A pure functional language like Haskell might offer type hierarchies, discriminated unions, quantification, existential types, and so on, but OOP languages typically lack these things.

• 象Haskell那样的纯函数型语言可以提供类型层次结构、区别合并、量化、存在类型等等,但OOP语言通常缺乏这些功能。