bug#17474: Making *unspecified* equivalent to (values) would seem convenient

[David Kastrup <dak@gnu.org>]

Mark H Weaver <mhw@netris.org> writes:

> ludo@gnu.org (Ludovic Courtès) writes:
>> I’m not completely convinced it makes sense to “specify” the zero values
>> case in this way, but I’d like to hear what others think.
>
> I'm strongly opposed to having core Guile mechanisms automatically
> convert between SCM_UNSPECIFIED and zero values, which is part of what
> David's patch set does.  I'd be glad to explain the reasons for my
> position in a later message, but I don't have time right now.
>
> However, I'm (cautiously) open to the idea of changing (if #f x) and
> some other things to return (values) instead of *unspecified*.  I agree
> that it would be cleaner, though I worry about backward compatibility
> issues.

The C API recommends using SCM_UNSPECIFIED here.  That was never
different.  Consolidating this will not really work without declaring
SCM_UNSPECIFIED and consequently *unspecified* the same as (values).
This strategy is also sketched in a comment by Andy where *unspecified*
is defined.

What this patch does for backward compatibility reasons is treating
*unspecified* as an immediate value in single-value contexts.

This will likely always be the case in the C API where all calls of
"values" are sort of half-transparent anyway.

It is conceivable to eventually deprecate this use in Scheme proper.
This won't be doable without a migration strategy.  The conversions this
patch uses in the Scheme/VM layer are such a migration strategy.  Step 1
would be this patch.  Step 2 would be putting out deprecation warnings.
Step 3 would be removing the automatic conversions.

Compatibility considerations will make the last two steps a large
hurdle.  I don't see a better step 1 towards the goal of letting
(if #f x) return (values).

I readily agree that this is a mess.  Where we disagree is in the
culprit.  You consider this patch the cause of the mess, I consider this
patch consolidation of the SCM_UNSPECIFIED/*unspecified* mess already
designed into GUILE.

> It would have to be done between major releases.

In the current form of the patch, it's surprisingly backwards compatible
but of course one would not call it a mere bug fix.

It will take a number of major releases to get more than step 1 done.
But every journey starts with the first step.

-- 
David Kastrup