How does creating handlers works? - go

Question

I am looking at this example
var name string
type helloWorldResponse struct {
Message string `json:"message"`
}
type helloWorldRequest struct {
Name string `json:"name"`
}
func main() {
port := 8080
handler := newValidationHandler(newHelloWorldHandler())
http.Handle("/helloworld", handler)
log.Printf("Server starting on port %v\n", port)
log.Fatal(http.ListenAndServe(fmt.Sprintf(":%v", port), nil))
}
type validationHandler struct {
next http.Handler
}
func newValidationHandler(next http.Handler) http.Handler {
return validationHandler{next: next}
}
func (h validationHandler) ServeHTTP(rw http.ResponseWriter, r *http.Request) {
var request helloWorldRequest
decoder := json.NewDecoder(r.Body)
err := decoder.Decode(&request)
if err != nil {
http.Error(rw, "Bad request", http.StatusBadRequest)
return
}
name = request.Name
h.next.ServeHTTP(rw, r)
}
type helloWorldHandler struct{}
func newHelloWorldHandler() http.Handler {
return helloWorldHandler{}
}
func (h helloWorldHandler) ServeHTTP(rw http.ResponseWriter, r *http.Request) {
response := helloWorldResponse{Message: "Hello " + name}
encoder := json.NewEncoder(rw)
encoder.Encode(response)
}
The author of the code explained that we are
going to be chaining handlers together, the first handler, which is our validation handler,
needs to have a reference to the next in the chain as it has the responsibility for calling
ServeHTTP or returning a response. I am newbe to Go and I do not understand this line
return validationHandler{next: next}
Which data structure next:next represents?

Answer 1

type validationHandler struct {
next http.Handler // 1
}
func newValidationHandler(next /* 2 */ http.Handler) http.Handler {
return validationHandler{next: next}
// 1 2
}
next number 1 is a field from validationHandler struct (a few lines above). And the other next is method's parameter (from the signature). All in all, this simply sets a field in a struct. No magic.
Which data structure next:next represents?
Not a data structure. It is struct initialization syntax. See more examples here: https://gobyexample.com/structs