{-# OPTIONS_GHC -fno-warn-missing-signatures #-}
{-# LANGUAGE EmptyDataDecls #-}
{-# LANGUAGE FlexibleContexts, FlexibleInstances, MultiParamTypeClasses #-}

{-
   The HList library

   A heterogeneous version of
        sequence :: (Monad m) => [m a] -> m [a]

   Only now we operate on heterogeneous lists, where different elements
   may have different types 'a'.
   In the argument list of monadic values (m a_i),
   although a_i may differ, the monad 'm' must be the same for all
   elements. That's why we need TypeCast. The typechecker will complain
   if we attempt to use hSequence on a HList of monadic values with different
   monads.

   The hSequence problem was posed by Matthias Fischmann
   in his message on the Haskell-Cafe list on Oct 8, 2006
   http://www.haskell.org/pipermail/haskell-cafe/2006-October/018708.html
   http://www.haskell.org/pipermail/haskell-cafe/2006-October/018784.html
 -}

module Data.HList.HSequence (hSequence) where

import Data.HList.HListPrelude -- (Apply(..), HCons(..), apply, hFoldr, HNil)
import Control.Monad (liftM2)

import Data.HList.TypeCastGeneric2                 -- For tests


data ConsM

instance (TypeCast (m1 l) (m l), Monad m)
    => Apply ConsM (m a, m1 l) (m (HCons a l)) where
    apply _ (me,ml) = liftM2 HCons me (typeCast ml)

-- Inferred type:
-- hSequence :: (Monad m, HFoldr ConsM (m HNil) l r) => l -> r
hSequence l = hFoldr (undefined::ConsM) (return HNil) l


-- Tests

hlist = HCons (Just (1 :: Integer)) (HCons (Just 'c') HNil) -- Maybe monad
hlist2 = HCons ([1]) (HCons (['c']) HNil)      -- List monad

testHSequence  = hSequence hlist
testHSequence2 = hSequence hlist2