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Re: [Haskell-cafe] Use Haskell to extract GXL representation
by Santoemma Enrico 20 Sep '05

20 Sep '05

Sara, I've used extensively HaXml and the tool DtdToHaskell to do xml processing. HaXml marshalls/demarshalls xml data and DtdToHaskell creates the "data" statements to handle xml in haskell fashion. I learnt how to write a palatable DTD for the delicate DtdToHaskell, but in your case I guess you're not the dtd's author. It's very simple, once everything is setup: import MYDTD myXml :: String -> IO MYDTD myXml fileName = return $ fReadXml fileName where MYDTD.hs is the output of: $ DtdToHaskell MYDTD.dtd MYDTD.hs Feel free to ask if you'll have troubles with DtdToHaskell. I remember I fought a lot :) Salus, Enrico > -----Messaggio originale----- > Da: haskell-cafe-bounces(a)haskell.org > [mailto:haskell-cafe-bounces@haskell.org]Per conto di Sara Kenedy > Inviato: sabato 17 settembre 2005 3.11 > A: Haskell-Cafe(a)haskell.org > Oggetto: [Haskell-cafe] Use Haskell to extract GXL representation > > > Dear you all, > > > Currently, I am working with Haskell and GXL file (Graph > eXchange Language). GXL > is a sublanguage of XML and its syntax is based on XML DTD. > > 1) In my work, I use GXL representation to represent a quantification > (e.g. forall(x:Z|x = 3 and x^2 - 3x + 2 =0)) > > 2) My objective is to write a Haskell module to extract the content of > the GXL file such that a prover theorem (e.g, ICS) and a computer > algebra system (e.g, Matlab) can read the content of GXL file. > > If any of you have experiences or any ideas about the problem, please > share with me. > > I really appreciate for that.Thanks a lot. > > Sara. > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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a new Monad
by Malcolm Wallace 20 Sep '05

20 Sep '05

Microsoft has announced the following: Developers can also expect a new scripting language for management applications, called Monad. Monad is an object-oriented language based on .NET, and provides command-line based management while enabling management services to be passed between different commands. http://www.theregister.co.uk/2005/09/16/microsoft_longhorn_server/ What a strange choice of name for a language...! :-) Malcolm

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RE: [Haskell-cafe] Template Haskell and Types
by Simon Peyton-Jones 20 Sep '05

20 Sep '05

| > putStrLn $(mysplice ''MyData) | > | | Thanks for responses. Is there any up-to-date documentation avaliable? Template Haskell is, alas, poorly documented. I would really welcome someone to volunteer to help write better documentation. Meanwhile, as the user manual says, the stuff about quoting names is described in a design note http://research.microsoft.com/~simonpj/tmp/notes2.ps Simon

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TH Q Monad and fail
by Gracjan Polak 20 Sep '05

20 Sep '05

Hi all, The Q Monad in template haskell has fail method. As I understand it, it throws some kind of exception. How do I catch this exception? Some code I'm trying to create: infoToCode :: Info -> Q Exp infoToCode (ClassI dec) = -- ClassI Dec fail "ClassI not supported" -- this will be implemented infoToCode (ClassOpI name xtype name2 fixity) = -- ClassOpI Name Type Name Fixity fail "ClassOpI not supported" infoToCode (TyConI dec) = -- TyConI Dec fail "TyConI not supported" infoToCode (PrimTyConI name int bool) = -- PrimTyConI Name Int Bool fail "PrimTyConI not supported" infoToCode (DataConI name xtype name2 fixity) = -- DataConI Name Type Name Fixity fail "DataConI not supported" infoToCode (VarI name xtype maybedec fixity) = -- VarI Name Type (Maybe Dec) Fixity fail "TVarI not supported" infoToCode (TyVarI name xtype) = -- TyVarI Name Type fail "TyVarI not supported" nameToCode1 :: Name -> Q Exp nameToCode1 name = do info <- reify name Code <- infoToCode info runIO $ putStrLn $ pprint Code return Code -- Here be dragons... nameToCode name = nameToCode1 name `catch` c where c e = do runIO $ putStrLn $ show e fail "refailed" -- Gracjan

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Dread __DISCARD__
by Steven Elkins 20 Sep '05

20 Sep '05

Hello everyone, I'm a Haskell newbie trying out various programs from the web. I'm trying to compile one called bjpop-ray (from Bernie Pope, I think) and I hit this at link-time: ghc --make -O0 -package wx Main.hs -o bjpop-ray Chasing modules from: Main.hs Skipping Data ( ./Data.hs, ./Data.o ) Skipping Sphere ( ./Sphere.hs, ./Sphere.o ) Skipping Parser ( ./Parser.hs, ./Parser.o ) Skipping Vector ( ./Vector.hs, ./Vector.o ) Skipping Polygon ( ./Polygon.hs, ./Polygon.o ) Skipping RayTrace ( ./RayTrace.hs, ./RayTrace.o ) Skipping Main ( Main.hs, Main.o ) Linking ... /usr/bin/ld: Undefined symbols: ___DISCARD__ collect2: ld returned 1 exit status I'm on Mac OS 10.4.2, using ghc 6.4 (from the haskell.org .dmg) and gcc 4.0.0. Other wxHaskell programs (the samples and my own experiments) compile without tripping over this. Googling __DISCARD__ led me to "...the binary on the ghc web site has the evil __DISCARD__ mangler problem..." on the darwinports bug mailing list, so I guess I'm not the first person to see this. Is there a workaround? Does the Darwin port have the same problem? Should I just work on the many other things I've yet to grasp? Usually ghc works fine. Thanks, Steve -- How wonderful it is that nobody need wait a single moment before starting to improve the world. -- Anne Frank Paradise is exactly where you are right now...only much, much better. -- Laurie Anderson

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RE: [Haskell-cafe] How to debug GHC
by Simon Marlow 19 Sep '05

19 Sep '05

On 10 September 2005 21:15, Frederik Eaton wrote: > On Fri, Sep 02, 2005 at 04:40:05PM +0400, Bulat Ziganshin wrote: >> Hello Nils, >> >> Friday, September 02, 2005, 10:47:05 AM, you wrote: >> >>> Compile your program with -prof -auto-all (make sure you have >>> the >> >>> I tried this out under GHC 6.4/Linux and got a segmentation fault >>> instead of a stack trace. Under GHC 6.2.2 it seemed to work, though. >> >> this error is already fixed in current pre-6.4.1 version > > I'm using a 2005/9/3 version of 6.4.1 and running into situations > where the "stack trace" has function A calling function B, where when > I look at the code, A never calls B. Is this normal? Is it some > side-effect of laziness? It sure makes the traces a lot less useful. It could be a bug - can you reduce the example and report it? GHC's profiler tries to overlay a lexical call graph on to the dynamic execution of the program. It does this more or less in the way you described before: every function gets an extra argument describing the call context. However, there are some tricky areas: notably CAFs. We don't as yet have a principled description of the mechanism, and I know of various cases where odd results are obtained with the current system. Also, the optimiser has to be careful not to change the shape of the call graph, and I suspect there are cases where it goes wrong. Cheers, Simon

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Re: [Haskell-cafe] Newbie syntax question
by Marc Ziegert 19 Sep '05

19 Sep '05

> > map (foo 5) my_list_of_lists_of_doubles > 1. map (flip foo 5) my_list_of_lists_of_doubles > 2. map (`foo` 5) my_list_of_lists_of_doubles > 3. map (\x->foo x 5) my_list_of_lists_of_doubles > 4. [foo x 5 | x <- my_list_of_lists_of_doubles] well, i've followed this discussion a while, but i did not see that solution i used for years. (i like solution 2, never thought of it before.) my solution is a variant of 1, that makes the reading a little bit easier -- at least to me. 5. map (foo `flip` 5) my_list_of_lists_of_doubles the `flip` shows the position behind foo, where to put the parameter. its a pitty that it only works if there is only one parameter behind `flip`. so, with abcd :: a->b->c->d->x abcd a b `flip` d :: c->x works, but abcd a `flip` c d :: b->x does not. experimenting with Data.Arrow, it could look like: (abcd a >>> ($ c) >>> ($ d)) :: b->x or with a flip-arrow combination: (abcd a `flip` c) >>> ($ d) :: b->x so, instead of hacking with arrows on it, i prefer solutions 3 and 4 whenever 5 does not work. - marc

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R: [Haskell-cafe] How to get all the paths of a tree
by Santoemma Enrico 19 Sep '05

19 Sep '05

Yeah! I've been struggling on concatMap for a noon and now here is the solution I've been unable to find. Thank you very much Enrico > -----Messaggio originale----- > Da: J. Garrett Morris [mailto:trevion@gmail.com] > Inviato: lunedì 19 settembre 2005 15.26 > A: Santoemma Enrico > Cc: Haskell-Cafe (E-mail) > Oggetto: Re: [Haskell-cafe] How to get all the paths of a tree > > > In this case, I think all you need is: > > > type Path = [String] > > > allPaths :: Path -> Tree -> [Path] > > allPaths p (Node s []) = [s:p] > > allPaths p (Node s subTrees) = concatMap (allPaths (s:p)) subTrees > > > paths = map reverse . allPaths [] > > In your test case, "root" will be at the beginning of each path > produced, but you can map tail over the list if you don't want it. > > /g >

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Re: [Haskell] Re: [Haskell-cafe] Haskell versus Lisp
by Wolfgang Jeltsch 19 Sep '05

19 Sep '05

Am Freitag, 16. September 2005 18:40 schrieben Sie: > Wolfgang Jeltsch wrote: > > > > > Bearing this in mind, and hoping you can see where I'm coming from, > > > > > I think my question is: shouldn't you guys be using Lisp? > > > > > > > > Lisp is impure, weakly typed and has way too many parentheses. Why > > > > would we use lisp? It seems to be lacking almost all the advantages > > > > of Haskell, and have an ugly, inflexible syntax to boot. > > > > > > The ability to dynamically generate, manipulate and analyse code in a > > > structured manner provides a flexibility which is unmatched by any > > > other language I know of. > > > > > > A good example is Emacs; lisp is entirely the right language for that, > > > IMHO. > > > > Could you explain this a bit more, please? To the moment, I cannot > > imagine cases where you need LISP's way of code analysis and manipulation > > because Haskell's capabilities are not sufficient. > > > > In Haskell, code is data too because code in the sense of imperative > > actions is described by IO values. You cannot analyse them. > > And thus they are not data. They are some kind of opaque data. > > But you can use your do > > expressions etc. to construct action descriptions with a more general > > type like MonadIO m => m a. Then you can instantiate m with a monad > > whose values store part of the action's structure so that this > > information can be used later. Or you use a monad which doesn't keep > > structural information to use it for later processing but which does the > > processing upon construction. > > Yeah, but this is heading in the direction of Greenspun's Tenth Rule > of Programming: > > Any sufficiently complicated C or Fortran program contains an > ad hoc informally-specified bug-ridden slow implementation of > half of Common Lisp." > > You could easily end up doing the same thing in Haskell. I'm not sure. Haskell is much more powerful than C, for instance concerning the type system. The point of the concept I described above is that the general MonadIO-based description of actions is very similar to what you would normally write when using IO directly. I doubt that you can do something similar with C. > [...] > Or, to put it another way: if Haskell is so flexible, why do we need > Template Haskell? I can't imagine a "Template Lisp"; it would just be > Lisp. Haskell alone is not so flexible but Haskell with Template Haskell is. The difference is that the Haskell/Template Haskell combination separates compile-time evaluation/code generation clearly from runtime evaluation which is, in my opinion, a very good thing. Best wishes, Wolfgang

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How to get all the paths of a tree
by Santoemma Enrico 19 Sep '05

19 Sep '05

Hello, I can't find the right recursive function to get a list of all the possible paths of a multi-way tree like this: data Tree= Node String [Tree] root = Node "root" [x, y] x = Node "x" [x1, x2] x1 = Node "x1" [] x2 = Node "x2" [x21, x22] x21 = Node "x21" [] x22 = Node "x22" [] y = Node "y" [] This is the Data.Tree.drawTree output: root | +- x | | | +- x1 | | | `- x2 | | | +- x21 | | | `- x22 | `- y And this is the list I'm trying to obtain: [ [x, x1] ,[x, x2, x21] ,[x, x2, x22] ,[y] ] Well, the actual problem I'm trying to solve is to print a set of lines like these: "leaf: x -> x1 \n" "leaf: x -> x2 -> x21 \n" ... and I thought I first need the list above. I'll be very grateful for any help Enrico

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