    {-# LANGUAGE CPP #-}
    {-# LANGUAGE FlexibleContexts #-}
    {-# LANGUAGE Trustworthy #-}
    {-# LANGUAGE ScopedTypeVariables #-}

    {-# OPTIONS_GHC -fno-warn-deprecations #-} -- Suppress warnings around Control.Monad.Trans.Error
    -- | Use lazy I\/O for consuming the contents of a source. Warning: All normal
    -- warnings of lazy I\/O apply. In particular, if you are using this with a
    -- @ResourceT@ transformer, you must force the list to be evaluated before
    -- exiting the @ResourceT@.
    module Data.Conduit.Lazy
        ( lazyConsume
        , MonadActive (..)
        ) where

    import Data.Conduit
    import Data.Conduit.Internal (Pipe (..), ConduitT (..))
    import System.IO.Unsafe (unsafeInterleaveIO)

    import Control.Monad.Trans.Class (lift)
    import Control.Monad.IO.Unlift (MonadIO, liftIO, MonadUnliftIO, withUnliftIO, unliftIO)

    import Control.Monad.Trans.Identity ( IdentityT)
    import Control.Monad.Trans.List     ( ListT    )
    import Control.Monad.Trans.Maybe    ( MaybeT   )
    import Control.Monad.Trans.Error    ( ErrorT, Error)
    import Control.Monad.Trans.Reader   ( ReaderT  )
    import Control.Monad.Trans.State    ( StateT   )
    import Control.Monad.Trans.Writer   ( WriterT  )
    import Control.Monad.Trans.RWS      ( RWST     )

    import qualified Control.Monad.Trans.RWS.Strict    as Strict ( RWST   )
    import qualified Control.Monad.Trans.State.Strict  as Strict ( StateT )
    import qualified Control.Monad.Trans.Writer.Strict as Strict ( WriterT )

    #if (__GLASGOW_HASKELL__ < 710)
    import Data.Monoid (Monoid)
    #endif
    import Control.Monad.ST (ST)
    import qualified Control.Monad.ST.Lazy as Lazy
    import Data.Functor.Identity (Identity)
    import Control.Monad.Trans.Resource.Internal (ResourceT (ResourceT), ReleaseMap (ReleaseMapClosed))
    import qualified Data.IORef as I

    -- | Use lazy I\/O to consume all elements from a @Source@.
    --
    -- This function relies on 'monadActive' to determine if the underlying monadic
    -- state has been closed.
    --
    -- Since 0.3.0
    lazyConsume
      :: forall m a.
         (MonadUnliftIO m, MonadActive m)
      => Source m a
      -> m [a]
    lazyConsume (ConduitT f0) =
        withUnliftIO $ \u ->
          let go :: Pipe () () a () m () -> IO [a]
              go (Done _) = return []
              go (HaveOutput src x) = do
                  xs <- unsafeInterleaveIO $ go src
                  return $ x : xs
              go (PipeM msrc) = unsafeInterleaveIO $ do
                  a <- unliftIO u monadActive
                  if a
                      then unliftIO u msrc >>= go
                      else return []
              go (NeedInput _ c) = go (c ())
              go (Leftover p _) = go p
          in go (f0 Done)

    -- | Determine if some monad is still active. This is intended to prevent usage
    -- of a monadic state after it has been closed.  This is necessary for such
    -- cases as lazy I\/O, where an unevaluated thunk may still refer to a
    -- closed @ResourceT@.
    --
    -- Since 0.3.0
    class Monad m => MonadActive m where
        monadActive :: m Bool

    instance (MonadIO m, MonadActive m) => MonadActive (ResourceT m) where
        monadActive = ResourceT $ \rmMap -> do
            rm <- liftIO $ I.readIORef rmMap
            case rm of
                ReleaseMapClosed -> return False
                _ -> monadActive -- recurse

    instance MonadActive Identity where
        monadActive = return True

    instance MonadActive IO where
        monadActive = return True

    instance MonadActive (ST s) where
        monadActive = return True

    instance MonadActive (Lazy.ST s) where
        monadActive = return True

    #define GO(T) instance MonadActive m => MonadActive (T m) where monadActive = lift monadActive
    #define GOX(X, T) instance (X, MonadActive m) => MonadActive (T m) where monadActive = lift monadActive
    GO(IdentityT)
    GO(ListT)
    GO(MaybeT)
    GOX(Error e, ErrorT e)
    GO(ReaderT r)
    GO(StateT s)
    GOX(Monoid w, WriterT w)
    GOX(Monoid w, RWST r w s)
    GOX(Monoid w, Strict.RWST r w s)
    GO(Strict.StateT s)
    GOX(Monoid w, Strict.WriterT w)
    #undef GO
    #undef GOX

    instance MonadActive m => MonadActive (Pipe l i o u m) where
        monadActive = lift monadActive
    instance MonadActive m => MonadActive (ConduitT i o m) where
        monadActive = lift monadActive