Hi Haskell gurus, I'm learning Haskell now and here I'm quite puzzled over some code about Monad Transformers. The code is like: type NDS a = StateT ProblemState [] a getVar :: Var -> NDS (Maybe Value) getVar v = do vs <- gets vars return $ lookup v vs What puzzles me is that, I think the Monad of the do block shall be the "NDS (Maybe Value)" in declaration, but the type of gets is gets :: (MonadState s m) => (s -> a) -> m a So "gets" returns a Monad of type m ([] is this case), which seems to be different from "NDS (Maybe Value)", but GHC does not complain about it. If I comment out the type declaration of "getVar :: Var -> NDS (Maybe Value)" and let GHC interpret the type, then the type of getVar is like: getVar :: (MonadState ProblemState m) => Var -> m (Maybe Value) So does it mean "StateT ProblemState m" and the "m" as in "MonadState ProblemState m" is the same thing? I guess I must missed something when trying to understand the Monad Transformers. Please give me some insights. Thanks, Fan