Subject: bug#25111: (Inaccurate documentation of inhibit-modification-hooks)

From: Alan Mackenzie <[email-masked]>
Date: Sun, 19 May 2019 20:31:19 +0000

Hello, Eli and Noam.

On Mon, Dec 12, 2016 at 18:06:34 +0200, Eli Zaretskii wrote:
> > From: [email-masked] (Phillip Lord)
> > Cc: [email-masked]
> > Date: Sun, 11 Dec 2016 22:11:14 +0000

> > So, instead of this:

> >      If these functions modify the buffer, they should bind
> >      ‘inhibit-modification-hooks’ to ‘t’ around doing so, to avoid
> >      confusing the internal mechanism that calls these hooks.

> > We could have:

> >     These functions should avoid unnecessarily modifying the buffer.
> >     Emacs binds 'inhibit-modification-hooks' to `t' during their
> >     evaluation, which means that any modifications will not be signalled
> >     to other hook functions listening for them.

> > Perhaps a better solution would be:

> >     These functions should avoid unnecessarily modifying the buffer; see
> >     Change Hooks for further details.

> > Then a new paragraph can be added to the Change Hooks section talking
> > about the complexity of modifying buffers on these hooks, with
> > alternatives.

> > I am happy to draft something if you wish.

> Sure, please do.

> Thanks.

OK, it was a while ago, and I'm not Phillip, but I recently got caught
out with the inaccurate documentation of inhibit-modification-hooks, so I
suggest the following, to get the discussion rolling again:

diff --git a/doc/lispref/display.texi b/doc/lispref/display.texi
index a2ed4b3891..b88702f2a2 100644
--- a/doc/lispref/display.texi
+++ b/doc/lispref/display.texi
@@ -1743,9 +1743,17 @@ Overlay Properties
 length is the number of characters deleted, and the post-change
 beginning and end are equal.)
 
-If these functions modify the buffer, they should bind
-@code{inhibit-modification-hooks} to @code{t} around doing so, to
-avoid confusing the internal mechanism that calls these hooks.
+@c If these functions modify the buffer, they should bind
+@c @code{inhibit-modification-hooks} to @code{t} around doing so, to
+@c avoid confusing the internal mechanism that calls these hooks.
+
+When these functions are called, @code{inhibit-modification-hooks} is
+bound to non-@code{nil}.  If the functions modify the buffer, you
+might want to bind @code{inhibit-modification-hooks} to nil, so as to
+cause the change hooks to run for these modifications.  @xref{Change
+Hooks}.  However, doing this can sometimes confuse the internal
+mechanism that calls all these hooks, leading, for example, to calling
+them recursively, which is usually unwanted.
 
 Text properties also support the @code{modification-hooks} property,
 but the details are somewhat different (@pxref{Special Properties}).
diff --git a/doc/lispref/text.texi b/doc/lispref/text.texi
index 278bc3c268..fd4338ecdb 100644
--- a/doc/lispref/text.texi
+++ b/doc/lispref/text.texi
@@ -3621,9 +3621,14 @@ Special Properties
 hook will only be run when removing some characters, replacing them
 with others, or changing their text-properties.
 
-If these functions modify the buffer, they should bind
-@code{inhibit-modification-hooks} to @code{t} around doing so, to
-avoid confusing the internal mechanism that calls these hooks.
+@c If these functions modify the buffer, they should bind
+@c @code{inhibit-modification-hooks} to @code{t} around doing so, to
+@c avoid confusing the internal mechanism that calls these hooks.
+
+When Emacs calls these functions, @code{inhibit-modification-hooks} is
+set to @code{nil}.  If the functions modify the buffer, you should
+consider binding this variable to non-@code{nil}, to avoid confusing
+the internal mechanism that calls these hooks.  @xref{Change Hooks}.
 
 Overlays also support the @code{modification-hooks} property, but the
 details are somewhat different (@pxref{Overlay Properties}).

-- 
Alan Mackenzie (Nuremberg, Germany).