Re: master c9a8a47ba4: Add new user option 'yank-transform-functions'

[Stefan Monnier <monnier@iro.umontreal.ca>]

Lars Ingebrigtsen [2022-05-24 18:02:11] wrote:
> Stefan Monnier <monnier@iro.umontreal.ca> writes:
>>>     * lisp/simple.el (yank-transform-functions): New user option.
>>>     (yank): Mention it.
>>
>> Hmm... why is this a user option?
>> [ Also, tho less importantly: Why is it a list of functions instead of
>>   being symmetric with Emacs-28's `kill-transform-function`?  ]
>>
>> Are users expected to be able to make use of this with Custom?  How?
>
> It's a list of functions because it's a defcustom, really.

[ FWIW, `kill-transform-function` is also a defcustom, tho I'd argue it
  would be better defined as a `defvar` for the same kinds of
  reasons.  ]

> I envisioned that people might be wanting to add other things to it to
> (say) record yanked text, for instance.

"Record yanked text" doesn't seem like a very compelling example.
I've never heard of anyone requesting such a feature or anything nearby.
And I'd expect such a feature to come in the form of a minor mode
instead (whose code then programmatically sets this var).

The problem with making it a defcustom is that packages can't
programmatically modify this var without risks of interfering
with Custom (e.g. the famous "CHANGED OUTSIDE CUSTOMIZE").

>>> +@lisp
>>> +(defun yank-with-clean-whitespace ()
>>> +  (interactive)
>>> +  (let ((yank-transform-functions
>>> +	 '(string-clean-whitespace)))
>>> +    (call-interactively #'yank)))
>>> +@end lisp
>>> +@end defopt
>> This example doesn't call for a `defcustom` but for a `defvar`, no?
> It's in the lispref manual.

Right, but it's the only example I can find (and any uses of Custom on
this var during the above command may result in an incorrect value
being saved in the user's init file).

[ FWIW, this discussion is prompted by my trying to use this var in
  `nhexl-mode` where it would be set rather than being let-bound.  ]


        Stefan