Validating OMDoc Documents

In Chapter 1 we have briefly discussed the basics of validating XML documents by document type definitions (DTDs) and schemata. In this chapter, we will instantiate this discussion with the particulars of validating

OMDoc

documents.

Generally, DTDs and schemata are context-free grammars for trees, that can be used by a

validating parser

to reject XML documents that do not conform to the constraints expressed in the

OMDoc

DTD or schemata discussed here.

Note that none of these grammars can enforce all constraints that the

OMDoc

specification in Part III of this book imposes on documents. Therefore grammar-based validation is only a necessary condition for

OMDoc

-

validity

. Still,

OMDoc

documents should be validated to ensure proper function of

OMDoc

tools, such as the ones discussed in Chapters 25 and 26. Validation against multiple grammars gives the best results. With the current state of validation technology, there is no clear recommendation, which of the validation approaches to prefer for

OMDoc

. DTD validation is currently best supported by standard XML applications and supports default values for attributes. This allows the author who writes

OMDoc

documents by hand to elide implicit attributes and make the marked-up text more readable. XML- and

RelaxNG

schema validation have the advantage that they are namespace-aware and support more syntactic constraints. Neither of these support mnemonic XML entities, such as the ones used for

Unicode

characters in Presentation-

MathML

, so that these have to be encoded as

Unicode

code points. Finally

RelaxNG

schemata do not fully support default values for attributes, so that

OMDoc

documents have to be normalized to be

RelaxNG

-valid.

We will now discuss the particulars of the respective validation formats. As the

RelaxNG

schema is the most expressive and readable for humans we consider it as the normative grammar formalism for

OMDoc

, and have included it in Appendix D for reference.