Hi John In this case a simple stack tracking the expression as it is reduced should work, the code below isn't entirely correct as it seems to need 'r' input twice to do the first roll-back, but it is along the right lines. The function 'evaluate' has been split to pass the stack into the function 'evalWithStack' which does the recursive work. Best wishes Stephen module RollbackEval where data Expression = Val Integer | Add Expression Expression | Subtract Expression Expression | Multiply Expression Expression | Divide Expression Expression deriving Show demo1 = (Add(Add(Add(Add(Val 6)(Val 5))(Val 10))(Val 7))(Val 30)) evalStep :: Expression -> Expression evalStep (Val x)= (Val x) evalStep (Add x y) = case x of (Val a) -> case y of (Val b) -> Val (a+b) left -> Add x (evalStep y) right -> Add (evalStep x)y type Stack = [Expression] evaluate :: Expression -> IO () evaluate exp = do stk <- evalWithStack exp [exp] putStrLn "Stack:" mapM_ (putStrLn . show) stk evalWithStack :: Expression -> Stack -> IO Stack -- Base case evalWithStack (Val a) stk = return stk -- Recursive case evalWithStack e stk = do putStrLn "Evaluating one more step" let e' = (evalStep e) putStrLn ("Result is "++(show e')) putStrLn "Do another step (y/n) or rollback (r)? :" c <- getLine case c of "y" -> evalWithStack e' (e':stk) "r" -> let (a,stk') = stackBack stk in evalWithStack a stk' _ -> do { putStrLn ("Ok you said" ++ show[c] ++ "so that's it" ++ show (getCount stk)) ; return (e':stk) } stackBack :: Stack -> (Expression,Stack) stackBack [a] = (a,[a]) stackBack (a:as) = (a,as) stackBack [] = error "Whoops empty - should be unreachable" getCount :: Stack -> Int getCount stk = length stk -------------------