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

From: Noam Postavsky <[email-masked]>
Date: Sat, 25 May 2019 10:36:55 -0400

Alan Mackenzie <[email-masked]> writes:

>>> @@ -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.
>
> Are you sure?  We're talking here about the text property (in which I
> think inhibit-modification-hooks IS at nil) as opposed to the overlay
> property (where inhibit-modification-hooks is bound to t).

Oh, you're quite right.  Here's some test code:

Which produces this:

mod-hook-text-prop (1 4), inhibit? nil
mod-hook-change-fun (1 4), inhibit? t
mod-hook-ov-prop (#<overlay from 1 to 5 in *test*> nil 1 4), inhibit? t
mod-hook-change-fun (1 1 3), inhibit? t
mod-hook-ov-prop (#<overlay from 1 to 2 in *test*> t 1 1 3), inhibit? t
mod-hook-change-fun (1 1), inhibit? t
mod-hook-change-fun (1 4 0), inhibit? t

I think we need to emphasize the difference in this case, it's rather
confusing.

> I'll answer the rest of your post later, I've got a lot on in Real Life
> at the moment.

No rush.  I've updated the patch based on your and Eli's feedback.

>From 7f6453596b7753af7704eaac7f27ebba8d03cfc4 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 (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.
(Special Properties): Emphasize that inhibit-modification-hooks is
left set to nil when executing text property change hooks.

Co-authored-by: Noam Postavsky <[email-masked]>
---
 doc/lispref/display.texi |  6 +++---
 doc/lispref/text.texi    | 12 ++++++++----
 2 files changed, 11 insertions(+), 7 deletions(-)

diff --git a/doc/lispref/display.texi b/doc/lispref/display.texi
index b07999432c..59d02d540a 100644
--- a/doc/lispref/display.texi
+++ b/doc/lispref/display.texi
@@ -1708,9 +1708,9 @@ 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.
+Similar to change hooks, when these functions are called,
+@code{inhibit-modification-hooks} is bound to @code{t}.  @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..c935cfe49b 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 @code{nil}, unlike for change hooks.  When writing a function
+which modifies the buffer, consider binding it @code{t}, to avoid
+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