Right now it's a trait and needs to be boxed. However, I don't expect that we will need to add future axes, and most structs are empty (and could probably just make it on the stack at run time, otherwise.

• Could make the parser tests more focused by asserting that a specific expression is found
• Could be used for printing the expression back as XPath
• Could be eventually used for optimization?

When we started, many things were public that didn't need to be. Dial it down for a real public release.

• Could be used for performance testing?
• Would be nice to reuse the structure in the parser...

http://exslt.org/

This should only report a single node:

$ echo '<a/>' > /tmp/foo.xml
$ evaluate --xpath '//* | //*' /tmp/foo.xml

For example:

$ evaluate --string 's=hello' --number 'n=1.23' --boolean 'b=true' --xpath '$s | $n | $b'

Instead of panicking, let's just return a failure. One example

Right now, we store a Nodeset as a collection of Node objects. However, all the nodes should belong to the same document, so it's possible we could split up the node and store the common information just once.

There isn't a conversion to nodeset unless you are already a nodeset. We should leverage pattern matching to force deciding on what to do at the call site.

XPath§4 lists a whole lot of functions. We currently have true, false, and not.

• last #23
• position #24
• count #22
• id #25
• local-name #26
• namespace-uri #27
• name #28
• string #29
• concat #30
• starts-with #31
• contains #32
• substring-before #35
• substring-after #36
• substring #37
• string-length #38
• normalize-space #39
• translate #40
• boolean #41
• lang #42
• number #43
• sum #44
• floor #45
• ceiling #46
• round #47

to_foo -> return a new object, leaving the original
as_foo -> return a "free" transformation of the object
into_foo -> consume the object, returning a new one

Specifically, to_node needs to be adapted.