On Tue, Nov 22, 2005 at 05:26:00PM -0700, Fan Wu wrote:

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?

No! Check out the instance: instance (Monad m) => MonadState s (StateT s m) where ... So you can see that the "m" in "MonadState ProblemState m" is actually "StateT ProblemState m'" where m' is the inner monad, [] in your case. So a correct type for gets as you use it is gets :: (ProblemState -> a) -> MonadState ProblemState [] a Andrew