Variable instances (Re: Trace on Variable assignment)

[Ulf Wiger]

On Fri, 3 Nov 2000, Ulf Wiger wrote:

>X = foo(...),  % [0] is implicit
>X[1] = bar(X),
>...
>
>or perhaps 
>
>X = foo(...),  % \0 is implicit
>X\1 = bar(X),
>...
>
>
>would allow trace on "changes" to a variable, and also introduce a
>standard way of describing something that occurs frequently in
>most Erlang code I've seen.
>
>/Uffe

Since I posted this without thinking much, I paused and thought about
it afterwards.

One common source of bugs (and very hard to find) is when "changing" a
variable and forgetting to pass the latest instance. An example from
application_controller.erl:

handle_call({unload_application, AppName}, _From, S) ->
    case keysearch(AppName, 1, S#state.running) of
        {value, _} -> {reply, {error, {running, AppName}}, S};
        false ->
            case get_loaded(AppName) of
                {true, _} ->
                    NewS = unload(AppName, S),
                    cntrl(AppName, S, 
                          {ac_application_unloaded, AppName}),
                    {reply, ok, NewS};

Now it possibly won't matter in this case that cntrl/3 is called
with S instead of NewS (cntrl will look at S#state.control, which
isn't changed by unload/2), but I'd say it'd be a lot better to pass
NewS anyway to avoid confusion.

If the function could be rewritten as:

handle_call({unload_application, AppName}, _From, S) ->
    case keysearch(AppName, 1, S#state.running) of
        {value, _} -> {reply, {error, {running, AppName}}, S};
        false ->
            case get_loaded(AppName) of
                {true, _} ->
                    S[1] = unload(AppName, S),
                    cntrl(AppName, S, 
                          {ac_application_unloaded, AppName}),
                    {reply, ok, S[1]};

a compiler could easily produce a warning stating that we're passing
an old instance of S, since this is almost always a bug, and can
easily be avoided in the cases when it it's intentional.

/Uffe
-- 
Ulf Wiger                                    tfn: +46  8 719 81 95
Senior System Architect                      mob: +46 70 519 81 95
Strategic Product & System Management    ATM Multiservice Networks
Data Backbone & Optical Services Division      Ericsson Telecom AB