Question $\pi$ $\sum_{n=0}^100n^2$ $\sqrt{a^2+b^2}=c^2$ test latex equation chain 就是 compose(join,map) functor: 算子 A functor is a container of type a that, when subjected to a function that maps from a→b, yields a container of type b.我们的 map 函数就是一个算子