SwapL.hs

{- | given two positions in a list, swap them.
     if (at least) one of the positions is invalid, return Nothing.

    Examples:

    >>> swapL 0 2 [0..5]
    Just [2,1,0,3,4,5]

    >>> fmap (take 6) $ swapL 0 2 [0..]
    Just [2,1,0,3,4,5]


    prop> \(x,y) -> swapL x y [] == Nothing

    prop> \(n,xs) -> swapL n n xs == Just xs
-}

swapL :: Int -> Int -> [a] -> Maybe [a]
swapL a b xs
 | min a b < 0                = Nothing
 | not $ atLeast (max a b) xs = Nothing
 | otherwise                  = Just $ zipWith swap xs [0..]

 where swap x i
         | i == a    = xs !! b
         | i == b    = xs !! a
         | otherwise = x

       atLeast 0  _  = True
       atLeast _ []  = False
       atLeast n xs' = atLeast (n - 1) (tail xs')

main :: IO ()
main = do
  print (swapL 0 2 [0..5] :: Maybe [Int])
  print $ fmap (take 6) (swapL 0 2 [0..] :: Maybe [Int])