> Deriving it instead of naming and explaining it will make it harder to understand any code using the derivation because you can’t look up what Promise.all does. If the code and explanation is written in terms of Arbitrary instead of Future then the explanation will be difficult to write and difficult to read.

To the contrary! Because I’ve already used that function (traverse) a million times and know exactly how to write it myself I understand exactly what it does without any knowledge about the details of your Future or Optional or whatever. True and meaningful abstraction. Which is truly an amazing super power!