Start working opds (#2)

- Relates to #2

src/Servant/XML.hs

@@ -1,9 +1,11 @@
 {-# Language OverloadedStrings #-}
 {-# Language FlexibleInstances #-}
 {-# Language MultiParamTypeClasses #-}
+{-# Language TypeApplications #-}
 module Servant.XML
   ( ToNode(..)
   , XML
+  , OPDS
   , Text.Hamlet.XML.xml
   , iso8601 )
   where
@@ -16,14 +18,22 @@ import Network.HTTP.Media.MediaType
 
 data XML
 
+data OPDS
+
 instance (ToNode a) => MimeRender XML a where
   mimeRender _ a =
     let [NodeElement root] = toNode a
     in renderLBS def (Document (Prologue [] Nothing []) root [])
 
+instance (ToNode a) => MimeRender OPDS a where
+  mimeRender _ a = mimeRender (Proxy @XML) a
+
 instance Accept XML where
   contentType _ = "application" // "xml" /: ("charset", "utf-8")
 
+instance Accept OPDS where
+  contentType _ = "application" // "atom+xml" /: ("charset", "utf-8") /: ("profile", "opds-catalog")
+
 iso8601 :: UTCTime -> Text
 iso8601 = pack . formatTime defaultTimeLocale "%Y-%m-%dT%H:%M:%SZ"