update: interface promoted methods

interfaces/08-promoted-methods/product.go

@@ -0,0 +1,23 @@
+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package main
+
+import "fmt"
+
+type product struct {
+	title string
+	price money
+}
+
+func (p *product) print() {
+	fmt.Printf("%-15s: %s\n", p.title, p.price.string())
+}
+
+func (p *product) discount(ratio float64) {
+	p.price *= money(1 - ratio)
+}