Programming Languages: Concepts and Implementation

C.4 Type Variables, Type Classes, and Qualified Types

To promote flexibility, Haskell has a hierarchy of type classes. A type class in Haskell is a set of types, unlike the concept of a class from object-oriented programming. Specifically, a type class in Haskell is a set of types, all of which define certain functions. The definition of a type class declares the names and types of the functions that all members of that class must define. Thus, a type class is like an interface from object-oriented programming, particularly an interface in Java. The concept of a class from object-oriented programming, which is the analog of a type (not a type class) in Haskell, can implement several interfaces, which means it must provide definitions for the ...