Add EmptyAPIClient and instance HasClient EmptyAPI

doc/tutorial/Client.lhs

@@ -62,6 +62,7 @@ Enough chitchat, let's see an example. Consider the following API type from the
 type API = "position" :> Capture "x" Int :> Capture "y" Int :> Get '[JSON] Position
       :<|> "hello" :> QueryParam "name" String :> Get '[JSON] HelloMessage
       :<|> "marketing" :> ReqBody '[JSON] ClientInfo :> Post '[JSON] Email
+      :<|> EmptyAPI
 ```
 
 What we are going to get with **servant-client** here is three functions, one to query each endpoint:
@@ -76,6 +77,8 @@ hello :: Maybe String -- ^ an optional value for "name"
 
 marketing :: ClientInfo -- ^ value for the request body
           -> ClientM Email
+
+emptyClient :: EmptyAPIClient
 ```
 
 Each function makes available as an argument any value that the response may
@@ -88,7 +91,7 @@ the function `client`. It takes one argument:
 api :: Proxy API
 api = Proxy
 
-position :<|> hello :<|> marketing = client api
+position :<|> hello :<|> marketing :<|> emptyClient = client api
 ```
 
 `client api` returns client functions for our _entire_ API, combined with `:<|>`, which we can pattern match on as above. You could say `client` "calculates" the correct type and number of client functions for the API type it is given (via a `Proxy`), as well as their implementations.

servant-client/src/Servant/Client.hs

@@ -24,6 +24,7 @@ module Servant.Client
   , ClientEnv (ClientEnv)
   , mkAuthenticateReq
   , ServantError(..)
+  , EmptyAPIClient(..)
   , module Servant.Common.BaseUrl
   ) where
 
@@ -88,6 +89,13 @@ instance (HasClient a, HasClient b) => HasClient (a :<|> b) where
     clientWithRoute (Proxy :: Proxy a) req :<|>
     clientWithRoute (Proxy :: Proxy b) req
 
+-- | TODO docs
+data EmptyAPIClient = EmptyAPIClient
+
+instance HasClient EmptyAPI where
+  type Client EmptyAPI = EmptyAPIClient
+  clientWithRoute Proxy _ = EmptyAPIClient
+
 -- | If you use a 'Capture' in one of your endpoints in your API,
 -- the corresponding querying function will automatically take
 -- an additional argument of the type specified by your 'Capture'.