Reverse Nodes in k-Group

reverseNodes.hs

{-# LANGUAGE NumDecimals #-}

swapNodes :: Int -> [a] -> [a]
swapNodes _ [] = []
swapNodes k z = swapNodes' z []
  where 
    swapNodes' [] acc = acc
    swapNodes'  x acc = 
      let (a,b)     = splitAt k x 
          firstHalf = if (length a < k) then 
                                        a 
                      else 
                        reverse a 
       in 
         swapNodes' b (acc ++ firstHalf)

main :: IO ()
main = do
  print $ swapNodes 3 [1..1e9]