XPathElem

XPath 3.1 AST. The root of the type hierarchy is XPathElem. It offers the ElemApi query API.

The requirements on this AST are as follows:

• It must represent the syntax tree of a successfully parsed XPath expression

• It is not annotated with more semantic information, like type information that is not included in the XPath expression

• It does not know anything about the context in which it runs, like bound namespaces etc.

• It is rich enough to be able to serialize the AST back to XPath, knowing exactly where to place parentheses, braces, etc.

• It is rich enough to contain operator precedence in the AST itself

• It must be readable in that object composition trees are not unnecessarily deep and therefore hard to comprehend

• Serialization of the AST to XPath may lead to differences in whitespace (and operator aliases), but other than that the result must be the same

• The AST class hierarchy does not have to use the exact same names as the XPath grammar

• Instances of the AST classes are easy to create in an ad-hoc manner, so they contain no parsing state

• The AST classes are case classes, with value equality and good support for pattern matching

It would be natural for the AST types to have IS-A relationships modeled as type inheritance, and HAS-A relationships modeled as composition. Where feasible, this approach has been followed. Consequently, sealed trait inheritance has been used a lot (IS-A instead of HAS-A), and unnecessarily deep object trees are prevented in this way.

Having such an AST of a successfully parsed XPath expression, it must be easy to reliably find used namespace prefixes, for example.

TODO Improve several class names.