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Re: Type Families: deleting from HList
by oleg＠okmij.org 04 Aug '10

04 Aug '10

Serguey Zefirov wrote: > Is it possible to delete an element from heterogenous list using type > families alone? > > I can do it using multiparameter type classes: > class Del a list result > instance Del a (a,list) list > instance Del a list list' => Del a (a',list) list' > instance Del a () () I'm afraid you'll find that class quite difficult to use: lots of type annotations will be required. For example, if we define del :: Del a li lo => a -> li -> lo del = undefined and write test1 = del True ('a',(True,())) the compiler will complain because it doesn't know which instance to choose. BTW, since GHC checks for overlapping lazily, the fact that GHC accepts a set of instance declarations does not tell that the instances `work'. The problems may lurk, becoming apparent when we define values that attempt to use the instances. In order for something like Del to be useful, functional dependencies are needed. It is easy to see that Del uses overlapping instances. That answers the question: since type families do not support overlapping instances, one cannot re-write Del using type families alone. One may ask is it possible to define some sort of type inequality using type families. And the answer is yes, recently discussed at http://www.haskell.org/pipermail/haskell-cafe/2010-July/080473.html

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Error Monad and strings
by Gerald Gutierrez 04 Aug '10

04 Aug '10

Reading the Control.Monad.Error documentation, I see that the Error class has noMsg and strMsg as its only two functions. Now, I understand that you can define your own Error instances such as in example 1 of the documentation, so why the need to always support strings via noMsg/strMsg ? What uses these? And if in my code, I will never throw an error with a string, am I supposed to implement these functions and then ignore them?

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Installation problem with latest version of haskell platform
by Pieter Laeremans 03 Aug '10

03 Aug '10

Hi, When I try to install haskell platform 64 bit on a x86_64 debian etch I get the following error after : "make install": /usr/bin/install -c -m 755 -d "/usr/local/lib/ghc-6.12.3/package.conf.d" "/usr/local/lib/ghc-6.12.3/ghc-pkg" --force --global-conf "/usr/local/lib/ghc-6.12.3/package.conf.d" update libffi/package.conf.install /usr/local/lib/ghc-6.12.3/ghc-pkg: error while loading shared libraries: libtinfo.so.5: cannot open shared object file: No such file or directory make[1]: *** [install_packages] Error 127 make: *** [install] Error 2 I have no good strategy to locate the needed library. A quick google search yielded ncurses, but I have already installed it, and it doesn't contain that binary. Furthermore configure doesn't yield any errors. And a dependency on ncurses does seem quite strange doesn't it ? thans in advance, Pieter -- Pieter Laeremans <pieter(a)laeremans.org> "The future is here. It's just not evenly distributed yet." W. Gibson

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ANNOUNCE: approximate-equality 1.0 -- Newtype wrappers for approximate equality
by Gregory Crosswhite 03 Aug '10

03 Aug '10

I am pleased to announce the release of the package "approximate-equality", which provides newtype wrappers that allow one to effectively override the equality operator of a value so that it is /approximate/ rather than /exact/. The wrappers use type annotations to specify the tolerance; the 'Digits' type constructor has been provided for specifying the tolerance using type-level natural numbers. Instances for all of the classes in the numerical hierarchy have been provided for the wrappers, so the wrapped values can mostly be used in the same way as the original values. (In fact, most of the time one doesn't even have to wrap the starting values, since expressions such as (1+sqrt 2/3) are automatically wrapped thanks to the 'fromIntegral' method of the 'Num' typeclass.) The motivation behind this package is that there are times when working with floating point numbers that one would like the equality operator to check for approximate equality rather than exact equality. For example, in one of my own projects I have a walker that spits out a list of coordinate values that I keep track of, and since floating point arithmetic is inexact it will often arrive at the same point from two different paths and thus obtain slightly different values for the coordinates of that point. I could have just written a function to do approximate matching of the point and be done with it, but then I can't leverage pre-built data structures such as Data.Set and Data.Map which use (==) for determining whether a key is a member. This package is compatible with Haskell 2010, as it only uses the EmptyDataDecls extension, and the only package dependency is type-level-natural-numbers which itself is Haskell 2010. Any feedback from the community is, of course, very welcome. Cheers, Greg

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ANN: scan-0.1.0.3, a Haskell style scanner
by Christian Maeder 03 Aug '10

03 Aug '10

Dear Haskell friends, I like to announce a Haskell style scanner at http://hackage.haskell.org/package/scan documented under http://projects.haskell.org/style-scanner/ It's best used in conjunction with hlint http://community.haskell.org/~ndm/hlint/ and gives many suggestions regarding the file format. Feedback is welcome. Thanks Christian (Discussions should only be posted to either haskell-cafe@ or beginners@)

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problem installing pivotal
by rustom 03 Aug '10

03 Aug '10

I tried to install pivotal According to the site I must do: ghc --make Main -fglasgow-exts -package plugins I get cant satisfy package plugins I believe the more general problem is that one has to choose between 'direct' cabal installs and using debian's packages. At least this is the same situation Ive seen before in python, ruby etc. What do I do? So this is really 2 questions: 1. How to remove this error and install pivotal 2. Is it better to install haskell-platform or is it better to have a minimal install of ghc and then use hackage/cabal [Or is this a type- wrong question :-) ]

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RE: what's the best environment for haskell work? (Rustom Mody)
by Luc TAESCH 03 Aug '10

03 Aug '10

On Sat, Jul 31, 2010 at 12:07 PM, Rustom Mody <rustompmody(a)gmail.com> wrote: > Do most people who work with haskell use emacs/vi/eclipse or something > else?? > > I used Emacs, as I used to in other langauages.( mainly on Linux) When switching to Mac Ox, I Re tried Leksah at 0.8 , and Now Am sticking to it. what I found of value: - In a Nutshell, increased focus. not only time, but focus. changes in my behavior , mainly : -when exploring unknown libraries, I found the one click access to libraries ( search Metadata) , then one click to read the source is easy (easier means then I do it more often). - the right click "type this ..." that seemlessly toogles betwwen ghi-ghci , is cool to paste types once a part of my code solidify - session saving means I find the same windows opened at the same place I left them the day before or a while ago... more focus again. ( when working in parallele on different things) ( Note : I have a 27 ' screen)

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Re: (kein Betreff)
by Janis Voigtländer 03 Aug '10

03 Aug '10

Stefan Holdermans wrote: >> The point of this discussion is that the Eval constraint needs to be >> on one >> of the functions. So I tried to specify that (x -> Int) and (y -> >> Int) are >> different types despite x and y being the same type, because one of >> them has >> an Eval constraint. This may be a shortcoming of Haskell (or System Fc?) >> types, although it may be doable with a newtype. > > That was kind of what my thinking out loud was getting at. If you want x > -> Int and y -> Int to be different types even if x and y actually are > the same type, then apparently you want x -> Int and y -> Int to be > built from different function-space constructors, say -> and ->*, > yielding x -> Int and y ->* Int. Replacing equals for equals again, you > get x -> Int and x ->* Int. So, basically, we are annotating function > types, what is IIRC exactly what Janis and David are doing. (I hope > Janis corrects me if I'm wrong here). Wrong only in that David's name is Daniel. :-) Other than that, I agree. Best, Janis. -- Jun.-Prof. Dr. Janis Voigtländer http://www.iai.uni-bonn.de/~jv/ mailto:jv@iai.uni-bonn.de

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Fw: Re: [Haskell-cafe] Re: Announce snm-0.0.2: The Simple Nice-Looking Manual Generator
by Benjamin L. Russell 03 Aug '10

03 Aug '10

The following message was sent to me, but was apparently intended for this thread, so I am forwarding it to the Haskell-Cafe Mailing List: On Mon, Aug. 2, 2010 at 11:44 PM, Gordon Sommers <gordon.sommers(a)gmail.com> wrote: > This looks pretty neat! The website would benefit from one or two > "full-size" examples I think. A good example is the classgen > documentation -- they have a series of pages on the specific features, > all with small examples, but then they also have some full examples > like so: http://classgen.sourceforge.net/docs/ex1.html, which provide > a great starting place. > On Thu, Jul 29, 2010 at 5:45 PM, Benjamin L. Russell <DekuDekuplex(a)yahoo.com> wrote: > > Johnny Morrice <spoon(a)killersmurf.com> writes: > > [...] > > > > snm allows you to write clean, web-friendly reports, user guides and > > manuals without having to edit fickle html. > Interesting project! Unlike some users, I happen to enjoy writing > documentation. Perhaps you should also announce this project on the main Haskell > mailing list. > > [...] > > > > Read the snm user guide here: > > http://www.killersmurf.com/static/snm_help.html > > 2. What snm does not do and is not > > > > [...] > > > > snm is not an adult activity. > Hmm ... the term "adult activity" could mean many different things > ... what one did you mean? > Thank you for the many examples in your user guide; they help clarify > usage. > -- Benjamin L. Russell > -- > Benjamin L. Russell / DekuDekuplex at Yahoo dot com > http://dekudekuplex.wordpress.com/ > Translator/Interpreter / Mobile: +011 81 80-3603-6725 > "Furuike ya, kawazu tobikomu mizu no oto." -- Matsuo Basho^ > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe

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ICFP 2010: Call for participation
by Wouter Swierstra 03 Aug '10

03 Aug '10

===================================================================== Call for Participation The 15th ACM SIGPLAN International Conference on Functional Programming (ICFP 2010) http://www.icfpconference.org/icfp2010/ Baltimore, Maryland September 25 – October 2 ===================================================================== ICFP 2010 provides a forum for researchers and developers to hear about the latest work on the design, implementations, principles, and uses of functional programming. The conference covers the entire spectrum of work, from practice to theory, including its peripheries. * Program: http://www.icfpconference.org/icfp2010/program.html * Invited speakers: - Mike Gordon ML: Metalanguage or Object Language? - Matthias Felleisen TeachScheme!: A Checkpoint - Guy Blelloch Functional Parallel Algorithms Schedule including related events: * September 25: Workshop on Mechanizing Metatheory (WMM) Workshop on High-Level Parallel Programming and Applications (HLPP) * September 26: Workshop on ML Workshop on Generic Programming (WGP) * September 27-29: ICFP 2010 * September 30: Haskell Symposium Erlang Workshop * October 1: Commercial Users of Functional Programming – Day 1 (CUFP Tutorials) Haskell Implementors' Workshop * October 2: Commercial Users of Functional Programming – Day 2 (CUFP Talks) Registration information: * Registration link: https://regmaster3.com/2010conf/ICFP10/register.php Local arrangements (including travel and accommodation): * http://www.icfpconference.org/icfp2010/local.html * Conference reservation/rate deadline: September 1st Conference organizers: * General Chair: Paul Hudak, Yale University * Program Chair: Stephanie Weirich, University of Pennsylvania * Local Arrangements Chair: Michael Hicks, University of Maryland * Workshop Co-Chairs: Derek Dreyer, Max Planck Institute for Software Systems Christopher Stone, Harvey Mudd College * Programming Contest Chair: Johannes Waldmann, Hochschule für Technik, Wirtschaft und Kultur, Leipzig * Video Chair: Scott Smith, Johns Hopkins University * Publicity Chair: Wouter Swierstra, Vector Fabrics =====================================================================

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