lift

maybe.erl

-spec lift(fun()) -> maybe(_, _).
%% @doc Lift F into maybe().
lift(F) ->
  try F() of
    ok                  -> {ok, ok};
    {ok, Res}           -> {ok, Res};
    error               -> {error, error};
    {error, Rsn}        -> {error, Rsn};
    Res                 -> {ok, Res}
  catch
    throw:{error,Rsn}   -> {error, Rsn};
    _:{aborted, Rsn}    -> exit({aborted, Rsn}); %Mnesia hack
    _:Exn               -> {error, {lifted_exn, Exn, erlang:get_stacktrace()}}
  end.

-spec unlift(fun(() -> broken())) -> _ | no_return().
%% @doc Call F and untag the result. Errors become exceptions.
unlift(F) ->
  case F() of
    ok           -> ok;
    {ok, Res}    -> Res;
    error        -> throw({error, error});
    {error, Rsn} -> throw({error, Rsn});
    Res          -> Res
  end.


%% @doc XXX this will be renamed
maybe(Thunk, Ok, Error) ->
  maybe([Thunk, Error, Ok]).

maybe(CPS) ->
  maybe(CPS, []).
maybe([Thunk, Error|Rest], Acc) ->
  case lift(Thunk()) of
    {error, Rsn} -> Error(Rsn);
    {ok, Res}    -> maybe(Rest, [Res|Acc])
  end;
maybe([Ok], Acc) -> apply(Ok, lists:reverse(Acc)).