bug#38406: 27.0.50; post-self-insert-hook does not hold its contract in cc-mode derived modes

[Alan Mackenzie <acm@muc.de>]

Hello, Ivan.

On Sun, Dec 01, 2019 at 12:02:56 +0200, yyoncho wrote:
> Hi Alan,

> > There are other possible "fixes", for example modifying these functions
> > so that they don't use self-insert-command at all, but somehow I don't
> > think that's what you want.

> I don't think that the code that is implemented against the contract listed
> in the hook documentation should be rewritten. If electric stuff is so
> that important and there is no way to disable it by default then at
> least a function to unbind the electric functionality the
> documentation of post-self-insert-hook should state: "Don't rely on
> this hook in cc derived modes because of {implementation details}. If
> you still want to use post-self-insert-hook disable use
> {implementation details} to turn electric off."

The problem you have stumbled over is more of a political problem than a
technical one.

post-self-insert-hook was introduced relatively recently as a quick and
dirty method of doing certain things.  Its implications weren't thought
through beforehand.  In particular, it breaks major modes which use
self-insert-command as part of their processing, including CC Mode.

If functions put onto post-self-insert-hook didn't violate the
definition of self-insert-command (inserting exactly one copy of the key
typed), there wouldn't be a problem.  An example of such a function is
blink-paren-post-self-insert-function (see lisp/simple.el L7801).

However, there are several functions put onto this hook that make
extensive buffer changes.  An example is
electric-pair-post-self-insert-function (in lisp/elec-pair.el).  These
mess up self-insert-command, and violate the principle that major modes
should be in charge of what text goes where in a window.

People like using post-self-insert-hook without worrying about the
problems it causes.  Binding post-self-insert-hook to nil in CC Mode,
while not good, was a pragmatic workaround from around a year ago.  This
allowed electric-pair-mode to function in CC Mode.  As I said, this
problem is primarily a political problem.  Forgive me not wanting to
draw too much attention to it at the moment.

Again, how does this binding of post-self-insert-hook to nil in CC Mode
affect you?  What is it you're trying to do that this binding makes
difficult?

> Thanks,
> Ivan

-- 
Alan Mackenzie (Nuremberg, Germany).