refactor: stringer interface

interfaces/09-stringer/book.go

@@ -9,43 +9,20 @@ package main
 
 import (
 	"fmt"
-	"strconv"
-	"time"
 )
 
 type book struct {
 	product
-	published interface{}
+	published timestamp
 }
 
 // book satisfies the fmt.Stringer
 func (b *book) String() string {
-	p := format(b.published)
-
 	// product.String() has a pointer receiver.
 	// That's why you need to manually take the product's address here.
 	//
 	// If you pass: "b.product", Go would pass it as a copy to Sprintf.
 	// In that case, Go can't deference b.product automatically.
 	// It's because: b.product would be different value—a copy.
-	return fmt.Sprintf("%s - (%v)", &b.product, p)
-}
-
-func format(v interface{}) string {
-	var t int
-
-	switch v := v.(type) {
-	case int:
-		t = v
-	case string:
-		t, _ = strconv.Atoi(v)
-	default:
-		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)
+	return fmt.Sprintf("%s - (%s)", &b.product, b.published)
 }

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)
 }

interfaces/09-stringer/timestamp.go

@@ -0,0 +1,51 @@
+// 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 (
+	"strconv"
+	"time"
+)
+
+// Mon Jan 2 15:04:05 -0700 MST 2006
+const layout = "2006/01"
+
+// unknown is the zero value of a timestamp.
+var unknown = timestamp(time.Time{})
+
+// Timestamp prints timestamps, it's a stringer.
+// Timestamp is useful even if it's zero.
+type timestamp time.Time
+
+// String makes the timestamp a stringer.
+func (ts timestamp) String() string {
+	t := time.Time(ts)
+
+	if t.IsZero() {
+		return "unknown"
+	}
+
+	return t.Format(layout)
+}
+
+// toTimestamp was "book.format" before.
+// Now it returns a timestamp value depending on the type of `v`.
+func toTimestamp(v interface{}) timestamp {
+	var t int
+
+	switch v := v.(type) {
+	case int:
+		t = v
+	case string:
+		t, _ = strconv.Atoi(v)
+	default:
+		return unknown
+	}
+
+	return timestamp(time.Unix(int64(t), 0))
+}