diff --git a/interfaces/08-composition/book.go b/interfaces/08-composition/book.go
index 4d1e5ac..c583442 100644
--- a/interfaces/08-composition/book.go
+++ b/interfaces/08-composition/book.go
@@ -7,19 +7,35 @@
 
 package main
 
-import "fmt"
+import (
+	"fmt"
+	"strconv"
+	"time"
+)
 
 type book struct {
-	title    string
-	price    money
-	readTime int
+	*product
+	published interface{}
 }
 
-func (b book) print() {
-	fmt.Printf("%-15s: %s\n", b.title, b.price.string())
+func (b *book) String() string {
+	p := format(b.published)
+	return fmt.Sprintf("%s - (%v)", b.product, p)
 }
 
-// TODO: NEW
-func (b book) sum() money {
-	return b.price
+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"
+	}
+
+	const layout = "2006/01"
+	u := time.Unix(int64(t), 0)
+	return u.Format(layout)
 }
diff --git a/interfaces/08-composition/discount.go b/interfaces/08-composition/discount.go
deleted file mode 100644
index 592c545..0000000
--- a/interfaces/08-composition/discount.go
+++ /dev/null
@@ -1,17 +0,0 @@
-package main
-
-// usually: include the discount method in the list.go
-// it is here for clarity
-
-// TODO: NEW
-func (l list) discount(ratio float64) {
-	type discounter interface {
-		discount(float64)
-	}
-
-	for _, it := range l {
-		if it, ok := it.(discounter); ok {
-			it.discount(ratio)
-		}
-	}
-}
diff --git a/interfaces/08-composition/game.go b/interfaces/08-composition/game.go
index 47ba4e6..6811170 100644
--- a/interfaces/08-composition/game.go
+++ b/interfaces/08-composition/game.go
@@ -7,27 +7,6 @@
 
 package main
 
-import "fmt"
-
 type game struct {
-	title    string
-	price    money
-	playTime int
-}
-
-func (g *game) print() {
-	fmt.Printf("%-15s: %s\n", g.title, g.price.string())
-}
-
-// func (g *game) String() string {
-// 	return fmt.Sprintf("%-15s: $%.2f", g.title, g.price)
-// }
-
-func (g *game) discount(ratio float64) {
-	g.price *= money(1 - ratio)
-}
-
-// TODO: NEW
-func (g *game) sum() money {
-	return g.price
+	*product
 }
diff --git a/interfaces/08-composition/list.go b/interfaces/08-composition/list.go
index 4bbf89d..bd87c8c 100644
--- a/interfaces/08-composition/list.go
+++ b/interfaces/08-composition/list.go
@@ -7,28 +7,51 @@
 
 package main
 
-import "fmt"
+import (
+	"fmt"
+	"strings"
+)
+
+type summer interface {
+	sum() money
+}
+
+type item interface {
+	summer       // same as: `sum() money`
+	fmt.Stringer // same as: `String() string`
+}
 
 type list []item
 
-func (l list) print() {
+func (l list) String() string {
 	if len(l) == 0 {
-		fmt.Println("Sorry. Our store is closed. We're waiting for the delivery 🚚.")
-		return
+		return "Sorry. We're waiting for delivery 🚚."
+	}
+
+	var str strings.Builder
+	for _, it := range l {
+		fmt.Fprintf(&str, "%s\n", it)
+	}
+	fmt.Fprintf(&str, "\tTOTAL  : $%.2f", l.sum())
+
+	return str.String()
+}
+
+func (l list) sum() (total money) {
+	for _, it := range l {
+		total += it.sum()
+	}
+	return
+}
+
+func (l list) discount(ratio float64) {
+	type discounter interface {
+		discount(float64)
 	}
 
 	for _, it := range l {
-		it.print()
+		if it, ok := it.(discounter); ok {
+			it.discount(ratio)
+		}
 	}
-
-	// TODO: NEW
-	fmt.Printf("\tTOTAL  : $%.2f\n", l.sum())
-}
-
-// TODO: NEW
-func (l list) sum() (n money) {
-	for _, it := range l {
-		n += it.sum()
-	}
-	return n
 }
diff --git a/interfaces/08-composition/main.go b/interfaces/08-composition/main.go
index 74d29f2..606bc06 100644
--- a/interfaces/08-composition/main.go
+++ b/interfaces/08-composition/main.go
@@ -7,37 +7,36 @@
 
 package main
 
-// import "fmt"
+import (
+	"fmt"
+	"sort"
+)
 
 func main() {
-	var (
-		mobydick  = book{title: "moby dick", price: 10, readTime: 10}
-		minecraft = game{title: "minecraft", price: 20, playTime: 5}
-		tetris    = game{title: "tetris", price: 5, playTime: 2}
-		rubik     = puzzle{title: "rubik's cube", price: 5}
-	)
+	store := list{
+		&book{&product{"moby dick", 10}, 118281600},
+		&book{&product{"odyssey", 15}, "733622400"},
+		&book{&product{"hobbit", 25}, nil},
+		&puzzle{&product{"rubik's cube", 5}},
+		&game{&product{"minecraft", 20}},
+		&game{&product{"tetris", 5}},
+		&toy{&product{"yoda", 150}},
+	}
 
-	var store list
-	store = append(store, &minecraft, &tetris, mobydick, rubik)
-	// tetris.discount(.8)
-	// store.print()
+	books := store[:3]
+	others := store[3:]
 
-	store.discount(.5)
-	store.print()
+	books.discount(.5)
+	fmt.Printf("%s\n", store)
 
-	// t := store.time()
-	// fmt.Printf("Total entertainment time: %d hours\n", t)
+	sort.Sort(byPrice(others))
+	fmt.Printf("\n%s\n", store)
 
-	// games := store[:2]
-	// other := store[2:]
-	// games.print()
-	// other.print()
-	// list{games, other}.print()
+	store = list{books, others}
+	fmt.Printf("\n%s\n", store)
 
-	// var b *book
-
-	// var np printer
-	// np = b
-	// // p := printer(puzzle{title: "sidewinder", price: 10})
-	// fmt.Println(np == nil)
+	// store.discount(.5)
+	// // sort.Sort(sort.Reverse(byPrice(store)))
+	// sort.Sort(byName(store))
+	// fmt.Println(store)
 }
diff --git a/interfaces/08-composition/money.go b/interfaces/08-composition/money.go
index 86f7d63..095a039 100644
--- a/interfaces/08-composition/money.go
+++ b/interfaces/08-composition/money.go
@@ -11,6 +11,6 @@ import "fmt"
 
 type money float64
 
-func (m money) string() string {
+func (m money) String() string {
 	return fmt.Sprintf("$%.2f", m)
 }
diff --git a/interfaces/08-composition/product.go b/interfaces/08-composition/product.go
new file mode 100644
index 0000000..f1b3383
--- /dev/null
+++ b/interfaces/08-composition/product.go
@@ -0,0 +1,27 @@
+// 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 "fmt"
+
+type product struct {
+	title string
+	price money
+}
+
+func (p *product) discount(ratio float64) {
+	p.price *= money(1 - ratio)
+}
+
+func (p *product) sum() money {
+	return p.price
+}
+
+func (p *product) String() string {
+	return fmt.Sprintf("%-15s: %s", p.title, p.price)
+}
diff --git a/interfaces/08-composition/puzzle.go b/interfaces/08-composition/puzzle.go
index 52dd4c1..bf3ecc4 100644
--- a/interfaces/08-composition/puzzle.go
+++ b/interfaces/08-composition/puzzle.go
@@ -7,18 +7,6 @@
 
 package main
 
-import "fmt"
-
 type puzzle struct {
-	title string
-	price money
-}
-
-func (p puzzle) print() {
-	fmt.Printf("%-15s: %s\n", p.title, p.price.string())
-}
-
-// TODO: NEW
-func (p puzzle) sum() money {
-	return p.price
+	*product
 }
diff --git a/interfaces/08-composition/sort.go b/interfaces/08-composition/sort.go
new file mode 100644
index 0000000..58d358c
--- /dev/null
+++ b/interfaces/08-composition/sort.go
@@ -0,0 +1,20 @@
+// 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
+
+type byPrice list
+
+func (l byPrice) Less(i, j int) bool { return l[i].sum() < l[j].sum() }
+func (l byPrice) Len() int           { return len(l) }
+func (l byPrice) Swap(i, j int)      { l[i], l[j] = l[j], l[i] }
+
+type byName list
+
+func (l byName) Less(i, j int) bool { return l[i].String() < l[j].String() }
+func (l byName) Len() int           { return len(l) }
+func (l byName) Swap(i, j int)      { l[i], l[j] = l[j], l[i] }
diff --git a/interfaces/08-composition/time.go b/interfaces/08-composition/time.go
deleted file mode 100644
index ef7ff17..0000000
--- a/interfaces/08-composition/time.go
+++ /dev/null
@@ -1,19 +0,0 @@
-package main
-
-// usually: include the time method in the list.go
-// it is here for clarity
-
-// TODO: NEW
-// you could include a time method in the book and game instead.
-// but sometimes it is not possible to do so.
-func (l list) time() (total int) {
-	for _, it := range l {
-		switch it := it.(type) {
-		case *game:
-			total += it.playTime
-		case book:
-			total += it.readTime
-		}
-	}
-	return total
-}
diff --git a/interfaces/08-composition/toy.go b/interfaces/08-composition/toy.go
new file mode 100644
index 0000000..73e99c4
--- /dev/null
+++ b/interfaces/08-composition/toy.go
@@ -0,0 +1,12 @@
+// 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
+
+type toy struct {
+	*product
+}
diff --git a/interfaces/08-composition/types.go b/interfaces/08-composition/types.go
deleted file mode 100644
index 8a10e3a..0000000
--- a/interfaces/08-composition/types.go
+++ /dev/null
@@ -1,23 +0,0 @@
-package main
-
-// don't separate your interfaces like this.
-// put them in the same file where you need to use them.
-// this is here for clarity
-
-type printer interface {
-	print()
-}
-
-// TODO: NEW
-type summer interface {
-	sum() money
-}
-
-// TODO: NEW
-// interface embedding
-// When an interface includes multiple methods,
-// choose a name that accurately describes its purpose.
-type item interface {
-	printer
-	summer
-}