refactor: marshaler unmarshaler iface

interfaces/13-io/json.go

@@ -24,11 +24,11 @@ func encode() ([]byte, error) {
 	return json.MarshalIndent(l, "", "\t")
 }
 
-func decode(data []byte) (l list, err error) {
+func decode(data []byte) (l list, _ error) {
 	if err := json.Unmarshal(data, &l); err != nil {
 		return nil, err
 	}
-	return l, nil
+	return
 }
 
 func decodeFile(path string) (list, error) {

interfaces/13-io/list.go

@@ -8,6 +8,7 @@
 package main
 
 import (
+	"sort"
 	"strings"
 )
 
@@ -34,16 +35,20 @@ func (l list) discount(ratio float64) {
 	}
 }
 
-// by default `list` sorts by `Title`.
+// implementation of the sort.Interface:
+
+// by default `list` sorts by `title`.
 func (l list) Len() int           { return len(l) }
-func (l list) Swap(i, j int)      { l[i], l[j] = l[j], l[i] }
 func (l list) Less(i, j int) bool { return l[i].Title < l[j].Title }
+func (l list) Swap(i, j int)      { l[i], l[j] = l[j], l[i] }
 
 // byRelease sorts by product release dates.
-type byRelease struct {
-	list
-}
+type byRelease struct{ list }
 
 func (bp byRelease) Less(i, j int) bool {
 	return bp.list[i].Released.Before(bp.list[j].Released.Time)
 }
+
+func byReleaseDate(l list) sort.Interface {
+	return &byRelease{l}
+}

interfaces/13-io/product.go

@@ -14,7 +14,7 @@ import (
 type product struct {
 	Title    string    `json:"title"`
 	Price    money     `json:"price"`
-	Released timestamp `json:"released,omitempty"`
+	Released timestamp `json:"released"`
 }
 
 func (p *product) String() string {

interfaces/13-io/timestamp.go

@@ -31,7 +31,7 @@ func (ts *timestamp) UnmarshalJSON(data []byte) error {
 	return nil
 }
 
-func (ts timestamp) MarshalJSON() (out []byte, err error) {
+func (ts timestamp) MarshalJSON() (out []byte, _ error) {
 	return strconv.AppendInt(out, ts.Unix(), 10), nil
 }