anchor.diff:  Let's have a look at a site that provides email authentication, storing
   passwords in a Persistent SQLite database.
   
  +NOTE: Even if you don't have an email server, for debugging purposes the
  +verification link is printed in the console.
  +
   [source, haskell]
   ----
   {-# LANGUAGE DeriveDataTypeable         #-}
  
positive.code:     addUnverified email verkey =
          runDB $ insert $ User email Nothing (Just verkey) False
  
      sendVerifyEmail email _ verurl =
          liftIO $ renderSendMail (emptyMail $ Address Nothing "noreply")
              { mailTo = [Address Nothing email]
              , mailHeaders =
  
negative.code: import           Control.Monad            (join)
  import           Control.Monad.Logger (runNoLoggingT)
  import           Data.Maybe               (isJust)
  import           Data.Text                (Text, unpack)
  import qualified Data.Text.Lazy.Encoding
  import           Data.Typeable            (Typeable)
  import           Database.Persist.Sqlite
  