This module, in combination with the RebindableSyntax and RecordWilCards extensions, allows you to write TwiML using do-notation. For example,

{-# LANGUAGE RebindableSyntax #-}
{-# LANGUAGE RecordWildCards #-}

import Prelude
import Text.XML.Twiml
import qualified Text.XML.Twiml.Syntax as Twiml

example :: VoiceTwiml
example =
  response $ do
    say "Hello World" def
    end
  where Twiml.Syntax{..} = def

This pattern is due to a suggestion from Adam Bergmark on Haskell-Cafe.