Committee Co-Chairs, Michael Codish and Eijiro Sumii ---------------------------------------------------------------------- Call For Papers =============== Twelfth International Symposium on Functional and Logic Programming (FLOPS 2014) June 4-6, 2014 Kanazawa, Japan http://www.jaist.ac.jp/flops2014/ (to be published) FLOPS is a forum for research on all issues concerning declarative programming, including functional programming and logic programming, and aims to promote cross-fertilization and integration between the two paradigms. Previous FLOPS meetings were held at Fuji Susono (1995), Shonan Village (1996), Kyoto (1998), Tsukuba (1999), Tokyo (2001), Aizu (2002), Nara (2004), Fuji Susono (2006), Ise (2008), Sendai (2010), and Kobe (2012). Topics ====== FLOPS solicits original papers in all areas of functional and logic programming, including (but not limited to): - Language issues: language design and constructs, programming methodology, integration of paradigms, interfacing with other languages, type systems, constraints, concurrency and distributed computing. - Foundations: logic and semantics, rewrite systems and narrowing, type theory, proof systems. - Implementation issues: compilation techniques, memory management, program analysis and transformation, partial evaluation, parallelism. - Applications: case studies, real-world applications, graphical user interfaces, Internet applications, XML, databases, formal methods and model checking. The proceedings will be published as an LNCS volume. The proceedings of the previous meetings (FLOPS 1999, 2001, 2002, 2004, 2006, 2008, 2010, and 2012) were published as LNCS 1722, 2024, 2441, 2998, 3945, 4989, 6009, and 7294. PC Co-Chairs ============ Michael Codish (Ben-Gurion University of the Negev) Eijiro Sumii (Tohoku University) PC Members ========== Lars Birkedal (Aarhus University) Michael Codish (Ben-Gurion University of the Negev) [co-chair] Marina De Vos (University of Bath) Moreno Falaschi (Universita degli studi di Udine) Carsten Fuhs (University College London) John Gallagher (Roskilde Universitet / IMDEA Software Institute) Samir Genaim (Universidad Complutense de Madrid) Laura Giordano (Universita del Piemonte Orientale) Ichiro Hasuo (University of Tokyo) Fritz Henglein (University of Copenhagen) Andy King (University of Kent) Oleg Kiselyov Vitaly Lagoon (MathWorks) Shin-Cheng Mu (Academia Sinica) Keiko Nakata (Institute of Cybernetics at Tallinn University of Technology) Luke Ong (University of Oxford) Peter Schachte (University of Melbourne) Takehide Soh (Kobe University) Eijiro Sumii (Tohoku University) [co-chair] Tachio Terauchi (Nagoya University) Joost Vennekens (KU Leuven) Janis Voigtlaender (Universitaet Bonn) Stephanie Weirich (University of Pennsylvania) Local Chair =========== Yuki Chiba (JAIST) Submission ========== Submissions must be unpublished and not submitted for publication elsewhere. Work that already appeared in unpublished or informally published workshops proceedings may be submitted. See also ACM SIGPLAN Republication Policy: http://www.sigplan.org/Resources/Policies/Republication Submissions should fall into one of the following categories: - Regular research papers: they should describe new results and will be judged on originality, correctness, and significance. - System descriptions: they should contain a link to a working system and will be judged on originality, usefulness, and design. - Declarative pearls: new and excellent declarative programs or theories with illustrative applications. System descriptions and declarative pearls must be explicitly marked as such in the title. Submissions must be written in English and can be up to 15 pages long including references, though pearls are typically shorter. Authors are required to use LaTeX2e and the Springer llncs class file, available at: http://www.springer.de/comp/lncs/authors.html Regular research papers should be supported by proofs and/or experimental results. In case of lack of space, this supporting information should be made accessible otherwise (e.g., a link to a Web page, or an appendix). Papers should be submitted electronically at: https://www.easychair.org/conferences/?conf=flops2014 Important Dates =============== Submission deadline: December 13, 2013 Author notification: February 10, 2014 Camera-ready copy: March 7, 2014 Venue ===== Main Hall, Ishikawa Prefectural Museum of Art, 2-1 Dewa-machi, Kanazawa, Ishikawa 920-0963 JAPAN. Some Previous FLOPS =================== FLOPS 2012, Kobe: http://www.org.kobe-u.ac.jp/flops2012/ FLOPS 2010, Sendai: http://www.kb.ecei.tohoku.ac.jp/flops2010/ FLOPS 2008, Ise: http://www.math.nagoya-u.ac.jp/~garrigue/FLOPS2008/ Sponsor ======= Japan Society for Software Science and Technology (JSSST), Special Interest Group on Programming and Programming Languages (SIG-PPL) In Cooperation With =================== ACM SIGPLAN Asian Association for Foundation of Software (AAFS) Association for Logic Programming (ALP)

1 0

Why GHC is written in Happy and not a monadic parser library?
by blackbox.dev.ml 04 Aug '13

04 Aug '13

Hi! Is there any specific reason why GHC is written in a parser GENERATOR (Happy) and not in MONADIC PARSER COMBINATOR (like parsec)? Is Happy faster / handles better errors / hase some great features or anything else? thank you :)

5 6

Re: [Haskell-cafe] [Haskell] Spam on the Haskell wiki
by Henk-Jan van Tuyl 03 Aug '13

03 Aug '13

On Mon, 16 Jul 2012 00:03:49 +0200, Henk-Jan van Tuyl <hjgtuyl(a)chello.nl> wrote: > > I am willing to do administrator tasks. > >> 4. ReCAPTCHA enabled for 'edits adding new, unrecognized external >> links' - which is all of the spam. > > This is already enabled. The HaskellWiki is still flooded with spam; we should take some measure to reduce the stream severely. Most spam seems to be created (semi-)automated; the pages do not contain links, the usernames end with two digits, most of the time. Some cures I have thought up: - Verify new wiki accounts, before granting them rights, based on e-mails in the Haskell mailing lists (or subscription of a Haskell mailing list) - Let new users only change pages, not create new pages - Block creation of usernames o ending with two or more digits o with more than one x or q o starting with "buy" o longer than 20 characters o with more than 4 consonants in a row - Block creation of pages with words in a certain list (Coach, Vuitton, Chanel, handbags, purses, outlet, luggage, "Nike Air Jordan") Regards, Henk-Jan van Tuyl -- http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --

9 13

ICFP Programming Contest (starts in < 1 week)
by Nikhil Swamy 02 Aug '13

02 Aug '13

Update: This year's ICFP programming contest is just 1 week away. You will have to register to participate 72 hours in advance. Do it now! Update: For updates about the contest, follow us on twitter at ICFPContest2013 If you fancy yourself a discriminating hacker, or just want to prove that your programming language of choice is clearly the best, you should plan to participate in the contest now! -- The contest will start at: 1700 PDT on August 8, 2013 (0000 UTC on August 9, 2013) -- The contest will end at: 1700 PDT on August 11, 2013 (0000 UTC August 12, 2013). -- You need to pre-register for the contest, 72 hours prior to the contest at the latest A hint for this year: The programming task will involve an element of program synthesis. If you have friends who use languages that are not too shabby, or are just extremely cool hackers, give them a heads up too! For more information, visit: http://research.microsoft.com/en-us/events/icfpcontest2013/ ----- The organizers, Peli de Halleux, Michal Moskal, Nikhil Swamy, and Nikolai Tillmann, are all members of the Research in Software Engineering (RiSE) group at MSR Redmond.

1 0

Learning GHC API
by blackbox.dev.ml 02 Aug '13

02 Aug '13

HI! Could you please give me hints where to find materials to learn GHC API? I know there is http://www.haskell.org/haskellwiki/GHC/As_a_library, but this contains only 2 examples and I would to learn how to create datatypes, classes, functions etc using GHC API, how to compile them later or generate Haskell code out of them (if its possible). I would be thnakful for any hint! :) Thank you, Wojtek

3 2

Proposal: Non-recursive let
by oleg＠okmij.org 02 Aug '13

02 Aug '13

Here is a snippet from a real code that could benefit from non-recursive let. The example is notable because it incrementally constructs not one but two structures (both maps), ast and headers. The maps are constructed in a bit interleaved fashion, and stuffing them into the same State would be ungainly. In my real code -- Add an update record to a buffer file do_update :: String -> Handle -> Parsed -> [Parsed] -> IO () do_update "TAF" h ast asts@(_:_) = do rv <- reflect $ get_trange "TRange" ast headers0 <- return . M.fromList =<< sequence (map (\ (n,fld) -> reflect (fld ast) >>= \v -> return (n,v)) fields_header) let headers = M.insert "_report_ranges" (format_two_tstamps rv) headers0 foldM write_period (rv,headers,(snd rv,snd rv)) asts return () where write_period (rv,headers,mv) ast = do pv@(p_valid_from,p_valid_until) <- reflect $ get_trange "TRange" ast check_inside pv rv let prevailing = M.lookup "PREVAILING" ast (mv,pv) <- case prevailing of Just _ -> return (pv,pv) -- set the major valid period -- Make sure each VAR period occurs later than the prevailing -- period. If exactly at the same time add 1 min Nothing -> case () of _ | fst mv < p_valid_from -> return (mv,pv) _ | fst mv == p_valid_from -> return (mv,(p_valid_from + 60, p_valid_until)) _ -> gthrow . InvalidData . unwords $ [ "VAR period begins before prevailing:", show ast, "; prevailing TRange", show mv] let token = maybe (M.findWithDefault "" "VAR" ast) id prevailing let ast1 = M.insert "_token" token . M.insert "_period_valid" (format_two_tstamps pv) . M.unionWith (\_ x -> x) headers $ ast let title = M.member "Title" ast let headers1 = if title then headers else M.delete "_limit_to " . M.delete "_limit_recd" $ headers write_fields h ast1 fields return (rv,headers1,mv)

6 6

Getting the 'ThreadId' of the calling thread
by Nikita Karetnikov 02 Aug '13

02 Aug '13

(I'm trying to update a package that was written with GHC 6.10.4 in mind.) I feel that a 'myThreadId' action, which is defined in this module [1], is useless, but I'm not sure. I think it will always return the "Map.find: element not in the map" exception because a 'threadMap' contains an empty 'ThreadMap'. Is it right, or am I missing something? [1] https://gitweb.torproject.org/tordnsel.git/blob/HEAD:/src/TorDNSEL/Control/…

2 2

[ANNOUNCE] fficxx : haskell-C++ FFI binding generator
by Ian-Woo Kim 02 Aug '13

02 Aug '13

Dear Haskellers, Hello. I am very happy to announce the first version of fficxx (http://ianwookim.org/fficxx, also, please look at http://github.com/wavewave/fficxx ) fficxx is a haskell Foreign Function Interface (FFI) generator to C++. This tool automatically generates haskell FFI package for a given C++ class structure. It has been used for generating my haskell binding to the ROOT library [1] called HROOT [2]. Now I made it as a separate tool to benefit everyone trying to make a haskell-C++ binding. I am going to explain how to use the library/tool one by one in my blog ( http://ianwookim.org/blog ). Currently, in the fficxx cabal package, I provide a very simple sample in the sample directory. If interested, please try this and give me some comments on it in the discussion mailing list: http://ianwookim.org/fficxx/discuss.html I hope that this is useful to many haskellers. Have fun with it! Thanks, Ian-Woo Kim (wavewave) [1] : http://root.cern.ch [2] : http://ianwookim.org/HROOT

3 2

Hoogle problems?
by Andrew Butterfield 01 Aug '13

01 Aug '13

I've just tried using Hoogle, but either get a 404 not found (http://haskell.org/hoogle/) or else I find I get a ELF 64-bit LSB executable being downloaded If I search using Google and click on the first link (shown as www.haskell.org/hoogle/ I get the following (spaces deliberately added to make it less dangerous) https://www.goo gle.com/url?sa=t&rct=j&q=&esrc=s&sou rce=web&cd=1&cad=rja&ved=0CC8QFjAA&url=http%3A%2F%2Fwww.haskell.org%2Fhoogle%2F&ei=y_njUdZAhY 7sBsGpgNAK&usg=AFQjCNFljq5Oyb4LT7VF-I5MUotq512AQg&sig2 =SVRE_6nF3bhho7-NKqEaCw&bvm=bv.487 05608,d.ZGU Using Mac OS X 10.6, Chrome and Safari Is it me, or is there a wider problem ? -------------------------------------------------------------------- Andrew Butterfield Tel: +353-1-896-2517 Fax: +353-1-677-2204 Lero@TCD, Head of Foundations & Methods Research Group Director of Teaching and Learning - Undergraduate, School of Computer Science and Statistics, Room G.39, O'Reilly Institute, Trinity College, University of Dublin http://www.scss.tcd.ie/Andrew.Butterfield/ --------------------------------------------------------------------

7 9

Haskell Weekly News: Issue 274
by Daniel Santa Cruz 01 Aug '13

01 Aug '13

Welcome to issue 274 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of July 14 to 27, 2013. Quotes of the Week * knuth: To help the reader drowning in a sea of abstraction, the following example may serve as a concrete life preserver >From the mail bag * Haskell for schools in Argentina [1] http://goo.gl/EAPHXx Top Reddit Stories * Parallel and Concurrent Programming in Haskell available now in ebook formats, print version can be preordered Domain: shop.oreilly.com, Score: 94, Comments: 14 On Reddit: [2] http://goo.gl/426HB0 Original: [3] http://goo.gl/anqIi * Lenses in pictures Domain: adit.io, Score: 84, Comments: 51 On Reddit: [4] http://goo.gl/e8Bc0N Original: [5] http://goo.gl/1kmqf * Monads made difficult Domain: stephendiehl.com, Score: 70, Comments: 11 On Reddit: [6] http://goo.gl/sQAmVD Original: [7] http://goo.gl/35EDt * Helm - FRP Game Engine Inspired by Elm Domain: github.com, Score: 67, Comments: 19 On Reddit: [8] http://goo.gl/1lJEIB Original: [9] http://goo.gl/4wDcHN * Ian Lynagh leaves GHC HQ Domain: haskell.org, Score: 64, Comments: 4 On Reddit: [10] http://goo.gl/XCnjrA Original: [11] http://goo.gl/Ox5KXW * Yesod in Production Domain: joelt.io, Score: 61, Comments: 13 On Reddit: [12] http://goo.gl/jlhvvm Original: [13] http://goo.gl/7pSCJ * A Comparison Between Perl and Haskell Domain: ocharles.org.uk, Score: 60, Comments: 22 On Reddit: [14] http://goo.gl/cwYXb0 Original: [15] http://goo.gl/P46D4e * Backpack: Retrofitting Haskell with Interfaces (Kilpatrick, Dreyer, SPJ, Marlow) [link to pdf] Domain: mpi-sws.org, Score: 58, Comments: 63 On Reddit: [16] http://goo.gl/sjjAE Original: [17] http://goo.gl/C06Ly * Parallel and Concurrent Programming in Haskell (online version, part of Atlas beta) Domain: chimera.labs.oreilly.com, Score: 56, Comments: 35 On Reddit: [18] http://goo.gl/x5dhf5 Original: [19] http://goo.gl/H7MeFa * Why Haskell at School matters Domain: sawafaso.blogspot.com.ar, Score: 52, Comments: 18 On Reddit: [20] http://goo.gl/2LHlsn Original: [21] http://goo.gl/8Dwy4R * Measuring the Haskell Gap :: PDF Domain: leafpetersen.com, Score: 49, Comments: 13 On Reddit: [22] http://goo.gl/6b4xdx Original: [23] http://goo.gl/nqDJZd * Automatic SIMD Vectorization for Haskell by Leaf Petersen, Dominic Orchard and Neal Glew [ICFP13][PDF] Domain: leafpetersen.com, Score: 47, Comments: 13 On Reddit: [24] http://goo.gl/dPCPio Original: [25] http://goo.gl/gDBajX * IRC Browse Domain: chrisdone.com, Score: 43, Comments: 12 On Reddit: [26] http://goo.gl/OPXVQK Original: [27] http://goo.gl/IOoJgT * How to Help Develop Hackage2 Domain: self.haskell, Score: 40, Comments: 46 On Reddit: [28] http://goo.gl/NQGmY Original: [29] http://goo.gl/NQGmY Top StackOverflow Questions * Can someone explain to me why the app function of ArrowApply makes them as powerful as monads? votes: 13, answers: 2 Read on SO: [30] http://goo.gl/V6I0fK * Haskell newtype with parentheses votes: 12, answers: 2 Read on SO: [31] http://goo.gl/k9A7YU * GHC: insert compilation date votes: 11, answers: 2 Read on SO: [32] http://goo.gl/MbWGqH * How can Haskell function guards operate on other values than the functions parameters? votes: 10, answers: 2 Read on SO: [33] http://goo.gl/sgI9i8 * Can Haskell functions be serialized? votes: 10, answers: 3 Read on SO: [34] http://goo.gl/fVsPvv * Calling Haskell library from C++ votes: 10, answers: 1 Read on SO: [35] http://goo.gl/6V2zqb * Debugging compile time performance issues caused by GHC's constraint solver votes: 10, answers: 1 Read on SO: [36] http://goo.gl/XPAqAy * Haskell performance when using classes and instances votes: 10, answers: 2 Read on SO: [37] http://goo.gl/EZNVMS * Could not deduce (m ~ m1) votes: 9, answers: 1 Read on SO: [38] http://goo.gl/vW0x6D * Running a subexpression once votes: 9, answers: 2 Read on SO: [39] http://goo.gl/MdRgkZ Until next time, [40]+Daniel Santa Cruz References 1. http://sawafaso.blogspot.com.ar/2013/07/why-haskell-at-school-matters.html?… 2. http://shop.oreilly.com/product/0636920026365.do 3. http://www.reddit.com/r/haskell/comments/1ioorw/parallel_and_concurrent_pro… 4. http://adit.io/posts/2013-07-22-lenses-in-pictures.html 5. http://www.reddit.com/r/haskell/comments/1isryj/lenses_in_pictures/ 6. http://www.stephendiehl.com/posts/monads.html 7. http://www.reddit.com/r/haskell/comments/1iafky/monads_made_difficult/ 8. https://github.com/z0w0/helm 9. http://www.reddit.com/r/haskell/comments/1j1fd8/helm_frp_game_engine_inspir… 10. http://www.haskell.org/pipermail/ghc-devs/2013-July/001677.html 11. http://www.reddit.com/r/haskell/comments/1ikc91/ian_lynagh_leaves_ghc_hq/ 12. http://joelt.io/entries/yesod-in-production 13. http://www.reddit.com/r/haskell/comments/1inf8a/yesod_in_production/ 14. http://ocharles.org.uk/blog/posts/2013-07-26-a-comparison-between-perl-and-… 15. http://www.reddit.com/r/haskell/comments/1j3jgd/a_comparison_between_perl_a… 16. http://www.mpi-sws.org/~skilpat/backpack/ 17. http://www.reddit.com/r/haskell/comments/1id0p7/backpack_retrofitting_haske… 18. http://chimera.labs.oreilly.com/books/1230000000929/index.html 19. http://www.reddit.com/r/haskell/comments/1iwr7x/parallel_and_concurrent_pro… 20. http://sawafaso.blogspot.com.ar/2013/07/why-haskell-at-school-matters.html 21. http://www.reddit.com/r/haskell/comments/1j3k19/why_haskell_at_school_matte… 22. http://www.leafpetersen.com/leaf/publications/hs2013/haskell-gap.pdf 23. http://www.reddit.com/r/haskell/comments/1j2k4a/measuring_the_haskell_gap_p… 24. http://www.leafpetersen.com/leaf/publications/icfp2013/vectorization-haskel… 25. http://www.reddit.com/r/haskell/comments/1izyok/automatic_simd_vectorizatio… 26. http://chrisdone.com/posts/ircbrowse 27. http://www.reddit.com/r/haskell/comments/1iy8nv/irc_browse/ 28. http://www.reddit.com/r/haskell/comments/1iki8p/how_to_help_develop_hackage… 29. http://www.reddit.com/r/haskell/comments/1iki8p/how_to_help_develop_hackage… 30. http://stackoverflow.com/questions/17668452/can-someone-explain-to-me-why-t… 31. http://stackoverflow.com/questions/17853232/haskell-newtype-with-parentheses 32. http://stackoverflow.com/questions/17796521/ghc-insert-compilation-date 33. http://stackoverflow.com/questions/17650866/how-can-haskell-function-guards… 34. http://stackoverflow.com/questions/17785916/can-haskell-functions-be-serial… 35. http://stackoverflow.com/questions/17815804/calling-haskell-library-from-c 36. http://stackoverflow.com/questions/17822539/debugging-compile-time-performa… 37. http://stackoverflow.com/questions/17834802/haskell-performance-when-using-… 38. http://stackoverflow.com/questions/17765690/could-not-deduce-m-m1 39. http://stackoverflow.com/questions/17900495/running-a-subexpression-once 40. https://plus.google.com/105107667630152149014/about

1 0

Template Haskell and Haddock
by Jose A. Lopes 31 Jul '13

31 Jul '13

Hi, Is there a way to access docstrings through Template Haskell ? For example, access the docstring of a function declaration ? Best regards, Jose

3 2

ANN: Angel 0.4.4
by Michael Xavier 31 Jul '13

31 Jul '13

I'm pleased to announce the release of Angel 0.4.4. angel is a daemon that runs and monitors other processes. It is similar to djb's daemontools or the Ruby project god. It's goals are to keep a set of services running, and to facilitate the easy configuration and restart of those services. New in 0.4.4 * Add env option to config to specify environment variables. * Inject ANGEL_PROCESS_NUMBER environment variable into processes started with count. This is intended for the purpose of logging, port enumeration, etc. Homepage: https://github.com/michaelxavier/angel HackageDB: http://hackage.haskell.org/package/angel -- Michael Xavier http://www.michaelxavier.net

3 2

ANNOUNCE: hdbi-1.0.0 and hdbi-postgresql-1.0.0
by Alexey Uimanov 31 Jul '13

31 Jul '13

Hello, haskellers. This is the first release of HDBI (Haskell Database Independent interface). It is the fork of HDBC. HDBI has some improvements in design, it also has better testing and performance (mainly because of using Text instead of String anywhere). HDBI designed to be more flexible and correct database interface. You can find out more information in the documentation. http://hackage.haskell.org/package/hdbi http://hackage.haskell.org/package/hdbi-postgresql I am planning to implement MySql and Sqlite drivers as well https://github.com/s9gf4ult/hdbi https://github.com/s9gf4ult/hdbi-postgresql Issues and pull requests are welcome, as well as collaboration.

4 8

ForeignPtr and Garbage Collection
by satvik chauhan 30 Jul '13

30 Jul '13

Hi Cafe, Is it guaranteed that the value of `ForeignPtr` will not be changed (for example due to compaction etc) by haskell garbage collector until it goes out of scope and finalizer is run. -Satvik

2 1

Call for talks: Haskell Implementors Workshop 2013, Sept 22, Boston
by Ryan Newton 30 Jul '13

30 Jul '13

Please pass on this announcement! The deadline is in two weeks.* * * * * Call for Talks* ACM SIGPLAN Haskell Implementors' Workshop http://haskell.org/haskellwiki/HaskellImplementorsWorkshop/2013 Boston, USA, September 22th, 2013 The workshop will be held in conjunction with ICFP 2013 http://www.icfpconference.org/icfp2013/ *Important dates* Proposal Deadline: *13th August 2013 (by midnight, any timezone)* Notification: 27th August 2013 Workshop: 22th September 2013 The Haskell Implementors' Workshop is to be held alongside ICFP 2013 this year in Boston. There will be no proceedings; it is an informal gathering of people involved in the design and development of Haskell implementations, tools, libraries, and supporting infrastructure. This relatively new workshop reflects the growth of the user community: there is a clear need for a well-supported tool chain for the development, distribution, deployment, and configuration of Haskell software. The aim is for this workshop to give the people involved with building the infrastructure behind this ecosystem an opportunity to bat around ideas, share experiences, and ask for feedback from fellow experts. We intend the workshop to have an informal and interactive feel, with a flexible timetable and plenty of room for ad-hoc discussion, demos, and impromptu short talks. Scope and target audience ------------------------- It is important to distinguish the Haskell Implementors' Workshop from the Haskell Symposium which is also co-located with ICFP 2013. The Haskell Symposium is for the publication of Haskell-related research. In contrast, the Haskell Implementors' Workshop will have no proceedings -- although we will aim to make talk videos, slides and presented data available with the consent of the speakers. In the Haskell Implementors' Workshop, we hope to study the underlying technology. We want to bring together anyone interested in the nitty-gritty details behind turning plain-text source code into a deployed product. Having said that, members of the wider Haskell community are more than welcome to attend the workshop -- we need your feedback to keep the Haskell ecosystem thriving. The scope covers any of the following topics. There may be some topics that people feel we've missed, so by all means submit a proposal even if it doesn't fit exactly into one of these buckets: * Compilation techniques * Language features and extensions * Type system implementation * Concurrency and parallelism: language design and implementation * Performance, optimisation and benchmarking * Virtual machines and run-time systems * Libraries and tools for development or deployment Talks ----- At this stage we would like to invite proposals from potential speakers for a relatively short talk. We are aiming for 20 minute talks with 10 minutes for questions and changeovers. We want to hear from people writing compilers, tools, or libraries, people with cool ideas for directions in which we should take the platform, proposals for new features to be implemented, and half-baked crazy ideas. Please submit a talk title and abstract of no more than 200 words. Submissions should be made via EasyChair. The website is: https://www.easychair.org/conferences/?conf=hiw2013 If you don't have an account you can create one here: https://www.easychair.org/account/signup.cgi Because the submission is an abstract only, please click the "abstract only" button when you make your submission. There is no need to attach a separate file. We will also have a lightning talks session which will be organised on the day. These talks will be 2-10 minutes, depending on available time. Suggested topics for lightning talks are to present a single idea, a work-in-progress project, a problem to intrigue and perplex Haskell implementors, or simply to ask for feedback and collaborators. Organisers ---------- * Ryan Newton (Indiana University) * Neal Glew (Intel Labs) * Edward Yang (Stanford University) * Thomas Schilling (University of Kent) * Geoffrey Mainland (Drexel University)

1 0

Dual-licensing the package on Hackage
by David Sorokin 30 Jul '13

30 Jul '13

Hi, Cafe! Probably, it was asked before but I could not find an answer with help of Google. I have a library which is hosted on Hackage. The library is licensed under BSD3. It is a very specialized library for a small target group. Now I'm going to relicense it and release a new version already under the dual-license: GPLv3 and commercial. In most cases GPL will be sufficient as this is not a library in common sense. Can I specify the GPL license in the .cabal file, or should I write OtherLicense? I'm going to add the information about dual-licensing in the description section of the .cabal file, though. Thanks, David

6 13

haskell.org down?
by Paul Koerbitz 29 Jul '13

29 Jul '13

Dear list, I am not sure if this is the right place to post this, but I just went to haskell.org and got a "Sorry! This site is experiencing technical difficulties." error message. cheers Paul

2 2

Re: [Haskell-cafe] CmdArgs non-flag arguments with newtypes
by wren ng thornton 29 Jul '13

29 Jul '13

On 7/23/13 9:01 PM, Michael Orlitzky wrote: > Obviously not what I want! Has anyone > else run into this? Figured out a workaround? I haven't run into this specific problem, but I do have a sort of workaround. Whenever dealing with CmdArgs (or any similar system) I typically define *two* record types. The first one just gets the raw input from CmdArgs; no more, no less. Thus, for your issue, this record would use String. For other issues mentioned recently about optionality, this record would use Maybe. The second one is the one actually used by the program as the configuration object. This one is generated from the first by performing various sanity checks, filling in defaults, converting types from their CmdArgs version to the version I actually want, etc. IME, regardless of language, trying to conflate these notions of an external-facing parser-state and an internal-facing configuration-record just causes problems and accidental complexity. It's best to keep them separate IMO. -- Live well, ~wren

2 1