https://twitter.com/1HaskellADay/status/486093012511252480

haskell_a_day_2014_07_07.hs

module Roll where
{--

Okay, shoot! Fell asleep while composing a problem; sorry for the delay!

So, this one is an easy one. We get a little Forth'y'. This is from P19
of the P99 problem-set.

 --}

-- | Roll a list @n@ elements to the left
-- >>> roll [1,2,3] 1
-- [2,3,1]
-- >>> roll [1,2,3] 2
-- [3,1,2]
-- >>> roll [1,2,3] (-1)
-- [3,1,2]
-- >>> roll [1,2,3] (-2)
-- [2,3,1]
-- >>> roll [1,2,3] 9
-- [1,2,3]
-- >>> roll [1,2,3] 8
-- [3,1,2]
roll :: [a] -> Int -> [a]
roll list n =
  let (hd, tl) = splitAt realN list
      len = length list
      realN = (if signum n < 0 then len + n else n) `mod` len
  in tl ++ hd

{--

roll rotates list n steps to the left.

roll [1,2,3] 1 ~> [2,3,1]

The argument n can be negative, indicating that the rotation is to occur
to the right, then, instead.

roll [1,2,3] (-1) ~> [3,1,2]

(Recall that we write '(-1)' and not just '-1' because '-1' is interpreted
as the binary operator (-) applied to the Int 1)

The list can be rotated multiple times, so:

roll [1,2,3] 9 ~> [1,2,3]

Have at it!

Explanation: the term ROLL comes from the FORTH programming language,
placing the n-th element of the stack on TOS ('top of stack').

 --}

-- A solution is provided at http://lpaste.net/107063