Dynamically constructing advice behaves strangely

Dynamically constructing advice behaves strangely

[Marcin Borkowski]

Hi list,

I want to be able to construct advice dynamically.  Here is a "minimal
example":

--8<---------------cut here---------------start------------->8---
;;;   -*- lexical-binding: t; -*-

(defun add-constructed-advice (fun)
  "Add a constructed advice to function foo."
  (let* ((fun-name (symbol-name fun))
	 (length-sym
	  (make-symbol (concat "length-of-" fun-name)))
	 (piece-of-advice-sym
	  (make-symbol (concat "advice-for-" fun-name)))
	 (piece-of-advice (lambda (orig-fun &rest args)
			    (apply orig-fun args)
			    (message "This function's name had length %s."
				     (symbol-value length-sym)))))
    
    (set length-sym (length (symbol-name fun)))
    (fset piece-of-advice-sym piece-of-advice)
    (advice-add fun :around piece-of-advice-sym)))

(defun foo-fun ()
  ""
  (message "Function foo-fun run."))

(add-constructed-advice 'foo-fun)
--8<---------------cut here---------------end--------------->8---

This works (i.e., M-: foo-fun properly runs the advice), but I expected
the symbol `advice-for-foo-fun' to have this piece of advice in its
function cell - and it does not.  Why?

TIA,

-- 
Marcin Borkowski
http://octd.wmi.amu.edu.pl/en/Marcin_Borkowski
Faculty of Mathematics and Computer Science
Adam Mickiewicz University

Re: Dynamically constructing advice behaves strangely

[Michael Heerdegen]

Marcin Borkowski <mbork@mbork.pl> writes:

> This works (i.e., M-: foo-fun properly runs the advice), but I expected
> the symbol `advice-for-foo-fun' to have this piece of advice in its
> function cell - and it does not.  Why?

You bind the function to a new uninterned symbol (`make-symbol'), which
is not what you want.  You want `intern'.

Michael.

Re: Dynamically constructing advice behaves strangely

[Marcin Borkowski]

On 2016-02-14, at 16:34, Michael Heerdegen <michael_heerdegen@web.de> wrote:

> Marcin Borkowski <mbork@mbork.pl> writes:
>
>> This works (i.e., M-: foo-fun properly runs the advice), but I expected
>> the symbol `advice-for-foo-fun' to have this piece of advice in its
>> function cell - and it does not.  Why?
>
> You bind the function to a new uninterned symbol (`make-symbol'), which
> is not what you want.  You want `intern'.

Stupid me.  Thanks for pointing that out.  My code works perfectly now.
I'm pretty sure I'll blog about it soon; also I'll post a message on
this ML in a few minutes.

> Michael.

Best,

-- 
Marcin Borkowski
http://octd.wmi.amu.edu.pl/en/Marcin_Borkowski
Faculty of Mathematics and Computer Science
Adam Mickiewicz University

Re: Dynamically constructing advice behaves strangely

[Michael Heerdegen]

Marcin Borkowski <mbork@mbork.pl> writes:

> (defun add-constructed-advice (fun)
>   "Add a constructed advice to function foo."
>   (let* ((fun-name (symbol-name fun))
> 	 (length-sym
> 	  (make-symbol (concat "length-of-" fun-name)))
> 	 (piece-of-advice-sym
> 	  (make-symbol (concat "advice-for-" fun-name)))
> 	 (piece-of-advice (lambda (orig-fun &rest args)
> 			    (apply orig-fun args)
> 			    (message "This function's name had length %s."
> 				     (symbol-value length-sym)))))

Also, `length-sym' doesn't have to be an uninterned symbol here.  You
use lexical-binding anyway (by referring to the value of the (interned!)
symbol named "length-sym".  You don't even have to use a symbol with a
unique name.  Just use a simple (lexical) variable for the length,
e.g. `advice-length', and

(message "This function's name had length %s." advice-length)

etc. (your piece of advice is a closure!).


Michael.

Re: Dynamically constructing advice behaves strangely

[Marcin Borkowski]

On 2016-02-14, at 16:44, Michael Heerdegen <michael_heerdegen@web.de> wrote:

> Marcin Borkowski <mbork@mbork.pl> writes:
>
>> (defun add-constructed-advice (fun)
>>   "Add a constructed advice to function foo."
>>   (let* ((fun-name (symbol-name fun))
>> 	 (length-sym
>> 	  (make-symbol (concat "length-of-" fun-name)))
>> 	 (piece-of-advice-sym
>> 	  (make-symbol (concat "advice-for-" fun-name)))
>> 	 (piece-of-advice (lambda (orig-fun &rest args)
>> 			    (apply orig-fun args)
>> 			    (message "This function's name had length %s."
>> 				     (symbol-value length-sym)))))
>
> Also, `length-sym' doesn't have to be an uninterned symbol here.  You
> use lexical-binding anyway (by referring to the value of the (interned!)
> symbol named "length-sym".  You don't even have to use a symbol with a
> unique name.  Just use a simple (lexical) variable for the length,
> e.g. `advice-length', and
>
> (message "This function's name had length %s." advice-length)
>
> etc. (your piece of advice is a closure!).

I know, and in fact I'm making use of lexical binding in my "real"
code.  This was just a trimmed-down version for the sake of asking here.

> Michael.

Best,

-- 
Marcin Borkowski
http://octd.wmi.amu.edu.pl/en/Marcin_Borkowski
Faculty of Mathematics and Computer Science
Adam Mickiewicz University