Excerpts from Evan Laforge's message of Mon Sep 06 13:30:43 -0400 2010:
I feel like the circular imports problem is worse in haskell than other languages. Maybe because there is a tendency to centralize all state, since you need to define it along with your state monad. But the state monad module must be one of the lower level ones, since all modules that use it must import it. However, the tendency for bits of typed data to migrate into the state means it's easy for it to eventually want to import one of its importers. And the state monad module gets larger and larger (the largest modules in my system are those that define state monads: 1186 lines, 706 lines, 1156 lines---the rest tend to be 100--300 lines).
I have used hs-boot files to this effect. I separated data and functionality, and typeclasses, which must be in the same module as data or are considered orphaned, get definitions via a circular import. Edward