有没有一个词描述数据类型
- 正好有两个构造函数;和
- 不是递归的吗?
即描述这些类型
data Bool = False | True
data Maybe a = Nothing | Just a
data Either l r = Left l | Right r
但不包括这些类型
data Ordering = LT | EQ | GT -- too many constructors
data () = () -- too few constructors
data [a] = a | a : [a] -- recursive definition