Predicates (of type A → Prop) in type theory give a model for Natural Deduction. A separation algebra gives a model for separation logic. We formalize these statements in Coq.

For a more expressive logic that permits general recursive types and quasi-self-reference, we use step-indexed models built with indirection theory. We will explain this in Part V; for now it suffices to say that indirection theory requires that the type T be ageable—elements of T must contain an approximation index. A given element of the model contains only a finite approximation to some ideal predicate; these approximations become weaker as we “age” them—which we do as the some operational semantics takes its steps.

To enforce that T is ageable we have a typeclass, ageable(T). Furthermore, when Separation is involved, the ageable mechanism must be compatible with the separating conjunction; this requirement is also expressed by a typeclass, Age_alg(T).

Theorem: Separation Algebras serve as a model of Separation Logic.

Proof. We express this theorem in Coq by saying that given type T, the function algNatDed models an instance of NatDed(pred T). Given a SepAlg over T, the function algSepLog models an instance of SepLog(pred T). The definability of algNatDed and algSepLog serve as a proof of the theorem.

What we show in this chapter is the indirection theory version (in the Coq file msl/alg_seplog.v), so ageable and Age-alg are mentioned from time to time.