-----------------------------------------------------------------------------
-- |
-- Module : Control.Comonad.Trans.Context
-- Copyright : (C) 2008-2011 Edward Kmett
-- License : BSD-style (see the file LICENSE)
--
-- Maintainer : Edward Kmett <ekmett@gmail.com>
-- Stability : provisional
-- Portability : portable
--
-- The environment comonad transformer (aka coreader).
-- This adds an extra value that can be accessed in the environment.
--
-- Left adjoint to the reader comonad.
----------------------------------------------------------------------------
module Control.Comonad.Trans.Env
( Env
, env
, runEnv
, EnvT(..)
, runEnvT
, ask
) where
import Control.Comonad
import Control.Comonad.Trans.Class
import Control.Comonad.Hoist.Class
import Data.Functor.Identity
type Env e = EnvT e Identity
data EnvT e w a = EnvT e (w a)
env :: e -> a -> Env e a
env e a = EnvT e (Identity a)
runEnv :: Env e a -> (e, a)
runEnv (EnvT e (Identity a)) = (e, a)
runEnvT :: EnvT e w a -> (e, w a)
runEnvT (EnvT e wa) = (e, wa)
instance Functor w => Functor (EnvT e w) where
fmap g (EnvT e wa) = EnvT e (fmap g wa)
instance Comonad w => Comonad (EnvT e w) where
extract (EnvT _ wa) = extract wa
duplicate p@(EnvT e wa) = EnvT e (p <$ wa)
instance ComonadTrans (EnvT e) where
lower (EnvT _ wa) = wa
instance ComonadHoist (EnvT e) where
cohoist (EnvT e wa) = EnvT e (Identity (extract wa))
ask :: EnvT e w a -> e
ask (EnvT e _) = e