We define a new, more semantic interpretation of gradual types and use it to ``gradualize'' two forms of polymorphism: subtyping polymorphism and implicit parametric polymorphism. In particular, we use the new interpretation to define three gradual type systems ---Hindley-Milner, with subtyping, and with union and intersection types--- in terms of two preorders, subtyping and materialization. We define these systems both declaratively ---by adding two subsumption-like rules--- which yields clearer, more intelligible, and streamlined definitions, and algorithmically by reusing existing techniques such as unification and tallying.

Polymorphic variants are a useful feature of the OCaml language whose current definition and implementation rely on kinding constraints to simulate a subtyping relation via unification. This yields an awkward formalization and results in a type system whose behaviour is in some cases unintuitive and/or unduly restrictive.