Learning Go: How to use http.HandlerFunc? - function

Question

Trying To Learn Go
My Test Project
I am building a simple API to communicate with a light SQL database.
I created this function, that gets all hosts from the DB table.
As I understand, my function should take a pointer to the DB and return me http.HandlerFunc.
func (h HostController) GetAllHosts(db *sql.DB) http.HandlerFunc {
return func(w http.ResponseWriter, r *http.Request) {
var host entities.Host
var hosts []entities.Host
var errorMessage entities.Error
rows, err := db.Query("SELECT * FROM hosts WHERE hosts = ?", host)
if err != nil {
errorMessage.Message = "Error: Get all hosts"
utils.SendError(w, http.StatusInternalServerError, errorMessage)
return
}
defer rows.Close()
for rows.Next() {
err := rows.Scan(&host.ID, &host.Uuid, &host.Name, &host.IPAddress)
if err != nil {
errorMessage.Message = "Error: (Get all hosts) Can't scan rows"
utils.SendError(w, http.StatusInternalServerError, errorMessage)
return
}
hosts = append(hosts, host)
}
////If hosts slice is empty -> no data in database
if len(hosts) == 0 {
errorMessage.Message = "No found hosts in database"
utils.SendError(w, http.StatusInternalServerError, errorMessage)
return
}
//Convert containers content to JSON representation
w.Header().Set("Content-Type", "application/json")
utils.SendSuccess(w, hosts)
}
}
Now, Until here everything should be good, but I don't know how to implement this on main.go
build layout:
.
├── controllers
│ └── hostcontroller.go
│
└── main.go
and this is how I am trying to implement it on the main.go
package main
import (
"examProg/config"
"examProg/controllers"
"fmt"
"log"
"net/http"
"github.com/gorilla/mux"
_ "github.com/mattn/go-sqlite3"
)
func main() {
db := config.GetDB() //Establishing connection to DBase
{
router := mux.NewRouter()
router.Handle("/host", controllers.HostController.GetAllHosts(db))
fmt.Println("Server is running at port: 2030")
log.Println("Server Running OK | Port: 2030")
log.Fatal(http.ListenAndServe(":2030", router))
}
}
I don't understand how to implement the http.HandlerFunc 🙃

Answer 1

The expression controllers.HostController.GetAllHosts is a method expression. The result of a method expression is a function that, in addition to the method's argument's, takes an instance of the receiver's type as its first argument.
Hence the "not enough arguments in call ..." error.
To illustrate:
f := controllers.HostController.GetAllHosts
fmt.Println(reflect.TypeOf(f))
// output: func(controllers.HostController, *sql.DB)
So, to make the code compile, you need to pass an instance of controllers.HostController to the call, i.e.
controllers.HostController.GetAllHosts(controllers.HostController{}, db)
As you can see, it ain't pretty. Method expressions have their use, I'm sure, but I haven't encountered them much. What I see most often instead, is the use of method values.
h := controllers.HostController{}
router.Handle("/host", h.GetAllHosts(db))