instance Data.Aeson.ToJSON InputDropboxFiles where
  Data.Aeson.toJSON x@(InputDropboxFiles {})
    = Data.Aeson.object
        (Data.Maybe.catMaybes
           [Just
              ((Data.Aeson..=) "root" (inputDropboxFilesRoot x)),
            Just
              ((Data.Aeson..=) "path" (inputDropboxFilesPath x)),
            fmap
              ((Data.Aeson..=) "rev") (inputDropboxFilesRev x),
            Just
              ((Data.Aeson..=)
                 "access_token" (inputDropboxFilesAccess_token x))])

Data.Aeson.toJSON should be toJSON at line 2