add a new 'overview' MVC example

Former-commit-id: f73cbf6010595c639f6c5b5119e2ec41bc9802a5

_examples/mvc/overview/service/greet_service.go

@@ -0,0 +1,50 @@
+package service
+
+import (
+	"app/database"
+	"app/environment"
+	"fmt"
+)
+
+// GreetService example service.
+type GreetService interface {
+	Say(input string) (string, error)
+}
+
+// NewGreetService returns a service backed with a "db" based on "env".
+func NewGreetService(env environment.Env, db database.DB) GreetService {
+	service := &greeter{db: db, prefix: "Hello"}
+
+	switch env {
+	case environment.PROD:
+		return service
+	case environment.DEV:
+		return &greeterWithLogging{service}
+	default:
+		panic("unknown environment")
+	}
+}
+
+type greeter struct {
+	prefix string
+	db     database.DB
+}
+
+func (s *greeter) Say(input string) (string, error) {
+	if err := s.db.Exec("simulate a query..."); err != nil {
+		return "", err
+	}
+
+	result := s.prefix + " " + input
+	return result, nil
+}
+
+type greeterWithLogging struct {
+	*greeter
+}
+
+func (s *greeterWithLogging) Say(input string) (string, error) {
+	result, err := s.greeter.Say(input)
+	fmt.Printf("result: %s\nerror: %v\n", result, err)
+	return result, err
+}