Conditional Attribute Grammars

Abstract

Attribute grammars are a useful formalism for the specification of computations on structured terms. The classical definition of attribute grammars, however, has no way of treating conditionals nonstrictly. Consequently, the natural way of expressing many otherwise well-behaved computations involves a circularity. This article presents conditional attribute grammars, an extension of attribute grammars that enables more precise analysis of conditionals. In conditional attribute grammars, attribute equations may have guards. Equations are active only when their guards are satisfied. The standard attribute grammar evaluation classes are definable for conditional attribute grammars, and the corresponding evaluation techniques can be easily adapted. However, determining membership in standard evaluation classes such as 1-SWEEP, OAG, and SNC is NP-hard.