Support for RSS extensions. Cf specification at http://web.resource.org/rss/1.0/modules/.

To implement an RSS extension:

• Create a data-type, that will be used as a tag to identify the extension. To allow stacking multiple extensions, your data-type should have kind * -> *

data MyExtension otherExtensions = MyExtension otherExtensions

• Implement extension types for <channel> and <item> elements:

data instance RssChannelExtension (MyExtension e) = MyExtensionChannel
  { -- ... add your fields ...
  , otherChannelExtensions :: RssChannelExtension e
  }

data instance RssItemExtension (MyExtension e) = MyExtensionItem
  { -- ... add your fields ...
  , otherItemExtensions :: RssItemExtension e
  }

• Implement ParseRssExtension and RenderRssExtension type classes:

-- Parser should rely on ZipConduit to be order-insensitive
instance ParseRssExtension e => ParseRssExtension (MyExtension e) where
  parseRssChannelExtension = getZipConduit $ MyExtensionChannel
    <$> ZipConduit -- ... parse fields
    <*> ZipConduit parseRssChannelExtension
  parseRssItemExtension = -- ... similarly

instance RenderRssExtension e => RenderRssExtension (MyExtension e) where
  renderRssChannelExtension (MyExtensionChannel {- fields -} otherChannelExtensions) = do
    -- ... render fields
    renderRssChannelExtension otherChannelExtensions
  renderRssItemExtension (MyExtensionItem {- fields -} otherItemExtensions) = -- ... similarly