Why is Scala’s immutable Set not covariant in its type?

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Set is invariant in its type parameter because of the concept behind sets as functions. The following signatures should clarify things slightly:

trait Set[A] extends (A=>Boolean) {
  def apply(e: A): Boolean
}

If Set were covariant in A, the apply method would be unable to take a parameter of type A due to the contravariance of functions. Set could potentially be contravariant in A, but this too causes issues when you want to do things like this:

def elements: Iterable[A]

In short, the best solution is to keep things invariant, even for the immutable data structure. You’ll notice that immutable.Map is also invariant in one of its type parameters.

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