emacs/lisp/emacs-lisp/nadvice.el

;;; nadvice.el --- Light-weight advice primitives for Elisp functions  -*- lexical-binding: t -*-

;; Copyright (C) 2012  Free Software Foundation, Inc.

;; Author: Stefan Monnier <monnier@iro.umontreal.ca>
;; Keywords: extensions, lisp, tools
;; Package: emacs

;; This program is free software; you can redistribute it and/or modify
;; it under the terms of the GNU General Public License as published by
;; the Free Software Foundation, either version 3 of the License, or
;; (at your option) any later version.

;; This program is distributed in the hope that it will be useful,
;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
;; GNU General Public License for more details.

;; You should have received a copy of the GNU General Public License
;; along with this program.  If not, see <http://www.gnu.org/licenses/>.

;;; Commentary:

;; This package lets you add behavior (which we call "piece of advice") to
;; existing functions, like the old `advice.el' package, but with much fewer
;; bells ans whistles.  It comes in 2 parts:
;;
;; - The first part lets you add/remove functions, similarly to
;;   add/remove-hook, from any "place" (i.e. as accepted by `setf') that
;;   holds a function.
;;   This part provides mainly 2 macros: `add-function' and `remove-function'.
;;
;; - The second part provides `advice-add' and `advice-remove' which are
;;   refined version of the previous macros specially tailored for the case
;;   where the place that we want to modify is a `symbol-function'.

;;; Code:

;;;; Lightweight advice/hook
(defvar advice--where-alist
  '((:around "\300\301\302\003#\207" 5)
    (:before "\300\301\002\"\210\300\302\002\"\207" 4)
    (:after "\300\302\002\"\300\301\003\"\210\207" 5)
    (:after-until "\300\302\002\"\206\013\000\300\301\002\"\207" 4)
    (:after-while "\300\302\002\"\205\013\000\300\301\002\"\207" 4)
    (:before-until "\300\301\002\"\206\013\000\300\302\002\"\207" 4)
    (:before-while "\300\301\002\"\205\013\000\300\302\002\"\207" 4))
  "List of descriptions of how to add a function.
Each element has the form (WHERE BYTECODE STACK) where:
  WHERE is a keyword indicating where the function is added.
  BYTECODE is the corresponding byte-code that will be used.
  STACK is the amount of stack space needed by the byte-code.")

(defvar advice--bytecodes (mapcar #'cadr advice--where-alist))

(defun advice--p (object)
  (and (byte-code-function-p object)
       (eq 128 (aref object 0))
       (memq (length object) '(5 6))
       (memq (aref object 1) advice--bytecodes)
       (eq #'apply (aref (aref object 2) 0))))

(defsubst advice--car   (f) (aref (aref f 2) 1))
(defsubst advice--cdr   (f) (aref (aref f 2) 2))
(defsubst advice--props (f) (aref (aref f 2) 3))

(defun advice--make-docstring (_string function)
  "Build the raw doc-string of SYMBOL, presumably advised."
  (let ((flist (indirect-function function))
        (docstring nil))
    (if (eq 'macro (car-safe flist)) (setq flist (cdr flist)))
    (while (advice--p flist)
      (let ((bytecode (aref flist 1))
            (where nil))
        (dolist (elem advice--where-alist)
          (if (eq bytecode (cadr elem)) (setq where (car elem))))
        (setq docstring
              (concat
               docstring
               (propertize (format "%s advice: " where)
                           'face 'warning)
               (let ((fun (advice--car flist)))
                 (if (symbolp fun) (format "`%S'" fun)
                   (let* ((name (cdr (assq 'name (advice--props flist))))
                          (doc (documentation fun t))
                          (usage (help-split-fundoc doc function)))
                     (if usage (setq doc (cdr usage)))
                     (if name
                         (if doc
                             (format "%s\n%s" name doc)
                           (format "%s" name))
                       (or doc "No documentation")))))
               "\n")))
      (setq flist (advice--cdr flist)))
    (if docstring (setq docstring (concat docstring "\n")))
    (let* ((origdoc (unless (eq function flist) ;Avoid inf-loops.
                      (documentation flist t)))
           (usage (help-split-fundoc origdoc function)))
      (setq usage (if (null usage)
                      (let ((arglist (help-function-arglist flist)))
                        (format "%S" (help-make-usage function arglist)))
                    (setq origdoc (cdr usage)) (car usage)))
      (help-add-fundoc-usage (concat docstring origdoc) usage))))

(defvar advice--docstring
  ;; Can't eval-when-compile nor use defconst because it then gets pure-copied,
  ;; which drops the text-properties.
  ;;(eval-when-compile
  (propertize "Advised function"
              'dynamic-docstring-function #'advice--make-docstring)) ;; )

(defun advice--make-interactive-form (function main)
  ;; TODO: Make it possible to do around-like advising on the
  ;; interactive forms (bug#12844).
  ;; TODO: make it so that interactive spec can be a constant which
  ;; dynamically checks the advice--car/cdr to do its job.
  ;; TODO: Implement interactive-read-args:
  ;;(when (or (commandp function) (commandp main))
  ;;  `(interactive-read-args
  ;;    (cadr (or (interactive-form function) (interactive-form main)))))
  ;; FIXME: This loads autoloaded functions too eagerly.
  (cadr (or (interactive-form function)
            (interactive-form main))))

(defsubst advice--make-1 (byte-code stack-depth function main props)
  "Build a function value that adds FUNCTION to MAIN."
  (let ((adv-sig (gethash main advertised-signature-table))
        (advice
         (apply #'make-byte-code 128 byte-code
                (vector #'apply function main props) stack-depth
                advice--docstring
                (when (or (commandp function) (commandp main))
                  (list (advice--make-interactive-form
                         function main))))))
    (when adv-sig (puthash advice adv-sig advertised-signature-table))
    advice))

(defun advice--make (where function main props)
  "Build a function value that adds FUNCTION to MAIN at WHERE.
WHERE is a symbol to select an entry in `advice--where-alist'."
  (let ((desc (assq where advice--where-alist)))
    (unless desc (error "Unknown add-function location `%S'" where))
    (advice--make-1 (nth 1 desc) (nth 2 desc)
                    function main props)))

(defun advice--member-p (function definition)
  (let ((found nil))
    (while (and (not found) (advice--p definition))
      (if (or (equal function (advice--car definition))
              (equal function (cdr (assq 'name (advice--props definition)))))
          (setq found t)
        (setq definition (advice--cdr definition))))
    found))

;;;###autoload
(defun advice--remove-function (flist function)
  (if (not (advice--p flist))
      flist
    (let ((first (advice--car flist))
          (props (advice--props flist)))
      (if (or (equal function first)
              (equal function (cdr (assq 'name props))))
          (advice--cdr flist)
        (let* ((rest (advice--cdr flist))
               (nrest (advice--remove-function rest function)))
          (if (eq rest nrest) flist
            (advice--make-1 (aref flist 1) (aref flist 3)
                            first nrest props)))))))

;;;###autoload
(defmacro add-function (where place function &optional props)
  ;; TODO:
  ;; - provide something like `around' for interactive forms.
  ;; - provide some kind of buffer-local functionality at least when `place'
  ;;   is a variable.
  ;; - obsolete with-wrapper-hook (mostly requires buffer-local support).
  ;; - provide some kind of control over ordering.  E.g. debug-on-entry, ELP
  ;;   and tracing want to stay first.
  ;; - maybe also let `where' specify some kind of predicate and use it
  ;;   to implement things like mode-local or eieio-defmethod.
  ;; :before is like a normal add-hook on a normal hook.
  ;; :before-while is like add-hook on run-hook-with-args-until-failure.
  ;; :before-until is like add-hook on run-hook-with-args-until-success.
  ;; Same with :after-* but for (add-hook ... 'append).
  "Add a piece of advice on the function stored at PLACE.
FUNCTION describes the code to add.  WHERE describes where to add it.
WHERE can be explained by showing the resulting new function, as the
result of combining FUNCTION and the previous value of PLACE, which we
call OLDFUN here:
`:before'	(lambda (&rest r) (apply FUNCTION r) (apply OLDFUN r))
`:after'	(lambda (&rest r) (prog1 (apply OLDFUN r) (apply FUNCTION r)))
`:around'	(lambda (&rest r) (apply FUNCTION OLDFUN r))
`:before-while'	(lambda (&rest r) (and (apply FUNCTION r) (apply OLDFUN r)))
`:before-until'	(lambda (&rest r) (or  (apply FUNCTION r) (apply OLDFUN r)))
`:after-while'	(lambda (&rest r) (and (apply OLDFUN r) (apply FUNCTION r)))
`:after-until'	(lambda (&rest r) (or  (apply OLDFUN r) (apply FUNCTION r)))
If FUNCTION was already added, do nothing.
PROPS is an alist of additional properties, among which the following have
a special meaning:
- `name': a string or symbol.  It can be used to refer to this piece of advice."
  (declare (debug t)) ;;(indent 2)
  `(advice--add-function ,where (gv-ref ,place) ,function ,props))

;;;###autoload
(defun advice--add-function (where ref function props)
  (unless (advice--member-p function (gv-deref ref))
    (setf (gv-deref ref)
          (advice--make where function (gv-deref ref) props))))

(defmacro remove-function (place function)
  "Remove the FUNCTION piece of advice from PLACE.
If FUNCTION was not added to PLACE, do nothing.
Instead of FUNCTION being the actual function, it can also be the `name'
of the piece of advice."
  (declare (debug t))
  (gv-letplace (getter setter) place
    (macroexp-let2 nil new `(advice--remove-function ,getter ,function)
      `(unless (eq ,new ,getter) ,(funcall setter new)))))

;;;; Specific application of add-function to `symbol-function' for advice.

(defun advice--subst-main (old new)
  (if (not (advice--p old))
      new
    (let* ((first (advice--car old))
           (rest (advice--cdr old))
           (props (advice--props old))
           (nrest (advice--subst-main rest new)))
      (if (equal rest nrest) old
        (advice--make-1 (aref old 1) (aref old 3)
                        first nrest props)))))

(defun advice--normalize (symbol def)
  (cond
   ((special-form-p def)
    ;; Not worth the trouble trying to handle this, I think.
    (error "advice-add failure: %S is a special form" symbol))
   ((and (symbolp def)
	 (eq 'macro (car-safe (ignore-errors (indirect-function def)))))
    (let ((newval (cons 'macro (cdr (indirect-function def)))))
      (put symbol 'advice--saved-rewrite (cons def newval))
      newval))
   ;; `f' might be a pure (hence read-only) cons!
   ((and (eq 'macro (car-safe def))
	 (not (ignore-errors (setcdr def (cdr def)) t)))
    (cons 'macro (cdr def)))
   (t def)))

(defsubst advice--strip-macro (x)
  (if (eq 'macro (car-safe x)) (cdr x) x))

(defun advice--defalias-fset (fsetfun symbol newdef)
  (when (get symbol 'advice--saved-rewrite)
    (put symbol 'advice--saved-rewrite nil))
  (setq newdef (advice--normalize symbol newdef))
  (let* ((olddef (advice--strip-macro
		  (if (fboundp symbol) (symbol-function symbol))))
         (oldadv
          (cond
	   ((null (get symbol 'advice--pending))
	    (or olddef
		(progn
		  (message "Delayed advice activation failed for %s: no data"
			   symbol)
		  nil)))
	   ((or (not olddef) (autoloadp olddef))
	    (prog1 (get symbol 'advice--pending)
	      (put symbol 'advice--pending nil)))
           (t (message "Dropping left-over advice--pending for %s" symbol)
              (put symbol 'advice--pending nil)
              olddef))))
    (let* ((snewdef (advice--strip-macro newdef))
	   (snewadv (advice--subst-main oldadv snewdef)))
      (funcall (or fsetfun #'fset) symbol
	       (if (eq snewdef newdef) snewadv (cons 'macro snewadv))))))
    

;;;###autoload
(defun advice-add (symbol where function &optional props)
  "Like `add-function' but for the function named SYMBOL.
Contrary to `add-function', this will properly handle the cases where SYMBOL
is defined as a macro, alias, command, ..."
  ;; TODO:
  ;; - record the advice location, to display in describe-function.
  ;; - change all defadvice in lisp/**/*.el.
  ;; - rewrite advice.el on top of this.
  ;; - obsolete advice.el.
  ;; To make advice.el and nadvice.el interoperate properly I see 2 different
  ;; ways:
  ;; - keep them separate: complete the defalias-fset-function setter with
  ;;   a matching accessor which both nadvice.el and advice.el will have to use
  ;;   in place of symbol-function.  This can probably be made to work, but
  ;;   they have to agree on a "protocol".
  ;; - layer advice.el on top of nadvice.el.  I prefer this approach.  the
  ;;   simplest way is to make advice.el build one ad-Advice-foo function for
  ;;   each advised function which is advice-added/removed whenever ad-activate
  ;;   ad-deactivate is called.
  (let* ((f (and (fboundp symbol) (symbol-function symbol)))
	 (nf (advice--normalize symbol f)))
    (unless (eq f nf) ;; Most importantly, if nf == nil!
      (fset symbol nf))
    (add-function where (cond
                         ((eq (car-safe nf) 'macro) (cdr nf))
                         ;; If the function is not yet defined, we can't yet
                         ;; install the advice.
                         ;; FIXME: If it's an autoloaded command, we also
                         ;; have a problem because we need to load the
                         ;; command to build the interactive-form.
                         ((or (not nf) (and (autoloadp nf))) ;; (commandp nf)
                          (get symbol 'advice--pending))
                         (t (symbol-function symbol)))
                  function props)
    (add-function :around (get symbol 'defalias-fset-function)
                  #'advice--defalias-fset))
  nil)

;;;###autoload
(defun advice-remove (symbol function)
  "Like `remove-function' but for the function named SYMBOL.
Contrary to `remove-function', this will work also when SYMBOL is a macro
and it will not signal an error if SYMBOL is not `fboundp'.
Instead of the actual function to remove, FUNCTION can also be the `name'
of the piece of advice."
  (when (fboundp symbol)
    (let ((f (symbol-function symbol)))
      ;; Can't use the `if' place here, because the body is too large,
      ;; resulting in use of code that only works with lexical-scoping.
      (remove-function (if (eq (car-safe f) 'macro)
                           (cdr f)
                         (symbol-function symbol))
                       function)
      (unless (advice--p
               (if (eq (car-safe f) 'macro) (cdr f) (symbol-function symbol)))
        ;; Not advised any more.
        (remove-function (get symbol 'defalias-fset-function)
                         #'advice--defalias-fset)
        (if (eq (symbol-function symbol)
                (cdr (get symbol 'advice--saved-rewrite)))
            (fset symbol (car (get symbol 'advice--saved-rewrite))))))
    nil))

;; (defun advice-mapc (fun symbol)
;;   "Apply FUN to every function added as advice to SYMBOL.
;; FUN is called with a two arguments: the function that was added, and the
;; properties alist that was specified when it was added."
;;   (let ((def (or (get symbol 'advice--pending)
;;                  (if (fboundp symbol) (symbol-function symbol)))))
;;     (while (advice--p def)
;;       (funcall fun (advice--car def) (advice--props def))
;;       (setq def (advice--cdr def)))))

;;;###autoload
(defun advice-member-p (advice function-name)
  "Return non-nil if ADVICE has been added to FUNCTION-NAME.
Instead of ADVICE being the actual function, it can also be the `name'
of the piece of advice."
  (advice--member-p advice
                    (or (get function-name 'advice--pending)
			(advice--strip-macro
			 (if (fboundp function-name)
			     (symbol-function function-name))))))


(provide 'nadvice)
;;; nadvice.el ends here