Hi, A bit of a detailed technical question. I'm writing a library which makes heavy use of the MTL (Monad.State, etc) and monad transformers. I have a monad transformer, MymonadT, which I want to inherit relevant type classes from the monads it transforms, i.e. MonadState s m => MonadState s (MymonadT m). The MTL itself uses such instance declarations, such as MonadState s m => MonadState s (ReaderT r m). When I compile my library however (using ghc-6.6.1 and -fglasgow-exts) I get an illegal instance declaration error: "The Coverage Condition fails for one of the functional dependencies; Use -fallow-undecidable-instances to permit this." The code compiles with -fallow-undecidable-instances. As a reminder, MonadState is declared as class Monad m => MonadState s m | m -> s The coverage condition is described in http://research.microsoft.com/~simonpj/papers/fd-chr/jfp06.pdf on page 12. My own analysis suggests that the declaration of MonadState for ReaderT violates this condition as well. The coverage condition exists, this paper claims, to prevent infinite loops in type inference---someone could use some instance declarations violating this condition to write a function whose type is undecidable. Does 'MonadState s m => MonadState s (ReaderT r m)', found in Control.Monad.Reader violate the coverage condition as I believe it does? Can one write a function using this library to force the type inference engine to loop indefinitely? If not, what mitigating conditions prevent it? Can I write a similar declaration in my library and compile it with -fallow-undecidable-instances and without worry? Thanks, Mike