small example of hlist from extensible library

extensible.hs

{-# LANGUAGE TypeOperators #-}
import           Control.Applicative
import           Data.Extensible
import           Control.Monad.Trans.Maybe
import           Data.Text

-- the shape of the problem
--
-- we want a list of Parsers, all of different types.
-- we will apply each of them, one after another,
-- in the context of asking questions in some monadic
-- context.

data QParser a = QParser
  { question :: Text
  , parse :: Text -> Maybe a
  }

collectAll :: Monad m
  => (Text -> m Text)
  -> QParser :* xs
  -> m (Maybe (AllOf xs))
collectAll ask = runMaybeT
  . htraverse (\(QParser q p) -> MaybeT $ fmap pure . p <$> ask q)