refactor: stringer interface

interfaces/10-marshaler/book.go

@@ -8,57 +8,14 @@
 package main
 
 import (
-	"encoding/json"
 	"fmt"
-	"strconv"
-	"time"
 )
 
 type book struct {
 	product
-	Published interface{}
+	Published timestamp
 }
 
 func (b book) String() string {
-	p := format(b.Published)
-	return fmt.Sprintf("%s - (%v)", &b.product, p)
-}
-
-// MarshalJSON is an implementation of the json.Marshaler interface.
-// json.Marshal and Decode call this method.
-func (b *book) MarshalJSON() ([]byte, error) {
-	// Calling Marshal creates an infinite loop because Marshal calls
-	// MarshalJSON again and again.
-	//
-	// Declaring a clone type without MarshalJSON fixes the problem.
-	type jbook book
-
-	// book is convertable to jbook
-	// jbook's underlying type is book.
-	jb := (*jbook)(b)
-
-	// Customize the formatting of the published field.
-	// This will result in the published field of the resulting json.
-	jb.Published = format(b.Published)
-
-	return json.Marshal(jb)
-}
-
-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)
 }