{-# LANGUAGE UndecidableInstances #-}
-- Search for UndecidableInstances to see why this is needed

-----------------------------------------------------------------------------
-- |
-- Module      :  Control.Monad.RWS.Class
-- Copyright   :  (c) Andy Gill 2001,
--                (c) Oregon Graduate Institute of Science and Technology, 2001
-- License     :  BSD-style (see the file libraries/base/LICENSE)
--
-- Maintainer  :  libraries@haskell.org
-- Stability   :  experimental
-- Portability :  non-portable (multi-param classes, functional dependencies)
--
-- Declaration of the MonadRWS class.
--
--      Inspired by the paper
--      /Functional Programming with Overloading and
--          Higher-Order Polymorphism/,
--        Mark P Jones (<http://web.cecs.pdx.edu/~mpj/>)
--          Advanced School of Functional Programming, 1995.
-----------------------------------------------------------------------------

module Control.Monad.RWS.Class (
    MonadRWS,
    module Control.Monad.Reader.Class,
    module Control.Monad.State.Class,
    module Control.Monad.Writer.Class,
  ) where

import Control.Monad.Reader.Class
import Control.Monad.State.Class
import Control.Monad.Writer.Class

import Control.Monad.Trans.Error(Error, ErrorT)
import Control.Monad.Trans.RWS.Lazy as Lazy (RWST)
import qualified Control.Monad.Trans.RWS.Strict as Strict (RWST)

import Data.Monoid

class (Monoid w, MonadReader r m, MonadWriter w m, MonadState s m)
   => MonadRWS r w s m | m -> r, m -> w, m -> s

instance (Monoid w, Monad m) => MonadRWS r w s (Lazy.RWST r w s m)

instance (Monoid w, Monad m) => MonadRWS r w s (Strict.RWST r w s m)
 
---------------------------------------------------------------------------
-- Instances for other mtl transformers
 
-- This instance needs UndecidableInstances, because
-- it does not satisfy the coverage condition
instance (Error e, MonadRWS r w s m) => MonadRWS r w s (ErrorT e m)