anchor.diff:  
   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, so some field types the lack of any input is actually
  +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.
   
  
positive.code:     <*> aopt urlField "Website" Nothing
  
  -- The GET handler displays the form
  getHomeR :: Handler RepHtml
  getHomeR = do
      -- Generate the form to be displayed
      (widget, enctype) <- generateFormPost personForm
  
negative.code: 
  -- The POST handler processes the form. If it is successful, it displays the
  -- parsed person. Otherwise, it displays the form again with error messages.
  postPersonR :: Handler Html
  postPersonR = do
      ((result, widget), enctype) <- runFormPost personForm
      case result of
  