reorganize: interfaces section

interfaces/04-interfaces/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
+
+func main() {
+	var (
+		mobydick  = book{title: "moby dick", price: 10}
+		minecraft = game{title: "minecraft", price: 20}
+		tetris    = game{title: "tetris", price: 5}
+		rubik     = puzzle{title: "rubik's cube", price: 5}
+	)
+
+	var store list
+	store = append(store, &minecraft, &tetris, mobydick, rubik)
+
+	tetris.discount(.8)
+	// printer(tetris).discount(.8)
+	store.print()
+
+	// p can store a value of any type that has a `print()` method
+	var p printer
+	p = puzzle{title: "sidewinder", price: 10}
+	p.print()
+}