Sub-protocol-evaluators for attribute grammars

This paper presents a new strategy for evaluating attribute grammars, sub-protocol attribute evaluation, and gives an algorithm for constructing sub-protocol-evaluators. Sub-protocol-evaluators can be built for any non-circular attribute grammar; this paper describes how to construct them for absolutely noncircular grammars [4]. Sub-protocol-evaluators are most easily understood as a simple optimization of another evaluator we call the protocol-evaluator. The protocol-evaluator has elements in common with the tree-walk evaluator of Kennedy-Warren [4] and with Nielson's direct evaluator [6]; it can be viewed as a refinement of each of these. Furthermore, the uniform AGs, proposed by Warren [9], and the ordered AGs, proposed by Kastens [3], are both subclasses of grammars for which especially efficient protocol-evaluators can be built.