1. func eval(a, b int, op string) int {
    2.     var result int
    3.     switch op {
    4.         case "+":
    5.             result = a + b
    6.         case "-":
    7.             result = a - b
    8.         case "*":
    9.             result = a * b
    10.         case "/":
    11.             result = a / b
    12.         default:
    13.             panic("unsupported operator:" + op)
    14.     }
    15.     return result
    16. }