add a README tutorial on the mvc/overview example

Former-commit-id: c563c4f1ffa98705829e14b189a6976c3a6aa898

_examples/mvc/overview/environment/environment.go

@@ -1,10 +1,50 @@
 package environment
 
-// Env is the environment type.
-type Env string
+import (
+	"os"
+	"strings"
+)
 
 // Available environments example.
 const (
 	PROD Env = "production"
 	DEV  Env = "development"
 )
+
+// Env is the environment type.
+type Env string
+
+// String just returns the string representation of the Env.
+func (e Env) String() string {
+	return string(e)
+}
+
+// ReadEnv returns the environment of the system environment variable of "key".
+// Returns the "def" if not found.
+// Reports a panic message if the environment variable found
+// but the Env is unknown.
+func ReadEnv(key string, def Env) Env {
+	v := Getenv(key, def.String())
+	if v == "" {
+		return def
+	}
+
+	env := Env(strings.ToLower(v))
+	switch env {
+	case PROD, DEV: // allowed.
+	default:
+		panic("unexpected environment " + v)
+	}
+
+	return env
+}
+
+// Getenv returns the value of a system environment variable "key".
+// Defaults to "def" if not found.
+func Getenv(key string, def string) string {
+	if v := os.Getenv(key); v != "" {
+		return v
+	}
+
+	return def
+}