[Haskell-cafe] Why can't we make an instance declaration on a type synonym?

This may be a dumb question, but why can we not declare a Monad instance of a type synonym? This question came to me while working with the State monad recently and feeling that the requirement that we wrap our functions in the State constructor is a bit... kludgy. Why can't the State monad be defined this way?:

type State s a = \s -> (a , s)

instance Monad (State s) where return a = \s -> (a, s) m >>= k = \s -> let (a, s') = m s in k a s'

It seems like Haskell's type system could understand the above, but I'm interested in hearing the technical reason which I'm sure exists for why we can't do this. Thanks for any clues, Brandon Simmons http://coder.bsimmons.name/blog/