-----------------------------------------------------------------------------
-- |
-- Module : Control.Comonad.Trans.Traced
-- Copyright : (C) 2008-2011 Edward Kmett
-- License : BSD-style (see the file LICENSE)
--
-- Maintainer : Edward Kmett <ekmett@gmail.com>
-- Stability : provisional
-- Portability : portable
--
-- The trace comonad transformer (aka the cowriter or exponential comonad transformer).
--
----------------------------------------------------------------------------
module Control.Comonad.Trans.Traced
(
-- * Traced comonad
Traced
, traced
, runTraced
-- * Traced comonad transformer
, TracedT(..)
-- * Operations
, trace
) where
import Control.Comonad
import Control.Comonad.Trans.Class
import Data.Functor.Identity
import Data.Monoid
type Traced m = TracedT m Identity
traced :: (m -> a) -> Traced m a
traced f = TracedT (Identity f)
runTraced :: Monoid m => Traced m a -> m -> a
runTraced (TracedT (Identity f)) = f
newtype TracedT m w a = TracedT { runTracedT :: w (m -> a) }
instance Functor w => Functor (TracedT m w) where
fmap g = TracedT . fmap (g .) . runTracedT
instance (Comonad w, Monoid m) => Comonad (TracedT m w) where
extract (TracedT wf) = extract wf mempty
extend f = TracedT . extend (\wf m -> f (TracedT (fmap (. mappend m) wf))) . runTracedT
instance Monoid m => ComonadTrans (TracedT m) where
lower = fmap ($mempty) . runTracedT
trace :: (Comonad w, Monoid m) => m -> TracedT m w a -> a
trace m (TracedT wf) = extract wf m