add: fmt.stringer to item interface

interfaces/09-stringer/book.go

@@ -22,11 +22,12 @@ type book struct {
 func (b *book) String() string {
 	p := format(b.published)
 
-	// product.String has a pointer receiver.
-	// that's why we need to take its address here.
+	// product.String() has a pointer receiver.
+	// That's why you need to manually take the product's address here.
 	//
-	// when you put a value in an interface, the interface
-	// cannot run that value's type's value-receiver methods.
+	// 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)
 }
 

interfaces/09-stringer/list.go

@@ -12,7 +12,18 @@ import (
 	"strings"
 )
 
-type list []fmt.Stringer
+type item interface {
+	discount(float64)
+
+	// item interface embeds the fmt.Stringer interface.
+	//
+	// Go adds all the methods of the fmt.Stringer
+	// to the item interface.
+	fmt.Stringer // same (see below): String() string
+	// String() string
+}
+
+type list []item
 
 // list satisfies the fmt.Stringer
 func (l list) String() string {
@@ -35,13 +46,7 @@ func (l list) String() string {
 }
 
 func (l list) discount(ratio float64) {
-	type discounter interface {
-		discount(float64)
-	}
-
 	for _, it := range l {
-		if it, ok := it.(discounter); ok {
-			it.discount(ratio)
-		}
+		it.discount(ratio)
 	}
 }