    -----------------------------------------------------------------------------
    -- |
    -- 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 LICENSE)
    --
    -- Maintainer  :  ross@soi.city.ac.uk
    -- Stability   :  experimental
    -- Portability :  non-portable (type families)
    --
    -- 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.Except(ExceptT)
    import Control.Monad.Trans.Maybe(MaybeT)
    import Control.Monad.Trans.Identity(IdentityT)
    import Control.Monad.Trans.RWS.Lazy as Lazy (RWST)
    import qualified Control.Monad.Trans.RWS.Strict as Strict (RWST)

    class (Monoid (WriterType m), MonadReader m, MonadWriter m, MonadState m) =>
        MonadRWS m

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

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

    ---------------------------------------------------------------------------
    -- Instances for other mtl transformers

    instance (MonadRWS m) => MonadRWS (ExceptT e m)

    instance (MonadRWS m) => MonadRWS (IdentityT m)

    instance (MonadRWS m) => MonadRWS (MaybeT m)