An attribute grammar view

Abstract

Controlled M-grammars, defined formally in Chapter 17, are essentially different from M-grammars without control. The set of well-formed derivation trees is defined at two levels instead of one. The control expressions define a superset of this set of derivation trees, and M-rule applications filter out the ill-formed ones from this superset. The difference is not one of formal power. A controlled M-grammar can be translated into one without control (cf. Chapter 19), and of course free grammars can be changed into controlled ones by adding a sterile control expression: a regular expression that generates the set of M-rules. Rather, the difference is one of grammar organisation. Control expressions function as the backbone of the grammar, and in some sense form an easy-to-grasp grammar summary. This makes the existence of control important for the design, development and maintenance of the grammar.