    {-# LANGUAGE UndecidableInstances #-}
    -- Search for UndecidableInstances to see why this is needed
    {- |
    Module      :  Control.Monad.Reader.Class
    Copyright   :  (c) Andy Gill 2001,
                   (c) Oregon Graduate Institute of Science and Technology 2001,
                   (c) Jeff Newbern 2003-2007,
                   (c) Andriy Palamarchuk 2007
    License     :  BSD-style (see the file LICENSE)

    Maintainer  :  libraries@haskell.org
    Stability   :  experimental
    Portability :  non-portable (multi-param classes, functional dependencies)

    [Computation type:] Computations which read values from a shared environment.

    [Binding strategy:] Monad values are functions from the environment to a value.
    The bound function is applied to the bound value, and both have access
    to the shared environment.

    [Useful for:] Maintaining variable bindings, or other shared environment.

    [Zero and plus:] None.

    [Example type:] @'Reader' [(String,Value)] a@

    The 'Reader' monad (also called the Environment monad).
    Represents a computation, which can read values from
    a shared environment, pass values from function to function,
    and execute sub-computations in a modified environment.
    Using 'Reader' monad for such computations is often clearer and easier
    than using the 'Control.Monad.State.State' monad.

      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.Reader.Class (
        MonadReader(..),
        asks,
        ) where

    import Control.Monad.Trans.Cont as Cont
    import Control.Monad.Trans.Error
    import Control.Monad.Trans.Identity
    import Control.Monad.Trans.List
    import Control.Monad.Trans.Maybe
    import Control.Monad.Trans.Reader (ReaderT)
    import qualified Control.Monad.Trans.Reader as ReaderT (ask, local, reader)
    import qualified Control.Monad.Trans.RWS.Lazy as LazyRWS (RWST, ask, local, reader)
    import qualified Control.Monad.Trans.RWS.Strict as StrictRWS (RWST, ask, local, reader)
    import Control.Monad.Trans.State.Lazy as Lazy
    import Control.Monad.Trans.State.Strict as Strict
    import Control.Monad.Trans.Writer.Lazy as Lazy
    import Control.Monad.Trans.Writer.Strict as Strict

    import Control.Monad.Trans.Class (lift)
    import Control.Monad
    import Data.Monoid

    -- ----------------------------------------------------------------------------
    -- class MonadReader
    --  asks for the internal (non-mutable) state.

    -- | See examples in "Control.Monad.Reader".
    -- Note, the partially applied function type @(->) r@ is a simple reader monad.
    -- See the @instance@ declaration below.
    class Monad m => MonadReader r m | m -> r where
        -- | Retrieves the monad environment.
        ask   :: m r

        -- | Executes a computation in a modified environment.
        local :: (r -> r) -- ^ The function to modify the environment.
              -> m a      -- ^ @Reader@ to run in the modified environment.
              -> m a

        -- | Retrieves a function of the current environment.
        reader :: (r -> a) -- ^ The selector function to apply to the environment.
               -> m a
        reader f = do
          r <- ask
          return (f r)

    -- | Retrieves a function of the current environment.
    asks :: MonadReader r m
        => (r -> a) -- ^ The selector function to apply to the environment.
        -> m a
    asks = reader

    -- ----------------------------------------------------------------------------
    -- The partially applied function type is a simple reader monad

    instance MonadReader r ((->) r) where
        ask       = id
        local f m = m . f
        reader    = id

    instance Monad m => MonadReader r (ReaderT r m) where
        ask = ReaderT.ask
        local = ReaderT.local
        reader = ReaderT.reader

    instance (Monad m, Monoid w) => MonadReader r (LazyRWS.RWST r w s m) where
        ask = LazyRWS.ask
        local = LazyRWS.local
        reader = LazyRWS.reader

    instance (Monad m, Monoid w) => MonadReader r (StrictRWS.RWST r w s m) where
        ask = StrictRWS.ask
        local = StrictRWS.local
        reader = StrictRWS.reader

    -- ---------------------------------------------------------------------------
    -- Instances for other mtl transformers
    --
    -- All of these instances need UndecidableInstances,
    -- because they do not satisfy the coverage condition.

    instance MonadReader r' m => MonadReader r' (ContT r m) where
        ask   = lift ask
        local = Cont.liftLocal ask local
        reader = lift . reader

    instance (Error e, MonadReader r m) => MonadReader r (ErrorT e m) where
        ask   = lift ask
        local = mapErrorT . local
        reader = lift . reader

    instance MonadReader r m => MonadReader r (IdentityT m) where
        ask   = lift ask
        local = mapIdentityT . local
        reader = lift . reader

    instance MonadReader r m => MonadReader r (ListT m) where
        ask   = lift ask
        local = mapListT . local
        reader = lift . reader

    instance MonadReader r m => MonadReader r (MaybeT m) where
        ask   = lift ask
        local = mapMaybeT . local
        reader = lift . reader

    instance MonadReader r m => MonadReader r (Lazy.StateT s m) where
        ask   = lift ask
        local = Lazy.mapStateT . local
        reader = lift . reader

    instance MonadReader r m => MonadReader r (Strict.StateT s m) where
        ask   = lift ask
        local = Strict.mapStateT . local
        reader = lift . reader

    instance (Monoid w, MonadReader r m) => MonadReader r (Lazy.WriterT w m) where
        ask   = lift ask
        local = Lazy.mapWriterT . local
        reader = lift . reader

    instance (Monoid w, MonadReader r m) => MonadReader r (Strict.WriterT w m) where
        ask   = lift ask
        local = Strict.mapWriterT . local
        reader = lift . reader