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Re: replacing a function with another one
From: |
lee |
Subject: |
Re: replacing a function with another one |
Date: |
Wed, 12 Mar 2014 15:48:46 +0100 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/24.3.50 (gnu/linux) |
Michael Heerdegen <michael_heerdegen@web.de> writes:
> 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
That sounds like a cryptic way to do the `callinstead' I just suggested
in my previous post :)
> Of course, you can't remove the advice with my simplified version, etc.
And advice-add is lacking what defadvice has with the ability to enable
and to disable the advice.
(callinstead orig-fn new-fn (orig-arg0..orig-argN) :named
"removable-callinstead")
(callinstead-remove "removable-callinstead")
(callinstead-add "removable-callinstead")
> 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.
I still don´t understand how it works. Here´s another example:
lexical-let is a Lisp macro in `cl.el'.
(lexical-let BINDINGS BODY)
Like `let', but lexically scoped.
The main visible difference is that lambdas inside BODY will create
lexical closures as in Common Lisp.
So what is that supposed to mean? `let' keeps driving me insane already
because it requires so many brackets. Then finally, I do something like
(let ((end-marker (concat "^" comment-start
lsl-hi-lock-patterns-end-marker))
((marker-pos (re-search-forward end-marker (point-max) t))))
(do-stuff))
and it doesn´t work because end-marker is undefined despite I just
defined it :( So I have
(let ((end-marker (concat "^" comment-start
lsl-hi-lock-patterns-end-marker)))
(let ((marker-pos (re-search-forward end-marker (point-max) t)))
(do-something)))
instead ...
And in the end, I´m left with the unanswerable question of how to
intentionally returning something in particular from a function:
(defun lsl-get-patterns-from-file (file)
"Read hi-lock-mode highlighting-patterns from a file and return
the patterns read."
(with-current-buffer
(find-file-noselect file)
(goto-char (point-min))
(let ((end-marker (concat "^" comment-start
lsl-hi-lock-patterns-end-marker)))
(let ((marker-pos (re-search-forward end-marker (point-max) t)))
(when marker-pos
(goto-char marker-pos)
(previous-line)
(end-of-line)
(setq marker-pos (point))
(goto-char (point-min))
(message "reading hi-lock patterns from %s (%d..%d)"
(buffer-name)
(point-min) marker-pos)
(let ((patterns nil))
(while (< (point) marker-pos)
(setq patterns (append (read (current-buffer)) patterns)))
(setq patterns patterns)))))))
I need this function to return `patterns'. Without the last line, it
seems to return nil because the setq is enclosed in the while.
This is what I made from the example implementation you made. That
really got me somewhere, and I don´t need to use advices anymore.
Then I found I want to be able to edit the patterns. Editing them is
easier when there is one pattern per line, so I made it write one per
line and then found that they are suddenly much more difficult to
read. Finally I got it to read them, just to find out that the function
doesn´t return them.
--
Knowledge is volatile and fluid. Software is power.
- Re: replacing a function with another one, (continued)
- Re: replacing a function with another one, lee, 2014/03/09
- Re: replacing a function with another one, Michael Heerdegen, 2014/03/09
- Re: replacing a function with another one, lee, 2014/03/09
- Re: replacing a function with another one, Michael Heerdegen, 2014/03/09
- Re: replacing a function with another one, lee, 2014/03/10
- Re: replacing a function with another one, Michael Heerdegen, 2014/03/10
- Re: replacing a function with another one, lee, 2014/03/11
- Re: replacing a function with another one, Michael Heerdegen, 2014/03/11
- Re: replacing a function with another one, Michael Heerdegen, 2014/03/12
- Re: replacing a function with another one, Michael Heerdegen, 2014/03/12
- Re: replacing a function with another one,
lee <=
- Re: replacing a function with another one, Michael Heerdegen, 2014/03/13
- Re: replacing a function with another one, lee, 2014/03/15
- Re: replacing a function with another one, Michael Heerdegen, 2014/03/17
- Re: replacing a function with another one, lee, 2014/03/12
- Re: replacing a function with another one, Stefan Monnier, 2014/03/12
- Re: replacing a function with another one, Michael Heerdegen, 2014/03/12
- Re: replacing a function with another one, Stefan Monnier, 2014/03/10
- Re: replacing a function with another one, lee, 2014/03/10
- Re: replacing a function with another one, Michael Heerdegen, 2014/03/10
- Re: replacing a function with another one, Michael Heerdegen, 2014/03/10