preferences:add-callback

• (preferences:add-callback p f) -> (-> void)
      p : symbol
      f : (symbol TST -> boolean)

  This function adds a callback which is called with a symbol naming a preference and it's value, when the preference changes. If the callback function returns #f, the preference is not changed. preferences:add-callback returns a thunk, which when invoked, removes the callback from this preference.

  The callbacks will be called in the order in which they were added.

  If you are adding a callback for a preference that requires marshalling and unmarshalling, you must set the marshalling and unmarshalling functions by calling preferences:set-un/marshall before adding a callback.

  This function raises exn:unknown-preference if the preference has not been set.