GNU Info

(elisp)Combined Definition

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The Combined Definition
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   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))

   Macros are redefined as macros, which means adding `macro' to the
beginning of the combined definition.

   The interactive form is present if the original function or some
piece of advice specifies one.  When an interactive primitive function
is advised, advice uses a special method: it calls the primitive with
`call-interactively' so that it will read its own arguments.  In this
case, the advice cannot access the arguments.

   The body forms of the various advice in each class are assembled
according to their specified order.  The forms of around-advice L are
included in one of the forms of around-advice L - 1.

   The innermost part of the around advice onion is

     apply original definition to ARGLIST

whose form depends on the type of the original function.  The variable
`ad-return-value' is set to whatever this returns.  The variable is
visible to all pieces of advice, which can access and modify it before
it is actually returned from the advised function.

   The semantic structure of advised functions that contain protected
pieces of advice is the same.  The only difference is that
`unwind-protect' forms ensure that the protected advice gets executed
even if some previous piece of advice had an error or a non-local exit.
If any around-advice is protected, then the whole around-advice onion
is protected as a result.