content: {-# LANGUAGE CPP #-}
  #if __GLASGOW_HASKELL__ >= 702
  {-# LANGUAGE Safe #-}
  #endif
  #if __GLASGOW_HASKELL__ >= 710
  {-# LANGUAGE AutoDeriveTypeable #-}
  #endif
  -----------------------------------------------------------------------------
  -- |
  -- Module      :  Control.Monad.Trans.Writer.CPS
  -- Copyright   :  (c) Daniel Mendler 2016,
  --                (c) Andy Gill 2001,
  --                (c) Oregon Graduate Institute of Science and Technology, 2001
  -- License     :  BSD-style (see the file LICENSE)
  --
  -- Maintainer  :  R.Paterson@city.ac.uk
  -- Stability   :  experimental
  -- Portability :  portable
  --
  -- The strict 'WriterT' monad transformer, which adds collection of
  -- outputs (such as a count or string output) to a given monad.
  --
  -- This monad transformer provides only limited access to the output
  -- during the computation. For more general access, use
  -- "Control.Monad.Trans.State" instead.
  --
  -- This version builds its output strictly and uses continuation-passing-style
  -- to achieve constant space usage. This transformer can be used as a
  -- drop-in replacement for "Control.Monad.Trans.Writer.Strict".
  -----------------------------------------------------------------------------
  
  module Control.Monad.Trans.Writer.CPS (
      -- * The Writer monad
      Writer,
      writer,
      runWriter,
      execWriter,
      mapWriter,
      -- * The WriterT monad transformer
      WriterT,
      writerT,
      runWriterT,
      execWriterT,
      mapWriterT,
      -- * Writer operations
      tell,
      listen,
      listens,
      pass,
      censor,
      -- * Lifting other operations
      liftCallCC,
      liftCatch,
    ) where
  
  import Control.Applicative
  import Control.Monad
  import Control.Monad.Fix
  import Control.Monad.IO.Class
  import Control.Monad.Trans.Class
  import Control.Monad.Signatures
  import Data.Functor.Identity
  
  #if !(MIN_VERSION_base(4,8,0))
  import Data.Monoid
  #endif
  
  #if MIN_VERSION_base(4,9,0)
  import qualified Control.Monad.Fail as Fail
  #endif
  
  -- ---------------------------------------------------------------------------
  -- | A writer monad parameterized by the type @w@ of output to accumulate.
  --
  -- The 'return' function produces the output 'mempty', while '>>='
  -- combines the outputs of the subcomputations using 'mappend'.
  type Writer w = WriterT w Identity
  
  -- | Construct a writer computation from a (result, output) pair.
  -- (The inverse of 'runWriter'.)
  writer :: (Monoid w, Monad m) => (a, w) -> WriterT w m a
  writer (a, w') = WriterT $ \ w ->
      let wt = w `mappend` w' in wt `seq` return (a, wt)
  {-# INLINE writer #-}
  
  -- | Unwrap a writer computation as a (result, output) pair.
  -- (The inverse of 'writer'.)
  runWriter :: (Monoid w) => Writer w a -> (a, w)
  runWriter = runIdentity . runWriterT
  {-# INLINE runWriter #-}
  
  -- | Extract the output from a writer computation.
  --
  -- * @'execWriter' m = 'snd' ('runWriter' m)@
  execWriter :: (Monoid w) => Writer w a -> w
  execWriter = runIdentity . execWriterT
  {-# INLINE execWriter #-}
  
  -- | Map both the return value and output of a computation using
  -- the given function.
  --
  -- * @'runWriter' ('mapWriter' f m) = f ('runWriter' m)@
  mapWriter :: (Monoid w, Monoid w') =>
      ((a, w) -> (b, w')) -> Writer w a -> Writer w' b
  mapWriter f = mapWriterT (Identity . f . runIdentity)
  {-# INLINE mapWriter #-}
  
  -- ---------------------------------------------------------------------------
  -- | A writer monad parameterized by:
  --
  --   * @w@ - the output to accumulate.
  --
  --   * @m@ - The inner monad.
  --
  -- The 'return' function produces the output 'mempty', while '>>='
  -- combines the outputs of the subcomputations using 'mappend'.
  
  newtype WriterT w m a = WriterT { unWriterT :: w -> m (a, w) }
  
  -- | Construct a writer computation from a (result, output) computation.
  -- (The inverse of 'runWriterT'.)
  writerT :: (Functor m, Monoid w) => m (a, w) -> WriterT w m a
  writerT f = WriterT $ \ w ->
      (\ (a, w') -> let wt = w `mappend` w' in wt `seq` (a, wt)) <$> f
  {-# INLINE writerT #-}
  
  -- | Unwrap a writer computation.
  -- (The inverse of 'writerT'.)
  runWriterT :: (Monoid w) => WriterT w m a -> m (a, w)
  runWriterT m = unWriterT m mempty
  {-# INLINE runWriterT #-}
  
  -- | Extract the output from a writer computation.
  --
  -- * @'execWriterT' m = 'liftM' 'snd' ('runWriterT' m)@
  execWriterT :: (Monad m, Monoid w) => WriterT w m a -> m w
  execWriterT m = do
      (_, w) <- runWriterT m
      return w
  {-# INLINE execWriterT #-}
  
  -- | Map both the return value and output of a computation using
  -- the given function.
  --
  -- * @'runWriterT' ('mapWriterT' f m) = f ('runWriterT' m)@
  mapWriterT :: (Monad n, Monoid w, Monoid w') =>
      (m (a, w) -> n (b, w')) -> WriterT w m a -> WriterT w' n b
  mapWriterT f m = WriterT $ \ w -> do
      (a, w') <- f (runWriterT m)
      let wt = w `mappend` w'
      wt `seq` return (a, wt)
  {-# INLINE mapWriterT #-}
  
  instance (Functor m) => Functor (WriterT w m) where
      fmap f m = WriterT $ \ w -> (\ (a, w') -> (f a, w')) <$> unWriterT m w
      {-# INLINE fmap #-}
  
  instance (Functor m, Monad m) => Applicative (WriterT w m) where
      pure a = WriterT $ \ w -> return (a, w)
      {-# INLINE pure #-}
  
      WriterT mf <*> WriterT mx = WriterT $ \ w -> do
          (f, w') <- mf w
          (x, w'') <- mx w'
          return (f x, w'')
      {-# INLINE (<*>) #-}
  
  instance (Functor m, MonadPlus m) => Alternative (WriterT w m) where
      empty = WriterT $ const mzero
      {-# INLINE empty #-}
  
      WriterT m <|> WriterT n = WriterT $ \ w -> m w `mplus` n w
      {-# INLINE (<|>) #-}
  
  instance (Monad m) => Monad (WriterT w m) where
  #if !(MIN_VERSION_base(4,8,0))
      return a = WriterT $ \ w -> return (a, w)
      {-# INLINE return #-}
  #endif
  
      m >>= k = WriterT $ \ w -> do
          (a, w') <- unWriterT m w
          unWriterT (k a) w'
      {-# INLINE (>>=) #-}
  
  #if !(MIN_VERSION_base(4,13,0))
      fail msg = WriterT $ \ _ -> fail msg
      {-# INLINE fail #-}
  #endif
  
  #if MIN_VERSION_base(4,9,0)
  instance (Fail.MonadFail m) => Fail.MonadFail (WriterT w m) where
      fail msg = WriterT $ \ _ -> Fail.fail msg
      {-# INLINE fail #-}
  #endif
  
  instance (Functor m, MonadPlus m) => MonadPlus (WriterT w m) where
      mzero = empty
      {-# INLINE mzero #-}
      mplus = (<|>)
      {-# INLINE mplus #-}
  
  instance (MonadFix m) => MonadFix (WriterT w m) where
      mfix f = WriterT $ \ w -> mfix $ \ ~(a, _) -> unWriterT (f a) w
      {-# INLINE mfix #-}
  
  instance MonadTrans (WriterT w) where
      lift m = WriterT $ \ w -> do
          a <- m
          return (a, w)
      {-# INLINE lift #-}
  
  instance (MonadIO m) => MonadIO (WriterT w m) where
      liftIO = lift . liftIO
      {-# INLINE liftIO #-}
  
  -- | @'tell' w@ is an action that produces the output @w@.
  tell :: (Monoid w, Monad m) => w -> WriterT w m ()
  tell w = writer ((), w)
  {-# INLINE tell #-}
  
  -- | @'listen' m@ is an action that executes the action @m@ and adds its
  -- output to the value of the computation.
  --
  -- * @'runWriterT' ('listen' m) = 'liftM' (\\ (a, w) -> ((a, w), w)) ('runWriterT' m)@
  listen :: (Monoid w, Monad m) => WriterT w m a -> WriterT w m (a, w)
  listen = listens id
  {-# INLINE listen #-}
  
  -- | @'listens' f m@ is an action that executes the action @m@ and adds
  -- the result of applying @f@ to the output to the value of the computation.
  --
  -- * @'listens' f m = 'liftM' (id *** f) ('listen' m)@
  --
  -- * @'runWriterT' ('listens' f m) = 'liftM' (\\ (a, w) -> ((a, f w), w)) ('runWriterT' m)@
  listens :: (Monoid w, Monad m) =>
      (w -> b) -> WriterT w m a -> WriterT w m (a, b)
  listens f m = WriterT $ \ w -> do
      (a, w') <- runWriterT m
      let wt = w `mappend` w'
      wt `seq` return ((a, f w'), wt)
  {-# INLINE listens #-}
  
  -- | @'pass' m@ is an action that executes the action @m@, which returns
  -- a value and a function, and returns the value, applying the function
  -- to the output.
  --
  -- * @'runWriterT' ('pass' m) = 'liftM' (\\ ((a, f), w) -> (a, f w)) ('runWriterT' m)@
  pass :: (Monoid w, Monoid w', Monad m) =>
      WriterT w m (a, w -> w') -> WriterT w' m a
  pass m = WriterT $ \ w -> do
      ((a, f), w') <- runWriterT m
      let wt = w `mappend` f w'
      wt `seq` return (a, wt)
  {-# INLINE pass #-}
  
  -- | @'censor' f m@ is an action that executes the action @m@ and
  -- applies the function @f@ to its output, leaving the return value
  -- unchanged.
  --
  -- * @'censor' f m = 'pass' ('liftM' (\\ x -> (x,f)) m)@
  --
  -- * @'runWriterT' ('censor' f m) = 'liftM' (\\ (a, w) -> (a, f w)) ('runWriterT' m)@
  censor :: (Monoid w, Monad m) => (w -> w) -> WriterT w m a -> WriterT w m a
  censor f m = WriterT $ \ w -> do
      (a, w') <- runWriterT m
      let wt = w `mappend` f w'
      wt `seq` return (a, wt)
  {-# INLINE censor #-}
  
  -- | Uniform lifting of a @callCC@ operation to the new monad.
  -- This version rolls back to the original state on entering the
  -- continuation.
  liftCallCC :: CallCC m (a, w) (b, w) -> CallCC (WriterT w m) a b
  liftCallCC callCC f = WriterT $ \ w ->
      callCC $ \ c -> unWriterT (f (\ a -> WriterT $ \ _ -> c (a, w))) w
  {-# INLINE liftCallCC #-}
  
  -- | Lift a @catchE@ operation to the new monad.
  liftCatch :: Catch e m (a, w) -> Catch e (WriterT w m) a
  liftCatch catchE m h = WriterT $ \ w ->
      unWriterT m w `catchE` \ e -> unWriterT (h e) w
  {-# INLINE liftCatch #-}
  