Area haskell

eval :: [Int] -> [Int] -> Double -> Double
eval [] [] x = 0
eval (a:as) (b:bs) x =
    da * (x**db) + eval as bs x
    where da = fromIntegral a
          db = fromIntegral b


solve' :: Double -> Double -> [Int] -> [Int] -> [Double]
solve' left right [] [] = [0, 0]
solve' left right a b =
  if left >= right
  then [0, 0]
  else [area, volume]
  where area = area_curr + area_rest
        volume = volume_curr + volume_rest
        area_curr = fx * dx
        volume_curr = pi * dx * fx*fx
        area_rest = head eval_rest
        volume_rest = last eval_rest
        eval_rest = solve' (left+dx) right a b
        dx = 0.001
        fx = eval a b left