    -- | The Snap.Snaplet.Test module contains primitives and combinators for
    -- testing Snaplets.
    module Snap.Snaplet.Test
      (
        -- ** Testing handlers
        evalHandler
      , evalHandler'
      , runHandler
      , runHandler'
      , getSnaplet
      , closeSnaplet
      , InitializerState
      , withTemporaryFile
      )
      where

    ------------------------------------------------------------------------------
    import           Control.Concurrent.MVar
    import           Control.Exception.Base (finally)
    import qualified Control.Exception as E
    import           Control.Monad.IO.Class
    import           Control.Monad (join)
    import           Data.Maybe (fromMaybe)
    import           Data.IORef
    import           Data.Text
    import           System.Directory
    import           System.IO.Error

    ------------------------------------------------------------------------------
    import           Snap.Core
    import           Snap.Snaplet
    import           Snap.Snaplet.Internal.Types
    import           Snap.Test hiding (evalHandler, runHandler)
    import qualified Snap.Test as ST
    import           Snap.Snaplet.Internal.Initializer

    ------------------------------------------------------------------------------
    -- | Remove the given file before running an IO computation. Obviously it
    -- can be used with 'Assertion'.
    withTemporaryFile :: FilePath -> IO () -> IO ()
    withTemporaryFile f = finally (removeFileMayNotExist f)

    ------------------------------------------------------------------------------
    -- | Utility function taken from Darcs
    removeFileMayNotExist :: FilePath -> IO ()
    removeFileMayNotExist f = catchNonExistence (removeFile f) ()
      where
        catchNonExistence :: IO a -> a -> IO a
        catchNonExistence job nonexistval =
            E.catch job $
            \e -> if isDoesNotExistError e then return nonexistval
                                          else ioError e

    ------------------------------------------------------------------------------
    -- | Helper to keep "runHandler" and "evalHandler" DRY.
    execHandlerComputation :: MonadIO m
                           => (RequestBuilder m () -> Snap v -> m a)
                           -> Maybe String
                           -> RequestBuilder m ()
                           -> Handler b b v
                           -> SnapletInit b b
                           -> m (Either Text a)
    execHandlerComputation f env rq h s = do
        app <- getSnaplet env s
        case app of
          (Left e) -> return $ Left e
          (Right (a, is)) -> execHandlerSnaplet a is f rq h

    ------------------------------------------------------------------------------
    -- | Helper to allow multiple calls to "runHandler" or "evalHandler" without
    -- multiple initializations.
    execHandlerSnaplet :: MonadIO m
                       => Snaplet b
                       -> InitializerState b
                       -> (RequestBuilder m () -> Snap v -> m a)
                       -> RequestBuilder m ()
                       -> Handler b b v
                       -> m (Either Text a)
    execHandlerSnaplet a is f rq h = do
      res <- f rq $ runPureBase h a
      closeSnaplet is
      return $ Right res

    ------------------------------------------------------------------------------
    -- | Given a Snaplet Handler and a 'RequestBuilder' defining
    -- a test request, runs the Handler, producing an HTTP 'Response'.
    --
    -- Note that the output of this function is slightly different from
    -- 'runHandler' defined in Snap.Test, because due to the fact running
    -- the initializer inside 'SnapletInit' can throw an exception.
    runHandler :: MonadIO m
               => Maybe String
               -> RequestBuilder m ()
               -> Handler b b v
               -> SnapletInit b b
               -> m (Either Text Response)
    runHandler = execHandlerComputation ST.runHandler

    ------------------------------------------------------------------------------
    -- | A variant of runHandler that takes the Snaplet and InitializerState as
    -- produced by getSnaplet, so those can be re-used across requests. It does not
    -- run cleanup actions, so closeSnaplet should be used when finished.
    runHandler' :: MonadIO m
                => Snaplet b
                -> InitializerState b
                -> RequestBuilder m ()
                -> Handler b b v
                -> m (Either Text Response)
    runHandler' a is = execHandlerSnaplet a is ST.runHandler

    ------------------------------------------------------------------------------
    -- | Given a Snaplet Handler, a 'SnapletInit' specifying the initial state,
    --  and a 'RequestBuilder' defining a test request, runs the handler,
    --  returning the monadic value it produces.
    --
    -- Throws an exception if the 'Snap' handler early-terminates with 'finishWith'
    -- or 'mzero'.
    --
    -- Note that the output of this function is slightly different from
    -- 'evalHandler defined in Snap.Test, because due to the fact running
    -- the initializer inside 'SnapletInit' can throw an exception.
    evalHandler :: MonadIO m
                => Maybe String
                -> RequestBuilder m ()
                -> Handler b b a
                -> SnapletInit b b
                -> m (Either Text a)
    evalHandler = execHandlerComputation ST.evalHandler

    ------------------------------------------------------------------------------
    -- | A variant of evalHandler that takes the Snaplet and InitializerState as
    -- produced by getSnaplet, so those can be re-used across requests. It does not
    -- run cleanup actions, so closeSnaplet should be used when finished.
    evalHandler' :: MonadIO m
                 => Snaplet b
                 -> InitializerState b
                 -> RequestBuilder m ()
                 -> Handler b b a
                 -> m (Either Text a)
    evalHandler' a is = execHandlerSnaplet a is ST.evalHandler

    ------------------------------------------------------------------------------
    -- | Run the given initializer, yielding a tuple where the first element is
    -- a @Snaplet b@, or an error message whether the initializer threw an
    -- exception. This is only needed for runHandler'/evalHandler'.
    getSnaplet :: MonadIO m
               => Maybe String
               -> SnapletInit b b
               -> m (Either Text (Snaplet b, InitializerState b))
    getSnaplet env (SnapletInit initializer) = liftIO $ do
        mvar <- newEmptyMVar
        let resetter f = modifyMVar_ mvar (return . f)
        runInitializer resetter (fromMaybe "devel" env) initializer

    ------------------------------------------------------------------------------
    -- | Run cleanup for an initializer. Should be run after finished using the
    -- state that getSnaplet returned. Only needed if using getSnaplet and
    -- evalHandler'/runHandler'.
    closeSnaplet :: MonadIO m
                 => InitializerState b
                 -> m ()
    closeSnaplet is = liftIO $ join (readIORef $ _cleanup is)