Example 7.

animal/animal.go

@@ -3,5 +3,14 @@ package animal
 // implementation of Animal
 type Dog struct{}
 
-func (a Dog) Speaks() string  { return "woof" }
-func (a Dog) IsTrained() bool { return true }
+func (d Dog) Speaks() string  { return "woof" }
+func (d Dog) IsTrained() bool { return true }
+func (d Dog) Jump() string    { return "jumps" }
+func (d Dog) Sit() string     { return "sit" }
+
+type Cat struct{}
+
+func (c Cat) IsTrained() bool { return false }
+func (c Cat) Speaks() string  { return "meow!" }
+func (c Cat) Jump() string    { return "meow!!" }
+func (c Cat) Sit() string     { return "meow!!!" }

circus/circus.go

@@ -2,6 +2,8 @@ package circus
 
 const (
 	ActVoice = iota
+	ActSit
+	ActJump
 )
 
 type Speaker interface {
@@ -11,6 +13,8 @@ type Speaker interface {
 type Animal interface {
 	Speaker
 	IsTrained() bool
+	Jump() string
+	Sit() string
 }
 
 type Tamer struct{}
@@ -21,6 +25,10 @@ func (t *Tamer) Command(action int, a Animal) string {
 	switch action {
 	case ActVoice:
 		return a.Speaks()
+	case ActSit:
+		return a.Sit()
+	case ActJump:
+		return a.Jump()
 	}
 	return ""
 }

main.go

@@ -8,10 +8,16 @@ import (
 )
 
 func main() {
-	d := &animal.Dog{}
 	t := &circus.Tamer{}
-	t2 := &circus.Tamer{}
+	d := &animal.Dog{}
 
-	fmt.Println(t.Command(circus.ActVoice, d))  // woof
-	fmt.Println(t.Command(circus.ActVoice, t2)) // Error: *circus.Tamer does not implement circus.Animal
+	fmt.Println(t.Command(circus.ActVoice, d)) // "woof"
+	fmt.Println(t.Command(circus.ActJump, d))  // "jumps"
+	fmt.Println(t.Command(circus.ActSit, d))   // "sit"
+
+	t2 := &circus.Tamer{}
+	c := &animal.Cat{}
+	fmt.Println(t2.Command(circus.ActVoice, c)) // "meow"
+	fmt.Println(t2.Command(circus.ActJump, c))  // "meow!!"
+	fmt.Println(t2.Command(circus.ActSit, c))   // "meow!!!"
 }