RE: Another Emacs incompatibilty

[Drew Adams <drew.adams@oracle.com>]

> > (put 'insert-char 'delete-selection t)
> 
> OK, so that's the syntax. This must be a very arcane
> way of configuration. I did use `put' but only to
> enable and disable, as in
> 
>   (put 'help-for-help 'disabled t)
> 
> But, what are properties and how do I know what
> properties a function has?

https://www.gnu.org/software/emacs/manual/html_node/elisp/Symbol-Properties.html

Ask Emacs: `C-h i m Elisp i symbol property'

A Lisp symbol is an object of sorts.  It has a name
and possibly other properties/attributes:

* a name           - `symbol-name'
* a function value - `symbol-function'
* a variable value - `symbol-value'
* a property list  - `symbol-plist'

And you can add any other properties you like.
Enjoy.

> And how do I write a function with properties? (I
> don't think I want to, in general, but I always want
> to do things I didn't do, at least once...)

As I think I said before, it's not really the function
that has properties.  It's the function symbol - the
symbol whose `symbol-function' is the function named
with the function's name (which is also the symbol's
name).

(defun foo ...)
(symbol-function 'foo) => foo ; symbol with name "foo"

(setq foo 42)
(symbol-value 'foo) => 42

See also `C-h f symbol-plist'.

> > Or if you instead want `insert-char' to kill the
> > selected text (so you can later yank it) instead of
> > just deleting it, add this to your init file, to
> > override the default behavior:
> >
> >   (put 'insert-char 'delete-selection 'kill)
> 
> `insert-char'? Sure, that's ONE way to insert, but
> there are many other ways to insert things, and not
> just a single char, 

It was an _example_.  A command whose `delete-selection'
property is defined to kill, not delete.  To show that
you can change the behavior to kill instead of only
delete.

> and then (if you desire this
> functionality to begin with, that is) then you
> _always_ want the selection to be killed, I think is
> the objective here?

It presumably is your objective, if you use `kill'.
Again, an _example_ of getting kill behavior.  Do I
think you'd likely really prefer to have `insert-char'
kill instead of delete?  No.  But you might.  Or some
library might.

> Anyway, I don't get it to work:
> 
>   (delete-selection-mode) ; t
>   (put 'insert-char 'delete-selection 'kill) ; kill
>   I type this
>   set-mark-command
>   beginning-of-line
>   set-mark-command
>   s ("I type this" disappears, the letter s appears)
>   yank
>   "I type this" does NOT appear

In your example, the kill behavior applies to
`insert-char', not to `self-insert-command', which
is what `s' is bound to.  So don't expect that when
you type `s' to replace the active region you can
then yank what was deleted.

If you instead select some text and use
`C-x 8 RET' `LATIN CAPITAL LETTER M', then the
selected text is replaced by `M', AND it's put in
the kill ring.

Maybe using `insert-char' to illustrate wasn't the
best choice.  I did so because it was the first one
in `delsel.el' after `self-insert-command' (which
is a special case).

> Here is a 19-liner how to do it:
> (delete-selection-mode)
> (defun delete-selection-pre-hook ()
>   (when (and delete-selection-mode
>              (use-region-p)
>              (not buffer-read-only) )
>     (if (member this-command '(self-insert-command insert-char insert) )
>         (delete-selection-helper 'kill)
>       (delete-selection-helper
>        (and (symbolp this-command)
>             (get this-command 'delete-selection) )))))

You don't need to redefine `delete-selection-pre-hook'.

You typically need only put a `delete-selection'
property on a command's symbol, to get the behavior
you want for it, which is just delete-the-selection
in most cases, i.e., value `t'.

And most people will never need to do even that.
They'll just use the out-of-the-box behavior.

People who add commands that insert or do some
other things might want to configure them to take
advantage of `delete-selection-mode'.  And even
then, they mostly just use `t': delete.

You didn't ask how to use `delete-selection-mode'.

I think you asked about its various behaviors and
configuring them by putting properties on command
symbols.  It can do more than your average "select
and type to replace".