A free monad example: converting a kind of functor instance to monad

maybe.hs

module Main where

import Control.Monad.Free

data Option a = Some a | None
  deriving Show

instance Functor Option where
  fmap f (Some a) = Some (f a)
  fmap f None = None

type OptionM = Free Option

some :: a -> OptionM a
some = return

none :: OptionM a
none = liftF None

twiceIfEven :: Int -> OptionM Int
twiceIfEven n
  | even n = some (2 * n)
  | otherwise = none

main :: IO ()
main = do
  print $ map (\i -> some i >>= twiceIfEven) [0..9]
  print $ none >>= twiceIfEven