golang中的map，的 key 可以是很多种类型，比如 bool,数字，string,指针,channel,还有 只包含前面几个类型的 interface types,structs,arrays

显然，slice， map 还有 function 是不可以了，因为这几个没法用 == 来判断

原文如下：

As mentioned earlier,map keys may be of any type that is comparable. Thelanguage specdefines this precisely,but in short,comparable types are boolean,numeric,string,pointer,and interface types,and structs or arrays that contain only those types. Notably absent from the list are slices,maps,and functions; these types cannot be compared using==,and may not be used as map keys.

type Key struct {
      Path,Country string
}

hits := make(map[Key]int)