Re: [Chicken-users] Bug with #!optional in Chicken 4.8.0

[John Croisant]

On 10/14/12 7:20 AM, Felix wrote:
> From: John Croisant <address@hidden>
> Subject: [Chicken-users] Bug with #!optional in Chicken 4.8.0
> Date: Sat, 13 Oct 2012 20:34:26 -0400
>
> > In Chicken 4.8.0, procedures defined with #!optional no longer signal
> > an exception if invoked with too many arguments. For example:
> >
> > (define (foo x #!optional y) (list x y))
> > (foo 'x 'y 'too 'many 'args)
> >
> > In Chicken 4.7, this would signal an exception, because the foo
> > procedure expects a maximum of 2 arguments. In Chicken 4.8, the extra
> > arguments are simply ignored.
> >
> > I think this is a bug, because the docs for "Extended DSSSL style
> > lambda lists" say:
> >
> > > If there is no rest-parameter  and no keyword-parameters in the
> > > parameter-list, then it shall be an error for any extra arguments to
> > > be passed to the procedure.
> That is correct. The check has been removed and conflicts with
> the documentation. I will update the docs accordingly.
>
> The check has been removed to accomodate a terrible hack for handling
> re-initialization of parameters with "parameterize", and for a slight
> performance win. The hack is so terrible, I hardly dare to talk about
> it in public. On the other hand, I see no problem in allowing extra
> parameters to lambdas with rest/optional arguments being silently
> dropped - it has been done before.
>
>
> cheers,
> felix
The new behavior may not break very much currently existing software 
(although it did break some of my unit tests), but future software 
written in Chicken will start to depend on the new behavior, so changing 
the behavior back in the future *would* break existing software. So, we 
should address this soon, while the pain of changing back is still minimal.

The old behavior was useful and valuable for (at least) two reasons:

1. It provided clear and immediate feedback if the procedure was called 
incorrectly, making the mistake much easier/cheaper to fix. (Even 
experienced professional programmers make this kind of mistake sometimes.)

2. It was consistent with the behavior of optional args in many other 
widely-used programming languages.

The new behavior can be useful in some situations, but it was already 
very easy to implement: just add a rest arg to any procedure that should 
ignore extra args. On the other hand, it takes a significant amount of 
code/effort for developers to (correctly) reimplement argument count 
checking in their own libraries and applications.

So, for all those reasons, I think this should be considered a bug and 
fixed as soon as possible. Perhaps if you describe the parameterize 
problem (or link to discussion about it elsewhere), someone can think of 
an alternative solution that will not require changing the behavior of 
optional args?

Regards,

- John