-- this works
elementAt :: [a] -> Int -> a
elementAt [] i = error "Empty list."
elementAt l i
    | i < 1 = head l
    | i > (length l) = last l
    | otherwise = l !! (i - 1)

-- but this throws an error. why?
elementAt :: (Integral b) => [a] -> b -> a
elementAt [] i = error "Empty list."
elementAt l i
    | i < 1 = head l
    | i > (length l) = last l
    | otherwise = l !! (i - 1)