Re: Unbalanced change hooks (part 2) [Documentation fix still remaining]

[Stefan Monnier]

>   Do @emph{not} expect the before-change hooks and the after-change
>   hooks be called in balanced pairs around each buffer change.  Also
>   don't expect the before-change hooks to be called for every chunk of
>   text Emacs is about to delete.  These hooks are provided on the
>   assumption that Lisp programs will use either before- or the
>   after-change hooks, but not both, and the boundaries of the region
>   where the changes happen might include more than just the actual
>   changed text, or even lump together several changes done piecemeal.

That looks OK, thank you.  It does make it sound like sometimes b-c-f
won't be called at all, whereas AFAIU it is *always* called before
a change, tho maybe "too early" (i.e. announcing several changes at the
same time), but I think it's good enough for now.

> One can (indeed, often MUST) expect b-c-f and a-c-f to work in pairs,

No.  I know you *want* to expect it, but that assumption has not been
true for many years and we don't want to impose it on ourselves.
Furthermore there is no evidence that you "MUST".
I gave various examples of ways you can live with only a-c-f.

> Editing text involves deletions and insertions.

It also involves replacements.

> Deletions can be known only in before-change-functions, insertions
> only in after-change-functions.

Not at all.  Deletions are definitely announced to a-c-f.

> CC Mode absolutely requires to know both deletions and insertions for
> its proper functioning.  Nobody has yet demonstrated that it can work
> properly knowing only insertions, despite Stefan loudly and repeatedly
> proclaiming it can.

All major modes using syntax-propertize demonstrate it.  C++ has its
share of syntactic quirks, but none that are fundamentally very
different from what we find in other languages such as Perl.

I also outlined in more detailed how to deal with a specific example you
seemed to think required absolutely to know what was there before
the deletion.

>   enabling state to be determined in a before-change function and used
>   in an after-change function.

That's exactly what I want to strongly discourage.  Based on my
experience with CC-mode, I'd even say that it's just a bad design, even
if the hooks were always called in pairs.  I don't know nor care about
how we ended up with such a design, and I understand that you might
prefer to try and live with it than to go back and rethink it, but it's
still a bad design (nor is it the only bad design we have in Emacs, of
course).


        Stefan