Saxon

Note also XPathSelector and XQueryEvaluator both have the capability to return an IEnumerator (should be IEnumerator<XdmItem> perhaps?), rather than XdmEnumerator. I suggest we move towards returning XdmEnumerator, while accepting IEnumerator<XdmItem>. But we should probably review more carefully all the usages of these classes.

This whole area looks very messy. I've spent some time looking at it, and I'm inclined to do the following:

(a) Wind back some of the changes we made in 9.9 to generify things. That is, get rid of the generified IXdmEnumerator<XdmItem>, as well as IXdmEnumerable. Make XdmValue.GetEnumerator() return non-generic IEnumerator as it did in 9.8. Having XdmValue.GetEnumerator() return IXdmEnumerator<XdmItem> rather than the non-generic IXdmEnumerator doesn't add value, given that we're not overriding it with methods that return a more specific type, e.g. XdmAtomicValue.GetEnumerator() isn't declared to return IXdmEnumerator<XdmAtomicValue>.

(b) In 10.0, get rid of IXdmEnumerator entirely. It's only used in ExtensionFunctionCall. Have its implementing classes implement IEnumerator<XdmItem> instead; and make GetEnumerator() on subtypes return a more specific type, e.g. IEnumerator<XdmAtomicValue>. Change the ExtensionFunctionCall.Call() method to accept and return IEnumerable<XdmItem>.

(c) For 9.9 as well as 10.0, ensure that an implementation of ExtensionFunctionCall.Call() can construct its return value using, for example, new XdmAtomicValue(123).GetEnumerator(). To do this, relax the return type of the method if necessary.

We've decided to clean this up, at the cost of taking a bit of a hit in terms of backwards compatibility.

The classes IXdmEnumerator and IXdmEnumerable disappear; in their place we use IEnumerator<XdmItem> and IEnumerable<XdmItem>.

Any code that implements ExtensionFunctionCall will need to change; in most cases it's just a matter of changing IXdmEnumerator to IEnumerator<XdmItem> in both the arguments and the result type of the Call() method.

Also affected is any 9.8 code that explicitly casts the results of (for example) XPathSelector.GetEnumerator() to IXdmEnumerator (as was done in some of the sample applications, which have now been rewritten).