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The advise macro can be thought of as a more
general version of trace. It allows code that
you specify to run before, after, or around a given function, for
the purpose of changing the behavior of the function. Each piece
of added code is called a piece of advice. Each piece of advice
has a unique name, so that you can have multiple pieces of advice
on the same function, including multiple
:before, :after, and
:around pieces of advice.
The :name and :when
keywords serve to identify the piece of advice. A later call to
advise with the same values of
:name and :when will replace
the existing piece of advice; a call with different values will not.
spec---
A specification of the function on which to put the
advice. This is either a symbol that is the name of a
function or generic function, or an expression of the
form (setf symbol), or a
specific method of a generic function in the form
(:method symbol {qualifiers} (specializer {specializer})).
form--- A form to execute before, after, or around the advised function. The form can refer to the variable arglist that is bound to the arguments with which the advised function was called. You can exit from form with (return).
name--- A name that identifies the piece of advice.
when---
An argument that specifies when the piece of advice is
run. There are three allowable values. The default is
:before, which specifies that form is
executed before the advised function is called. Other
possible values are :after, which
specifies that form is executed after the advised
function is called, and :around,
which specifies that form is executed around the call to
the advised function. Use (:do-it)
within form to indicate invocation of the original
definition.
The function foo, already defined, does
something with a list of numbers. The following code uses a
piece of advice to make foo return zero if any of its
arguments is not a number. Using :around advice, you can do
the following:
(advise foo (if (some #'(lambda (n) (not (numberp n))) arglist) 0 (:do-it)) :when :around :name :zero-if-not-nums)
To do the same thing using a :before piece of advice:
(advise foo (if (some #'(lambda (n) (not (numberp n))) arglist) (return 0)) :when :before :name :zero-if-not-nums)
The unadvise macro removes the piece or pieces of advice
matching spec, when,
and name. When the value of
spec is t and the values of when
and name are nil, unadvise
removes every piece of advice; when spec is
t, the argument when is nil, and
name is non-nil, unadvise removes all
pieces of advice with the given name.
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