The universal quantification works for defining the _implementation_, but (by itself) not for defining the constraints. Yes, if you write an implementation for

The universal quantification works for defining the _implementation_, but (by itself) not for defining the constraints. Yes, if you write an implementation for `Optional<A>` then it _is_ implemented for any `A` — but it's impossible for an implementation to _require_ it to be implemented for every `A` (and thereby be able to take advantage of that fact, e.g. to be able to know that `Optional<A> → Optional<B>` is well-defined for any `A` and `B`). :)