swift改动所造成的错误

• if let operand = operandEvaluation.result { return (operation(operand), operandEvaluation.result) }

出错原因：initializer for conditional binding must have optional type ,not "Double"

• return (nil, ops)

  出错原因：nil is incompatible with reture type "Double"

• return evaluate()

出错原因：unexpected non-void return value in void function

在Xcode6编译可以成功，但是Xcode7.1出错了，请问应该怎么改动才能正确