Polymorphic rewriting conserves algebraic strong normalization and confluence

Abstract

We study combinations of many-sorted algebraic term rewriting systems and polymorphic lambda term rewriting. Algebraic and lambda terms are mixed by adding the symbols of the algebraic signature to the polymorphic lambda calculus, as higher-order constants.

We show that if a many-sorted algebraic rewrite system R is strongly normalizing (terminating, noetherian), then R + β + η + type-β + type-η rewriting of mixed terms is also strongly normalizing. We obtain this results using a technique which generalizes Girard's “candidats de reductibilité”, introduced in the original proof of strong normalization for the polymorphic lambda calculus.

We also show that if a many-sorted algebraic rewrite system R has the Church-Rosser property (is confluent), then R + β + type-β + type-η rewriting of mixed terms has the Church-Rosser property too. Combining the two results, we conclude that if R is canonical (complete) on algebraic terms, then R + β + type-β + type-η is canonical on mixed terms.

η reduction does not commute with algebraic reduction, in general. However, using long η-normal forms, we show that if R is canonical then R + β + η + type-β + type-η convertibility is still decidable.

Partially supported by ONR Grant NOOO14-88-K-0634 and by ARO Grant DAAG29-84-K-0061

Partially supported by ONR Grant NOOO14-88-K-0593.