On Sun, Feb 22, 2009 at 9:20 AM, Luis O'Shea
test3 :: MonadState Integer m => String -> m String
Good point. It's interesting that this allows the signature of test5b to become MonadState Integer m => m Integer (instead of (Monad m) => StateT Integer (StateT String m) Integer) which is more general, and (surprisingly to me) does not mention String.
Odd. If I break up test5b like so:
test5b = flip execStateT 0 . flip evalStateT "" $ test5bImpl
test5bImpl = do
modifyM test3
lift . modify $ \x -> x*2 + 1
modifyM test3
lift . modify $ \x -> x*x
modifyM test3
and ask GHCi for the types, I get:
*Main> :t test5bImpl
test5bImpl :: (MonadState Integer m) => StateT String m ()
*Main> :t test5b
test5b :: (Monad m) => m Integer
--
Dave Menendez