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

[Kevin Rodgers <kevin.d.rodgers@gmail.com>]

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