refactor: stringer

interfaces/09-stringer/main.go

@@ -10,10 +10,15 @@ package main
 import "fmt"
 
 func main() {
+	// The money type is a stringer.
+	// You don't need to call the String method when printing a value of it.
+	// var pocket money = 10
+	// fmt.Println(pocket)
+
 	store := list{
 		&book{product{"moby dick", 10}, toTimestamp(118281600)},
 		&book{product{"odyssey", 15}, toTimestamp("733622400")},
-		&book{product{"hobbit", 25}, unknown},
+		&book{product{"hobbit", 25}, toTimestamp(nil)},
 		&puzzle{product{"rubik's cube", 5}},
 		&game{product{"minecraft", 20}},
 		&game{product{"tetris", 5}},
@@ -22,12 +27,12 @@ func main() {
 
 	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.
+	// The list is a stringer.
+	// The `fmt.Print` function can print the `store`
+	// by calling `store`'s `String()` method.
+	//
+	// Underneath, `fmt.Print` uses a type switch to
+	// detect whether a type is a Stringer:
+	// https://golang.org/src/fmt/print.go#L627
 	fmt.Print(store)
-
-	// timestamp is useful even if it's zero.
-	var ts timestamp
-	fmt.Println(ts)
 }