Saxon

For both XQuery and XSLT, we keep information about the context item requirement in a GlobalContextRequirement object (whose fields are not especially well documented or clearly named). The fields (with their apparent meaning) are:

requiredItemTypes (XQuery allows more than one, in different modules)
defaultValue (XQuery only)
mayBeOmitted
absentFocus

In XSLT, use=required sets mayBeOmitted=false, absentFocus=false. use=optional sets mayBeOmitted=true, absentFocus=false. use=absent sets mayBeOmitted=true, absentFocus=true.

In XQuery, it seems there is a combination not possible in XSLT: there is a context item, but it cannot be specified externally. I'm not sure we can actually represent that combination in the GlobalContextRequirement object currently.

The XQuery spec isn't very clear what happens if you specify "declare context item external" with no default value, and no value is supplied externally. The analogy with external variables suggests that it's an error. So the possible options seem to be:

• := value --> there will always be a context item, the value cannot be supplied externally
• external := value --> there will always be a context item, if none is supplied externally then the default is used
• external --> there will always be a context item, it must be supplied externally.

This turns out not to be a very good correspondence to the XSLT options:

• req --> there will always be a context item, it must be supplied externally.
• opt --> there may or may not be a context item, if there is then it will be supplied externally
• pro --> there will be no context item

The options in the two languages are so different it's probably a mistake to try and fit them into the same mould.

A progress report on this: I attempted a rewrite of GlobalContextRequirement to refactor the different options and make them consistent. But it caused a surprising number of problems. (A lot of difficulty with load-xquery-module(), for example.) So for the time being I've rolled the changes back. Will focus now on getting 9.9 out, and come back to this later with a less radical approach to the solution.