That's interesting, I did not know that.

From a quick glance it appears they do not have bounded polymorphism ("protocol-constrained generic parameters"), associated types, or existential types ("values of protocol type"). So for example if you have a generic Set data type you would have to pass in an equality and hash function to each operation instead of saying that the element type is Hashable, etc.

The explicit instantiation looks quaint, but it reminds me of ML functors for some reason:

    procedure Instance_Swap is new Swap (Float);