How can I reduce redundant code of duplicate function in Golang? - function

Question

I have a Rest API application to list all json data to browser. as long as I have more modules my code is more redundant. and complex.
func UserList(w http.ResponseWriter, r *http.Request) {
list := []models.User{}
db.Find(&list)
json.NewEncoder(w).Encode(list)
}
func ProductList(w http.ResponseWriter, r *http.Request) {
list := []models.Product{}
db.Find(&list)
json.NewEncoder(w).Encode(list)
}
func OrderList(w http.ResponseWriter, r *http.Request) {
list := []models.Order{}
db.Find(&list)
json.NewEncoder(w).Encode(list)
}
Is there any better solution to make this code into just one function
Example
func List(w http.ResponseWriter, r *http.Request) {
list := ??? List of struct here ???
db.Find(&list)
json.NewEncoder(w).Encode(list)
}

Answer 1

you can do something like this:
func List(list interface{}, w http.ResponseWriter, r *http.Request,) {
db.Find(list)
json.NewEncoder(w).Encode(list)
}

Answer 2

If you pass the model type as a request param, this should do it (including error handling):
func List(w http.ResponseWriter, r *http.Request) {
var list interface{}
switch r.FormValue("model") {
case "user":
list = new([]models.User)
case "product":
list = new([]models.Product)
case "order":
list = new([]models.Order)
default:
http.Error(w, "invalid type", http.StatusBadRequest)
return
}
if err := db.Find(list); err != nil {
http.Error(w, "db error", http.StatusInternalServerError)
return
}
if err := json.NewEncoder(w).Encode(list); err != nil {
log.Printf("json encoding error: %v", err)
}
}
Another option is to build a registry of types, and even the slice creation can be factored out, using the help of reflect:
var reg = map[string]reflect.Type{
"user": reflect.TypeOf((*models.User)(nil)).Elem(),
"product": reflect.TypeOf((*models.Product)(nil)).Elem(),
"order": reflect.TypeOf((*models.Order)(nil)).Elem(),
}
func List(w http.ResponseWriter, r *http.Request) {
etype := reg[r.FormValue("model")]
if etype == nil {
http.Error(w, "invalid type", http.StatusBadRequest)
return
}
list := reflect.New(reflect.SliceOf(etype)).Interface()
if err := db.Find(list); err != nil {
http.Error(w, "db error", http.StatusInternalServerError)
return
}
if err := json.NewEncoder(w).Encode(list); err != nil {
log.Printf("json encoding error: %v", err)
}
}

Answer 3

Given that you are calling db.Find(&list) I assume that they share a common interface. In which case you could wrap your handler calls like so;
func ListHandler(list <YOUR_INTERFACE>) func(w http.ResponseWriter, r *http.Request) {
return func(w http.ResponseWriter, r *http.Request) {
db.Find(&list)
json.NewEncoder(w).Encode(list)
}
}
In your call;
http.HandleFunc("/user/list", ListHandler([]models.User{}))
http.HandleFunc("/product/list", ListHandler([]models.Product{}))
http.HandleFunc("/order/list", ListHandler([]models.Order{}))