Map and then call filter, Haskell won't know which function to use. Here's how we solve this.

如filter和null，装入Data.Map模块之后再调用filter,haskell就不知道它究竟是哪个函数。

youdao

Moving out, we call the filter function.

向外移，我们会调用filter函数。

youdao

Moving out, we call the filter function.

向外移，我们会调用filter函数。

youdao