Decidability of Type-checking in the Calculus of Algebraic Constructions with Size Annotations

Abstract

Since Val Tannen's pioneer work on the combination of simply-typed lambda-calculus and first-order rewriting (LICS'88), many authors have contributed to this subject by extending it to richer typed lambda-calculi and rewriting paradigms, culminating in calculi like the Calculus of Algebraic Constructions. These works provide theoretical foundations for type-theoretic proof assistants where functions and predicates are defined by oriented higher-order equations. This kind of definitions subsumes induction-based definitions, is easier to write and provides more automation. On the other hand, checking that user-defined rewrite rules are strongly normalizing and confluent, and preserve the decidability of type-checking when combined with beta-reduction, is more difficult. Most termination criteria rely on the term structure. In a previous work, we extended to dependent types and higher-order rewriting, the notion of ``sized types'' studied by several authors in the simpler framework of ML-like languages, and proved that it preserves strong normalization. The main contribution of the present paper is twofold. First, we prove that, in the Calculus of Algebraic Constructions with size annotations, the problems of type inference and type-checking are decidable, provided that the sets of constraints generated by size annotations are satisfiable and admit most general solutions. Second, we prove the later properties for a size algebra rich enough for capturing usual induction-based definitions and much more.