Re: Lisp primitives and their calling of the change hooks

[Alan Mackenzie <acm@muc.de>]

Hello, Eli.

On Fri, Jan 05, 2018 at 08:55:20 +0200, Eli Zaretskii wrote:
> > Date: Thu, 4 Jan 2018 21:11:54 +0000
> > From: Alan Mackenzie <acm@muc.de>
> > Cc: emacs-devel@gnu.org

> >     The primitives which atomically insert or delete a contiguous chunk
> >     of text into or from a buffer will call `before-change-functions'
> >     and `after-change-functions' in balanced pairs, once for each
> >     change.  The arguments to these hooks will exactly delimit the
> >     change being made.  Calls to these primitives comprise the vast bulk
> >     of buffer changes.

> >     Other, more complex primitives aim to call `before-change-functions'
> >     once before making any changes, then to call
> >     `after-change-functions' zero, one, or several times, depending on
> >     how many individual changes the primitive makes.  The `BEG' and
> >     `END' arguments to `before-change-functions' will enclose a region
> >     in which the individual changes are made, but won't necessarily be
> >     the minimal such region.  The `BEG', `END', and `OLD-LEN' arguments
> >     to each successive call of `after-change-functions' will accurately
> >     delimit the current change.

> How will the reader know to distinguish between these two classes of
> primitives?  Without such an ability, the extra accuracy in this text
> is not useful.

A good point.  Wherein lies the difference, from a programmers point of
view?  Briefly I think that we have a "complex primitive" when we don't
have an exact match between one b-c-f and one a-c-f.  How about adding
the following paragraph to what comes above:

    The "complex primitive" case can be distinguised from the "atomic
    primitive" case because either the call to `after-change-functions'
    is missing (i.e. there are two consecutive calls to
    `before-change-functions'), or in the first call to
    `after-change-functions', `OLD-LEN' is less then `END' - `BEG' in
    `before-change-functions'.

The above leaves unsaid what happens when a "complex primitive" happens
to call b-c-f and a-c-f as though it were an "atomic primitive".  This
doesn't seem important enough to take up the space.

Personally, I think that when we come to rationalise and refactor
insdel.c and related files sometime in the medium future, we should
arrange to have b-c-f and a-c-f called only as "atomic" changes.  There
is no longer any need to optimise the calling of these hooks, and the
irregularites of these optimisations imposes an overhead on the use of
these hooks.

-- 
Alan Mackenzie (Nuremberg, Germany).