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Re: Lisp primitives and their calling of the change hooks

From: Eli Zaretskii
Subject: Re: Lisp primitives and their calling of the change hooks
Date: Fri, 05 Jan 2018 21:53:25 +0200

> From: Stefan Monnier <address@hidden>
> Cc: Alan Mackenzie <address@hidden>,  address@hidden
> Date: Fri, 05 Jan 2018 11:50:56 -0500
> I find it useful in order to explain why naively observing the behavior
> may give one the impression that all b-c-f and a-c-f calls are
> "balanced".

We don't normally include such "preemptive" explanations in the
manual.  If the text doesn't say one should expect balanced calls, the
reader has no reason to expect balanced calls.  The current text even
makes a point of saying that explicitly.

>     The vast bulk of buffer changes will call `before-change-functions'
>     and `after-change-functions' in balanced pairs, once for each
>     change where the arguments to these hooks will exactly delimit the
>     change being made.  Yet, hook functions should not rely on this
>     being always the case:
>     Other, more complex primitives may call `before-change-functions'
>     once before making changes and then 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 more accurately
>     delimit the current change.

This basically says the calls are mostly balanced, but don't rely on
that, because sometimes they aren't".  The text about
after-change-functions being called zero or more times adds
non-trivial information, but what is its practical usefulness?  Same
with the text about BEG and END.

Maybe I don't understand what are we trying to accomplish with these
changes, and that's why I fail to see why the proposed changes are for
the better.

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