Seeeduino XIAO Ultrasonic obstacle avoidance robot

#include <Servo.h>
#include <SparkFunMiniMoto.h>
#include <Ultrasonic.h>

Servo servo_2;

MiniMoto motor_CH1(0xD0);
MiniMoto motor_CH2(0xC1);
Ultrasonic ult(7);

long ult_dis;
long L_dis;
long R_dis;

void setup(){
  servo_2.attach(0);

  servo_2.write(80);
  delay(100);
  motor_CH1.stop();
  motor_CH2.stop();
}

//forward
void line(){
  motor_CH1.drive(150);
  motor_CH2.drive(150);

}
//stop

void lull(){
  motor_CH1.stop();motor_CH2.stop();
}

//retreat
void retreat(){
  motor_CH1.drive(-150);
  motor_CH2.drive(-150);
}

//Turn left
void left(){
  motor_CH1.drive(0);
  motor_CH2.drive(150);

}
//Turn right
void right(){
  motor_CH1.drive(150);
  motor_CH2.drive(0);
}

//Motion function
void robot(){

  ultcenter();
  if ((ult_dis > 50)) {
    line();
  }
  if ((ult_dis < 30)) {
    lull();
    delay(600);
    retreat();
    delay(600);
    servo_2.write(150);
    delay(100);
    ultleft();
    delay(200);
    servo_2.write(30);
    delay(100);
    ultright();
    delay(200);
    servo_2.write(90);
    delay(100);
    if ((L_dis > R_dis)) {
      left();
      delay(600);
    } else {
      right();
      delay(600);
    }
  }

}

//Ranging function
void ultcenter(){
  ult_dis = ult.MeasureInCentimeters();
}

void ultleft(){
  L_dis = ult.MeasureInCentimeters();
}

void ultright(){
  R_dis = ult.MeasureInCentimeters();
}
void loop(){
  robot();

}