http://hackage.haskell.org/package/free-functors

functor-free.hs

import Data.Functor.Free
import Data.Monoid

type CSV a = Free Monoid [a]

cell :: a -> CSV a
cell = unit . return

fromList :: [a] -> CSV a
fromList = unit

toList :: CSV a -> [a]
toList = counit

showCSV :: (Show a) => CSV a -> String
showCSV = rightAdjunct show' where
  show' :: (Show a) => [a] -> String
  show' [] = ""
  show' (c:cs) = show c ++ "," ++ show' cs

main = do
  putStrLn $ showCSV $ fromList [1..20]
  -- #=> 1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,

  putStrLn $ showCSV $ fmap (++ ["s", "s2", "s3"]) $ fromList ["aaa", "bbb", "ccc"]
  -- #=> "aaa","bbb","ccc","s","s2","s3",