type Interval = (Double, Double)

m3 :: [Int] -> Maybe Int
m3 _ = Nothing

op :: [Maybe Int] -> Maybe Int
op _ =  Nothing

closest :: [Interval] -> Double -> Maybe Interval
closest _ _ = Nothing

imapper :: (Interval-> Maybe Interval) -> [Interval] -> [Interval]
imapper _ _ = []


ck_m3 = [m3 [1,2] == Nothing,
         m3 [10,20,30,40,50] == Just 30,
         m3 [] == Nothing,
         m3 [3] == Nothing,
         m3 [3,6,9] == Just 9,
         m3 [3,6,9,12] == Just 9,
         m3 [101..] == Just 103]
  

ck_op = [op [Just 3, Just 4, Just 10, Just 20] == Just 80,
          op [Just 5, Nothing, Just 2, Just 1] == Nothing,
          op [Nothing, Just 5] == Just 5,
          op [Just 7] == Just 1 -- product of 0 things is 1
          ]



check_closest = [
     closest [(5,2),(7,1)] 12 == Nothing,
     closest [(5,2),(7,1)] 6.5 == Just (7,1),
     closest [(5,2),(7,1)] 6 == Just (7,1),
     closest [(5,2),(7,2)] 5.5 == Just (5,2)]

fff :: Interval -> Maybe Interval
fff (center, radius)
      | radius >= 4   = Just (center, radius/2)
      | otherwise     = Nothing

check_i = imapper fff [(10,8), (20,2), (30,4)] == [(10,4),(30,2)]

check = do
  putStrLn "check_m3"
  putStrLn $ show ck_m3
  putStrLn "check_op"
  putStrLn $ show ck_op
  putStrLn "check_closest"
  putStrLn $ show check_closest
  putStrLn "check: imapper"
  putStrLn $ show check_i