Hi, folks. I've got a working class and instances thereof. I would now like to change the class such that error behaviour is specified by MonadError, for the moment throwing String errors. When I try to add MonadError into the types I eventually hit the requirement for allow-undecidable-instances. Is there some way I can I avoid having to use this extension? My starting point consists of: class (Num i, Bounded i, Monad m) => MonadSource i m | m -> i where ... newtype SourceT i m a = SourceT (StateT (Store i) m a) deriving (Functor, Monad, MonadIO, MonadTrans) instance (Num i, Bounded i, Monad m) => MonadSource i (SourceT i m) where ... I changed it to: class (Num i, Bounded i, Monad m, MonadError String m) => MonadSource i m | m -> i where .... newtype SourceT i m a = SourceT (StateT (Store i) m a) deriving (Functor, Monad, MonadIO, MonadTrans, MonadError e) instance (Num i, Bounded i, Monad m, MonadError String m) => MonadSource i (SourceT i m) where ... Thanks Daniel