diff --git a/x/io/_demo/asyncdemo/async.go b/x/io/_demo/asyncdemo/async.go
index ff75e943..17138fc3 100644
--- a/x/io/_demo/asyncdemo/async.go
+++ b/x/io/_demo/asyncdemo/async.go
@@ -27,15 +27,13 @@ func (r *Response) TextCompiled() *io.PromiseImpl[string] {
 	P := &io.PromiseImpl[string]{}
 	P.Debug = "Text"
 	P.Func = func(resolve func(string, error)) {
-		for {
-			switch P.Next {
-			case 0:
-				P.Next = -1
-				resolve(r.Body, nil)
-				return
-			default:
-				panic("Promise already done")
-			}
+		switch P.Next {
+		case 0:
+			P.Next = -1
+			resolve(r.Body, nil)
+			return
+		default:
+			panic("Promise already done")
 		}
 	}
 	return P
@@ -65,15 +63,13 @@ func AsyncHttpGetCompiled(url string) *io.PromiseImpl[*Response] {
 	P := &io.PromiseImpl[*Response]{}
 	P.Debug = "HttpGet"
 	P.Func = func(resolve func(*Response, error)) {
-		for {
-			switch P.Next {
-			case 0:
-				P.Next = -1
-				Http("GET", url, resolve)
-				return
-			default:
-				panic("Promise already done")
-			}
+		switch P.Next {
+		case 0:
+			P.Next = -1
+			Http("GET", url, resolve)
+			return
+		default:
+			panic("Promise already done")
 		}
 	}
 	return P
@@ -88,15 +84,13 @@ func AsyncHttpPostCompiled(url string) *io.PromiseImpl[*Response] {
 	P := &io.PromiseImpl[*Response]{}
 	P.Debug = "HttpPost"
 	P.Func = func(resolve func(*Response, error)) {
-		for {
-			switch P.Next {
-			case 0:
-				P.Next = -1
-				Http("POST", url, resolve)
-				return
-			default:
-				panic("Promise already done")
-			}
+		switch P.Next {
+		case 0:
+			P.Next = -1
+			Http("POST", url, resolve)
+			return
+		default:
+			panic("Promise already done")
 		}
 	}
 	return P
@@ -142,55 +136,52 @@ func GetUserCompiled(name string) *io.PromiseImpl[User] {
 	P := &io.PromiseImpl[User]{}
 	P.Debug = "GetUser"
 	P.Func = func(resolve func(User, error)) {
-		for {
-			switch P.Next {
-			case 0:
-				P.Next = 1
-				state1 = AsyncHttpGetCompiled("http://example.com/user/" + name)
-				state1.Exec = P.Exec
-				state1.Parent = P
-				state1.Call()
+		switch P.Next {
+		case 0:
+			P.Next = 1
+			state1 = AsyncHttpGetCompiled("http://example.com/user/" + name)
+			state1.Exec = P.Exec
+			state1.Parent = P
+			state1.Call()
+			return
+		case 1:
+			P.Next = 2
+			resp, err := state1.Value(), state1.Err()
+			log.Printf("resp: %v, err: %v\n", resp, err)
+			if err != nil {
+				resolve(User{}, err)
 				return
-			case 1:
-				P.Next = 2
-				resp, err := state1.Value, state1.Err
-				log.Printf("resp: %v, err: %v\n", resp, err)
-				if err != nil {
-					resolve(User{}, err)
-					return
-				}
-
-				if resp.StatusCode != 200 {
-					resolve(User{}, fmt.Errorf("http status code: %d", resp.StatusCode))
-					return
-				}
-
-				state2 = resp.TextCompiled()
-				state2.Exec = P.Exec
-				state2.Parent = P
-				state2.Call()
-				log.Printf("TextCompiled state2: %+v\n", state2)
-				return
-			case 2:
-				P.Next = -1
-				log.Printf("TextCompiled state2: %+v\n", state2)
-				body, err := state2.Value, state2.Err
-				if err != nil {
-					resolve(User{}, err)
-					return
-				}
-				user := User{}
-				log.Printf("body: %v\n", body)
-				if err := json.Unmarshal([]byte(body), &user); err != nil {
-					resolve(User{}, err)
-					return
-				}
-
-				resolve(user, nil)
-				return
-			default:
-				panic(fmt.Sprintf("Promise already done, %+v", P))
 			}
+
+			if resp.StatusCode != 200 {
+				resolve(User{}, fmt.Errorf("http status code: %d", resp.StatusCode))
+				return
+			}
+
+			state2 = resp.TextCompiled()
+			state2.Exec = P.Exec
+			state2.Parent = P
+			state2.Call()
+			return
+		case 2:
+			P.Next = -1
+			body, err := state2.Value(), state2.Err()
+			if err != nil {
+				resolve(User{}, err)
+				return
+			}
+			user := User{}
+			log.Printf("body: %v\n", body)
+			if err := json.Unmarshal([]byte(body), &user); err != nil {
+				resolve(User{}, err)
+				return
+			}
+
+			log.Printf("resolve user: %+v\n", user)
+			resolve(user, nil)
+			return
+		default:
+			panic(fmt.Sprintf("Promise already done, %+v", P))
 		}
 	}
 	return P
@@ -230,52 +221,50 @@ func GetScoreCompiled() *io.PromiseImpl[float64] {
 	P := &io.PromiseImpl[float64]{}
 	P.Debug = "GetScore"
 	P.Func = func(resolve func(float64, error)) {
-		for {
-			switch P.Next {
-			case 0:
-				P.Next = 1
-				state1 = AsyncHttpGetCompiled("http://example.com/score/")
-				state1.Exec = P.Exec
-				state1.Parent = P
-				state1.Call()
+		switch P.Next {
+		case 0:
+			P.Next = 1
+			state1 = AsyncHttpGetCompiled("http://example.com/score/")
+			state1.Exec = P.Exec
+			state1.Parent = P
+			state1.Call()
+			return
+		case 1:
+			P.Next = 2
+			resp, err := state1.Value(), state1.Err()
+			if err != nil {
+				resolve(0, err)
 				return
-			case 1:
-				P.Next = 2
-				resp, err := state1.Value, state1.Err
-				if err != nil {
-					resolve(0, err)
-					return
-				}
-
-				if resp.StatusCode != 200 {
-					resolve(0, fmt.Errorf("http status code: %d", resp.StatusCode))
-					return
-				}
-
-				state2 = resp.TextCompiled()
-				state2.Exec = P.Exec
-				state2.Parent = P
-				state2.Call()
-
-				return
-			case 2:
-				P.Next = -1
-				body, err := state2.Value, state2.Err
-				if err != nil {
-					resolve(0, err)
-					return
-				}
-
-				score := 0.0
-				if _, err := fmt.Sscanf(body, "%f", &score); err != nil {
-					resolve(0, err)
-					return
-				}
-				resolve(score, nil)
-				return
-			default:
-				panic("Promise already done")
 			}
+
+			if resp.StatusCode != 200 {
+				resolve(0, fmt.Errorf("http status code: %d", resp.StatusCode))
+				return
+			}
+
+			state2 = resp.TextCompiled()
+			state2.Exec = P.Exec
+			state2.Parent = P
+			state2.Call()
+
+			return
+		case 2:
+			P.Next = -1
+			body, err := state2.Value(), state2.Err()
+			if err != nil {
+				resolve(0, err)
+				return
+			}
+
+			score := 0.0
+			if _, err := fmt.Sscanf(body, "%f", &score); err != nil {
+				resolve(0, err)
+				return
+			}
+			resolve(score, nil)
+			return
+		default:
+			panic("Promise already done")
 		}
 	}
 	return P
@@ -303,33 +292,131 @@ func DoUpdateCompiled(op string) *io.PromiseImpl[io.Void] {
 	P := &io.PromiseImpl[io.Void]{}
 	P.Debug = "DoUpdate"
 	P.Func = func(resolve func(io.Void, error)) {
-		for {
-			switch P.Next {
-			case 0:
-				P.Next = 1
-				state1 = AsyncHttpPostCompiled("http://example.com/update/" + op)
-				state1.Exec = P.Exec
-				state1.Parent = P
-				state1.Call()
+		switch P.Next {
+		case 0:
+			P.Next = 1
+			state1 = AsyncHttpPostCompiled("http://example.com/update/" + op)
+			state1.Exec = P.Exec
+			state1.Parent = P
+			state1.Call()
+			return
+		case 1:
+			P.Next = -1
+			resp, err := state1.Value(), state1.Err()
+			if err != nil {
+				resolve(io.Void{}, err)
 				return
-			case 1:
-				P.Next = -1
-				resp, err := state1.Value, state1.Err
-				if err != nil {
-					resolve(io.Void{}, err)
-					return
-				}
-
-				if resp.StatusCode != 200 {
-					resolve(io.Void{}, fmt.Errorf("http status code: %d", resp.StatusCode))
-					return
-				}
-
-				resolve(io.Void{}, nil)
-				return
-			default:
-				panic("Promise already done")
 			}
+
+			if resp.StatusCode != 200 {
+				resolve(io.Void{}, fmt.Errorf("http status code: %d", resp.StatusCode))
+				return
+			}
+
+			resolve(io.Void{}, nil)
+			return
+		default:
+			panic("Promise already done")
+		}
+	}
+	return P
+}
+
+func GenInts() (yield io.Promise[int]) {
+	yield(3, nil)
+	yield(2, nil)
+	yield(5, nil)
+	return
+}
+
+func GenIntsCompiled() *io.PromiseImpl[int] {
+	P := &io.PromiseImpl[int]{}
+	P.Debug = "GenInts"
+	P.Func = func(resolve func(int, error)) {
+		switch P.Next {
+		case 0:
+			P.Next = 1
+			resolve(3, nil)
+			return
+		case 1:
+			P.Next = 2
+			resolve(2, nil)
+			return
+		case 2:
+			P.Next = 3
+			resolve(5, nil)
+			return
+		case 3:
+			P.Next = -1
+			resolve(0, fmt.Errorf("stop"))
+			return
+		default:
+			panic("Generator already done")
+		}
+	}
+	return P
+}
+
+func GenUsers() (yield io.Promise[User]) {
+	u, _ := GetUser("Alice").Await()
+	yield(u, nil)
+	u, _ = GetUser("Bob").Await()
+	yield(u, nil)
+	u, _ = GetUser("Cindy").Await()
+	yield(u, nil)
+	log.Printf("genUsers done\n")
+	return
+}
+
+func GenUsersCompiled() (resolve *io.PromiseImpl[User]) {
+	var state1, state2, state3 *io.PromiseImpl[User]
+
+	P := &io.PromiseImpl[User]{}
+	P.Debug = "GenUsers"
+	P.Func = func(resolve func(User, error)) {
+		switch P.Next {
+		case 0:
+			P.Next = 1
+			state1 = GetUserCompiled("Alice")
+			state1.Exec = P.Exec
+			state1.Parent = P
+			state1.Call()
+			return
+		case 1:
+			P.Next = 2
+			u, _ := state1.Value(), state1.Err()
+			resolve(u, nil)
+			return
+		case 2:
+			P.Next = 3
+			state2 = GetUserCompiled("Bob")
+			state2.Exec = P.Exec
+			state2.Parent = P
+			state2.Call()
+			return
+		case 3:
+			P.Next = 4
+			u, _ := state2.Value(), state2.Err()
+			resolve(u, nil)
+			return
+		case 4:
+			P.Next = 5
+			state3 = GetUserCompiled("Cindy")
+			state3.Exec = P.Exec
+			state3.Parent = P
+			state3.Call()
+			return
+		case 5:
+			P.Next = 6
+			u, _ := state3.Value(), state3.Err()
+			resolve(u, nil)
+			return
+		case 6:
+			P.Next = -1
+			resolve(User{}, fmt.Errorf("stop"))
+			return
+		default:
+			panic("Generator already done")
 		}
 	}
 	return P
@@ -348,6 +435,27 @@ func Demo() (resolve io.Promise[io.Void]) {
 	user, score, _, err := io.Await3[User, float64, io.Void](GetUser("8"), GetScore(), DoUpdate("update sth."))
 	log.Println(user, score, err)
 
+	// for loop with generator
+	g := GenInts()
+	for {
+		g.Call()
+		if g.Done() {
+			break
+		}
+		log.Println("genInt:", g.Value(), g.Done())
+	}
+
+	// for loop with async generator
+	g1 := GenUsers()
+	for {
+		g.Call()
+		u, err := io.Await[int](g)
+		if g1.Done() {
+			break
+		}
+		log.Println("genUser:", u, err)
+	}
+
 	// TODO(lijie): select from multiple promises without channel
 	// select {
 	// case user := <-GetUser("123").Chan():
@@ -365,65 +473,81 @@ func DemoCompiled() *io.PromiseImpl[io.Void] {
 	var state2 *io.PromiseImpl[User]
 	var state3 *io.PromiseImpl[[]io.Result[User]]
 	var state4 *io.PromiseImpl[io.Await3Result[User, float64, io.Void]]
+	var g1 *io.PromiseImpl[int]
+	var g2 *io.PromiseImpl[User]
 
 	P := &io.PromiseImpl[io.Void]{}
 	P.Debug = "Demo"
 	P.Func = func(resolve func(io.Void, error)) {
-		for {
-			switch P.Next {
-			case 0:
-				P.Next = 1
-				state1 = GetUserCompiled("1")
-				state1.Exec = P.Exec
-				state1.Parent = P
-				state1.Call()
-				return
-			case 1:
-				P.Next = 2
-				user, err := state1.Value, state1.Err
-				log.Printf("user: %+v, err: %v\n", user, err)
+		switch P.Next {
+		case 0:
+			P.Next = 1
+			state1 = GetUserCompiled("1")
+			state1.Exec = P.Exec
+			state1.Parent = P
+			state1.Call()
+			return
+		case 1:
+			P.Next = 2
+			user, err := state1.Value(), state1.Err()
+			log.Printf("user: %+v, err: %v\n", user, err)
 
-				state2 = io.Race[User](GetUserCompiled("2"), GetUserCompiled("3"), GetUserCompiled("4"))
-				log.Printf("state2: %+v\n", state2)
-				state2.Exec = P.Exec
-				state2.Parent = P
-				state2.Call()
-				return
-			case 2:
-				log.Printf("case 2, state2: %+v\n", state2)
+			state2 = io.Race[User](GetUserCompiled("2"), GetUserCompiled("3"), GetUserCompiled("4"))
+			state2.Exec = P.Exec
+			state2.Parent = P
+			state2.Call()
+			return
+		case 2:
+			P.Next = 3
+			user, err := state2.Value(), state2.Err()
+			log.Printf("race user: %+v, err: %v\n", user, err)
 
-				P.Next = 3
-				user, err := state2.Value, state2.Err
-				log.Printf("race user: %+v, err: %v\n", user, err)
+			state3 = io.All[User]([]io.AsyncCall[User]{GetUserCompiled("5"), GetUserCompiled("6"), GetUserCompiled("7")})
+			state3.Exec = P.Exec
+			state3.Parent = P
+			state3.Call()
+			return
+		case 3:
 
-				state3 = io.All[User]([]io.AsyncCall[User]{GetUserCompiled("5"), GetUserCompiled("6"), GetUserCompiled("7")})
-				log.Printf("state3: %+v\n", state3)
-				state3.Exec = P.Exec
-				state3.Parent = P
-				state3.Call()
-				return
-			case 3:
+			P.Next = 4
+			users, err := state3.Value(), state3.Err()
+			log.Println(users, err)
 
-				P.Next = 4
-				users, err := state3.Value, state3.Err
-				log.Println(users, err)
+			state4 = io.Await3Compiled[User, float64, io.Void](GetUserCompiled("8"), GetScoreCompiled(), DoUpdateCompiled("update sth."))
+			state4.Exec = P.Exec
+			state4.Parent = P
+			state4.Call()
+			return
+		case 4:
+			P.Next = 5
+			user, score, _, err := state4.Value().V1, state4.Value().V2, state4.Value().V3, state4.Value().Err
+			log.Println(user, score, err)
 
-				state4 = io.Await3Compiled[User, float64, io.Void](GetUserCompiled("8"), GetScoreCompiled(), DoUpdateCompiled("update sth."))
-				log.Printf("state4: %+v\n", state4)
-				state4.Exec = P.Exec
-				state4.Parent = P
-				state4.Call()
-				return
-			case 4:
+			g1 = GenIntsCompiled()
+			for {
+				g1.Call()
+				if g1.Done() {
+					break
+				}
 
+				log.Printf("genInt: %+v, done: %v\n", g1.Value(), g1.Done())
+			}
+
+			g2 = GenUsersCompiled()
+			g2.Exec = P.Exec
+			g2.Parent = P
+			g2.Call()
+			return
+		case 5:
+			if g2.Err() != nil {
 				P.Next = -1
-				user, score, _, err := state4.Value.V1, state4.Value.V2, state4.Value.V3, state4.Value.Err
-				log.Println(user, score, err)
 				resolve(io.Void{}, nil)
 				return
-			default:
-				panic("Promise already done")
 			}
+			log.Printf("genUser: %+v, done: %v\n", g2.Value(), g2.Done())
+			g2.Call()
+		default:
+			panic("Promise already done")
 		}
 	}
 	return P
diff --git a/x/io/extra.go b/x/io/extra.go
index 163b20e6..7ea8df75 100644
--- a/x/io/extra.go
+++ b/x/io/extra.go
@@ -88,7 +88,7 @@ func Race[OutT any](acs ...AsyncCall[OutT]) *PromiseImpl[OutT] {
 						log.Printf("io.Race done: %+v won the race\n", p)
 					}
 					returned = true
-					resolve(p.Value, p.Err)
+					resolve(p.value, p.err)
 					return
 				}
 			}
@@ -134,7 +134,7 @@ func All[OutT any](acs []AsyncCall[OutT]) *PromiseImpl[[]Result[OutT]] {
 
 			ret := make([]Result[OutT], len(acs))
 			for idx, p := range ps {
-				ret[idx] = Result[OutT]{p.Value, p.Err}
+				ret[idx] = Result[OutT]{p.value, p.err}
 			}
 			if debugAsync {
 				log.Printf("io.All done: %+v\n", ret)
@@ -164,7 +164,50 @@ type Await2Result[T1 any, T2 any] struct {
 func Await2Compiled[OutT1, OutT2 any](
 	ac1 AsyncCall[OutT1], ac2 AsyncCall[OutT2],
 	timeout ...time.Duration) (ret *PromiseImpl[Await2Result[OutT1, OutT2]]) {
-	return
+	p1 := ac1.(*PromiseImpl[OutT1])
+	p2 := ac2.(*PromiseImpl[OutT2])
+	remaining := 2
+	P := &PromiseImpl[Await2Result[OutT1, OutT2]]{}
+	P.Debug = "Await2"
+	P.Func = func(resolve func(Await2Result[OutT1, OutT2], error)) {
+		switch P.Next {
+		case 0:
+			P.Next = 1
+			p1.Exec = P.Exec
+			p1.Parent = P
+			p1.Call()
+
+			p2.Exec = P.Exec
+			p2.Parent = P
+			p2.Call()
+			return
+		case 1:
+			remaining--
+			if remaining > 0 {
+				return
+			}
+			P.Next = -1
+			if !p1.Done() || !p2.Done() {
+				log.Fatalf("io.Await2: not done: %+v, %+v\n", p1, p2)
+			}
+
+			var err error
+			if p1.err != nil {
+				err = p1.err
+			} else if p2.err != nil {
+				err = p2.err
+			}
+
+			resolve(Await2Result[OutT1, OutT2]{
+				V1: p1.value, V2: p2.value,
+				Err: err,
+			}, err)
+			return
+		default:
+			panic("unreachable")
+		}
+	}
+	return P
 }
 
 // llgo:link Await3 llgo.await
@@ -218,16 +261,16 @@ func Await3Compiled[OutT1, OutT2, OutT3 any](
 			}
 
 			var err error
-			if p1.Err != nil {
-				err = p1.Err
-			} else if p2.Err != nil {
-				err = p2.Err
-			} else if p3.Err != nil {
-				err = p3.Err
+			if p1.err != nil {
+				err = p1.err
+			} else if p2.err != nil {
+				err = p2.err
+			} else if p3.err != nil {
+				err = p3.err
 			}
 
 			resolve(Await3Result[OutT1, OutT2, OutT3]{
-				V1: p1.Value, V2: p2.Value, V3: p3.Value,
+				V1: p1.value, V2: p2.value, V3: p3.value,
 				Err: err,
 			}, err)
 			return
@@ -273,7 +316,7 @@ func PAwait3[OutT1, OutT2, OutT3 any](ac1 AsyncCall[OutT1], ac2 AsyncCall[OutT2]
 func PAwait3Compiled[OutT1, OutT2, OutT3 any](
 	ac1 AsyncCall[OutT1], ac2 AsyncCall[OutT2], ac3 AsyncCall[OutT3]) *PromiseImpl[Await3Result[OutT1, OutT2, OutT3]] {
 	P := &PromiseImpl[Await3Result[OutT1, OutT2, OutT3]]{}
-	P.Debug = "Parallel3"
+	P.Debug = "PAwait3"
 	P.Func = func(resolve func(Await3Result[OutT1, OutT2, OutT3], error)) {
 		ret := Await3Result[OutT1, OutT2, OutT3]{}
 		wg := sync.WaitGroup{}
diff --git a/x/io/io.go b/x/io/io.go
index 86ebaef1..b82c4fcf 100644
--- a/x/io/io.go
+++ b/x/io/io.go
@@ -80,7 +80,7 @@ func Run[OutT any](ac AsyncCall[OutT]) (OutT, error) {
 		e.acs = e.acs[1:]
 		ac.Call()
 		if ac.Done() && ac == rootAc {
-			return p.Value, p.Err
+			return p.value, p.err
 		}
 	}
 }
@@ -91,32 +91,40 @@ type Promise[OutT any] func(OutT, error)
 
 // llgo:link Promise.Await llgo.await
 func (p Promise[OutT]) Await(timeout ...time.Duration) (ret OutT, err error) {
-	return
+	panic("should not called")
 }
 
 func (p Promise[OutT]) Call() {
-
+	panic("should not called")
 }
 
 func (p Promise[OutT]) Chan() <-chan OutT {
-	return nil
+	panic("should not called")
 }
 
 func (p Promise[OutT]) Done() bool {
-	return false
+	panic("should not called")
+}
+
+func (p Promise[OutT]) Err() error {
+	panic("should not called")
+}
+
+func (p Promise[OutT]) Value() OutT {
+	panic("should not called")
 }
 
 // -----------------------------------------------------------------------------
 
 type PromiseImpl[TOut any] struct {
+	Debug  string
 	Next   int
 	Exec   *executor
 	Parent asyncCall
-	Debug  string
 
 	Func  func(resolve func(TOut, error))
-	Err   error
-	Value TOut
+	err   error
+	value TOut
 	c     chan TOut
 }
 
@@ -137,23 +145,31 @@ func (p *PromiseImpl[TOut]) Done() bool {
 
 func (p *PromiseImpl[TOut]) Call() {
 	p.Func(func(v TOut, err error) {
+		p.value = v
+		p.err = err
 		if debugAsync {
 			log.Printf("Resolve task: %+v, %+v, %+v\n", p, v, err)
 		}
-		p.Value = v
-		p.Err = err
 		if p.Parent != nil {
 			p.Parent.Resume()
 		}
 	})
 }
 
+func (p *PromiseImpl[TOut]) Err() error {
+	return p.err
+}
+
+func (p *PromiseImpl[TOut]) Value() TOut {
+	return p.value
+}
+
 func (p *PromiseImpl[TOut]) Chan() <-chan TOut {
 	if p.c == nil {
 		p.c = make(chan TOut, 1)
 		p.Func(func(v TOut, err error) {
-			p.Value = v
-			p.Err = err
+			p.value = v
+			p.err = err
 			p.c <- v
 		})
 	}
@@ -163,5 +179,3 @@ func (p *PromiseImpl[TOut]) Chan() <-chan TOut {
 func (p *PromiseImpl[TOut]) Await(timeout ...time.Duration) (ret TOut, err error) {
 	panic("should not called")
 }
-
-// -----------------------------------------------------------------------------