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Re: Lisp primitives and their calling of the change hooks

From: Stefan Monnier
Subject: Re: Lisp primitives and their calling of the change hooks
Date: Fri, 05 Jan 2018 11:50:56 -0500
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/27.0.50 (gnu/linux)

> How will the reader know to distinguish between these two classes of
> primitives?

He won't and shouldn't attempt to (the boundary between those two is an
internal implementation detail that is subject to change).

> Without such an ability, the extra accuracy in this text
> is not useful.

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

Maybe the first paragraph should be reworded a bit so it doesn't sound
like a promise of behavior?  How 'bout:

    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.

-- Stefan

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