add: money type to interfaces

interfaces/04-interfaces/game.go

@@ -11,13 +11,13 @@ import "fmt"
 
 type game struct {
 	title string
-	price float64
+	price money
 }
 
 func (g *game) print() {
-	fmt.Printf("%-15s: $%.2f\n", g.title, g.price)
+	fmt.Printf("%-15s: %s\n", g.title, g.price.string())
 }
 
 func (g *game) discount(ratio float64) {
-	g.price *= (1 - ratio)
-}
+	g.price *= money(1 - ratio)
+}