update: interface promoted methods

interfaces/08-promoted-methods/book.go

@@ -0,0 +1,60 @@
+// 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"
+	"strconv"
+	"time"
+)
+
+type book struct {
+	// embed the product type into the book type.
+	// all the fields and methods of the product will be
+	// available in this book type.
+	product
+	published interface{}
+}
+
+// book type's print method takes priority.
+//
+// + when you call it on a book value, the following method will
+//   be executed.
+//
+// + if it wasn't here, the product type's print method would
+//   have been executed.
+func (b *book) print() {
+	// the book can also call the embedded product's print method
+	// if it wants to, as in here:
+	b.product.print()
+
+	p := format(b.published)
+	fmt.Printf("\t - (%v)\n", p)
+}
+
+func format(v interface{}) string {
+	var t int
+
+	switch v := v.(type) {
+	case int:
+		// book{title: "moby dick", price: 10, published: 118281600},
+		t = v
+	case string:
+		// book{title: "odyssey", price: 15, published: "733622400"},
+		t, _ = strconv.Atoi(v)
+	default:
+		// book{title: "hobbit", price: 25},
+		return "unknown"
+	}
+
+	// Mon Jan 2 15:04:05 -0700 MST 2006
+	const layout = "2006/01"
+
+	u := time.Unix(int64(t), 0)
+	return u.Format(layout)
+}