-----------------------------------------------------------------------------
-- |
-- 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.Lazy
  ( 
  -- * The environment comonad
    Env
  , env
  , runEnv
  -- * The environment comonad transformer
  , EnvT
  , runEnvT
  -- * Combinators
  , ask
  , asks
  , local
  ) where

import Control.Comonad
import Control.Comonad.Trans.Class
import Control.Comonad.Hoist.Class
import Data.Functor.Identity
import Data.Monoid

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 (Monoid e, FunctorApply w) => FunctorApply (EnvT e w) where
  ~(EnvT ef wf) <.> ~(EnvT ea wa) = EnvT (ef `mappend` ea) (wf <.> wa)

instance (Monoid e, ComonadApply w) => ComonadApply (EnvT e w)

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

asks :: (e -> f) -> EnvT e w a -> f
asks f = f . ask

local :: (e -> e) -> EnvT e w a -> EnvT e w a
local f ~(EnvT e wa) = EnvT (f e) wa