small example of hlist from extensible library

extensible.hs

module Parser where

import Control.Applicative
-- this is from the extensible package
import Data.Extensible.HList
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 Text (Text -> Maybe a)

parse :: (Text -> Maybe a) -> Text -> Maybe a
parse = undefined

-- collectAll2 :: Monad m
--             => (Text -> m Text)
--             -> QParser a -> QParser b
--             -> m (Maybe (a,b))
-- collectAll2 ask (q1,p1) (q2,p2) =
--   (liftA2 (,))
--        <$> (parse p1 <$> ask q1)
--        <*> (parse p2 <$> ask q2)

-- collectAll3 :: Monad m
--             => (Text -> m Text)
--             -> QParser a -> QParser b -> QParser c
--             -> m (Maybe (a,b,c))
-- collectAll3 ask (q1,p1) (q2,p2) (q3,p3) =
--   liftA3 (,,)
--      <$> (parse p1 <$> ask q1)
--      <*> (parse p2 <$> ask q2)
--      <*> (parse p3 <$> ask q3)


-- liftA4 doesn't exist :(
-- collectAll4 :: Monad m
--             => (Text -> m Text)
--             -> QParser a -> QParser b -> QParser c -> QParser d
--             -> m (Maybe (a,b,c,d))
-- collectAll4 ask (q1,p1) (q2,p2) (q3,p3) (q4,p4) =
--   liftA4 (,,,)
--   <*> (parse p1 <$> ask q1)
--   <*> (parse p2 <$> ask q2)
--   <*> (parse p3 <$> ask q3)
--   <*> (parse p4 <$> ask q4)

collectAll
  :: Monad m
  => (Text -> m Text)
  -> HList QParser es
  -> m (HList Maybe es)
collectAll ask = htraverse $ \(QParser text parser) -> fmap parser (ask text)