add a field in Delayed that lets us specify a clean up action that can use the result of bodyD to perform some IO clean up operation

servant-server/src/Servant/Server/Internal.hs

@@ -400,7 +400,7 @@ instance HasServer Raw context where
   type ServerT Raw m = Application
 
   route Proxy _ rawApplication = RawRouter $ \ env request respond -> do
-    r <- runDelayed rawApplication env request
+    (r, _) <- runDelayed rawApplication env request
     case r of
       Route app   -> app request (respond . Route)
       Fail a      -> respond $ Fail a

servant-server/src/Servant/Server/Internal/RoutingApplication.hs

@@ -6,10 +6,13 @@
 {-# LANGUAGE KindSignatures             #-}
 {-# LANGUAGE RecordWildCards            #-}
 {-# LANGUAGE StandaloneDeriving         #-}
+{-# LANGUAGE TupleSections              #-}
 module Servant.Server.Internal.RoutingApplication where
 
 import           Control.Monad                      (ap, liftM)
 import           Control.Monad.Trans                (MonadIO(..))
+import           Control.Monad.Trans.Except         (runExceptT)
+import           Data.IORef                         (newIORef, readIORef, writeIORef)
 import           Network.Wai                        (Application, Request,
                                                      Response, ResponseReceived)
 import           Prelude                            ()
@@ -103,6 +106,10 @@ data Delayed env c where
              , authD     :: DelayedIO auth
              , bodyD     :: DelayedIO body
              , serverD   :: captures -> auth -> body -> Request -> RouteResult c
+             , cleanupD  :: body -> IO ()
+               -- not in DelayedIO because:
+               -- - most likely should not depend on the request
+               -- - simpler
              } -> Delayed env c
 
 instance Functor (Delayed env) where
@@ -142,7 +149,7 @@ instance MonadIO DelayedIO where
 -- | A 'Delayed' without any stored checks.
 emptyDelayed :: RouteResult a -> Delayed env a
 emptyDelayed result =
-  Delayed (const r) r r r (\ _ _ _ _ -> result)
+  Delayed (const r) r r r (\ _ _ _ _ -> result) (const $ return ())
   where
     r = return ()
 
@@ -198,6 +205,7 @@ addBodyCheck Delayed{..} new =
   Delayed
     { bodyD    = (,) <$> bodyD <*> new
     , serverD  = \ c a (z, v) req -> ($ v) <$> serverD c a z req
+    , cleanupD = cleanupD . fst -- not sure that's right
     , ..
     } -- Note [Existential Record Update]
 
@@ -240,13 +248,19 @@ passToServer Delayed{..} x =
 runDelayed :: Delayed env a
            -> env
            -> Request
-           -> IO (RouteResult a)
+           -> IO (RouteResult a, IO ())
-runDelayed Delayed{..} env = runDelayedIO $ do
+runDelayed Delayed{..} env req = do
-  c <- capturesD env
+  cleanupRef <- newIORef (return ())
+  routeRes <- runDelayedIO
+    (do c <- capturesD env
         methodD
         a <- authD
         b <- bodyD
-  DelayedIO (\ req -> return $ serverD c a b req)
+        liftIO (writeIORef cleanupRef $ cleanupD b)
+        DelayedIO $ \ req -> return (serverD c a b req)
+    )
+    req
+  fmap (routeRes,) $ readIORef cleanupRef
 
 -- | Runs a delayed server and the resulting action.
 -- Takes a continuation that lets us send a response.
@@ -258,8 +272,11 @@ runAction :: Delayed env (Handler a)
           -> (RouteResult Response -> IO r)
           -> (a -> RouteResult Response)
           -> IO r
-runAction action env req respond k =
+runAction action env req respond k = do
-  runDelayed action env req >>= go >>= respond
+  (routeResult, cleanup) <- runDelayed action env req
+  resp <- go routeResult
+  cleanup
+  respond resp
   where
     go (Fail e)      = return $ Fail e
     go (FailFatal e) = return $ FailFatal e