6.3 The Advice Facility

6.3.2 Advice examples

;; Define a function that adds 1 to its argument.
> (defun add-one (x) (+ x 1))
ADD-ONE

> (add-one 3)               ; 3+1 = 4
4

;; Put some advice on it that gives a default value for the
;; argument.
> (defadvice (add-one default-to-zero) (&optional (x 0))
      (advice-continue x))
#<Advice DEFAULT-TO-ZERO B00D37

> (add-one)                 ; No argument supplied, so 0+1 = 1
1

;; Put some more advice on to double the result.
> (defadvice (add-one double-result) (&rest args)
             (* 2 (apply-advice-continue args)))
#<Advice DOUBLE-RESULT B261BF

> (add-one)                 ; (default 0+1)*2 = 2
2

> (add-one 5)               ; (5+1)*2 = 12
12

;; See what is there now.
> (describe-advice 'add-one)
Advice DOUBLE-RESULT: 
 #<Interpreted-Function (:ADVICE ADD-ONE DOUBLE-RESULT) B362D7>
  Advice DEFAULT-TO-ZERO: 
   #<Interpreted-Function (:ADVICE ADD-ONE DEFAULT-TO-ZERO)
                  B3619F>
    Original definition: #<Interpreted-Function (NAMED-LAMBD
 ADD-ONE (X) 
(BLOCK ADD-ONE (+ X 1))) AF1A57>
NIL

;; Get rid of one of the pieces of advice.
> (remove-advice 'add-one 'default-to-zero)
Removing (:ADVICE ADD-ONE DEFAULT-TO-ZERO)

;; Now install more advice to add 1 to the result.  Put it outside
;; the advice that does the doubling.
> (defadvice (add-one add-one-to-result 
             (:outside double-result)) (x)
             (+ 1 (advice-continue x)))
#<Advice ADD-ONE ADD-ONE-TO-RESULT)B7732F>
> (add-one 2)               ; ((2+1)*2)+1 = 7
7

;; Replace that advice with the same piece of advice but put it 
;; inside the doubling advice instead.
> (defadvice (add-one add-one-to-result 
             (:inside double-result)) (x)
             (+ 1 (advice-continue x)))
#<ADVICE ADD-ONE-TO-RESULT B7ED9F>

> (add-one 2)               ; ((2+1)+1)*2 = 8
8

;; Redefine the basic function inside the advice to return its 
;; argument unchanged.
> (defun add-one (x) x)
;;; Warning: Redefining ADD-ONE, keeping advice DOUBLE-RESULT, 
;;; ADD-ONE-TO-RESULT
ADD-ONE

> (add-one 2)               ; (2+1)*2 = 6
6

;; Remove all the advice from the function.
> (remove-advice 'add-one)
Removing 2 pieces of advice from ADD-ONE
NIL

> (add-one 1)
1

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