如果没有隐式定义，如何返回None或引发异常？

Question

I have this function:

case object Evaluator {
    import Ordering.Implicits._
    def eval[T: Ordering](x: T, y: T): Boolean = Some(x < y)
}

I want that in case that the use sends unsupported object to the eval function to return None. eg:

case object Bar
assert(Evaluator.eval(Bar, 1) == None)

How can I do that ?

Answer 1

Try providing the default value for the implicit argument.

case object Evaluator{
    import Ordering.Implicits._
    def eval[T](x: T, y: T)(implicit ev:Ordering[T] = null):Boolan =
      if(ev == null)
        None 
      else
        Some(x < y)
}