Client.hs

class MonadClient m where
  performRequestCT :: Proxy ct -> method -> Req -> m a

instance MonadClient ClientM where
 .... 

instance OVERLAPPING_
  -- Note [Non-Empty Content Types]
  ( MonadClient m, MimeUnrender ct a, BuildHeadersTo ls, ReflectMethod method, cts' ~ (ct ': cts)
  ) => HasClient (Verb method status cts' (Headers ls a)) where
  type Client (Verb method status cts' (Headers ls a))
    = m (Headers ls a)
  clientWithRoute Proxy req = do
    let method = reflectMethod (Proxy :: Proxy method)
    (hdrs, resp) <- performRequestCT (Proxy :: Proxy ct) method req
    return $ Headers { getResponse = resp
                     , getHeadersHList = buildHeadersTo hdrs
                     }