April 2007 - Haskell-Cafe

HXT namespace problem
by Martin Huschenbett 25 Apr '07

25 Apr '07

Hi all, I'm currently trying to generate XML documents with HXT. Everything went well but I can't figure out how to generate the "xmlns:..." attributes for the namespaces. My code looks like: runX $ constA (request TableListRequest) >>> root [] [writeA] >>> writeDocument [(a_indent,v_1)] "-" where writeA generates the actual content. This code generates something like: <?xml version="1.0" encoding="UTF-8"?> <soapenv:Envelope> <soapenv:Body> <request:tablelist/> </soapenv:Body> </soapenv:Envelope> But I want it with XML namespaces, i.e.: <?xml version="1.0" encoding="UTF-8"?> <soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:request="http://www.officematic.de/domas/request" > <soapenv:Body> <request:tablelist/> </soapenv:Body> </soapenv:Envelope> I tried using "uniqueNamespaces" and "uniqueNamespacesFromDeclAndQNames" but neither worked. Maybe I used them in the wrong part of code. The tree representation my document looks like: ---XTag "/" | +---XTag "{http://schemas.xmlsoap.org/soap/envelope/}soapenv:Envelope" | +---XTag "{http://schemas.xmlsoap.org/soap/envelope/}soapenv:Body" | +---XTag "{http://www.officematic.de/domas/request}request:tablelist" and for me this looks like if there is enough namespace information provided. I would appreciate any help, Martin.

2 2

Why Perl is more learnable than Haskell
by kynn 25 Apr '07

25 Apr '07

Perl is a large, ugly, messy language filled with quirks and eccentricities, while Haskell is an extremely elegant language whose design is guided by a few overriding ideas. (Or so I'm told.) Based on this one would think that it would be much easier to learn Haskell than to learn Perl, but my experience is exactly the opposite. I've been wanting to learn Haskell for years, literally, but it has been a case of Sisyphus and the Rock. Despite my efforts, I never get to the level of expertise that would make Haskell useful to me. (I don't need elegant factorial or Fibonacci functions in my everyday work.) Sooner or later life intervenes: big project due, long trip abroad, etc., and when I finally return to learning Haskell, I have forgotten almost everything I learned and I have to start all over again. (BTW, I've heard similar stories from many wannabe Haskell programmers.) Arguably, this experience means that I have no business learning Haskell, because it's just not relevant to my work. Maybe so, but I still cling to the fanciful notion that if I knew Haskell well enough, I would find plenty of stuff to do with it in my daily work... Anyway, in contrast to my struggle with Haskell, I learned Perl incrementally over the years, by using it in daily little projects, ranging at first from command-line snippets to 100-line self-contained scripts, and moving on to larger, hairier projects. This daily reinforcement of the little bits of Perl I was picking up was crucial to my being able to retain it and move forward. Perhaps Haskell will never lend itself to something like a Perl one-liner, but still I wish that there were books on Haskell that focused on making Haskell useful to the learner as quickly as possible... If such already exist and I've missed it, please let me know. Or I can always wait until I retire; then I'll probably have a sufficiently long stretch of free time in my hands (barring any operations, strokes, heart attacks, hip fractures, etc.). I bet I could start a Haskell Wannabes Club at the nursing home... kj -- View this message in context: http://www.nabble.com/Why-Perl-is-more-learnable-than-Haskell-tf3559193.htm… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

48 77

Re: [Haskell] ANN: IOSpec 0.1
by Wouter Swierstra 25 Apr '07

25 Apr '07

Hi Stefan, > in the Teletype module, it would probably be better to reflect > IOTeletype operations as: > > data Action a = Finish a | Put Char (Action a) | Get (Char -> > Action a) > > That way, there is no need to use (unobservable) laziness. I'm afraid I've lost you. How does this differ from the current definition of IOTeletype? Thanks, Wouter This message has been checked for viruses but the contents of an attachment may still contain software viruses, which could damage your computer system: you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

1 0

Re: [Haskell] ANN: IOSpec 0.1
by Wouter Swierstra 25 Apr '07

25 Apr '07

Hi Twan, > Wouter Swierstra wrote: >> Test.IOSpec Version 1.0 > Shouldn't that be 0.1? You're right of course. > You use Dynamic for the data type in IORefs, this has the > unfortunate consequence of needing Typeable constraints. You could > try to use unsafeCoerce instead, > > type Data = () > > unsafeToData :: a -> Data > > unsafeToData = unsafeCoerce > > unsafeFromData :: Data -> a > > unsafeFromData = unsafeCoerce > > I think this should be just as safe as Dynamic, since internally > Dynamic uses unsafeCoerce as well. Just steal the details from there. I'm not entirely happy with the "unsafeFromDynamic" approach I use at the moment. GHC can derive instances of Typeable, so the price you have to pay is not too high. The real problem is that Haskell's type system is not expressive enough to cleanly describe the type of the heap (I know, I could be using HList, but still...). If you're interested, I also have a dependently typed version of the code for IORefs that allows heterogeneous, type-safe, well-scoped pointers. I should probably mention that an ulterior motive for this work is integrating IO in dependently typed languages such as Epigram. It is all, of course, still very much subject to active research. Wouter This message has been checked for viruses but the contents of an attachment may still contain software viruses, which could damage your computer system: you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

1 0

Re: [Haskell] ANN: IOSpec 0.1
by Wouter Swierstra 25 Apr '07

25 Apr '07

Hi Isaac, <Moving to Haskell-cafe> > How does or might this relate to the typeclass-based Restricted IO > monad > http://article.gmane.org/gmane.comp.lang.haskell.general/14526 > ? RIO's technique has the advantage that multiple capabilities can be > easily combined in one use of the monad - but this manner of using > typeclasses might interfere with how testable it is? (I think that > should be okay... Also that this is a really neat project - > incremental > progress on taming Haskell IO in practice!) Stefan is absolutely right. The RIO, as far as I remember, restricts which parts of the IO monad you use. The same could be said for the Test.IOSpec modules. However, the Test.IOSpec also give you a pure specification of the functions you are using. This is very important if you want to trace/debug an execution of you code - you can see how the state of your machine changes at every step. There's also a bit finer control over how you test your functions: concurrent code is parameterised over the scheduler, for instance. Both these aren't possible with the RIO approach. > P.S. why >> *If you're an ICFP referee, you may want to avoid reading any >> further* ICFP has a double-blind review process: if I were to submit a paper, I should do so anonymously. The idea behind it all is that if the reviewers don't know who the author of a paper is, they can't be prejudiced. The code I just released is described in a paper submitted to ICFP. All the best, Wouter This message has been checked for viruses but the contents of an attachment may still contain software viruses, which could damage your computer system: you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

1 0

ICFP Programming Contest 2007
by Johan Jeuring 25 Apr '07

25 Apr '07

Want to show off your programming skills? Your favorite programming language? Your best programming tools? Join the ICFP Programming Contest 2007! The 10th ICFP Programming Contest celebrates a decade of contests. This is one of the world's most advanced and prestiguous programming contest you can enter. For free! Book July 20 - 23, 2007. Check out http://www.icfpcontest.org/.

1 0

Is there a best *nix or BSD distro for Haskell hacking?
by David Cabana 24 Apr '07

24 Apr '07

I have a spare Windows machine I want to put to better use. I want to turn it into a Haskell hacking box, and was wondering whether any particular *nix or BSD distribution is best (or worst) suited for this. Any thoughts? Thank you, David Cabana

13 14

About functional programing, type theory and a master thesis topic
by Glauber Cabral 24 Apr '07

24 Apr '07

Hi everybody =) First time I write to the list. My name is Glauber and I'm doing my master course at UNICAMP, Brazil, under supervisor of Prof. Dr. Arnaldo Vieira Moura. I'm interested in Haskell, type theory and algebraic specification (formal methods). I've been studying these subjects to my Computer-Science-Bachelor-degree-final work. It was an implementation of an homomorphism between a little constructive algebraic specification code (based mainly in Loeckx's Specification of Abstract Data Types) and a Haskell code. SableCC was used as tool to implement the interpreter because I've already used it before. One theme to work one should be implementing this in Haskell itself, but there is already HasCASL. I've already contacted the HasCASL professors and I'm reading more about the project. At the same time, I want to look for some theme under type theory, too. The main concern about HasCASL is that I want to get in deep touch with functional programming during my master thesis and I'm not sure (yet) if studying HasCASL would get me there. I've reading some papers about type theory and Haskell (mainly the ones suggested in the haskell.org site) and as I could see the topics are very interesting. I'm not sure, however, if there is something that can be done in a 2-year-master course. Mainly, they propose new extensions that have became new languages (Cayenne, Omega and Epigram). Other topic I'd like to get in touch is theorem proving. As I could see, Coq and Isabelle are the most used with Haskell. Indeed, HasCASL use Isabelle and this could be a nice thing to work on, as was suggested by HasCASL professors. As I know that lots of you here are researchers, I'd like to ask some opinions about what can be done as a master thesis having in mind that I want to continue in a PhD course after the master one. Sorry if the email is too large. And thank for the patience and for any opinion you can give me. Yours, Glauber.

2 4

Newbie seeking advice regarding data structure for a tricky algorithm
by Toby Hutton 24 Apr '07

24 Apr '07

Hi, I'm trying to implement a fast kd-tree in Haskell. http://en.wikipedia.org/wiki/Kd-tree It's a binary tree representing space partitions. One of the fastest ways to build kd-trees is outlined in this paper: http://www.cgg.cvut.cz/~havran/ARTICLES/ingo06rtKdtree.pdf (I'm trying to implement Algorithm 5 on page 5 and the steps outlined in section 4.3.1.) I'll try to outline a simplified analogous algorithm to demonstrate my problem. It's all about classifying which children should go in a left or right branch using heuristics. Say I want to put the words 'foo', 'bar' and 'baz' into a binary tree. The heuristic requires I split the words into letters and sort them: 'aabbfoorz'. The heuristic then may decide, based on the sorted letters, that 'bar' and 'foo' should go in the left child and 'baz' goes in the right. Typically we'd then simply recurse and for example, the left child's words would be re-sorted into 'abfoor' and the heuristic is reapplied. If we assume that sorting is relatively expensive, we can avoid the re-sort for the children by unmerging the parent's sorted list of letters. Two sublists of a sorted list should already be sorted. If we know which word each letter belongs to it would be more efficient to tag the letters with 'left' or 'right' as the words are classified. Then we can iterate down the sorted letter list and produce new sorted sublists rather simply. So it's not actually that complicated, and I can imagine exactly how it could be done in C but I really don't know how to approach this in Haskell. The problem I'm having is how to keep a map between the words and its letters (which in the real problem is a map between a list of vectors and 6 tuples containing Doubles and enumerations) while keeping in mind you can't just map any letter to a word, but specific letters. e.g., the 'b's in the example must remember specifically whether they belong to 'bar' or 'baz'. Thanks for any insights, Toby.

3 2

Haskell version of Norvig's Python Spelling Corrector
by Pete Kazmier 24 Apr '07

24 Apr '07

Recently I read an interesting article by Peter Norvig[1] on how to write a spelling corrector in 21-lines of Python. I wanted to try and implement it in Haskell. My implementation is terribly slow and I was hoping for some tips on how to improve it and make it idiomatic. I'd love to see other Haskell implementations as well if anyone has a few moments to spare. Admittedly, it took me several hours to get my version working, but I'm a Haskell newbie. Unfortunately, I think it runs as slow as it took me to write it! There is definitely something wrong with it, a memory leak, because I can't correct more than a few words without a great deal of memory being consumed. Thanks, Pete [1] http://norvig.com/spell-correct.html

10 21