which class of programming languages, XML Schema language belongs

    I've been thinking on this topic, since a while now, and thought of seeking opinion of list members here. As the subject of this mail says, I'm interested to know, which class of programming languages, XML Schema language belongs.

As far as I know, the computer language theory says that, a language is a set of strings (or, a set of words as few books say). In that sense, XML Schema is definitely a formal computer language.

Most of the time, I tend to compare XML Schema's type system with inheritance based OO language type system. That alone, doesn't make XML Schema exactly equivalent to the nature of an OO language. XML Schema, misses the procedural semantics of an OO language. XML Schema is more like a constraint specification language. An XML schema, specifies a set of constraints (specified as, element declarations, attribute declarations, type definitions etc), that certain set of XML instance documents should follow.

Any thoughts, about this topic shall be enlightening to know.