[Haskell-beginners] one more step backwards

[Stephen Tetley]

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

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