The popularity of XML has motivated the development of novel XML processing tools many of which embed the XPath language for XML querying, transformation, constraint specification, etc. XPath developers (as well as less technical users) have access to commercial tools to help them use the language effectively. Example tools include debuggers that return the result of XPath subexpressions visualized in the context of the input XML document. This paper introduces XPlainer, a language that provides explanations of why XPath expressions return a specific answer. An explanation returns precisely the nodes in the input XML document that contribute to the answer. We provide a complete formalization for explanation queries based on the semantics of XPath. This enables the use of XPath engines for the evaluation of explanation queries. We describe a tool that uses XPlainer queries to provide visual explanations. The XPlainer-Eclipse tool is built on an extensible development environment that includes editors for visualizing both XML documents and XPath expressions as trees together with the explanation of the answers.

XML is an important practical paradigm in information technology and has a broad range of applications. How to access and retrieve the XML data is crucial to these applications. There are two standard ways for accessing and manipulating XML data, the Simple API for XML (SAX) and the Document Object Model (DOM). However, when an application needs to traverse through XML data, it is not easy to retrieve the required data with these two standard ways. XML data is impossible to be retrieved back and forth by SAX, and the graph-oriented DOM notation is not easy to work with. With such limitation, the W3C supervises the development of three important languages: XPath [3], XQuery and XSLT for exploring and querying XML. Among these three languages, XPath is the key and cornerstone language for the other two. XPath defines expressions for traversing an XML document and specifies the set of nodes (XPath 1.0) or the sequence of nodes (XPath 2.0) in the XML document.