How to do it…

1. Install github.com/asaskevich/govalidator and the github.com/gorilla/schema package using the go get command, as follows:

$ go get github.com/asaskevich/govalidator
$ go get github.com/gorilla/schema

2. Create html-form-validation.go, where we will read an HTML form, decode it using github.com/gorilla/schema, and validate each field of it against a tag defined in the User struct using github.com/asaskevich/govalidator, as follows:

package main
import 
(
  "fmt"
  "html/template"
  "log"
  "net/http"
  "github.com/asaskevich/govalidator"
  "github.com/gorilla/schema"
)
const 
(
  CONN_HOST = "localhost"
  CONN_PORT = "8080"
  USERNAME_ERROR_MESSAGE = "Please enter a valid Username"
  PASSWORD_ERROR_MESSAGE = "Please enter a valid Password"
  GENERIC_ERROR_MESSAGE = "Validation Error"
)
type User struct 
{
  Username string `valid:"alpha,required"`
  Password string `valid:"alpha,required"`
}
func readForm(r *http.Request) *User 
{
  r.ParseForm()
  user := new(User)
  decoder := schema.NewDecoder()
  decodeErr := decoder.Decode(user, r.PostForm)
  if decodeErr != nil 
  {
    log.Printf("error mapping parsed form data to struct : ",
    decodeErr)
  }
  return user
}
func validateUser(w http.ResponseWriter, r *http.Request, user *User) (bool, string) 
{
  valid, validationError := govalidator.ValidateStruct(user)
  if !valid 
  {
    usernameError := govalidator.ErrorByField(validationError,
    "Username")
    passwordError := govalidator.ErrorByField(validationError,
    "Password")
    if usernameError != "" 
    {
      log.Printf("username validation error : ", usernameError)
      return valid, USERNAME_ERROR_MESSAGE
    }
    if passwordError != "" 
    {
      log.Printf("password validation error : ", passwordError)
      return valid, PASSWORD_ERROR_MESSAGE
    }
  }
  return valid, GENERIC_ERROR_MESSAGE
}
func login(w http.ResponseWriter, r *http.Request) 
{
  if r.Method == "GET" 
  {
    parsedTemplate, _ := template.ParseFiles("templates/
    login-form.html")
    parsedTemplate.Execute(w, nil)
  } 
  else 
  {
    user := readForm(r)
    valid, validationErrorMessage := validateUser(w, r, user)
    if !valid 
    {
      fmt.Fprintf(w, validationErrorMessage)
      return
    }
    fmt.Fprintf(w, "Hello "+user.Username+"!")
  }
}
func main() 
{
  http.HandleFunc("/", login)
  err := http.ListenAndServe(CONN_HOST+":"+CONN_PORT, nil)
  if err != nil 
  {
    log.Fatal("error starting http server : ", err)
    return
  }
}

3. Run the program with the following command:

$ go run html-form-validation.go