add: interfaces assertion and composition (wip)

interfaces/05-composition/time.go

@@ -0,0 +1,19 @@
+package main
+
+// usually: include the time method in the list.go
+// it is here for clarity
+
+// TODO: NEW
+// you could include a time method in the book and game instead.
+// but sometimes it is not possible to do so.
+func (l list) time() (total int) {
+	for _, it := range l {
+		switch it := it.(type) {
+		case *game:
+			total += it.playTime
+		case book:
+			total += it.readTime
+		}
+	}
+	return total
+}