December 2007 - Haskell

Announcing Djinn, version 2004-12-11, a coding wizard
by Lennart Augustsson 07 Apr '11

07 Apr '11

Howdy, y'all! I've written a small program that takes a (Haskell) type and gives you back a function of that type if one exists. It's kind of fun, so I thought I'd share it. It's probably best explained with a sample session. calvin% djinn Welcome to Djinn version 2005-12-11. Type :h to get help. # Djinn is interactive if not given any arguments. # Let's see if it can find the identity function. Djinn> f ? a->a f :: a -> a f x1 = x1 # Yes, that was easy. Let's try some tuple munging. Djinn> sel ? ((a,b),(c,d)) -> (b,c) sel :: ((a, b), (c, d)) -> (b, c) sel ((_, v5), (v6, _)) = (v5, v6) # We can ask for the impossible, but then we get what we # deserve. Djinn> cast ? a->b -- cast cannot be realized. # OK, let's be bold and try some functions that are tricky to write: # return, bind, and callCC in the continuation monad Djinn> type C a = (a -> r) -> r Djinn> returnC ? a -> C a returnC :: a -> C a returnC x1 x2 = x2 x1 Djinn> bindC ? C a -> (a -> C b) -> C b bindC :: C a -> (a -> C b) -> C b bindC x1 x2 x3 = x1 (\ c15 -> x2 c15 (\ c17 -> x3 c17)) Djinn> callCC ? ((a -> C b) -> C a) -> C a callCC :: ((a -> C b) -> C a) -> C a callCC x1 x2 = x1 (\ c15 _ -> x2 c15) (\ c11 -> x2 c11) # Well, poor Djinn has a sweaty brow after deducing the code # for callCC so we had better quit. Djinn> :q Bye. To play with Djinn do a darcs get http://darcs.augustsson.net/Darcs/Djinn or get http://darcs.augustsson.net/Darcs/Djinn/Djinn.tar.gz Then just type make. (You need a Haskell 98 implementation and some libraries.) And then start djinn. For the curious, Djinn uses a decision procedure for intuitionistic propositional calculus due to Roy Dyckhoff. It's a variation of Gentzen's LJ system. This means that (in theory) Djinn will always find a function if one exists, and if one doesn't exist Djinn will terminate telling you so. This is the very first version, so expect bugs. :) Share and enjoy. -- Lennart

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Haskell-mode 2.0
by Stefan Monnier 02 Feb '09

02 Feb '09

I have recently taken over maintainership of Haskell-mode, and after making a bunch of changes, I figured it would be a good idea to make a new release. You can find this new release at: http://www-perso.iro.umontreal.ca/~monnier/elisp/ If you think it should also be found at http://www.haskell.org/haskell-mode/, then please tell John Peterson to get back to me (my email to him seem to go straight to the bitbucket. John?). This release has several changes that have all been only lightly tested, especially w.r.t compatibility with various Emacsen. See appended the NEWS file that describes some of the notewirthy changes. Stefan Changes since 1.45: * keybindings C-c <char> have been replaced by C-c C-<char> so as not to collide with minor modes. * The following modules are now automatically activated without having to add anything to haskell-mode-hook: haskell-font-lock (just turn on global-font-lock-mode). haskell-decl-scan (just bind `imenu' to some key). * In recent Emacsen, haskell-doc hooks into eldoc-mode. * haskell-hugs and haskell-ghci are superceded by inf-haskell. * Indentation rules have been improved when using layout inside parens/braces. * Symbols like -> and \ can be displayed as actual arrows and lambdas. See haskell-font-lock-symbols. * Tweaks to the font-lock settings. Among other things paren-matching with things like \(x,y) should work correctly now. * New maintainer <monnier(a)gnu.org>.

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The real Monad Transformer
by Henning Thielemann 13 Nov '08

13 Nov '08

It was argued that people avoid Haskell because of terms from Category theory like 'Monad'. This problem can now be solved by a wrapper which presents all the WWW without monads! Start e.g. at http://saxophone.jpberlin.de/MonadTransformer?source=http%3A%2F%2Fwww%2Ehas… Of course the tool is written in Haskell, that is, Haskell helps solving problems which only exist because of Haskell. Bug reports and feature requests can be tracked at https://sourceforge.net/projects/parallelweb

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ANN: MacOSX - A library some Haskell users on OS X might find useful.
by Corey O'Connor 27 Feb '08

27 Feb '08

I've been working on integrating haskell code with some Objective-C/C++ Cocoa Mac OS X applications. Nothing fancy like a Haskell/Cocoa bridge. Just trying to get the build process smooth. The default cabal build system was incomplete to do this effectively;* Extending XCode was no good either. With some effort I created some Cabal build hooks to fix up the behavior of build on OS X and add support for packaging an executable into a ".app" package. That's not much, but I packaged it up, plus some extras, into a library in case other projects would like to use it: http://tothepowerofdisco.com/repo/HaskellLibraries/ Of course, this is all hackish and very incomplete. EG: * No fat binary support. I only have a PPC Mac and the only fat-binary process usable with GHC I know of requires executing the x86 GHC. * The build hook in MacOSX.Build will only work on the Cabal distributed with GHC 6.6.1 and *completely* replaces the standard GHC build hook. * GHC compiled to use the framework version of GMP installed to /Library/Frameworks is required. * The name is vague... Still, I've used it to add haskell support and replace XCode for building for 3 exisitng Cocoa applications. Maybe somebody else will find it useful as well. -- -Corey O'Connor * I neglected to make a good list of why the default Cabal build hook doesn't work.

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garbage collection of Concurrent Haskell threads?
by Conal Elliott 03 Jan '08

03 Jan '08

The classic paper "The Incremental Garbage Collection of Processes" ( http://citeseer.ist.psu.edu/baker77incremental.html) describes "futures" and how particularly garbage collecting them when their pending result is no longer referenced. I've been playing with an implementation of futures in Concurrent Haskell (http://haskell.org/haskellwiki/Reactive) using MVars, and I'm stumped about how to GC non-winning threads in a race between futures ("parallel or"). I'm having winner kill loser, which seems to work fine, though is potentially dangerous w.r.t locked resources. Still, the elegance of a GC-based solution appeals to me. Has anyone explored process GC ideas for Concurrent Haskell (or STM)? Futures are implemented using Concurrent Haskell's MVars. I first tried using STM and TVars, simply using orElse to implement mappend for futures. However, I didn't see how to avoid nesting "atomically", which yielded a run-time error. If anyone has ideas about using STM & TVars for futures, I'd love to hear. Thanks, - Conal

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Empty instance declaration
by Hugo Macedo 03 Jan '08

03 Jan '08

Dear all A student from a beginners course on Functional Programming came to me with a problem, he declared a type like: data WeekDay = Mon | Tue | Fri -- ... He had forgot to complete a Show instance definition. instance Show WeekDay where (empty) Then he complained about getting *** Exception: stack overflow when he tried to invoke some function on that type. After checking the Haskell98 grammar I found out that this is allowed syntactically and probably semantically too. Is there any reason to do that? Now I'm wondering: Shouldn't a class be an warranty of a well defined interface? Thus forcing the user to define the show function or showPrec? Am I missing any point? My best regards, Hugo

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announcing qtHaskell-1.1.1, the first preview release of qtHaskell
by David Harley 03 Jan '08

03 Jan '08

For all interested in Haskell GUIs, there is now a first preview release of qtHaskell - a set of Haskell bindings for Trolltech's Qt available at http://qthaskell.sourceforge.net Any feedback, comments etc welcome Merry Christmas David Harley P.S. if anyone can get the system to build and work on a non-linux or Windows environment - in particular MacOs - could the let me know the precise install steps required.

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Postdoctoral position at INRIA Nancy (France)
by Eric Kow 28 Dec '07

28 Dec '07

Linguistic Field(s): Computational Linguistics Topic: Surface realisation and large scale over-generation detection Deadline: February 15, 2008. Employer: INRIA (French National Institute for Research in Computer Science) Nancy Grand Est (France) Job Description: The Lorraine Laboratory of IT Research and its Applications (Nancy, France) has a position for a Postdoctoral fellow to work on the development of a surface realiser for French. Applicants must have a ** recent doctoral degree ** (Doctoral thesis less than one year old before the recruitment's date) or defend their PhD before the end of 2008. They must have expertise in an area relevant to the project (linguistics, computational linguistics, computer science), strong hands-on experience in Natural Language Processing and a particular interest in NL generation. A good knowledge of Haskell is a necessity. Further particulars and details of how to apply are available at: http://www.loria.fr/~gardent The official closing date is February 15, 2007, but applications will be processed until the position is filled. Contact: Claire Gardent <claire.gardent(a)loria.fr> -- Eric Kow PGP Key ID: 08AC04F9

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Summer School LERNET 2008 - Call for Participation
by Alberto Pardo - INCO 27 Dec '07

27 Dec '07

=================================================================== Call for Participation International Summer School on Language Engineering and Rigorous Software Development LERNET 2008, Piriapolis, Uruguay February 25 to March 1, 2008 http://www.fing.edu.uy/inco/eventos/lernet2008 email: lernet(a)fing.edu.uy *** REGISTRATION IS OPEN *** =================================================================== The aim of the school is the dissemination of advanced scientific knowledge and the promotion of international contacts among scientists. The school is directed to students and researchers working in computer science and interested in formal techniques for the design and construction of software systems and programming languages. The school is partially supported by the EU LerNet ALFA project (http://www.di.uminho.pt/lernet/) which implements a joint PhD programme on Software Design and Programming Language Engineering, based on a co-tutoring scheme and conformed by 6 european universities and 5 from Latin America. PROGRAM LERNET 2008 will consist of six courses: - Introduction to Type Theory Herman Geuvers (Radboud University Nijmegen, The Netherlands) - Theory and Applications of the PF-transform Jose Nuno Oliveira (University of Minho, Portugal) - Embedded Domain Specific Languages: Combinator Parsers Doaitse Swierstra (Utrecht University, The Netherlands) - Dependent Types at Work Peter Dybjer and Ana Bove (Chalmers University, Sweden) - Formal Programming Language Semantics with Inductive Types Yves Bertot (INRIA Sophia-Antipolis, France) - Verification Methods for Software Security and Correctness Gilles Barthe (INRIA Sophia-Antipolis, France) In addition, there will be some PhD sessions where students from the LerNet project will expose advances of their PhD thesis. VENUE LERNET 2008 will be held at Argentino Hotel (http://www.argentinohotel.com.uy) located in Piriápolis, a seaside city, 100 kms east from Montevideo. REGISTRATION To register, fill in the registration form at http://www.cs.chalmers.se/Cs/Research/Logic/LerNet08/registration.html The deadline for registration is January 31, 2008. Early registration is encouraged. FEES Details about the registration fees can be found in the school's webpage (http://www.fing.edu.uy/inco/eventos/lernet2008) GRANTS We may be able to offer a reduction in the value of the registration fee to a limited number of students. Priority will be given to students from Latin America. Details of the fee reduction application can be found in the school's webpage (http://www.fing.edu.uy/inco/eventos/lernet2008) Deadline for grant application is also January, 31 2008. Early application is encouraged. SCHOOL ORGANISERS Luis Barbosa (Univerity of Minho, Portugal) Ana Bove (Chalmers University, Sweden) Alberto Pardo (Universidad de la República, Uruguay) Jorge Sousa Pinto (Univerity of Minho, Portugal) LOCAL ORGANISATION Alberto Pardo Luis Sierra Carlos Luna Instituto de Computación Facultad de Ingeniería Universidad de la República Montevideo, Uruguay FURTHER INFORMATION For further details on the school, visit the webpage http://www.fing.edu.uy/inco/eventos/lernet2008

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Re: [Haskell] Re: ANNOUNCE: GHC version 6.8.2
by Benjamin L. Russell 26 Dec '07

26 Dec '07

--- Juanma Barranquero <lekktu(a)gmail.com> wrote: ... > > Though, if you're using Emacs on Windows and you do > want it to use a > different HOME you can set > HOME=C:\this\is\my\emacs\home as a new > string value in the HKLM\Software\Gnu\Emacs key of > the registry. The registry ... I hadn't thought of that one. Thanks for the tip. However, usually I steer clear of the registry unless absolutely necessary because making a mistake there can render Windows unbootable. In that case, though, would you happen to know which value in the registry would allow me to override HOMEPATH for GHCi? Benjamin L. Russell

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