October 2010 - Haskell-Cafe

Re: [Haskell-cafe] Map constructor in a DSL
by Brandon Moore 26 Oct '10

26 Oct '10

You have to figure out here how you can use the function which is the argument to map. It seems you want to end up with a function of type a -> Evaluator b, to be used as an argument to mapM. One idea is to add a new constructor Var to represent variables, and something like evalVar :: Obs b -> a -> Evaluator b which handles Var by "return"ing the second argument, and everything else as in evalObs. Then you can evaluate Map like evalObs (Map f) as = evalObs as >>= mapM (evalVar (f Var)) Except, this won't quit work because the type for evalVar is wrong. That can be fixed by adding an extra parameter for the variable type: data Obs varType a where ... Var :: Obs v v Map :: (Obs a a -> Obs a b) -> Obs v [a] -> Obs v [b] or something like that. Brandon

1 0

problem with happy
by Roman Dzvinkovsky 26 Oct '10

26 Oct '10

Hi, I've tried to make simple monadic happy parser with monadic lexer, but even trivial parser (attached) outputs: Ok "\r\n" instead of string from 'sample' file. Seems I can't spot error by myself, so I need your help, guys. Thanks in advance, Roman.

2 4

Parsec in Haskell platform
by Roman Cheplyaka 26 Oct '10

26 Oct '10

Since Parsec 2 and Parsec 3 are usually considered to be different packages, it's ambiguous to say that 'parsec' is included in Haskell Platform [1]. Could someone please: 1. Clarify which version(s) are included? 2. Fix the page [1] It would be convenient to have a page which would list all the HP packages with their versions. The release page [2] only has a list of packages whose versions has changed since the last release, as I understood. [1] http://hackage.haskell.org/platform/contents.html [2] http://hackage.haskell.org/platform/changelog.html -- Roman I. Cheplyaka :: http://ro-che.info/ "Don't let school get in the way of your education." - Mark Twain

9 12

[ANNOUNCE] MissingPy 0.10.5
by Matt Brown 26 Oct '10

26 Oct '10

Hello all, I've recently taken over maintenance of MissingPy, and pushed version 0.10.5 to hackage[1]. This includes two notable improvements: - fix build errors for python >= 2.5 - allow calling into multi-threaded python code (requires python >= 2.3). I'm trying to provide support for as many python versions as possible, though I still have more testing to do. Help in this area would be most appreciated. Please report any bugs via my github repo [2]. Examples can be found in the testsrc directory. A good introduction by Tim Lopez is also available [3]. withGIL provides the easiest way to call into threaded python code. I haven't added any examples of its usage yet, but the gist of it is: > withGIL $ do <some python stuff> I'll try to add some actual examples soon. Thanks, -matt [1]: http://hackage.haskell.org/package/MissingPy [2]: http://github.com/softmechanics/missingpy/issues [3]: http://www.brool.com/index.php/using-missingpy

1 0

Functional Programming in Industry
by Jeroen Janssen 26 Oct '10

26 Oct '10

Dear all, I am pleased to announce to you that, on the occasion of BelHac, the first Belgian Haskell Hackathon from 5-7 Nov 2010, the Ghent Functional Programming Group is organizing a symposium on "Functional Programming in Industry", to which you are all cordially invited. These talks will be held on Friday, November 5th at 17:00 in the Jozef Plateauzaal in the Plateau building of Ghent University (Jozef Plateaustraat 22, 9000 Gent). The program is as follows: - Duncan Coutts* (Well-Typed, http://www.well-typed.com/, BelHac sponsor) who will be talking on how Well-Typed are providing consultancy services for Haskell, a pure functional programming language. - Romain Slootmaekers (Incubaid, http://www.incubaid.com, BelHac sponsor) who will be talking on the use of functional programming in Incubaid, a company based in the Ghent area. - Don Stewart (Galois, http://www.galois.com) who will be talking on how Galois is using Haskell in many of their projects, including government contracts. The talks will be followed by a reception to which all attendees are cordially invited. * Actual speaker has yet to be confirmed, but the talk will take place. Kind Regards, The GhentFPG Organizing Committee.

1 2

Working Generic/Polytypic Haskell extension
by mulhern 26 Oct '10

26 Oct '10

Hi all, I would like to teach a small section on polytypism/genericity in the "functional programming using Haskell" course I'm teaching. I won't, though, unless I can assign an actual programming exercise in polytypic programming, however brief. Can anybody recommend a functioning compiler that I can reasonably require the students to use in doing this assignment? I am not up-to-date on the current state of research in this topic in Haskell and hope that some people on the list can help me out. Thanks! - mulhern

2 1

Call for Contributions - Haskell Communities and Activities Report, November 2010 edition
by Janis Voigtländer 26 Oct '10

26 Oct '10

Dear all, It is time to collect contributions for the 19th edition of the ================================================================ Haskell Communities & Activities Report http://www.haskell.org/communities/ Submission deadline: 1 November 2010 (please send your contributions to hcar at haskell.org, in plain text or LaTeX format) ================================================================ This is the short story: * If you are working on any project that is in some way related to Haskell, please write a short entry and submit it. Even if the project is very small or unfinished or you think it is not important enough -- please reconsider and submit an entry anyway! * If you are interested in an existing project related to Haskell that has not previously been mentioned in the HCAR, please tell me, so that I can contact the project leaders and ask them to submit an entry. * Feel free to pass on this call for contributions to others that might be interested. More detailed information: The Haskell Communities & Activities Report is a bi-annual overview of the state of Haskell as well as Haskell-related projects over the last, and possibly the upcoming six months. If you have only recently been exposed to Haskell, it might be a good idea to browse the May 2010 edition -- you will find interesting topics described as well as several starting points and links that may provide answers to many questions. Contributions will be collected until the submission deadline. They will then be compiled into a coherent report that is published online as soon as it is ready. As always, this is a great opportunity to update your webpages, make new releases, announce or even start new projects, or to talk about developments you want every Haskeller to know about! Looking forward to your contributions, Janis (current editor) FAQ: Q: What format should I write in? A: The required format is a LaTeX source file, adhering to the template that is available at: http://haskell.org/communities/11-2010/template.tex There is also a LaTeX style file at http://haskell.org/communities/11-2010/hcar.sty that you can use to preview your entry. If you do not know LaTeX, then use plain text. If you modify an old entry that you have written for an earlier edition of the report, you should already have received your old entry as a template (provided I have your valid email address). Please modify that template, rather than using your own version of the old entry as a template. Q: Can I include images? A: Yes, you are even encouraged to do so. Please use .jpg format, then. Q: Should I send files in .zip archives or similar? A: No, plain file attachements are the way. If you have several entries to submit, it helps if you send them in separate emails. Q: How much should I write? A: Authors are asked to limit entries to about one column of text. This corresponds to approximately one page, or 40 lines of text, with the above style and template. A general introduction is helpful. Apart from that, you should focus on recent or upcoming developments. Pointers to online content can be given for more comprehensive or "historic" overviews of a project. Images do not count towards the length limit, so you may want to use this opportunity to pep up entries. There is no minimum length of an entry! The report aims for being as complete as possible, so please consider writing an entry, even if it is only a few lines long. Q: Which topics are relevant? A: All topics which are related to Haskell in some way are relevant. We usually had reports from users of Haskell (private, academic, or commercial), from authors or contributors to projects related to Haskell, from people working on the Haskell language, libraries, on language extensions or variants. We also like reports about distributions of Haskell software, Haskell infrastructure, books and tutorials on Haskell. Reports on past and upcoming events related to Haskell are also relevant. Finally, there might be new topics we do not even think about. As a rule of thumb: if in doubt, then it probably is relevant and has a place in the HCAR. You can also ask the editor. Q: Is unfinished work relevant? Are ideas for projects relevant? A: Yes! You can use the HCAR to talk about projects you are currently working on. You can use it to look for other developers that might help you. You can use it to write "wishlist" items for libraries and language features you would like to see implemented. Q: If I do not update my entry, but want to keep it in the report, what should I do? A: Tell the editor that there are no changes. The old entry will typically be reused in this case, but it might be dropped if it is older than a year, to give more room and more attention to projects that change a lot. Do not resend complete entries if you have not changed them.

1 1

Re: [Haskell-cafe] running and understanding a lifting program
by Patrick Browne 25 Oct '10

25 Oct '10

Patrick, Thanks for taking the time to get the program running. It seems fine, but I cannot get the *md* to print out, probably missing the Show class somewhere. Thanks again, Pat Patrick LeBoutillier wrote: > Patrick, > > I found this program interesting and decided to spend a bit of time on > it, even though I'm still a newbie. > I did a few things to simplify the code, here are some comments: > > 1) I chose to rename the arithmetic functions in the Number class > instead of trying to overload the "real" ones, I'm not that good at > Haskell yet... > > 2) The program had some errors, namely I think the definition of the > Point type should be: > > data Point a = Point a a > > to allow for different types of Points. > > 3) The Points class seemed useless in the end, I simply defined the > dist function at the top level. > > 4) If you import Control.Monad, it makes functions (and therefore > "Changing v") into a Monad (maybe my terminology is off here...) and > allows you to use the general liftM and liftM2 lifting functions > instead of defining your own. > > > Here's the complete program: > > {-# LANGUAGE TypeSynonymInstances, FlexibleInstances #-} > > data Point a = Point { x ::a, y :: a } > type Time = Float > > -- Functor Changing, which adds time parameter t to its input value. > -- For example, Changing Float indicates a changing floating number > -- (i.e. a function of time). > type Changing v = Time -> v > > -- Lifting functions > lift1 op a = \t -> op (a t) > lift2 op a b = \t -> op (a t) (b t) > > class Number a where > add, sub, mul :: a -> a -> a > square, squareRoot :: a -> a > square a = a `mul` a > > instance Number Float where > add = (+) > sub = (-) > mul = (*) > squareRoot = sqrt > > instance Number (Changing Float) where > add = lift2 add > sub = lift2 sub > mul = lift2 mul > squareRoot = lift1 squareRoot > > -- The distance operation is defined as follow > dist :: Number a => Point a -> Point a -> a > dist a b = squareRoot $ square((x a) `sub` (x b)) `add` square ((y a) > `sub` (y b)) > > -- Running the code > -- If p1 and p2 are two 2D static points, > -- their distance d is calculated as follows: > p1, p2 :: Point Float > p1 = Point 3.4 5.5 > p2 = Point 4.5 4.5 > > -- distance between p1 and p2 --> 1.55 > d = dist p1 p2 > > -- For 2D moving points mp1 and mp2, their distance md, > -- which is a function of time, is calculated as follows: > mp1, mp2 :: Point (Changing Float) > mp1 = Point (\t -> 4.0 + 0.5 * t) (\t -> 4.0 - 0.5 * t) > mp2 = Point (\t -> 0.0 + 1.0 * t) (\t -> 0.0 - 1.0 * t) > -- distance between mp1 and mp2 > md = dist mp1 mp2 > -- distance md for time 2 ----> 5.83 > > >> >> This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> > > Patrick > This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie

2 1

Re: [Haskell-cafe] Is it possible to "easily" connect Haskell to JavaScript/JavaFX in the browser and use a browser as a Windows GUI? :)
by caseyh＠istar.ca 25 Oct '10

25 Oct '10

Quoting Aleksandar Dimitrov <aleks.dimitrov(a)googlemail.com>: > Hi, > >> Is it possible to "easily" connect Haskell to JavaScript/JavaFX in >> the browser and use a browser as a Windows GUI? :) > > I don't exactly know what you mean by "connect," but here's an LtU > thread that might give you some hints: > http://lambda-the-ultimate.org/node/1836 > > Regards, > Aleks > I meant call Haskell functions from a browser; so the browser handles the GUI widgets and Haskell handles the processing. :)

6 11

type level program
by Patrick Browne 25 Oct '10

25 Oct '10

Hi, I hypothesize that at type level Haskell offers a form of equational logic. At type level the following program[1] could be interpreted as a first order program in equational logic where; 1)Data types represent declarations of constructors (both constants and functions) 2)Type synonyms represent equations assigning constructors to variables 3)Each class contains a non-constructor operation signature with dependencies 4) instances contain equations that define the operations (similar the term rewriting systems (TRS) of Maude/CafeOBJ). 5):t acts as a reduction or evaluation at type level Is the a reasonable description of this program? Regards, Pat [1] From Fritz Ruehr http://www.willamette.edu/~fruehr/haskell/evolution.html -- static Peano constructors and numerals data Zero data Succ n type One = Succ Zero type Two = Succ One type Three = Succ Two type Four = Succ Three -- dynamic representatives for static Peanos zero = undefined :: Zero one = undefined :: One two = undefined :: Two three = undefined :: Three four = undefined :: Four -- addition, a la Prolog class Add a b c | a b -> c where add :: a -> b -> c instance Add Zero b b instance Add a b c => Add (Succ a) b (Succ c) -- multiplication, a la Prolog class Mul a b c | a b -> c where mul :: a -> b -> c instance Mul Zero b Zero instance (Mul a b c, Add b c d) => Mul (Succ a) b d -- factorial, a la Prolog class Fac a b | a -> b where fac :: a -> b instance Fac Zero One instance (Fac n k, Mul (Succ n) k m) => Fac (Succ n) m -- try, for "instance" (sorry): -- -- :t fac four This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie

2 1