diff --git a/interfaces/12-marshaler/json.go b/interfaces/12-marshaler/json.go
index b4018db..b26a7df 100644
--- a/interfaces/12-marshaler/json.go
+++ b/interfaces/12-marshaler/json.go
@@ -7,9 +7,7 @@
 
 package main
 
-import (
-	"encoding/json"
-)
+import "encoding/json"
 
 func encode() ([]byte, error) {
 	l := list{
@@ -17,13 +15,12 @@ func encode() ([]byte, error) {
 		{Title: "odyssey", Price: 15, Released: toTimestamp("733622400")},
 		{Title: "hobbit", Price: 25},
 	}
-
 	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
 }
diff --git a/interfaces/12-marshaler/list.go b/interfaces/12-marshaler/list.go
index b501e9f..e313b8f 100644
--- a/interfaces/12-marshaler/list.go
+++ b/interfaces/12-marshaler/list.go
@@ -8,6 +8,7 @@
 package main
 
 import (
+	"sort"
 	"strings"
 )
 
@@ -34,16 +35,20 @@ func (l list) discount(ratio float64) {
 	}
 }
 
+// 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}
+}
diff --git a/interfaces/12-marshaler/main.go b/interfaces/12-marshaler/main.go
index 985fc12..a238fd9 100644
--- a/interfaces/12-marshaler/main.go
+++ b/interfaces/12-marshaler/main.go
@@ -9,13 +9,45 @@ package main
 
 import (
 	"fmt"
+	"log"
 )
 
 func main() {
-	// removed error handling for brevity (normally you shouldn't do this).
-	out, _ := encode()
-	fmt.Println(string(out))
+	// First encode products as JSON:
+	data, err := encode()
+	if err != nil {
+		log.Fatalln(err)
+	}
+	fmt.Println(string(data))
 
-	l, _ := decode(out)
+	// Then decode them back from JSON:
+	l, err := decode(data)
+	if err != nil {
+		log.Fatalln(err)
+	}
+
+	// Let the list value print itself:
 	fmt.Print(l)
 }
+
+/*
+Summary:
+
+- json.Marshal() and json.MarshalIndent() can only encode primitive types.
+  - It cannot encode custom types.
+  - Implement the json.Marshaler interface and teach it how to encode your custom types.
+  - See time.Time code: It satisfies the json.Marshaler interface.
+
+- json.Unmarshal() can only decode primitive types.
+  - It cannot decode custom types.
+  - Implement the json.Unmarshaler interface and teach it how to decode your custom types.
+  - See time.Time code: It satisfies the json.Unmarshaler interface as well.
+
+- strconv.AppendInt() can append an int value to a []byte.
+  - There are several other functions in the strconv package for other primitive types as well.
+  - Do not make unnecessary string <-> []byte conversions.
+
+- log.Fatalln() can print the given error message and terminate the program.
+  - Use it only from the main(). Do not use it in other functions.
+  - main() is should be the main driver of a program.
+*/
diff --git a/interfaces/12-marshaler/product.go b/interfaces/12-marshaler/product.go
index 356b0d7..20d51ab 100644
--- a/interfaces/12-marshaler/product.go
+++ b/interfaces/12-marshaler/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 {
diff --git a/interfaces/12-marshaler/timestamp.go b/interfaces/12-marshaler/timestamp.go
index ebc9c0a..b23a1bd 100644
--- a/interfaces/12-marshaler/timestamp.go
+++ b/interfaces/12-marshaler/timestamp.go
@@ -16,6 +16,8 @@ type timestamp struct {
 	time.Time
 }
 
+// implementation of the fmt.Stringer interface:
+
 func (ts timestamp) String() string {
 	if ts.IsZero() {
 		return "unknown"
@@ -26,8 +28,9 @@ func (ts timestamp) String() string {
 	return ts.Format(layout)
 }
 
+// implementation of the json.Marshaler and json.Unmarshaler interfaces:
+
 // timestamp knows how to decode itself from json.
-// UnmarshalJSON is an implementation of the json.Unmarshaler interface.
 // json.Unmarshal and json.Decode call this method.
 func (ts *timestamp) UnmarshalJSON(data []byte) error {
 	*ts = toTimestamp(string(data))
@@ -35,11 +38,9 @@ func (ts *timestamp) UnmarshalJSON(data []byte) error {
 }
 
 // timestamp knows how to encode itself to json.
-// MarshalJSON is an implementation of the json.Marshaler interface.
 // json.Marshal and json.Encode call this method.
-func (ts timestamp) MarshalJSON() (out []byte, err error) {
-	// return ts.Time.MarshalJSON()
-	return strconv.AppendInt(out, ts.Unix(), 10), nil
+func (ts timestamp) MarshalJSON() (data []byte, _ error) {
+	return strconv.AppendInt(data, ts.Unix(), 10), nil
 }
 
 func toTimestamp(v interface{}) (ts timestamp) {
diff --git a/interfaces/13-io/json.go b/interfaces/13-io/json.go
index 7507c96..deca712 100644
--- a/interfaces/13-io/json.go
+++ b/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) {
diff --git a/interfaces/13-io/list.go b/interfaces/13-io/list.go
index b501e9f..94f5c04 100644
--- a/interfaces/13-io/list.go
+++ b/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}
+}
diff --git a/interfaces/13-io/product.go b/interfaces/13-io/product.go
index 356b0d7..20d51ab 100644
--- a/interfaces/13-io/product.go
+++ b/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 {
diff --git a/interfaces/13-io/timestamp.go b/interfaces/13-io/timestamp.go
index 940fc11..791bbe1 100644
--- a/interfaces/13-io/timestamp.go
+++ b/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
 }
 
diff --git a/interfaces/14-composition/json.go b/interfaces/14-composition/json.go
index 7507c96..deca712 100644
--- a/interfaces/14-composition/json.go
+++ b/interfaces/14-composition/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) {
diff --git a/interfaces/14-composition/list.go b/interfaces/14-composition/list.go
index b501e9f..94f5c04 100644
--- a/interfaces/14-composition/list.go
+++ b/interfaces/14-composition/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}
+}
diff --git a/interfaces/14-composition/product.go b/interfaces/14-composition/product.go
index 356b0d7..20d51ab 100644
--- a/interfaces/14-composition/product.go
+++ b/interfaces/14-composition/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 {
diff --git a/interfaces/14-composition/timestamp.go b/interfaces/14-composition/timestamp.go
index 940fc11..791bbe1 100644
--- a/interfaces/14-composition/timestamp.go
+++ b/interfaces/14-composition/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
 }