    {-# 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 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.Class
    import Control.Monad.Trans.Error(Error, ErrorT)
    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)

    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
    --
    -- All of these instances need UndecidableInstances,
    -- because they do not satisfy the coverage condition.

    instance (Error e, MonadRWS r w s m) => MonadRWS r w s (ErrorT e m)
    instance MonadRWS r w s m => MonadRWS r w s (IdentityT m)
    instance MonadRWS r w s m => MonadRWS r w s (MaybeT m)