add test for using the Config machinery

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

@@ -50,8 +50,8 @@ instance (Eq a, Eq (Config as)) => Eq (Config (a ': as)) where
 e .:. cfg = ConsConfig (ConfigEntry e) cfg
 infixr 5 .:.
 
-class HasConfigEntry (cfg :: [*]) (a :: k) (val :: *) | cfg a -> val where
-    getConfigEntry :: proxy a -> Config cfg -> val
+class HasConfigEntry (cfg :: [*]) (tag :: k) (val :: *) | cfg tag -> val where
+    getConfigEntry :: proxy tag -> Config cfg -> val
 
 instance OVERLAPPABLE_
          HasConfigEntry xs tag val => HasConfigEntry (notIt ': xs) tag val where

servant-server/test/Servant/Server/UsingConfigSpec.hs

@@ -0,0 +1,71 @@
+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE FlexibleContexts #-}
+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE TypeOperators #-}
+{-# LANGUAGE UndecidableInstances #-}
+
+module Servant.Server.UsingConfigSpec where
+
+import           Network.Wai
+import           Test.Hspec (Spec, describe, it)
+import           Test.Hspec.Wai
+
+import           Servant
+import           Servant.Server.Internal.Config
+import           Servant.Server.Internal.Router
+import           Servant.Server.Internal.RoutingApplication
+
+-- * custom test combinator
+
+data CustomCombinator
+
+data CustomConfig = CustomConfig String
+
+data Tag
+
+instance forall subApi (c :: [*]) .
+  (HasServer subApi) =>
+  HasServer (CustomCombinator :> subApi) where
+
+  type ServerT (CustomCombinator :> subApi) m =
+    CustomConfig -> ServerT subApi m
+  type HasCfg (CustomCombinator :> subApi) c =
+    (HasConfigEntry c Tag CustomConfig, HasCfg subApi c)
+
+  route Proxy config delayed =
+    route subProxy config (fmap (inject config) delayed :: Delayed (Server subApi))
+    where
+      subProxy :: Proxy subApi
+      subProxy = Proxy
+
+      inject config f = f (getConfigEntry (Proxy :: Proxy Tag) config)
+
+-- * API
+
+type API =
+  CustomCombinator :> Get '[JSON] String
+
+api :: Proxy API
+api = Proxy
+
+testServer :: Server API
+testServer (CustomConfig s) = return s
+
+app :: Application
+app =
+  serve api config testServer
+  where
+    config :: Config '[ConfigEntry Tag CustomConfig]
+    config = CustomConfig "configValue" .: EmptyConfig
+
+-- * tests
+
+spec :: Spec
+spec = do
+  describe "using Config in a custom combinator" $ do
+    with (return app) $ do
+      it "allows to retrieve the ConfigEntry" $ do
+        get "/" `shouldRespondWith` "\"configValue\""