data Expr = Lit Int |
            Add Expr Expr |
            Sub Expr Expr

{-
    (3 + 2)-4
    ((Lit 3) + (Lit 2) )-(Lit 4)
    (Add (Lit 3) (Lit 2) )-(Lit 4)
  eval  (Sub (Add (Lit 3) (Lit 2)) (Lit 4)) = 5 - 4 = 1
eval (Add (Lit 3) (Lit 2)) = 3 + 2 = 5

-} 

eval :: Expr -> Int
eval (Lit n) = n
eval (Add e1 e2) = (eval e1) + (eval e2)
eval (Sub e1 e2) = (eval e1) -(eval e2)