update haskell servant samples

samples/server/petstore/haskell-servant/lib/OpenAPIPetstore/API.hs

@@ -27,6 +27,8 @@ module OpenAPIPetstore.API
   , OpenAPIPetstoreClientError(..)
   -- ** Servant
   , OpenAPIPetstoreAPI
+  -- ** Plain WAI Application
+  , serverWaiApplicationOpenAPIPetstore
   ) where
 
 import           OpenAPIPetstore.Types
@@ -284,7 +286,13 @@ runOpenAPIPetstoreMiddlewareServer Config{..} middleware backend = do
   let warpSettings = Warp.defaultSettings
         & Warp.setPort (baseUrlPort url)
         & Warp.setHost (fromString $ baseUrlHost url)
-  liftIO $ Warp.runSettings warpSettings $ middleware $ serve (Proxy :: Proxy OpenAPIPetstoreAPI) (serverFromBackend backend)
+  liftIO $ Warp.runSettings warpSettings $ middleware $ serverWaiApplicationOpenAPIPetstore backend
+
+-- | Plain "Network.Wai" Application for the OpenAPIPetstore server.
+--
+-- Can be used to implement e.g. tests that call the API without a full webserver.
+serverWaiApplicationOpenAPIPetstore :: OpenAPIPetstoreBackend (ExceptT ServerError IO) -> Application
+serverWaiApplicationOpenAPIPetstore backend = serve (Proxy :: Proxy OpenAPIPetstoreAPI) (serverFromBackend backend)
   where
     serverFromBackend OpenAPIPetstoreBackend{..} =
       (coerce addPet :<|>