什么是Haskell的多态性风格？

Question

With Haskell's type classes it almost seems that it enables ad hoc polymorphism, but its functions declarations seem parametric polymorphism. Am I mixing my understanding of different things?

Answer 1

Indeed, Haskell supports both (higher rank) parametric polymorphism, and ad hoc (or bounded ) polymorphism. Parametric polymorphism in Haskell is supported via its Hindley-Milner/ System F type system. Ad hoc polymorphism is supported via type classes.

For the origin of type classes and ad hoc polymorphism, see Wadler's papers:

• How to make ad-hoc polymorphism less ad hoc , Philip Wadler and Stephen Blott. 16'th Symposium on Principles of Programming Languages, ACM Press, Austin, Texas, January 1989.

For the origin of the distinction between parametric and ad hoc polymorphism, you can dig up Strachey's papers,

• C. Strachey, Fundamental concepts in programming languages . Lecture notes for the International Summer School in Computer Programming, Copenhagen, August 1967