Open Proofs and Open Terms: A Basis for Interactive Logic

Abstract

When proving a theorem, one makes intermediate claims, leaving parts temporarily unspecified. These ‘open’ parts may be proofs but also terms. In interactive theorem proving systems, one prominently deals with these ‘unfinished proofs’ and ‘open terms’. We study these ‘open phenomena’ from the point of view of logic. This amounts to finding a correctness criterion for ‘unfinished proofs’ (where some parts may be left open, but the logical steps that have been made are still correct). Furthermore we want to capture the notion of ‘proof state’. Proof states are the objects that interactive theorem provers operate on and we want to understand them in terms of logic.

In this paper we define ‘open higher order predicate logic’, an extension of higher order logic with unfinished (open) proofs and open terms. Then we define a type theoretic variant of this open higher order logic together with a formulas-as-types embedding from open higher order logic to this type theory. We show how this type theory nicely captures the notion of ‘proof state’, which is now a type-theoretic context.