[rockostrich]

I think you may have misunderstood their point. They're just saying that specifying `Functor` in the return type of `map` isn't enough to resolve the issue because you could still have a case of `List#map` returning a `Maybe` or `Either` since they are all implementations of `Functor`.

This gets handle by the Cats library in Scala (https://typelevel.org/cats/typeclasses/functor.html) by defining the type class the functor is being defined for as an abstract type on the functor itself.