diff --git a/16_interfaces/casting.v b/16_interfaces/casting.v
index 7b4c443..79398f1 100644
--- a/16_interfaces/casting.v
+++ b/16_interfaces/casting.v
@@ -2,21 +2,19 @@
 
 struct Dog{
 	breed string
-	speak() string
 }
 
-fn (d Dog) speak() string {
-	println('Woof')
+fn (d Dog) speak() string{
+	return 'Woof'
 }
 
 
 struct Cat{
 	breed string
-	speak() string
 }
 
 fn (c Cat) speak() string{
-	println('Meow')
+	return 'Meow'
 }
 
 interface Something{}
@@ -24,7 +22,7 @@ interface Something{}
 
 fn announce(s Something){
 	if s is Dog {
-		println('a ${s.breed] dog')
+		println('a ${s.breed} dog')
 	}else if s is Cat{
 		println('a cat speaks ${s.speak()}')
 	}else {
diff --git a/16_interfaces/dynamic_casts.v b/16_interfaces/dynamic_casts.v
new file mode 100644
index 0000000..d03c1b6
--- /dev/null
+++ b/16_interfaces/dynamic_casts.v
@@ -0,0 +1,34 @@
+struct Moon{}
+
+struct Mars{}
+
+struct Venus{}
+
+type World = Mars | Moon | Venus
+
+fn (m Mars) dust_storm() bool {
+	return true
+}
+
+fn main(){
+	mut w := World(Moon{})
+
+	assert w is Moon
+	w = Mars{}
+
+	mars := w as Mars
+	
+	if mars.dust_storm(){
+		println('bad weather!')
+	}
+
+	//w1 := World(Mars{})
+
+	if mut w is Mars{
+		assert typeof(w).name == typeof(Mars{}).name
+		
+		if w.dust_storm(){
+			println('bad weather!')
+		}
+	}
+}
\ No newline at end of file
diff --git a/16_interfaces/embedded_interface.v b/16_interfaces/embedded_interface.v
new file mode 100644
index 0000000..7d26426
--- /dev/null
+++ b/16_interfaces/embedded_interface.v
@@ -0,0 +1,29 @@
+pub interface Reader{
+mut:
+	read(mut buf []u8) ?int
+}
+
+pub interface Write{
+mut:
+	write(buf []u8) ?int
+}
+
+pub interface ReaderWriter{
+	Reader
+	Write
+}
+
+
+struct Moon{}
+struct Mars{}
+struct Venus{}
+
+type World = Mars | Moon | Venus
+
+fn main(){
+
+	sum := World(Moon{})
+	assert sum.type_name() == 'Moon'
+	println(sum)
+	
+}
\ No newline at end of file
diff --git a/16_interfaces/interface_example4.v b/16_interfaces/interface_example4.v
new file mode 100644
index 0000000..91b71e1
--- /dev/null
+++ b/16_interfaces/interface_example4.v
@@ -0,0 +1,36 @@
+interface IFoo{
+	foo()
+}
+
+interface IBar{
+	bar()
+}
+
+struct SFoo{}
+
+fn (sf SFoo) foo() {}
+
+
+struct SFooBar {}
+
+fn (sfb SFooBar) foo() {}
+
+
+fn (sfb SFooBar) bar() {
+	dump('This implements IBar')
+}
+
+fn main(){
+
+	mut arr := []IFoo{}
+	arr << SFoo{}
+	arr << SFooBar{}
+
+	for a in arr{
+		dump(a)
+	
+		if a is IBar{
+			a.bar()
+		}
+	}
+}
\ No newline at end of file
diff --git a/16_interfaces/interface_example5.v b/16_interfaces/interface_example5.v
new file mode 100644
index 0000000..49776bb
--- /dev/null
+++ b/16_interfaces/interface_example5.v
@@ -0,0 +1,34 @@
+interface Adoptable{}
+
+fn (a Adoptable) speak() string {
+	return 'adopt me!'
+}
+
+struct Cat{}
+
+fn (c Cat) speak() string{
+	return 'meow!'
+}
+
+struct Dog{}
+
+fn main(){
+	cat := Cat{}
+
+	assert dump(cat.speak()) == 'meow!'
+
+	a := Adoptable(cat)
+
+	assert dump(a.speak()) == 'adopt me!'
+
+	if a is Cat{
+		dump(a.speak())
+	}
+
+	b := Adoptable(Dog{})
+	assert dump(b.speak()) == 'adopt me!'
+
+	//if b is Dog{
+	//	dump(b.speak())
+	//}
+}
\ No newline at end of file
diff --git a/16_interfaces/interface_example6.v b/16_interfaces/interface_example6.v
new file mode 100644
index 0000000..0c5c795
--- /dev/null
+++ b/16_interfaces/interface_example6.v
@@ -0,0 +1,24 @@
+struct Empty{}
+
+struct Node{
+	value f64
+	left Tree
+	right Tree
+}
+
+type Tree = Empty | Node
+
+fn sum(tree Tree) f64{
+	return match tree {
+		Empty { 0 }
+		Node { tree.value + sum(tree.left) + sum(tree.right)}
+	}
+}
+
+fn main(){
+	left := Node{0.2,Empty{}, Empty{}}
+	right := Node{0.3, Empty{}, Node{0.4, Empty{},Empty{},}}
+	tree := Node{0.5, left, right}
+
+	println(sum(tree))
+}
\ No newline at end of file
diff --git a/16_interfaces/matching_sum.v b/16_interfaces/matching_sum.v
new file mode 100644
index 0000000..3e6faea
--- /dev/null
+++ b/16_interfaces/matching_sum.v
@@ -0,0 +1,44 @@
+struct Moon{}
+
+struct Mars{}
+
+struct Venus{}
+
+type World = Mars | Moon | Venus
+
+fn open_parachutes(n int){
+	println(n)
+}
+
+fn land(w World){
+	match w {
+		Moon{}
+		Mars{
+			open_parachutes(3)
+		}
+		Venus{
+			open_parachutes(1)
+		}
+	}
+}
+
+
+fn (m Moon) moon_walk(){}
+fn(m Mars) shiver(){}
+fn(v Venus) sweat(){}
+
+fn pass_time(w World){
+	match w{
+		Moon { w.moon_walk() }
+		Mars { w.shiver()    }
+		else {}
+	}
+}
+
+fn main(){
+	
+	mut world := World(Mars{})
+	
+	land(world)
+	pass_time(world)
+}
\ No newline at end of file
diff --git a/README.md b/README.md
index 6cf551b..9345695 100644
--- a/README.md
+++ b/README.md
@@ -1 +1,3 @@
 ### Codes of my learning process in V Programming Language
+
+[V Documentation](https://github.com/vlang/v/blob/master/doc/docs.md)
\ No newline at end of file