I don't know if I asked my question as clearly as possible. Mocking your DB, the file system, the remote API, etc are all a given when doing tests. My question was aimed at trying to find out if it's feasible to replace the imperative Presenter/Service/etc patterns with a more functional approach.

What I'd like to see is testing only the returned value of a function, as opposed to calling a void function and checking whether the injected mocks got the proper method invocations, i.e. whether the desired side effects occurred.