enum pattern matching in Swift

I just started learn Swift and trying to understand pattern matching.

I found the next example:

private enum Entities{
  case Operand(Double)
  case UnaryOperation(Double -> Double)
  case BinaryOperation((Double, Double) -> Double)
}

and later I use pattern matching to figure out the type of Entity

func evaluate(entity: Entities) -> Double? {
    switch entity{
    case .Operand(let operand):
        return operand;

    case .UnaryOperation(let operation):
        return operation(prevExtractedOperand1);

    case .BynaryOperation(let operation):
        return operation(prevExtractedOperand1, prevExtractedOperand2);
    }
}

Syntax of getting associated value seems little bit weird, but it works fine.

After that I found, that it is possible to use pattern matching in if statement, so I tried to do the same with if

if case entity = .Operand(let operand){
    return operand
}

but compiler throws error Expected ',' separator, which, I suspect, has nothing common with real reason of the error.

Could you pls help me to understand, what is wrong with my attempt to use pattern matching in if statement?