This also pops up in modeling sets. You can define an immutable set by its explicit list of members (intrinsic) or by function that tests membership (extrinsic)

This also pops up in modeling sets. You can define an immutable set by its explicit list of members (intrinsic) or by function that tests membership (extrinsic). If you want to have only one unified type Set that permits both methods of construction, it has to be invariant with respect to the type of its members. This is because the intrinsic approach is covariant (just as a list is) but the extrinsic approach is contravariant (just as a function is in its inputs). I wish I had a really clear example of this, but it escapes me right now.