Strong Normalization for the Simply-Typed Lambda Calculus in Constructive Type Theory Using Agda

We consider a pre-existing formalization in Constructive Type Theory of the pure Lambda Calculus in its presentation in first order syntax with only one sort of names and alpha-conversion based upon multiple substitution, as well as of the system of assignment of simple types to terms. On top of it, we formalize a slick proof of strong normalization given by Joachimski and Matthes whose main lemma proceeds by complete induction on types and subordinate induction on a characterization of the strongly normalizing terms which is in turn proven sound with respect to their direct definition as the accessible part of the relation of one-step beta reduction. The proof of strong normalization itself is thereby allowed to consist just of a direct induction on the type system. The whole development has been machine-checked using the system Agda. We assess merits and drawbacks of the approach.