data Val a = Val a | Fkt a | Inc a | Abs a | Neg a

instance Num (Val a) where
	negate (Val a) = Fkt a
	negate (Fkt a) = Inc a
	negate (Inc a) = Abs a
	negate (Abs a) = Neg a
	negate (Neg a) = Val a

eval (Val a) = a
eval (Fkt a) = fkt a where
	fkt a = if a > 0 then a * fkt (a-1) else 1
eval (Inc a) = a + 1
eval (Abs a) = abs a
eval (Neg a) = - a

v = Val 4

main = mapM_ (print . eval) [v, -v, -(-v), -(-(-v)), -(-(-(-v))), -(-(-(-(-v))))]