all messages for Emacs-related lists mirrored at yhetil.org
 help / color / mirror / code / Atom feed
From: Michael Heerdegen <michael_heerdegen@web.de>
To: help-gnu-emacs@gnu.org
Subject: Re: replacing a function with another one
Date: Wed, 12 Mar 2014 07:11:17 +0100	[thread overview]
Message-ID: <87bnxcrloa.fsf@web.de> (raw)
In-Reply-To: 874n34ticd.fsf@web.de

Michael Heerdegen <michael_heerdegen@web.de> writes:

> Because it's trivial.

I have the feeling that this might sound strange - but it's true!  So,
here is some pseudo code that shows, in a simplified manner, what
defining an around advice with `advice-add' does:

(defun my-add-around-advice (fun-to-advice advice)
  (lexical-let ((oldfun (symbol-function fun-to-advice))
		(function advice))
    (fset fun-to-advice
	  (lambda (&rest r) (apply function oldfun r)))))
          
FUN-TO-ADVICE is the function to advice, ADVICE is the piece of advice
you want to add.  Do you recognize the line I cite all the time?  It's
what you get as resulting combined function.

Actually, it's not pseudo code but fully functional.

An example.  Let's define the faculty (a non-recursive version):

(defun my-fac (n) (reduce '* (number-sequence 1 n)))

Suppose we want to make it return the faculty of -n for negative
integers n by adding an around advice (currently it returns 1 for negative
arguments).  With the above simple implementation, you would do it
like that:

(my-add-around-advice
 'my-fac
 (lambda (orig-fun n) (funcall orig-fun (abs n))))

Then, e.g.

  (my-fac -5)

    ==> 120

Of course, you can't remove the advice with my simplified version, etc.

With `advice-add' you would do

(advice-add 'my-fac :around
  (lambda (orig-fun n) (funcall orig-fun (abs n))))

Let's add another around advice that makes `my-fac' print the result in
the echo area.  This time using a named function as advice:

(defun my-fac--print-result-around-advice (orig-fun n)
  "Print result in the echo area."
  (let ((result (funcall orig-fun n)))
    (message "The faculty of %d is %d" n result)
    (sit-for 3)
    result))

(my-add-around-advice 'my-fac #'my-fac--print-result-around-advice)

These are not very useful examples, but hopefully they show a bit how it
works, and one can play with them.  You also see how to deal with
arguments and the return value of the original function.

The other advice types can be implemented similarly.


Regards,

Michael.




  reply	other threads:[~2014-03-12  6:11 UTC|newest]

Thread overview: 62+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-03-08 19:43 replacing a function with another one lee
2014-03-08 19:54 ` Eli Zaretskii
2014-03-09  1:17   ` lee
2014-03-08 22:30 ` Michael Heerdegen
2014-03-09 17:58   ` lee
2014-03-09 19:10     ` Michael Heerdegen
2014-03-09 20:57       ` lee
2014-03-09 22:02         ` Michael Heerdegen
2014-03-10  0:53           ` lee
2014-03-10  2:18             ` Michael Heerdegen
2014-03-10 15:29               ` lee
2014-03-11  0:03                 ` Michael Heerdegen
2014-03-11 13:34                   ` lee
2014-03-11 23:40                     ` Michael Heerdegen
2014-03-12  6:11                       ` Michael Heerdegen [this message]
2014-03-12  7:07                         ` Michael Heerdegen
2014-03-12 14:48                         ` lee
2014-03-13  7:19                           ` Michael Heerdegen
2014-03-15 19:51                             ` lee
2014-03-17 12:00                               ` Michael Heerdegen
2014-03-12 14:04                       ` lee
2014-03-12 18:26                         ` Stefan Monnier
2014-03-12  4:10                     ` Michael Heerdegen
2014-03-10 12:44             ` Stefan Monnier
2014-03-10 23:35               ` lee
2014-03-11  0:41                 ` Michael Heerdegen
2014-03-11  1:45                   ` Michael Heerdegen
2014-03-11 19:05                     ` lee
2014-03-11 22:58                       ` Michael Heerdegen
2014-03-12 15:11                         ` lee
2014-03-12 18:15                           ` Stefan Monnier
2014-03-12 21:43                             ` lee
2014-03-13  7:22                               ` Michael Heerdegen
2014-03-15 20:02                                 ` lee
2014-03-11  4:11                   ` lee
2014-03-11  5:01                     ` Michael Heerdegen
2014-03-11 14:25                       ` lee
2014-03-11 23:51                         ` Michael Heerdegen
2014-03-12 15:22                           ` lee
2014-03-13  7:33                             ` Michael Heerdegen
2014-03-13 12:29                               ` Stefan Monnier
2014-03-15 20:05                                 ` lee
2014-03-16 17:20                                   ` Stefan
2014-03-11  6:51                     ` Michael Heerdegen
2014-03-11 15:41                       ` lee
2014-03-11 23:21                         ` Michael Heerdegen
2014-03-12 17:33                           ` lee
2014-03-12 19:34                             ` Florian Beck
2014-03-12 19:51                               ` Florian Beck
2014-03-13  7:54                             ` Michael Heerdegen
2014-03-15 20:14                               ` lee
2014-03-12 12:45                         ` Stefan Monnier
2014-03-12 17:42                           ` lee
2014-03-13  2:43                             ` Jambunathan K
2014-03-15 20:17                               ` How to propose an emacs patch (Re: replacing a function with another one) lee
2014-03-16  3:21                                 ` Jambunathan K
2014-03-17  4:35                                   ` lee
2014-03-10 13:45           ` replacing a function with another one lee
2014-03-10 23:31             ` Michael Heerdegen
2014-03-12 13:16 ` Jambunathan K
2014-03-12 13:18   ` Jambunathan K
2014-03-15 20:22   ` lee

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=87bnxcrloa.fsf@web.de \
    --to=michael_heerdegen@web.de \
    --cc=help-gnu-emacs@gnu.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
Code repositories for project(s) associated with this external index

	https://git.savannah.gnu.org/cgit/emacs.git
	https://git.savannah.gnu.org/cgit/emacs/org-mode.git

This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.