add an example to show how to apply a WAI middleware

servant-examples/servant-examples.cabal

@@ -27,3 +27,17 @@ executable hackage
     , transformers
   hs-source-dirs:      hackage
   default-language:    Haskell2010
+
+executable wai-middleware
+  main-is: wai-middleware.hs
+  build-depends:
+      aeson >= 0.8
+    , base >= 4.7
+    , servant
+    , servant-server
+    , text
+    , wai
+    , wai-extra
+    , warp
+  hs-source-dirs: wai-middleware
+  default-language: Haskell2010

servant-examples/wai-middleware/wai-middleware.hs

@@ -0,0 +1,51 @@
+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE DeriveGeneric #-}
+{-# LANGUAGE OverloadedStrings #-}
+import Data.Aeson
+import Data.Text
+import GHC.Generics
+import Network.Wai
+import Network.Wai.Handler.Warp
+import Network.Wai.Middleware.RequestLogger
+import Servant
+
+data Product = Product
+  { name              :: Text
+  , brand             :: Text
+  , current_price_eur :: Double
+  , available         :: Bool
+  } deriving (Eq, Show, Generic)
+
+instance ToJSON Product
+
+products :: [Product]
+products = [p1, p2]
+
+  where p1 = Product "Haskell laptop sticker"
+                     "GHC Industries"
+                     2.50
+                     True
+
+        p2 = Product "Foldable USB drive"
+                     "Well-Typed"
+                     13.99
+                     False
+
+type SimpleAPI = Get '[JSON] [Product]
+
+simpleAPI :: Proxy SimpleAPI
+simpleAPI = Proxy
+
+server :: Server SimpleAPI
+server = return products
+
+-- logStdout :: Middleware
+-- i.e, logStdout :: Application -> Application
+-- serve :: Proxy api -> Server api -> Application
+-- so applying a middleware is really as simple as
+-- applying a function to the result of 'serve'
+app :: Application
+app = logStdout (serve simpleAPI server)
+
+main :: IO ()
+main = run 8080 app