gs_up.erl

-module(gs_up).
-export([init/1, handle_call/3, terminate/2]).

-record(state, {
    own_fields,
    field1,
    field2,
    field3
  }).

init([F1, F3]) ->
  {ok, #state{own_fields = record_info(fields, state),
      field1 = F1, field3 = F3}}.

terminate(_,_) -> ok.

handle_call(Call, From, State) ->
  NewState = case element(2, State) == record_info(fields, state) of
    true -> State;
    false -> migrate(State)
  end,
  do_handle_call(Call, From, NewState).

do_handle_call(_, _, #state{} = State) ->
  {reply, state_to_pl(State), State}.


state_to_pl(State) when is_tuple(State) ->
  Fields = element(2, State),
  tl(lists:zip(Fields, tl(tuple_to_list(State)))).

state_from_pl(PL) ->
  ZeroState = #state{own_fields = record_info(fields, state)},
  FieldsToInherit = state_to_pl(ZeroState),
  InheritedValues = [proplists:get_value(Field, PL, Default) || {Field, Default} <- FieldsToInherit],
  list_to_tuple([element(1, ZeroState), element(2, ZeroState) | InheritedValues]).

migrate(OldState) ->
  state_from_pl(state_to_pl(OldState)).