I propose to remove the MonadFix instance for the ExceptT transformer, because it gives the illusion that it can handle exceptions, which it cannot. The current implementation is: instance (MonadFix m) => MonadFix (ExceptT e m) where mfix f = ExceptT $ mfix $ \ a -> runExceptT $ f $ case a of Right x -> x Left _ -> error "mfix ExceptT: Left" You see, that it cannot handle the exception case. Whenever someone needs an mfix on ExceptT I strongly suggest that he first handles the exception, thus turning (ExceptT e m a) into plain (m a) and then calls 'mfix' only on (m a). I further propose to declare the MonadFix instance as non-implementable in a way suggested in [1]: class NoMonadFix m where instance (NoMonadFix m) => MonadFix (ExceptT e m) where mfix = error "unimplementable" where no instance of NoMonadFix exist and NoMonadFix is not exported. Whenever someone tries to call 'mfix' on 'ExceptT e m' he will get the type error, that a NoMonadFix instance on 'm' is missing and he will not be able to add it. [1] http://ghc.haskell.org/trac/ghc/ticket/9334#comment:9