tttt.hs

data Door = Door {
      width    :: Width
    , height   :: Height
    , material :: Material
}

getHeightOfTheDoor :: House -> NumberOfDoor -> Maybe Height
getHeightOfTheDoor (House _ [] _)    _            = Nothing :: Maybe Height
getHeightOfTheDoor (House _ doors _) numberOfDoor = 
    if numberOfDoor < 1
        then Nothing :: Maybe Height
        else
            if numberOfDoor > quantityOfDoors
                then Nothing :: Maybe Height
                else 
                    let ourDoor = doors !! (numberOfDoor - 1)
                    in Just (height ourDoor) :: Maybe Height
  where
    quantityOfDoors = length doors

main :: IO ()
main =
    case heightOfOurDoor of
        Nothing        -> значит что-то пошло не так, и высоту двери получить не удалось
        Just ourHeight -> print ourHeight
  where
    heightOfOurDoor :: Maybe Height
    heightOfOurDoor = getHeightOfTheDoor ourHouse 2
    
    ourHouse = House ...