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

From: Noam Postavsky <[email-masked]>
Date: Sat, 25 May 2019 08:39:39 -0400

Alan Mackenzie <[email-masked]> writes:

> @@ -1743,9 +1743,17 @@ Overlay Properties
>  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.

> @@ -3621,9 +3621,14 @@ Special Properties

> +When Emacs calls these functions, @code{inhibit-modification-hooks} is
> +set to @code{nil}.

As Phillip mentioned in the OP, Emacs in fact binds it to t.

> 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}.

I find this "confusing the internal mechanism" thing unhelpful, how
about this instead:

>From 568e640f962cd9a59d695351ded11c4a8e781f06 Mon Sep 17 00:00:00 2001
From: Alan Mackenzie <[email-masked]>
Date: Sun, 19 May 2019 20:31:19 +0000
Subject: [PATCH] Clarify elisp ref for inhibit-modification-hooks (Bug#25111)

* doc/lispref/display.texi (Overlay Properties):
* doc/lispref/text.texi (Special Properties)
(Change Hooks): Explain that inhibit-modification-hooks is bound to t
while executing change hooks, and suggest binding to nil with suitable
precautions when modifying buffer from a change hook.

Co-authored-by: Noam Postavsky <[email-masked]>
---
 doc/lispref/display.texi |  9 ++++++---
 doc/lispref/text.texi    | 12 ++++++++----
 2 files changed, 14 insertions(+), 7 deletions(-)

diff --git a/doc/lispref/display.texi b/doc/lispref/display.texi
index b07999432c..f7140f444e 100644
--- a/doc/lispref/display.texi
+++ b/doc/lispref/display.texi
@@ -1708,9 +1708,12 @@ 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.
+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.  However, doing
+this may call your own change hook recursively, so be sure to prepare
+for that.  @xref{Change Hooks}.
 
 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 f3d222b708..5954161fcf 100644
--- a/doc/lispref/text.texi
+++ b/doc/lispref/text.texi
@@ -3514,9 +3514,10 @@ 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.
+When Emacs calls these functions, @code{inhibit-modification-hooks} is
+set to non-@code{nil}.  If the functions modify the buffer, you should
+consider binding this variable to @code{nil}, but in that case you
+must be prepared for recursive calls.  @xref{Change Hooks}.
 
 Overlays also support the @code{modification-hooks} property, but the
 details are somewhat different (@pxref{Overlay Properties}).
@@ -5093,5 +5094,8 @@ Change Hooks
 a modification hook does not cause other modification hooks to be run.
 If you do want modification hooks to be run in a particular piece of
 code that is itself run from a modification hook, then rebind locally
-@code{inhibit-modification-hooks} to @code{nil}.
+@code{inhibit-modification-hooks} to @code{nil}.  However, doing this
+may cause recursive calls to the modification hooks, so be sure to
+prepare for that (for example, by binding some variable which tells
+your hook to do nothing).
 @end defvar
-- 
2.11.0