Abstract

We propose a modest conservative extension to ML that allows semi-explicit first-class polymorphism while preserving the essential properties of type inference. In our proposal, the introduction of polymorphic types is fully explicit, that is, both introduction points and exact polymorphic types are to be specified. However, the elimination of polymorphic types is semi-implicit: only elimination points are to be specified as polymorphic types themselves are inferred. This extension is particularly useful in Objective ML where polymorphism replaces subtyping. Introduction The success of the ML language is due to its combination of several attractive features. Undoubtedly, the polymorphism of ML [Damas and Milner, 1982] ---or polymorphism `a la ML--- with the type inference it allows, is a major advantage. The ML type system stays in close correspondence with the rules of logic, following the Curry-Howard isomorphism between types and formulas, which provides a simple intuition, ...

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