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Started playing with nxml-mode, which makes editing XML much nicer in emacs (psgml-1.3 does an okay job, but the indenter and tag closer sometimes get confused by empty elements). There is a nice article about nxml-mode on xmlhack which gives an introduction to the mode.

The first thing that struck me about nxml in comparison to psgml was the lack of syntax highlighting. It turned out that the reason for this was that colours were only specified for the light background case, and I was using a dark background. After setting the colours appropriately (customise faces matching the regexp ^nxml-), I could see that the highlighting was a lot better than what psgml did.

One of the big differences between nxml and psgml is that it uses RELAX-NG schemas rather than DTDs. It comes with schemas for most of the common formats I want to edit (xhtml, docbook, etc), but I also wanted to edit documents in a few custom formats (the module description files I use for jhbuild being a big one).

Writing RELAX-NG schemas in the compact syntax is very easy to do (the tutorial helps a lot). I especially like the interleave feature, since it makes certain constraints much easier to express (in a lot of cases, your code doesn’t care what order the child elements occur in, as long as particular ones appear). While it is possible to express the same constraint without the interleave operator, you end up with a combinatorial explosion (I guess that’s why XML Schema people don’t like RELAX-NG people making use of it). For example, A & B & C would need to be expressed as:

(A, B, C) | (A, C, B) | (B, A, C) | (B, C, A) | (C, A, B) | (C, B, A)

(for n interleaved items, you’d end up with n! groups in the resulting pattern).

After writing a schema, it was a simple matter of dropping a schemas.xml file in the same directory as my XML documents to associate the schema with the documents. This is required because RELAX-NG doesn’t specify a way to associate a schema with a document, so nxml has its own method. Matching rules can be based on file extensions, document element names, XML namespaces or public IDs, but I used the document element name for simplicity. You can specify other locations for schema locator rules, but putting it in the same directory is the easiest with multiple developers.

Once that is done, you get background revalidation of the document, and highlighting of invalid portions of the document (something that psgml doesn’t seem to be able to do). It also says whether the document is valid or not in the modeline, which is helpful when editing documents.

Now all we need is for libxml2 to be able to parse RELAX-NG compact syntax schemas …