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Adapting code using the old defadvice

A lot of code uses the old defadvice mechanism, which is largely made obsolete by the new advice-add, whose implementation and semantics is significantly simpler.

An old piece of advice such as:

(defadvice previous-line (before next-line-at-end
                                 (&optional arg try-vscroll))
  "Insert an empty line when moving up from the top line."
  (if (and next-line-add-newlines (= arg 1)
           (save-excursion (beginning-of-line) (bobp)))
      (progn
        (beginning-of-line)
        (newline))))

could be translated in the new advice mechanism into a plain function:

(defun previous-line--next-line-at-end (&optional arg try-vscroll)
  "Insert an empty line when moving up from the top line."
  (if (and next-line-add-newlines (= arg 1)
           (save-excursion (beginning-of-line) (bobp)))
      (progn
        (beginning-of-line)
        (newline))))

Obviously, this does not actually modify previous-line. For that the old advice needed:

(ad-activate 'previous-line)

whereas the new advice mechanism needs:

(advice-add 'previous-line :before #'previous-line--next-line-at-end)

Note that ad-activate had a global effect: it activated all pieces of advice enabled for that specified function. If you wanted to only activate or deactivate a particular piece, you needed to enable or disable it with ad-enable-advice and ad-disable-advice. The new mechanism does away with this distinction.

Around advice such as:

(defadvice foo (around foo-around)
  "Ignore case in `foo'."
  (let ((case-fold-search t))
    ad-do-it))
(ad-activate 'foo)

could translate into:

(defun foo--foo-around (orig-fun &rest args)
  "Ignore case in `foo'."
  (let ((case-fold-search t))
    (apply orig-fun args)))
(advice-add 'foo :around #'foo--foo-around)

Regarding the advice’s class, note that the new :before is not quite equivalent to the old before, because in the old advice you could modify the function’s arguments (e.g., with ad-set-arg), and that would affect the argument values seen by the original function, whereas in the new :before, modifying an argument via setq in the advice has no effect on the arguments seen by the original function. When porting before advice which relied on this behavior, you’ll need to turn it into new :around or :filter-args advice instead.

Similarly old after advice could modify the returned value by changing ad-return-value, whereas new :after advice cannot, so when porting such old after advice, you’ll need to turn it into new :around or :filter-return advice instead.