I believe that the type constructor for State is now lowercase: "state" not "State". 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) play = do push 1 push 2 push 3 pop main = print $ runState play [] L. On Sun, Apr 22, 2012 at 01:40:17AM -0700, Paul Higham wrote:

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

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