Extending ML with Semi-Explicit Higher-Order Polymorphism

We propose a modest conservative extension to ML that allows semi-explicit higher-order polymorphism while preserving the essential properties of ML. In our proposal, the introduction of polymorphic types remains fully explicit, that is, both the introduction and the exact polymorphic type must be specified. However, the elimination of polymorphic types is now semi-implicit: only the elimination itself must be specified as the polymorphic type is inferred. This extension is particularly useful in Objective ML where polymorphism replaces subtyping.

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