computedGoto编译指示

calculateGoto 编译指示可用于告诉编译器如何编译在 while true 语句中中的Nim case 语句。 从语法上讲，它必须在循环中用作语句：

type
  MyEnum = enum
    enumA, enumB, enumC, enumD, enumE
 
proc vm() =
  var instructions: array[0..100, MyEnum]
  instructions[2] = enumC
  instructions[3] = enumD
  instructions[4] = enumA
  instructions[5] = enumD
  instructions[6] = enumC
  instructions[7] = enumA
  instructions[8] = enumB
 
  instructions[12] = enumE
  var pc = 0
  while true:
    {.computedGoto.}
    let instr = instructions[pc]
    case instr
    of enumA:
      echo "yeah A"
    of enumC, enumD:
      echo "yeah CD"
    of enumB:
      echo "yeah B"
    of enumE:
      break
    inc(pc)
 
vm()

如示例所示， computedGoto 对解释器最有用。 如果底层后端（C编译器）不支持计算的goto扩展，则简单地忽略编译指示。