错误处理中间件-将err转换为字符串以进行响应

I have this middleware func:

func errorMiddleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        defer func() {
            if err := recover(); err != nil {
                log.Error("Caught error in defer/recover middleware: ", err)
                w.WriteHeader(http.StatusInternalServerError)
                json.NewEncoder(w).Encode(struct {
                    ID string
                }{
                    err.Error(),
                })
            }
        }()
        next.ServeHTTP(w, r)
    })
}

I use it like so:

router := mux.NewRouter()
router.Use(errorMiddleware)

however I am getting a compilation error, it says:

Anybody know what that's about? I am just trying to convert err to a string, ultimately, serialize it for the client etc.

recover() returns an interface with no methods to proxy any value sent by panic(). In the defer block, you're trying to access the Error() method of a pure, has-no-method interface. If you want to distinguish the built-in error type, you'd have to assert its type like:

realErr, ok := err.(error)
if ok {
    // here you can use realErr.Error().
}

So that it'll give you a real value of type error. If you check out the built-in types, you'll see that error is to implement an Error() string method.

Type assertions: https://tour.golang.org/methods/15