The standard formalism for explaining abstract types is existential quantification. While it provides a sufficient model for type abstraction in entirely statically typed languages, it proves to be too weak for languages enriched with forms of dynamic typing, where parametricity is violated. As an alternative approach to type abstraction that addresses this shortcoming we present a calculus for dynamic type generation. It features an explicit construct for generating new type names and relies on coercions for managing abstraction boundaries between generated types and their designated representation. Sealing is represented as a generalized form of these coercions. The calculus maintains abstractions dynamically without restricting type analysis.
Extended version of PPDP'2003 paper of the same title.