A typeful and tagless representation for XML documents

When constructing programs to process XML documents, we immediately face the question as to how XML documents should be represented internally in the programming language we use. Currently, most representations for XML documents are typeless in the sense that the type information of an XML document cannot be reflected in the type of the representation of the document (if the representation is assumed to be typed). Though convenient to construct, a typeless representation for XML documents often makes use of a large number of representation tags, which not only require some significant amount of space to store but may also incur numerous run-time tag checks when the represented documents are processed. Moreover, with a typeless representation for XML documents, it becomes difficult or even impossible to statically capture program invariants that are related to the type information of XML documents. Building upon our recent work on guarded recursive datatypes, we present an approach to representing XML documents in this paper that not only allows the type information of an XML document to be reflected in the type of the representation of the document but also significantly reduces the need for representation tags that are required in typeless representations. With this approach, we become able to process XML documents in a typeful manner, thus reaping various well-known software engineering benefits from the presence of types.