Re: Xml Schema and deriving Haskell2Xml, was Re: haxr - marshalling of Variants in Records

Hi, As Shae says, HAIFA can do XML Schema -> Haskell Mapping (if you can call it Haskell); well at least it could, I've since done some refactoring on the XML Serializer class so it won't work out of box, but should work with some tweaking. I stopped working on it because in order to map XML Schema properly I needed to break Haskell's type-system in just about every way possible and the code was quite horrible. In order to represent Sequences and Choices (the staple part of XML Schema Complex-Types), you need HList and Union types, since structures within a single complex-type can be quite ... err ... complex. As a result the whole thing is undecidable and context stack overflows are not uncommon. Nevertheless - it does work; it takes an XML Schema and produces a Haskell module called things like Org.W3.N2001.XMLSchema with serializers, but the data-types produced are pretty horrible and undecidable - see the attachment generated from Google Search. Fortunately as Google Search doesn't use either Sequences(/Interleaving) or Union types in awkward places the types aren't too bad in this case. The other problem you're likely to run up against is a lack of automatic mutual module recursion support for Haskell, assuming you want to map each schema namespace to a single module - there exists quite a lot of examples of mutually recursive schemas. Certainly I haven't even attempted to work round this problem yet. If I get round to it, I'll try and get XML Schema serialization and type-mapping work again; although it's pretty non-trivial to use. There aren't any source distributions of HAIFA atm, though I can easily bring the darcs repositories back online. To be honest though, it may be better to do the type-mapping outside of Haskell (anathema I know, but other languages do already have full schema support) - this project seems to have made a reasonable start: http://www.cin.ufpe.br/~haskell/hwsproxygen/, although the mapping they do seems to be quite primitive at the moment, and even if the data-types were created outside of Haskell, you'd still have to work out how to represent Unions/embedded Sequences effectively. Oh, and so far as XML Serializer derivation goes, HAIFA can do that quite nicely with a number of xmlify* TH functions. Although I know other libraries have this too and may do it better. Thanks, -Si. On Mon, 2005-12-05 at 20:43 +0100, Shae Matijs Erisson wrote:

Johannes Waldmann writes:

...

If the Haskell app is the defining side, I'd love to just "derive Haskell2Xml" for my data, and then I'd want a "haskell2scheme" (not: DTD) so that the Java side can use a Schema->Java compiler (like xjc).

On the other hand if I have to use a given XML schema, I want some "xhc" tool (Schema->Haskell).

Perhaps some of these projects are already out there?

See the new XmlContent type/module in HaXml 1.14: http://nhc98.blogspot.com/2005/10/unifying-haskell2xml-and-xml2haskell.html

Relax NG was prototyped in Haskell, not sure if that helps any.

HAIFA does some sort of Xml Schema work. There's an HAIFA article in the first issue of The Monad.Reader: http://www.haskell.org/hawiki/TheMonadReader/IssueOne -- Shae Matijs Erisson - http://www.ScannedInAvian.com/ - Sockmonster once said: You could switch out the unicycles for badgers, and the game would be the same.

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