Abstract Predicates and Mutable ADTs in Hoare Type Theory
Hoare Type Theory (HTT) combines a dependently typed, higher-order language with monadically-encapsulated, stateful computations. The type system incorporates pre- and post-conditions, in a fashion similar to Hoare and Separation Logic, so that programmers can modularly specify the requirements and effects of computations within types. This paper extends HTT with quantification over abstract predicates (i.e., higher-order logic), thus embedding into HTT the Extended Calculus of Constructions. When combined with the Hoare-like specifications, abstract predicates provide a powerful way to define and encapsulate the invariants of private state that may be shared by several functions, but is not accessible to their clients.