Scott's Representation Theorem and the Univalent Karoubi Envelope

Abstract

Lambek and Scott constructed a correspondence between simply-typed lambda calculi and Cartesian closed categories. Scott's Representation Theorem is a cousin to this result for untyped lambda calculi. It states that every untyped lambda calculus arises from a reflexive object in some category. We present a formalization of Scott's Representation Theorem in univalent foundations, in the (Rocq-)UniMath library. Specifically, we implement two proofs of that theorem, one by Scott and one by Hyland. We also explain the role of the Karoubi envelope -- a categorical construction -- in the proofs and the impact the chosen foundation has on this construction. Finally, we report on some automation we have implemented for the reduction of $\lambda$-terms.