Isomorphism of "Functional" Intersection Types

Type isomorphism for intersection types is quite odd, since it is not a congruence and it does not extend type equality in the standard interpretation of types. The lack of congruence is due to the proof theoretic nature of the intersection introduction rule, which requires the same term to be the subject of both premises. A partial congruence can be recovered by introducing a suitable notion of type similarity. Type equality in standard models becomes included in type isomorphism whenever atomic types have “functional” interpretations, i.e. they are equivalent to arrow types. This paper characterises type isomorphism for a type system in which the equivalence between atomic types and arrow types is induced by the initial projections of the Scott D∞ model via the correspondence between inverse limit models and filter λ-models. 1998 ACM Subject Classification F.4.1 Mathematical Logic, F.3.3 Studies of Program Constructs, D.1.1 Applicative (Functional) Programming