First-class polymorphism for ML

Polymorphism in ML is implicit: type variables are silently introduced and eliminated. The lack of an explicit declaration of type variables restricts the expressiveness of parameterised modules (functors). Certain polymorphic functions cannot be expressed as functors, because implicit type parameters of polymorphic functions are in one respect more powerful than formal type parameters of functors. The title suggests that this lack of expressiveness is due to a restricted ability to abstract --- polymorphism is restricted. Type variables can only be abstracted from value declarations, but not from other forms of declarations, especially not from structure declarations. The paper shows in the case of Standard ML how (syntax and) semantics can be modified to fill this language gap. This is not so much a question of programming language design as a contribution for better understanding the relationship between polymorphic functions, polymorphic types, and functors.