add: money type to interfaces

interfaces/03-nonstructs/game.go

@@ -11,24 +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())
 }
 
-// + discount gets a copy of `*game`.
-// + discount can update the original `game` through the game pointer.
-// + it's better to use the same receiver type: `*game` for all methods.
 func (g *game) discount(ratio float64) {
-	g.price *= (1 - ratio)
+	g.price *= money(1 - ratio)
 }
-
-// PREVIOUS CODE:
-
-// ----------------------------------------------------------------------------
-// + `g` is a copy: `discount` cannot change the original `g`.
-// func (g game) discount(ratio float64) {
-// 	g.price *= (1 - ratio)
-// }