Intersection Types for Unboundedness Problems

Abstract

Intersection types have been originally developed as an extension of simple types, but they can also be used for refining simple types. In this survey we concentrate on the latter option; more precisely, on the use of intersection types for describing quantitative properties of simply typed lambda-terms. We present two type systems. The first allows to estimate (by appropriately defined value of a derivation) the number of appearances of a fixed constant a in the beta-normal form of a considered lambdaterm. The second type system is more complicated, and allows to estimate the maximal number of appearances of the constant a on a single branch. * Work supported by the National Science Centre, Poland (grant no. 2016/22/E/ST6/00041). 1 Following the convention in this area, we use the word "sort" for simple types, and the word "type" for intersection types refining them.