I don't know Alice ML but one advantage of using abstractions like Applicative is that you can write code which is polymorphic in the particular applicative you choose. So, you can mock your concurrent requests using the Identity or ZipList applicative, and use the type class laws to prove things which are true about both. I'm guessing, but it looks like the spawn syntax is built-in here. Not that it's not as elegant as the Concurrent version, but likely not as powerful from an abstraction point of view.
