In a recent course based on Scala I found a hint that the Scala types
Future[T] are dual. This was explained only informally and the reader was referred to duality in category theory for a more precise definition.
I have some basic knowledge of category theory but I cannot see how these two types can correspond to categories that are dual to each other.
So, what categories correspond to the Scala types
Future[T], and why are these categories dual?