add: type assertion

interfaces/05-assertion/list.go

@@ -11,6 +11,9 @@ import "fmt"
 
 type printer interface {
 	print()
+
+	// use type assertion when you cannot change the interface.
+	// discount(ratio float64)
 }
 
 type list []printer
@@ -26,59 +29,50 @@ func (l list) print() {
 	}
 }
 
+// type assertion can extract the wrapped value.
+// or: it can put the value into another interface.
 func (l list) discount(ratio float64) {
+	// you can declare an interface in a function/method as well.
+	// interface is just a type.
 	type discounter interface {
 		discount(float64)
 	}
 
 	for _, it := range l {
+		// you can assert to an interface.
+		// and extract another interface.
 		if it, ok := it.(discounter); ok {
 			it.discount(ratio)
 		}
 	}
 }
 
-//
-// type assertion can pull out the real value behind an interface value.
-//
-// it can also check whether the value convertable to a given type.
-//
+// ----
+
 // func (l list) discount(ratio float64) {
 // 	for _, it := range l {
-// 		// remember: interface is a type
-// 		if it, ok := it.(interface{ discount(float64) }); ok {
-// 			it.discount(ratio)
+// 		// you can inline-assert interfaces
+// 		// interface is just a type.
+// 		g, ok := it.(interface{ discount(float64) })
+// 		if !ok {
+// 			continue
 // 		}
+
+// 		g.discount(ratio)
 // 	}
 // }
 
-//
-// type switch can pull out the real value behind an interface value.
-//
-// func (l list) discount(ratio float64) {
-// 	for _, it := range l {
-// 		switch it := it.(type) {
-// 		case *game:
-// 			it.discount(ratio)
-// 		case *puzzle:
-// 			it.discount(ratio)
-// 		}
-// 	}
-// }
+// ----
 
-//
-// type switch can find the type behind an interface value.
-//
+// // type assertion can pull out the real value behind an interface value.
+// // it can also check whether the value convertable to a given type.
 // func (l list) discount(ratio float64) {
 // 	for _, it := range l {
-// 		switch it.(type) {
-// 		case *game:
-// 			fmt.Print("it's a *game!   --> ")
-// 		case *puzzle:
-// 			fmt.Print("it's a *puzzle! --> ")
-// 		default:
-// 			fmt.Print("neither         --> ")
+// 		g, ok := it.(*game)
+// 		if !ok {
+// 			continue
 // 		}
-// 		it.print()
+
+// 		g.discount(ratio)
 // 	}
 // }