"void-variable" error when adding advice to `kill-ring-save`

Question

I am trying to add an advice to kill-ring-save and kill-region, so that when I call them with no active regions, instead of throwing an error, the line the point is currently on would be marked automatically. I am requireing the package simple.el because this is where kill-ring-save and kill-region are defined.

(require 'simple)

(defun mark-line-if-no-active-region ()
  "When the adviced function is called in an interactive context
with no active region, mark the current line as the active region."
  (interactive
   (if mark-active (list (region-beginning) (region-end))
     (list (line-beginning-position)
           (line-beginning-position 2)))))

(add-function :before kill-ring-save (lambda () (mark-line-if-no-active-region)))
(add-function :before kill-region (lambda () (mark-line-if-no-active-region)))

However, it appears that emacs doesn't recognize kill-ring-save. When launching with --debug-init, Emacs produces the following error.

Debugger entered--Lisp error: (void-variable kill-ring-save)
  default-value(kill-ring-save)
  (lambda nil (default-value 'kill-ring-save))()
  advice--add-function(:before ((lambda nil (default-value 'kill-ring-save)) lambda (gv--val) (set-default 'kill-ring-save gv--val)) (lambda n$
  eval-buffer(#<buffer  *load*-570600> nil "/Users/nalzok/.emacs.d/custom/setup-editing.el" nil t)  ; Reading at buffer position 2025
  load-with-code-conversion("/Users/nalzok/.emacs.d/custom/setup-editing.el" "/Users/nalzok/.emacs.d/custom/setup-editing.el" nil t)
  require(setup-editing)
  eval-buffer(#<buffer  *load*> nil "/Users/nalzok/.emacs.d/init.el" nil t)  ; Reading at buffer position 443
  load-with-code-conversion("/Users/nalzok/.emacs.d/init.el" "/Users/nalzok/.emacs.d/init.el" t t)
  load("/Users/nalzok/.emacs.d/init" t t)
  #f(compiled-function () #<bytecode 0x4009f5e9>)()
  command-line()
  normal-top-level()

I have tried replacing kill-ring-save, 'kill-ring-save, and #'kill-ring-save, but the same error persists. I'm using Emacs 26.1 installed from Homebrew on macOS.

By the way, I would appreciate it if you can explain what does the hashtag (#) preceding a quote (') does.

Answer 1

add-function is designed to work with variables holding functions. You probably want advice-add instead. See Advising Functions