content: {-# LANGUAGE CPP #-}
  #if __GLASGOW_HASKELL__ >= 702
  {-# LANGUAGE Safe #-}
  #endif
  #if __GLASGOW_HASKELL__ >= 710
  {-# LANGUAGE AutoDeriveTypeable #-}
  #endif
  -----------------------------------------------------------------------------
  -- |
  -- Module      :  Control.Monad.Trans.Writer.Strict
  -- Copyright   :  (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; for a lazy version with
  -- the same interface, see "Control.Monad.Trans.Writer.Lazy".
  -- Although the output is built strictly, it is not possible to
  -- achieve constant space behaviour with this transformer: for that,
  -- use "Control.Monad.Trans.Writer.CPS" instead.
  -----------------------------------------------------------------------------
  
  module Control.Monad.Trans.Writer.Strict (
      -- * The Writer monad
      Writer,
      writer,
      runWriter,
      execWriter,
      mapWriter,
      -- * The WriterT monad transformer
      WriterT(..),
      execWriterT,
      mapWriterT,
      -- * Writer operations
      tell,
      listen,
      listens,
      pass,
      censor,
      -- * Lifting other operations
      liftCallCC,
      liftCatch,
    ) where
  
  import Control.Monad.IO.Class
  import Control.Monad.Trans.Class
  import Data.Functor.Classes
  #if MIN_VERSION_base(4,12,0)
  import Data.Functor.Contravariant
  #endif
  import Data.Functor.Identity
  
  import Control.Applicative
  import Control.Monad
  #if MIN_VERSION_base(4,9,0)
  import qualified Control.Monad.Fail as Fail
  #endif
  import Control.Monad.Fix
  import Control.Monad.Signatures
  #if MIN_VERSION_base(4,4,0)
  import Control.Monad.Zip (MonadZip(mzipWith))
  #endif
  import Data.Foldable
  import Data.Monoid
  import Data.Traversable (Traversable(traverse))
  import Prelude hiding (null, length)
  
  -- ---------------------------------------------------------------------------
  -- | 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 :: (Monad m) => (a, w) -> WriterT w m a
  writer = WriterT . return
  {-# INLINE writer #-}
  
  -- | Unwrap a writer computation as a (result, output) pair.
  -- (The inverse of 'writer'.)
  runWriter :: Writer w a -> (a, w)
  runWriter = runIdentity . runWriterT
  {-# INLINE runWriter #-}
  
  -- | Extract the output from a writer computation.
  --
  -- * @'execWriter' m = 'snd' ('runWriter' m)@
  execWriter :: Writer w a -> w
  execWriter m = snd (runWriter m)
  {-# INLINE execWriter #-}
  
  -- | Map both the return value and output of a computation using
  -- the given function.
  --
  -- * @'runWriter' ('mapWriter' f m) = f ('runWriter' m)@
  mapWriter :: ((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 { runWriterT :: m (a, w) }
  
  instance (Eq w, Eq1 m) => Eq1 (WriterT w m) where
      liftEq eq (WriterT m1) (WriterT m2) = liftEq (liftEq2 eq (==)) m1 m2
      {-# INLINE liftEq #-}
  
  instance (Ord w, Ord1 m) => Ord1 (WriterT w m) where
      liftCompare comp (WriterT m1) (WriterT m2) =
          liftCompare (liftCompare2 comp compare) m1 m2
      {-# INLINE liftCompare #-}
  
  instance (Read w, Read1 m) => Read1 (WriterT w m) where
      liftReadsPrec rp rl = readsData $
          readsUnaryWith (liftReadsPrec rp' rl') "WriterT" WriterT
        where
          rp' = liftReadsPrec2 rp rl readsPrec readList
          rl' = liftReadList2 rp rl readsPrec readList
  
  instance (Show w, Show1 m) => Show1 (WriterT w m) where
      liftShowsPrec sp sl d (WriterT m) =
          showsUnaryWith (liftShowsPrec sp' sl') "WriterT" d m
        where
          sp' = liftShowsPrec2 sp sl showsPrec showList
          sl' = liftShowList2 sp sl showsPrec showList
  
  instance (Eq w, Eq1 m, Eq a) => Eq (WriterT w m a) where (==) = eq1
  instance (Ord w, Ord1 m, Ord a) => Ord (WriterT w m a) where compare = compare1
  instance (Read w, Read1 m, Read a) => Read (WriterT w m a) where
      readsPrec = readsPrec1
  instance (Show w, Show1 m, Show a) => Show (WriterT w m a) where
      showsPrec = showsPrec1
  
  -- | Extract the output from a writer computation.
  --
  -- * @'execWriterT' m = 'liftM' 'snd' ('runWriterT' m)@
  execWriterT :: (Monad m) => 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 :: (m (a, w) -> n (b, w')) -> WriterT w m a -> WriterT w' n b
  mapWriterT f m = WriterT $ f (runWriterT m)
  {-# INLINE mapWriterT #-}
  
  instance (Functor m) => Functor (WriterT w m) where
      fmap f = mapWriterT $ fmap $ \ (a, w) -> (f a, w)
      {-# INLINE fmap #-}
  
  instance (Foldable f) => Foldable (WriterT w f) where
      foldMap f = foldMap (f . fst) . runWriterT
      {-# INLINE foldMap #-}
  #if MIN_VERSION_base(4,8,0)
      null (WriterT t) = null t
      length (WriterT t) = length t
  #endif
  
  instance (Traversable f) => Traversable (WriterT w f) where
      traverse f = fmap WriterT . traverse f' . runWriterT where
         f' (a, b) = fmap (\ c -> (c, b)) (f a)
      {-# INLINE traverse #-}
  
  instance (Monoid w, Applicative m) => Applicative (WriterT w m) where
      pure a  = WriterT $ pure (a, mempty)
      {-# INLINE pure #-}
      f <*> v = WriterT $ liftA2 k (runWriterT f) (runWriterT v)
        where k (a, w) (b, w') = (a b, w `mappend` w')
      {-# INLINE (<*>) #-}
  
  instance (Monoid w, Alternative m) => Alternative (WriterT w m) where
      empty   = WriterT empty
      {-# INLINE empty #-}
      m <|> n = WriterT $ runWriterT m <|> runWriterT n
      {-# INLINE (<|>) #-}
  
  instance (Monoid w, Monad m) => Monad (WriterT w m) where
  #if !(MIN_VERSION_base(4,8,0))
      return a = writer (a, mempty)
      {-# INLINE return #-}
  #endif
      m >>= k  = WriterT $ do
          (a, w)  <- runWriterT m
          (b, w') <- runWriterT (k a)
          return (b, w `mappend` 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 (Monoid w, Fail.MonadFail m) => Fail.MonadFail (WriterT w m) where
      fail msg = WriterT $ Fail.fail msg
      {-# INLINE fail #-}
  #endif
  
  instance (Monoid w, MonadPlus m) => MonadPlus (WriterT w m) where
      mzero       = WriterT mzero
      {-# INLINE mzero #-}
      m `mplus` n = WriterT $ runWriterT m `mplus` runWriterT n
      {-# INLINE mplus #-}
  
  instance (Monoid w, MonadFix m) => MonadFix (WriterT w m) where
      mfix m = WriterT $ mfix $ \ ~(a, _) -> runWriterT (m a)
      {-# INLINE mfix #-}
  
  instance (Monoid w) => MonadTrans (WriterT w) where
      lift m = WriterT $ do
          a <- m
          return (a, mempty)
      {-# INLINE lift #-}
  
  instance (Monoid w, MonadIO m) => MonadIO (WriterT w m) where
      liftIO = lift . liftIO
      {-# INLINE liftIO #-}
  
  #if MIN_VERSION_base(4,4,0)
  instance (Monoid w, MonadZip m) => MonadZip (WriterT w m) where
      mzipWith f (WriterT x) (WriterT y) = WriterT $
          mzipWith (\ (a, w) (b, w') -> (f a b, w `mappend` w')) x y
      {-# INLINE mzipWith #-}
  #endif
  
  #if MIN_VERSION_base(4,12,0)
  instance Contravariant m => Contravariant (WriterT w m) where
      contramap f = mapWriterT $ contramap $ \ (a, w) -> (f a, w)
      {-# INLINE contramap #-}
  #endif
  
  -- | @'tell' w@ is an action that produces the output @w@.
  tell :: (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 :: (Monad m) => WriterT w m a -> WriterT w m (a, w)
  listen m = WriterT $ do
      (a, w) <- runWriterT m
      return ((a, w), w)
  {-# 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 :: (Monad m) => (w -> b) -> WriterT w m a -> WriterT w m (a, b)
  listens f m = WriterT $ do
      (a, w) <- runWriterT m
      return ((a, f w), w)
  {-# 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 :: (Monad m) => WriterT w m (a, w -> w) -> WriterT w m a
  pass m = WriterT $ do
      ((a, f), w) <- runWriterT m
      return (a, f w)
  {-# 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 :: (Monad m) => (w -> w) -> WriterT w m a -> WriterT w m a
  censor f m = WriterT $ do
      (a, w) <- runWriterT m
      return (a, f w)
  {-# INLINE censor #-}
  
  -- | Lift a @callCC@ operation to the new monad.
  liftCallCC :: (Monoid w) => CallCC m (a,w) (b,w) -> CallCC (WriterT w m) a b
  liftCallCC callCC f = WriterT $
      callCC $ \ c ->
      runWriterT (f (\ a -> WriterT $ c (a, mempty)))
  {-# 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 $ runWriterT m `catchE` \ e -> runWriterT (h e)
  {-# INLINE liftCatch #-}
  