Cycle.hs

module Cycle where

import Data.List (elemIndex)
import Data.Functor ((<$>))

data Direction = L | R deriving (Show)

cycleList :: (Eq a) => Direction -> [a] -> a -> Maybe a
cycleList _ [] x = Nothing
cycleList L xs x = ((rotate R xs) !!) <$> elemIndex x xs
cycleList R xs x = ((rotate L xs) !!) <$> elemIndex x xs

rotate :: Direction -> [a] -> [a]
rotate L (x:xs) = xs ++ [x]
rotate R xs     = last xs : init xs