Type class-Typeclass-泛型基础上的二次抽象---随意多态

对泛型的类型添加约束，从而使泛型类型的变量具有某种通用操作。

再使用这些操作，参与到其它操作中。

In computer science, a type class is a type system construct that supports ad hoc polymorphism. This is achieved by adding constraints to type variables in parametrically polymorphic types. Such a constraint typically involves a type class T and a type variable a, and means that a can only be instantiated to a type whose members support the overloaded operations associated with T.

The programmer defines a type class by specifying a set of function or constant names, together with their respective types, that must exist for every type that belongs to the class. In Haskell, types can be parameterized; a type class Eq intended to contain types that admit equality would be declared in the following way:

class Eq a where
  (==) :: a -> a -> Bool
  (/=) :: a -> a -> Bool