scala: check presence and absence of certain types in List-CodePudding

I have a list. I want to check that it contains an element of certain type, but not element of another type. Is this a idiomatic way:

myList.contains(NeededClass) && !myList.contains(NotNeededClass)

CodePudding user response：

I guess you may do this:

implicit class ListOps[A](private val list: List[A]) extends AnyVal {
  def containsType[B <: A](implicit ev: ClassTag[B]): Boolean
    list.exists {
      case _: B => true
      case _ => false
    }
}

myList.containsType[NeededClass] && !myList.containsType[NotNeededClass]

Disclaimer: Since this is a class check and not a type check, it is prone to errors due to type erasure.

CodePudding user response：

contains takes an exemplar object, you want a function that takes a predicate.

myList.exists(_.isInstanceOf[NeededClass]) 
&& myList.forall(_.isInstanceOf[NotNeededClass] != true)

Of course that traverses the list twice. We can traverse once, at the cost of much less readable code.

val (c1, c2) = myList.foldLeft((0, 0))((b, a) 
  => (b._1   if (a.isInstanceOf[NeededClass]) 1 else 0, 
      b._2   if (a.isInstanceOf[NotNeededClass]) 1 else 0))

c1 >= 1 && c2 == 0