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8.3 Using DTDs

What I didn't appreciate in my first XML project (this one) was how much error checking I was doing just to verify the format of incoming XML. During testing I'd go looking for attributes or elements that should have been there but for various reasons were not. Because I was coding fast and furious I overlooked some and ignored others. Testing quickly ferreted out my carelessnes and my application started throwing exceptions faster than election officials throw chads.

The cure, at least for formatting, is having a DTD, or Document Type Definition describing the XML format. You can read more about the syntax of DTDs in the XML specification.

There's not a lot programmers are able to do with DTDs in VisualWorks, except requiring incoming XML to include DOCTYPE statements. There is something programmers need to do to handle the exceptions the XML parser throws when it finds errors.

I'm not an expert at writing Smalltalk exception handling code, and I haven't decided on what those exceptions should look like to the client who sent the poorly formatted XML in the first place. The code below does a decent job of catching the errors and putting the description of the error into an XML response. It's also a fairly decent example of XML document building as discussed earlier.

     replyDoc := XML Document new.
     replyDoc addNode: (XML Element tag: 'response').
         doc := XML XMLParser processDocumentString: (anIsdMessage message copyWithout: 0) asString
     ] on: Exception do: [ :ex |
         replyDoc root
             addAttribute: (XML Attribute name: 'type' value: 'Exception');
             addNode: ((XML Element tag: 'description')
                 addNode: (XML Text text: ex signal description));
             addNode: ((XML Element tag: 'message')
                 addNode: (XML Text text: ex messageText))

I said before there's not a lot programmers can do with DTDs, but there are some things I wish VW's XML library would do: