free_monad.hs

data Toy b next =
  Output b next
  | Bell next
  | Done
  deriving (Show)

data FixE f e = Fix (f (FixE f e)) | Throw e

catch :: (Functor f) => FixE f e1 -> (e1 -> FixE f e2) -> FixE f e2
catch (Fix x) f = Fix (fmap (flip catch f) x)
catch (Throw e) f = f e

instance (Functor f) => Monad (Free f) where
  return = Pure
  (Free x) >>= f = Free (fmap (>>= f) x)
  (Pure r) >>= f = f r