RE: [External] : What is the difference between (deactivate-mark) and (setq deactivate-mark t)?

[Drew Adams]

> Well, I understand what each of them does, but whay was the variable
> introduced and why would I want to  say `(setq deactivate-mark t)' in
> a command instead of just calling `(deactivate-mark)'?  I can't think of
> any possible reasons.  Any ideas?

I may be repeating what you say you already
understand, but...

Function `deactivate-mark' deactivates the mark
(duh), and it does so right away.

After a command finishes and returns, the command
loop normally automatically deactivates the mark.
IOW, for the next command the mark is inactive.

But if you set variable `deactivate-mark' to `nil'
in your command then the command loop won't
deactivate it when your command is done.

As the manual says:

  To write Lisp code that modifies the buffer
  without causing deactivation of the mark at
  the end of the command, bind 'deactivate-mark'
  to 'nil' around the code that does the modification.
  For example:

          (let (deactivate-mark)
            (insert " "))

Setting the variable to nil says do NOT deactivate
the mark.  See (elisp) `The Mark'.

https://www.gnu.org/software/emacs/manual/html_node/elisp/The-Mark.html