Question

I’m writing an XSD for a rather complex XML file format. At many points, the XML can either contain an actual literal value or a script yielding the value. (Think Excel: 42 vs. =21+21)

For example, to specify a size, it should be valid to write either of the following:

<size>42</size>
<size><script>2*21</script></size>

My first thought was using <choice> to allow either an int or a script within the size element. However, then I would have to give the string element a name, leading to a clumsy syntax like <size><value>42</value></size>. Since literals will be used far more often than scripts, I don’t want to bloat their syntax.

Any ideas how to express the initial syntax in XSD?

Answer 1

I think there are two decent options.

1. Don’t put any subelements inside <size> in either case. Instead, you could set a ‘type’ attribute on your size element, indicating to a consumer whether it’s a literal or script.
2. Use one of the two techniques described here (both based on using the ‘mixed=true’ attribute on complexContent: XML Schema: Element that can contain elements or text?

To choose between these options, you would put yourself in the shoes of the consumer app, and decide how it would most likely want to get this data. In general, I think option one is best if the primary consumer is a developer parsing the XML as a one-off integration task (because both the output and the XSD are more terse, and in my opinion more readable); option two is probably better if you see a XML tool consuming your schema and therefore it may have an easier time doing something special if there are concrete elements to parse out. Ultimately though I’m just guessing what your use-case is…