refactor: stringer interface

interfaces/09-stringer/main.go

@@ -11,9 +11,9 @@ import "fmt"
 
 func main() {
 	store := list{
-		&book{product{"moby dick", 10}, 118281600},
-		&book{product{"odyssey", 15}, "733622400"},
-		&book{product{"hobbit", 25}, nil},
+		&book{product{"moby dick", 10}, toTimestamp(118281600)},
+		&book{product{"odyssey", 15}, toTimestamp("733622400")},
+		&book{product{"hobbit", 25}, unknown},
 		&puzzle{product{"rubik's cube", 5}},
 		&game{product{"minecraft", 20}},
 		&game{product{"tetris", 5}},
@@ -26,4 +26,8 @@ func main() {
 	// but the Print function can print it.
 	// it's because, the list satisfies the stringer.
 	fmt.Print(store)
+
+	// timestamp is useful even if it's zero.
+	var ts timestamp
+	fmt.Println(ts)
 }