Can't figure out how to use 'getter callback'

Despite reading around in code + documentation + examples, and quite a few attempts, I am unable to use the 'getter callback' for new data types in a proper way.

Situation: I want to use a custom function for retrieving a property for a custom data type. Kind of like this.

/**
 * Defines info for the properties of the views_display data structure.
 */
function _rb_views_views_display_info() {
  return array(
    'field' => array(
      'type' => 'list<views_field>',
      'label' => t('Fields'),
      'getter callback' => 'my_custom_function',
    ),
  );
}

Problem: Every time I add the 'getter callback' entry, I get fatal errors on my site. (White screen, no messages shown. I should turn on my error messages, by the way.)

Wanted: A description of how to use 'getter callback', and 'setter callback'. I suspect that there may be problems if the object they act on has internal callbacks, but I'm not sure.