Fix servant-docs code sample in README (#1335)

Author: Szabo Gergely

servant-docs/README.md

@@ -2,24 +2,56 @@
 
 ![servant](https://raw.githubusercontent.com/haskell-servant/servant/master/servant.png)
 
-Generate API docs for your *servant* webservice. Feel free to also take a look at [servant-pandoc](https://github.com/mpickering/servant-pandoc) which uses this package to target a broad range of output formats using the excellent **pandoc**.
+Generate API docs for your _servant_ webservice. Feel free to also take a look at [servant-pandoc](https://github.com/mpickering/servant-pandoc) which uses this package to target a broad range of output formats using the excellent **pandoc**.
 
 ## Example
 
 See [here](https://github.com/haskell-servant/servant/blob/master/servant-docs/example/greet.md) for the output of the following program.
 
-``` haskell
+```haskell
 {-# LANGUAGE DataKinds #-}
-{-# LANGUAGE PolyKinds #-}
-{-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE DeriveGeneric #-}
-{-# LANGUAGE TypeOperators #-}
 {-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE PolyKinds #-}
+{-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE TypeOperators #-}
 
-import Data.Proxy
-import Data.Text
+import Data.Aeson (FromJSON, ToJSON)
+import Data.Proxy (Proxy (..))
+import Data.String.Conversions (cs)
+import Data.Text (Text)
+import GHC.Generics (Generic)
+import Servant.API
+  ( (:<|>),
+    (:>),
+    Capture,
+    Delete,
+    Get,
+    JSON,
+    MimeRender,
+    PlainText,
+    Post,
+    QueryParam,
+    ReqBody,
+    mimeRender,
+  )
 import Servant.Docs
+  ( API,
+    DocCapture (..),
+    DocQueryParam (..),
+    ParamKind (..),
+    ToCapture,
+    ToParam,
+    ToSample,
+    docs,
+    markdown,
+    singleSample,
+    toCapture,
+    toParam,
+    toSamples,
+  )
 
 -- our type for a Greeting message
 data Greet = Greet { _msg :: Text }
@@ -29,22 +61,23 @@ data Greet = Greet { _msg :: Text }
 -- 'MimeRender' instance for 'JSON'.
 instance FromJSON Greet
 instance ToJSON Greet
+instance ToSample ()
 
 -- We can also implement 'MimeRender' explicitly for additional formats.
 instance MimeRender PlainText Greet where
     mimeRender Proxy (Greet s) = "<h1>" <> cs s <> "</h1>"
 
 -- we provide a sample value for the 'Greet' type
 instance ToSample Greet where
-  toSample = Just g
-
+  toSamples _ = singleSample g
     where g = Greet "Hello, haskeller!"
 
 instance ToParam (QueryParam "capital" Bool) where
   toParam _ =
     DocQueryParam "capital"
                   ["true", "false"]
                   "Get the greeting message in uppercase (true) or not (false). Default is false."
+                  Normal
 
 instance ToCapture (Capture "name" Text) where
   toCapture _ = DocCapture "name" "name of the person to greet"
@@ -55,8 +88,8 @@ instance ToCapture (Capture "greetid" Text) where
 -- API specification
 type TestApi =
        "hello" :> Capture "name" Text :> QueryParam "capital" Bool :> Get '[JSON,PlainText] Greet
-  :<|> "greet" :> RQBody '[JSON] Greet :> Post '[JSON] Greet
-  :<|> "delete" :> Capture "greetid" Text :> Delete '[] ()
+  :<|> "greet" :> ReqBody '[JSON] Greet :> Post '[JSON] Greet
+  :<|> "delete" :> Capture "greetid" Text :> Delete '[JSON] ()
 
 testApi :: Proxy TestApi
 testApi = Proxy