Dynamically constructing advice behaves strangely

[Marcin Borkowski <mbork@mbork.pl>]

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