SPath: an extensible query-language for Scala

Scala combines the functional and object-oriented paradigms and is good at supporting embedded domain-specific languages. Scala presents therefore an opportunity for designing a new kind of query-language that belongs to the family of XPath-like query-languages, for querying semistructured data that is stored within internal memory. In the terminology of XPath, a relation between the nodes of a tree, is called an axis and their purpose is to provide a way of navigating between the nodes within a tree. An XPath query is then, approximately, a path of axis-steps, resembling a directory path in a file-system. XPath has 13 axes. However, it is natural to extend the notion of axis to any relation on tree-nodes, by allowing queries with user-defined axes. The effect of the approach that has been taken in this article is an alternative syntax and semantics for user-defined axes, compared with XPath 2.0. It also turns out that separating the query-language from specific types of trees, enables reuse with any tree data-structure that conforms to the composite design-pattern. SPath is an XPath-like query-language based on Linear Temporal Logic that has been implemented as a domain-specific language, embedded into Scala.