ReadP as a list

-- Okay, ReadP should be a MonadFix. It can, if you have the internal representation, be converted to `ReadNT []`, where:
newtype ReadNT m a = ReadNT { runReadNT :: String -> m (a, Either Int String) }

deriving instance Functor m => Functor (ReadNT m)

instance Monad m => Applicative (ReadNT m) where { pure a = ReadNT $ const $ pure (a, Left 0); (<*>) = ap }

instance Monad m => Monad (ReadNT m) where
  ReadNT m >>= f = ReadNT $ \s -> do
    (a, r) <- m s
    case r of
      Left i -> do
        (b, t) -> runReadNT (f a) $ drop i s
        case t of
          Left j -> (b, Left (i + j))
          Right sn -> (b, Right sn)
      Right sr -> do
        (b, t) -> runReadNT (f a) sr
        case t of
          Left j -> (b, Right (drop j sr))
          Right sn -> (b, Right sn)

-- It has the obvious MonadFix instance.
instance MonadFix m => MonadFix (ReadNT m) where
  mfix f = ReadNT $ \s -> mfix $ \ ~(a, _) -> runReadNT (f a) s