Re: around advice: why does (and ad-do-it nil) return t?

[Kevin Rodgers]

On 2/13/12 6:03 AM, Jambunathan K wrote:
> Leo Alekseyev<dnquark@gmail.com>  writes:
>
> > Consider the following code:
> >
> > (defun foobar () t)
> > (defadvice foobar (around foobar-advice activate)
> >    (and ad-do-it nil))
> >
> > Evaluating the advised foobar seems to return t.  Why?  Naively, one
> > expects (and [whatever] nil) to evaluate to nil!
>
> One possible reason could be this:
>
> (info "(elisp) Around-Advice")
> ,----
> |  -- Variable: ad-do-it
> |      This is not really a variable, rather a place-holder that looks
> |      like a variable.  You use it in around-advice to specify the place
> |      to run the function's original definition and other "earlier"
> |      around-advice.
> `----
>
> May be you are looking for ad-return-value.
>
> (info "(elisp) Defining Advice")
> ,----
> |  -- Variable: ad-return-value
> |      While advice is executing, after the function's original
> |      definition has been executed, this variable holds its return
> |      value, which will ultimately be returned to the caller after
> |      finishing all the advice.  After-advice and around-advice can
> |      arrange to return some other value by storing it in this variable.
> `----

Exactly.  The advised function always returns ad-return-value (which the
various pieces of advice may alter).  As explained in the "Combined Definition"
node of the Elisp manual:

----------------------------------------------------------------------
Suppose that a function has N pieces of before-advice (numbered from 0
through N-1), M pieces of around-advice and K pieces of after-advice.
Assuming no piece of advice is protected, the combined definition
produced to implement the advice for a function looks like this:

     (lambda ARGLIST
       [ [ADVISED-DOCSTRING] [(interactive ...)] ]
       (let (ad-return-value)
         before-0-body-form...
              ....
         before-N-1-body-form...
         around-0-body-form...
            around-1-body-form...
                  ....
               around-M-1-body-form...
                  (setq ad-return-value
                        apply original definition to ARGLIST)
               end-of-around-M-1-body-form...
                  ....
            end-of-around-1-body-form...
         end-of-around-0-body-form...
         after-0-body-form...
               ....
         after-K-1-body-form...
         ad-return-value))
----------------------------------------------------------------------

Basically, ad-do-it expands to

                  (setq ad-return-value
                        apply original definition to ARGLIST)

--
Kevin Rodgers
Denver, Colorado, USA