Type Classes are Signatures of Abstract Types
The generalization of type classes to abstract types has the advantage of greater expressiveness: We can model polymorphic abstract types and heterogeneous data structures, concepts which cannot be expressed in Haskell. An example of a polymorphic abstract type is ∀α.Bag α, the abstract type of all bags with elements of type α. In Haskell, we would either have to fix the element type, or we would have to fix the implementation of Bag.
Our extension shares the desirable properties of the type class approach in that it is fully static and in that type reconstruction is feasible.
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