The thing is that while the implementation of the fake can get ugly, that's effectively library code that's written once - the actual application code is incred

The thing is that while the implementation of the fake can get ugly, that's effectively library code that's written once - the actual application code is incredibly clean since all you have to do to it is tag it as being World instead of IO. Likewise, for tests, the actual interface in practice is clean - we have a few fake-World-only setup functions, and when testing things like the database we can just use the regular insert interface to do our setup.