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)
 }