diff --git a/builtin/function.go b/builtin/function.go
index e240fa16..897c5078 100644
--- a/builtin/function.go
+++ b/builtin/function.go
@@ -1,11 +1,26 @@
 package builtin
 
-import "time"
+import (
+	"math/rand"
+	"time"
+)
 
 var Functions = map[string]interface{}{
-	"sleep": Sleep,
+	"sleep":             sleep,
+	"gen_random_string": genRandomString,
 }
 
-func Sleep(nSecs int) {
+func sleep(nSecs int) {
 	time.Sleep(time.Duration(nSecs) * time.Second)
 }
+
+func genRandomString(n int) string {
+	const letters = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890"
+	const lettersLen = len(letters)
+
+	b := make([]byte, n)
+	for i := range b {
+		b[i] = letters[rand.Intn(lettersLen)]
+	}
+	return string(b)
+}
diff --git a/parser.go b/parser.go
index b9023a9c..c27a8e30 100644
--- a/parser.go
+++ b/parser.go
@@ -169,7 +169,9 @@ func mergeVariables(variables, overriddenVariables map[string]interface{}) map[s
 	return mergedVariables
 }
 
-func callFunction(funcName string, params []interface{}) (interface{}, error) {
+// callFunction call function with arguments
+// only support return at most one result value
+func callFunction(funcName string, arguments []interface{}) (interface{}, error) {
 	function, ok := builtin.Functions[funcName]
 	if !ok {
 		// function not found
@@ -182,13 +184,25 @@ func callFunction(funcName string, params []interface{}) (interface{}, error) {
 		return nil, fmt.Errorf("function %s is invalid", funcName)
 	}
 
-	paramsValue := make([]reflect.Value, len(params))
-	for index, param := range params {
-		paramsValue[index] = reflect.ValueOf(param)
+	argumentsValue := make([]reflect.Value, len(arguments))
+	for index, argument := range arguments {
+		argumentsValue[index] = reflect.ValueOf(argument)
 	}
 
-	log.Printf("[callFunction] function: %v, params: %v", funcName, params)
-	result := funcValue.Call(paramsValue)
-	log.Printf("[callFunction] result: %v", result)
+	log.Printf("[callFunction] func: %v, input arguments: %v", funcName, arguments)
+	resultValues := funcValue.Call(argumentsValue)
+	log.Printf("[callFunction] output results: %v", resultValues)
+
+	if len(resultValues) == 0 {
+		return nil, nil
+	} else if len(resultValues) > 1 {
+		// function should return at most one value
+		err := fmt.Errorf("function %s should return at most one value", funcName)
+		log.Printf("[callFunction] error: %s", err.Error())
+		return nil, err
+	}
+
+	// convert reflect.Value to interface{}
+	result := resultValues[0].Interface()
 	return result, nil
 }
diff --git a/parser_test.go b/parser_test.go
index 9cd7a026..674971ea 100644
--- a/parser_test.go
+++ b/parser_test.go
@@ -281,6 +281,7 @@ func TestMergeVariables(t *testing.T) {
 }
 
 func TestCallFunction(t *testing.T) {
+	// call function without arguments
 	funcName := "sleep"
 	params := []interface{}{1}
 	timeStart := time.Now()
@@ -291,4 +292,15 @@ func TestCallFunction(t *testing.T) {
 	if !assert.Greater(t, time.Since(timeStart), time.Duration(1)*time.Second) {
 		t.Fail()
 	}
+
+	// call function with one argument
+	funcName = "gen_random_string"
+	params = []interface{}{10}
+	result, err := callFunction(funcName, params)
+	if !assert.Nil(t, err) {
+		t.Fail()
+	}
+	if !assert.Equal(t, 10, len(result.(string))) {
+		t.Fail()
+	}
 }