add: interfaces assertion and composition (wip)

interfaces/05-composition/puzzle.go

@@ -0,0 +1,24 @@
+// 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 puzzle struct {
+	title string
+	price float64
+}
+
+func (p puzzle) print() {
+	fmt.Printf("%-15s: $%.2f \n", p.title, p.price)
+}
+
+// TODO: NEW
+func (p puzzle) sum() float64 {
+	return p.price
+}