[Haskell-beginners] state monad

I am trying to come to grips with the state monad and as a starting point I copied the some code directly from the book Learn You a Haskell for Great Good then added the type synonym for Stack import Control.Monad.State type Stack = [Int] pop :: State Stack Int pop = State $ \(x:xs) -> (x,xs) push :: Int -> State Stack () push a = State $ \xs -> ((),a:xs) However, attempting to load this into ghci produces the errors stackPlay.hs:6:7: Not in scope: data constructor `State' stackPlay.hs:9:10: Not in scope: data constructor `State' Please excuse the cognitive deficit, but I cannot figure out why this should not work as advertised. ::paul