An Intuitionistic Theory of Types with Assumptions of High-Arity Variables

Abstract Bossi, A. and S. Valentini, An intuitionistic theory of types with assumptions of high-arity variables, Annals of Pure and Applied Logic 57 (1992) 93–149. After an introductory discussion on Martin-Lof's Intuitionistic Theory of Types (ITT), the paper introduces the notion of assumption of high-arity variable. Then the original theory is extended (HITT) in a very uniform way by means of the new assumptions. Some improvements allowed by high-arity variables are shown. The main result of the paper is a normal form theorem for HITT. The detailed proof follows a computability method ‘a la Tait’. The main consequences of the normal form theorem are: the consistency of HITT, which also implies the consistency of ITT, and the computability of any judgement derived within HITT. Besides that, a canonical form theorem is shown: to any derivable judgement we can associate a canonical one whose derivation ends with an introductory rule. The standard disjunction and existential properties follow. Moreover, by using the computational interpretation of types it immediately follows that the execution of any proved correct program terminates.