{-# OPTIONS -XGADTs #-}

data Reader s a where 
    R :: (s -> a) -> Reader s a

instance Monad (Reader s) where
    return x = R (\ _ -> x)
    (R f) >>= g = R (\s -> case g (f s) of
                             (R f') -> f' s)

readst :: Reader s s
readst = R (\s -> s)

modify :: (s' -> s) -> Reader s a -> Reader s' a
modify m (R f) = R (\s' -> f (m s'))

run :: Reader s a -> s -> a
run (R f) s = f s

data Exp where
    Var   :: String -> Exp
    Const :: Integer -> Exp
    Plus  :: Exp -> Exp -> Exp
    Let   :: String -> Exp -> Exp -> Exp

type Env = String -> Integer

bind :: String -> Integer -> Env -> Env
bind x i old findthis = if x == findthis then i else (old x)

eval :: Exp -> Reader Env Integer
eval (Var x) = do env <- readst 
                  return (env x)
eval (Const x) = return x
eval (Plus e1 e2) = do i1 <- eval e1
                       i2 <- eval e2
                       return (i1 + i2)
eval (Let x e1 e2) = do i1 <- eval e1
                        modify (bind x i1) (eval e2)

e1 :: Exp
e1 = Let "y" (Plus (Const 1) (Const 2)) (Plus (Let "y" (Const 100) (Var "y")) (Var "y"))

initenv :: Env
initenv s = undefined