simple user-interface

def main():
    """
        simple user-interface
    """
    print("\t\tCalculator\n\n")
    while True:
        user_input = input("expression or exit: ")
        if user_input == "exit":
            break
        try:
            print("The result is {0}".format(evaluate(user_input)))
        except Exception:
            print("invalid syntax!")
            user_input = input("expression or exit: ")
    print("program end")