Enable minor-mode during function call

Question

Similar to the somewhat neat, but in the end too clunky, hydra-goto-line, I'd like to customize goto-line to enable linum-mode, but only during the function call.

I tried add-advice, :before and :after, but nothing kinda happened. So what's the proper way to enable a minor-mode during a single function call?

Answer 1

I probably borrowed this from somewhere, but darned if I remember where:

(defun goto-line-with-feedback ()
  "Show line numbers temporarily, while prompting for the line number input."
  (interactive)
  (defvar goto/line 0)
  (unwind-protect
      (progn
        (linum-mode 1)
        (setq goto/line (read-number "Goto line: "))
        (goto-char (point-min))
        (forward-line (1- goto/line)))
    (linum-mode -1)))

I rebind M-g to that; works a treat.

(Note that if you're using Emacs 26, you should use display-line-numbers-mode instead of linum-mode -- just swapping the two function calls is all you need to do.)

As a general rule — to try to address your question in a more generic, "teach you to fish" kind of way — any function of the form foo-mode in Emacs will take a positive argument to unconditionally enable the mode, and a negative (or sometimes nil) argument to unconditionally disable it. The best way to validate that is to read the documentation for the function (C-h f then the function name).