anchor.diff:      <*> aopt urlField "Website" Nothing
   
   -- The GET handler displays the form
  -getHomeR :: Handler RepHtml
  +getHomeR :: Handler Html
   getHomeR = do
       -- Generate the form to be displayed
       (widget, enctype) <- generateFormPost personForm
  
positive.code:                 <p>Invalid input, let's try again.
                  <form method=post action=@{PersonR} enctype=#{enctype}>
                      ^{widget}
                      <input type=submit>
              |]
  
  main :: IO ()
  
negative.code: 
  That last case might sound surprising. It would seem that Yesod can
  automatically know that no information is supplied when the input list is
  empty. But in reality, fomr some field types, the lack of any input is actually
  valid input. Checkboxes, for instance, indicate an unchecked state by sending
  in an empty list.
  
  