I am becoming extremely frustrated now. The task I want to perform is simple, yet I simply cannot make Haskell do what I want. I've given up hope of ever getting my program to handle infinite result sets. That means I can make do with just ListT. So I have the following monad: type MyMonad x = StateT MyState (ListT Identity) x Now I'm trying to run two computations, starting from _the same state_, and combine the two resulting lists. The trouble is, I am literally losing the will to live trying to comprehend the whinings of the type checker. The operation I'm trying to perform is perfectly simple; I don't understand why this has to be so damned *difficult*! >_< Any suggestions? I found that by using the brief and easily memorable construction "runIdentity $ runListT $ runStateT foo state" I can get at the result set for each action, and combine them. But nothing in hell seems to transform this from [((), MyState)] back into MyMonad ().