{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FunctionalDependencies #-}

module Control.Monad.MultiLift (-- * MultiLift class
                                MultiLift(..)
                               -- * Example
                               -- $example
                               )where

import Control.Monad.Trans (MonadTrans(..), MonadIO(..))

-- | 'MultiLift' provides an 'mlift' method that invokes the 'lift'
-- method of 'MonadTrans' multiple times, based on the requested
-- argument and result type.  You will usually have to specify the
-- type of the argument explicitly.
--
-- Note that 'mlift' only works with up to @N@ levels of nested monad
-- transformer.
class MultiLift mIn mOut where
    mlift :: mIn a -> mOut a

instance MultiLift m m where
    mlift = id


{- $example

> module Main where
> 
> import Control.Monad.MultiLift
> import Control.Monad.MultiWrap
> import Control.Monad.Reader
> import Control.Monad.Trans
> import Control.Monad.Wrap
> 
> newtype Type1 = Type1 { unType1 :: String }
> type Reader1 = ReaderT Type1 IO
> 
> newtype Type2 = Type2 { unType2 :: String }
> type Reader2 = ReaderT Type2 Reader1
> 
> r2 :: Reader2 ()
> r2 = do
>   -- Note that you have to specify the inner type
>   s1 <- mlift (asks unType1 :: Reader1 String)
>   liftIO $ putStrLn $ "s1: " ++ s1 ++ "\n"
>   s2 <- asks unType2
>   liftIO $ putStrLn $ "s2: " ++ s2 ++ "\n"
> 
> r1 :: Reader1 ()
> r1 = do
>   runReaderT r2 $ Type2 "this is the Reader2 contents"
> 
> main :: IO ()
> main = do
>   runReaderT r1 $ Type1 "this is the Reader1 contents"

-}