在Golang中将interface {}直接转换为int，其中interface将数字存储为字符串

I got a map[string]interface{} because decoding to JSON; with normal data, interface most be only a number but in type string, like this :

var a interface{}
a="3"

And then all data will be stored into a struct.

type someStruct struct {
   ID string
   Number1 int
   Number2 int
   Number3 int
   Number4 int
}

So I need to convert interface to int, but can´t do it easily and efficiently because only the code would be https://play.golang.org/p/oktbvTUbk93, pretty annoying and code does not seem to be readable if you take in mind the fact that I should handle all of possible errors

I would want to convert it directly to int, I´ve been searching for solutions but any of this convertions works as I want https://play.golang.org/p/Dw67U6kZwHC

In case you wondering why I don´t just decode it into struct directly, it´s because it´s dynamic data, the actual decoding occurs like this :

type dataIn struct {
   Code int         `json:"code"`
   ID   string      `json:"id"`
   Data interface{} `json:"data"`
}

And then I handle Data according to code and id, and they all are different data structures, so I can´t directly handle it with JSON