add: string interface

interfaces/09-stringer/main.go

@@ -0,0 +1,29 @@
+// 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"
+
+func main() {
+	store := list{
+		&book{product{"moby dick", 10}, 118281600},
+		&book{product{"odyssey", 15}, "733622400"},
+		&book{product{"hobbit", 25}, nil},
+		&puzzle{product{"rubik's cube", 5}},
+		&game{product{"minecraft", 20}},
+		&game{product{"tetris", 5}},
+		&toy{product{"yoda", 150}},
+	}
+
+	store.discount(.5)
+
+	// the store doesn't have a print method anymore.
+	// but the Print function can print it.
+	// it's because, the list satisfies the stringer.
+	fmt.Print(store)
+}