Check if a list is infinite

CycleList.hs

module Main where
  data List a = Empty | Cons a (List a)
  instance Show a => Show (List a) where
    show (Cons a next) = show a ++ "-" ++ show next
    show Empty = "*"


  simpleList :: List Integer
  simpleList = Cons 1 . Cons 2 . Cons 3 . Cons 4 $ Empty

  closedList :: List Integer
  closedList = Cons 1 . Cons 2 . Cons 3 . Cons 4 $ loop

  loop :: List Integer
  loop = Cons 5 knot
    where knot = Cons 6 loop

  takeFirst :: Integer -> List Integer -> List Integer
  takeFirst 0 _  = Empty
  takeFirst n (Cons a rest) = Cons a $ takeFirst (n - 1) rest

  isCyclic :: List Integer -> String

  isCyclic (Cons a (Cons b rest)) = isCyclic' rest (Cons a . Cons b $ rest)
  isCyclic _ = "No"

  isCyclic' Empty _ = "No"
  isCyclic' (Cons fast (Cons _ fastRest)) (Cons slow slowRest)
    | fast == slow = "Yes"
    | otherwise = isCyclic' fastRest slowRest

  main :: IO ()
  main = do
    print simpleList
    print $ "simpleList is infinite ? " `mappend` isCyclic simpleList
    print $ takeFirst 10 loop
    print $ "cyclicList is infinite ? " `mappend` isCyclic loop
    print $ takeFirst 10 closedList
    print $ "closedList is infinite ? " `mappend` isCyclic closedList