We present a typing system for the λ-calculus, with non-idempotent intersection types. As it is the case in (some) systems with idempotent intersections, a λ-term is typable if and only if it is strongly normalising. Nonidempotency brings some further information into typing trees, such as a bound on the longest β-reduction sequence reducing a term to its normal form. We actually present these results in Klop’s extension of λ-calculus, where the bound that is read in the typing tree of a term is refined into an exact measure of the longest reduction sequence. This complexity result is, for longest reduction sequences, the counterpart of de Carvalho’s result for linear head-reduction sequences.

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Abstract. We examine a logical foundation for the intersection and union types assignment system (IUT). The proposed system is Intersection and Union Logic (IUL), an extension of Intersection Logic (IL) with the canonical rules for union. We investigate two different formalisms for IUL, as well as its properties and its relation with minimal intuitionistic logic.