Re: Should mode commands be idempotent?

[Philipp Stephani <p.stephani2@gmail.com>]

[-- Attachment #1.1: Type: text/plain, Size: 1032 bytes --]

John Wiegley <jwiegley@gmail.com> schrieb am Di., 26. Sep. 2017 um
20:55 Uhr:

> >>>>> Drew Adams <drew.adams@oracle.com> writes:
>
> > Good luck with such a guideline. The state of an Emacs session is _never_
> > exactly the same after each time you turn on a mode - any mode, any
> session.
> > So many things change...
>
> I think we can reduce the scope of what we're saying:
>
> The statement I made only applies if the command is called in immediate
> succession. That is, the function is *itself* idempotent; it doesn't
> guarantee
> that intervening effects are somehow negated.
>
> That is, the following should be functionally equivalent:
>
>     (foo-mode 1)
>     (progn (foo-mode 1) (foo-mode 1))
>
> I don't see what great difficulty this poses. If a mode author is doing
> thing
> inside `foo-mode' like changing the system or the disk, that's exactly the
> sort of thing our guideline seeks to prevent.
>

Given that all current and past maintainers seem to agree, how about the
following patch for the Lisp manual?

[-- Attachment #1.2: Type: text/html, Size: 1474 bytes --]

[-- Attachment #2: 0001-Document-that-mode-commands-should-be-idempotent.txt --]
[-- Type: text/plain, Size: 1363 bytes --]

From 466ee5853a07f90593149e4de07f92845e076170 Mon Sep 17 00:00:00 2001
From: Philipp Stephani <phst@google.com>
Date: Sun, 8 Oct 2017 17:25:31 +0200
Subject: [PATCH] Document that mode commands should be idempotent.

* doc/lispref/modes.texi (Major Mode Conventions, Minor Mode
Conventions): Document that the mode commands should be idempotent.
---
 doc/lispref/modes.texi | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/doc/lispref/modes.texi b/doc/lispref/modes.texi
index f7013da943..f2fc742495 100644
--- a/doc/lispref/modes.texi
+++ b/doc/lispref/modes.texi
@@ -313,6 +313,10 @@ Major Mode Conventions
 Data}, for other possible forms).  The name of the mode appears
 in the mode line.
 
+@item
+Calling the major mode command twice in direct succession should not
+fail and should do the same thing as calling the command only once.
+
 @item
 @cindex functions in modes
 Since all global names are in the same name space, all the global
@@ -1412,6 +1416,10 @@ Minor Mode Conventions
 @noindent
 However, this is not very commonly done.
 
+  Enabling or disabling a minor mode twice in direct succession should
+not fail and should do the same thing as enabling or disabling it only
+once.
+
 @item
 Add an element to @code{minor-mode-alist} for each minor mode
 (@pxref{Definition of minor-mode-alist}), if you want to indicate the
-- 
2.14.2