bug#69454: Not possible to insert an empty vtable

[Adam Porter <adam@alphapapa.net>]

Hi Eli,

 > P.S. Adam, I took the liberty of adding you to this discussion, since
 > you seem lately to be interested in vtable.

Thanks for adding me.  Indeed, I've found vtable to be very useful in my 
new listen.el package.

On 3/9/24 02:54, Eli Zaretskii wrote:
>> Date: Wed, 28 Feb 2024 15:29:11 +0100
>> From: Eric Marsden <eric.marsden@risk-engineering.org>
>>
>> Hello,
>>
>> The following generates an error. It seems to me that it would be
>> preferable to insert the header line and show zero rows for the vtable.
>>
>>      (require 'vtable)
>>      (make-vtable :columns '("tweedle" "dum") :objects (list))
>>
>> Debugger entered--Lisp error: (wrong-number-of-arguments #<subr max> 0)
>>     max()
>>     apply(max nil)
>>     seq-max(nil)
> 
> I'm not sure we want to support zero-size vtables.  A better error
> message would be nice, though.  What do others think?

I tend to agree with Eric that it would be helpful if vtable could 
handle having an empty objects collection value to insert, because it 
saves the application from having to wrap the rather large `make-vtable' 
form in a `when' block, like here:

https://github.com/alphapapa/listen.el/blob/e9ea67350cf3b6cd870561c5e52d4b5255b04d34/listen-queue.el#L135

Also, it's possible that, after inserting a vtable, the collection of 
objects may be modified so that the collection is empty--then if the the 
vtable is reverted, it should be able to handle the case of the 
collection being empty.

AFAICT there's not much the application could do to avoid errors in that 
case, other than working outside of vtable's revert API and calling the 
function that tested the collection and conditionally inserted the 
vtable in the first place--in which case the vtable revert API would 
seem useless.

So IMO, when inserting or reverting a vtable, vtable ought to check 
whether the collection is empty; and if so, handle it gracefully, 
meaning that an "empty vtable" (whatever that would mean; maybe just one 
line of text saying that it's an empty collection) would still be 
inserted, and that if the collection became non-nil, it could be 
reverted and displayed properly.