add: money type to interfaces

interfaces/04-interfaces/puzzle.go

@@ -11,9 +11,9 @@ import "fmt"
 
 type puzzle struct {
 	title string
-	price float64
+	price money
 }
 
 func (p puzzle) print() {
-	fmt.Printf("%-15s: $%.2f \n", p.title, p.price)
+	fmt.Printf("%-15s: %s\n", p.title, p.price.string())
 }