Revisiting Iso-Recursive Subtyping

The Amber rules are well-known and widely used for subtyping iso-recursive types. They were first briefly and informally introduced in 1985 by Cardelli in a manuscript describing the Amber language. Despite their use over many years, important aspects of the metatheory of the iso-recursive style Amber rules have not been studied in depth or turn out to be quite challenging to formalize.

This article aims to revisit the problem of subtyping iso-recursive types. We start by introducing a novel declarative specification for Amber-style iso-recursive subtyping. Informally, the specification states that two recursive types are subtypes if all their finite unfoldings are subtypes. The Amber rules are shown to have equivalent expressive power to this declarative specification. We then show two variants of sound, complete and decidable algorithmic formulations of subtyping with respect to the declarative specification, which employ the idea of double unfoldings. Compared to the Amber rules, the double unfolding rules have the advantage of: (1) being modular; (2) not requiring reflexivity to be built in; (3) leading to an easy proof of transitivity of subtyping; and (4) being easily applicable to subtyping relations that are not antisymmetric (such as subtyping relations with record types). This work sheds new insights on the theory of subtyping iso-recursive types, and the new rules based on double unfoldings have important advantages over the original Amber rules involving recursive types. All results are mechanically formalized in the Coq theorem prover.