Is XML Schema a constraint language, a specification language, or agramm

Hi Folks,

Thank you for your excellent inputs! To recap, here are three ways to view XML Schema:

XML Schema is a constraint language. An XML Schema states constraints that must be satisfied by instances for them to be “valid”. Determining if an instance is valid is a constraint satisfaction problem.

              Enormous technical breakthroughs in recent years have led to
             amazingly good ways to approach the satisfiability problem.
             We now have algorithms that are much more efficient than
             anyone had dared believe possible before the year 2000. These
             so-called “SAT solvers” are able to handle industrial-strength
             problems, involving millions of variables.
              “Satisfiability and the Art of Computer Programming” by Donald Knuth

An XML Schema is a specification language. An XML Schema is a specification that informs instance document authors how to construct instances. Determining if an instance is valid is a matter of checking that the instance adheres to the specification.

              Specification languages are generally not directly executed. They
             are meant to describe the what, not the how. Indeed, it is considered
             as an error if a requirement specification is cluttered with unnecessary
             implementation detail.
             “Specification language”, Wikipedia

XML Schema is a regular tree grammar language. An application structures – parses – XML instance documents in accordance with a given grammar (XML Schema).

Parsing is the process of structuring a linear representation
             in accordance with a given grammar.
             For each grammar, there are generally an infinite number of
             linear representations (“sentences”) that can be structured with
             it. That is, a finite-sized grammar can supply structure to an infinite
             number of  sentences. This is the main strength of the grammar
             paradigm and indeed the main source of the importance of grammars:
             they summarize succinctly the structure of an infinite number of
             objects of a certain class.
             “Parsing Techniques” by Dick Grune and Ceriel J. H. Jacobs

/Roger