Scala列表匹配类型和大小

Question

I'm trying to do some patting matching in Scala and I'm running into some problems. Here's my code (minus the fluff):

realtionships.filter(...) match {
    case (parent: Relationship) :: Nil => parent.endNode
    case _ => throw new Exception
}

The gist is that I want a list that is only a single item that is of type Relationship . If it's anything else, throw an Exception. But the compiler is giving me this error:

constructor cannot be instantiated to expected type
found: scala.collection.immutable.::[B]
required: scala.collection.immutable.Set[Relationship]

Why exactly am I getting this error? Am I not allowed to match the elements of a list while also trying to match the type? Or do I just have the wrong syntax? (Side note: Relationship is a case class.)

Answer 1

The compiler will refuse a pattern match if it can deduce that you are trying to match types which cannot be equal. From the error message is appears that relationships is a Set and you are trying to match for a List . Now a Set can never be a List , so the compiler rightfully rejects that code.

If you want a List actually, you can enforce it:

relationships.filter(...).toList match {
  ..
}

If you want to stick to a Set , you would need an extractor on Set . That doesn't exist, however... Here is a question related with an answer that shows you to write a custom extractor. If you don't want to go through this effort, a poor man's solution would be

val f = relationships.filter(...)
f.headOption match {
  case Some(parent: Relationship) if f.size == 1 => parent.endNode
  case _ => throw new Exception
}

(Note however that size is an O(N) operation)