First-class polymorphism with existential types

Abstract datatypes can be typed conveniently using existential types. Even though it is a powerful abstraction mechanism, current type inference systems based on Hindley-Milner do not allow existential types as first-class citizens – every existential type must be explicitly declared, packed, and unpacked using a data constructor. We present an extension of theMLF type system with first-class existential types. Existential types are simply introduced by an annotation, and eliminated automatically in applications without the need for a special open construct. The system is fully implemented in the experimental Morrow interpreter.datatypes can be typed conveniently using existential types. Even though it is a powerful abstraction mechanism, current type inference systems based on Hindley-Milner do not allow existential types as first-class citizens – every existential type must be explicitly declared, packed, and unpacked using a data constructor. We present an extension of theMLF type system with first-class existential types. Existential types are simply introduced by an annotation, and eliminated automatically in applications without the need for a special open construct. The system is fully implemented in the experimental Morrow interpreter.