Loose and strict XPath variables/namespaces

Loose keys

XPath namespaces and variables doesn't accept symbol keys. It'll be convenient if symbol keys are accepted.
Nokogiri accepts both string and symbol keys.

namespaces = { ns: 'url' }
variables = { id: 'a' }
xml = '<foo xmlns="url"><bar id="a"/></foo>'
Nokogiri::XML.parse(xml).xpath('//ns:bar[@id=$id]', namespaces, variables)

Strict value check

XPath variables have value validation. Namespaces can also have validation

Nokogiri::XML.parse('<a/>').xpath('//a', { ns: 42 }) #=> TypeError

Strict variable existence check

There's two options

• Reference time check
  • Pros: easy to implement
  • Cons: error is document dependent
• Pre-check all variables in xpath
  • Pros: can detect variable name typo easily
  • discussed in https://github.com/ruby/rexml/pull/342#discussion_r3586024018

Nokogiri(libxml2) implements reference time check.

Nokogiri::XML.parse('<b/>').xpath('//a[@id=$foo]', nil, {fooo: '1'}) # => []
Nokogiri::XML.parse('<a/>').xpath('//a[@id=$foo]', nil, {fooo: '1'}) # => ERROR: Undefined variable

XPath 1.0 spec:

A VariableReference evaluates to the value to which the variable name is bound in the set of variable bindings in the context. It is an error if the variable name is not bound to any value in the set of variable bindings in the expression context.

I'm not sure whether it should be an error in evaluation context (reference-time) or in pre-check phase.
In XPath 2.0, it's explicitly defined to be a static-error, so XPath 1.0 spec might be ambiguous.

Start at REXML's XPath entry point and trace how namespace and variable bindings are handled. Compare the requested symbol-key and namespace-value behavior with the Nokogiri examples and the XPath 1.0 specification. Decide whether undefined variables are checked during reference-time evaluation or before evaluation, then cover the chosen behavior with tests.