I'm working through Learn You a Haskell for Great Good [1] and getting a compiler error while playing around with some of the code. I have this: -- BEGIN state.hs import Control.Monad.State type Stack = [Int] pop :: State Stack Int pop = State $ \(x:xs) -> (x,xs) push :: Int -> State Stack () push x = State $ \xs -> ((), x:xs) stackManip :: State Stack Int stackManip = do push 3 pop pop -- END state.hs But when I try to load it into ghci I get the following errors: Prelude> :load "/home/admi/.pe/state.hs" [1 of 1] Compiling Main ( /home/admi/.pe/state.hs, interpreted ) /home/admi/.pe/state.hs:6:6: Not in scope: data constructor `State' /home/admi/.pe/state.hs:9:9: Not in scope: data constructor `State' Failed, modules loaded: none. Prelude> Now, I'm not exactly sure how to read the documentation for Control.Monad.State [2] but it seems that newtype State s a = State {...} defines a constructor, or am I wrong on that point too? So, what am I missing here? In case it matters, I am using mtl-2.0.1.0 and ghci 6.12.3. [1] http://learnyouahaskell.com/for-a-few-monads-more [2] http://cvs.haskell.org/Hugs/pages/libraries/mtl/Control-Monad-State.html -- Adam Miezianko