Lisp Def Method Structure

Question

(defmethod update :before ((a AGENT) (e UPDATE))
  (when (null (timestamps a))
    (push 0 (pls a))
    (push 0 (fitnesses a)))
  (push (timestamp e) (timestamps a))
  (push (price e) (revalprices a))
  (preprocess a e)
  (format T ":BEFORE completed for agent  ̃A and event  ̃A ̃%" a e))

The above method came from a book and I wanted to inquire about what is the difference between update and :before. From my understanding from defining methods in CLOS Lisp, the update is the name of the method, but what does :before do?

Lastly what does the last line do? (format T ":BEFORE completed for agent ̃A and event ̃A ̃%" a e))

Answer 1

What you're looking at is the definition of a :before auxiliary method. When the method update is called with arguments that satisfy the types agent and update, this piece of code will run before the main method body. It's also possible to define :after and :around auxiliaries. Have a look at this PCL chapter for more information.

As to the format string, I assume that's mistranscribed. The form you've got there will just issue a warning that you're passing it too many format arguments. I think what's meant is

(format t ":BEFORE completed for agent ~a and event ~a~%" a e)

which will print ":BEFORE completed for agent " followed by the value of a, followed by " and event " followed by the value of e, followed by a newline, to the stream *standard-output*. For more information on CL's format directives, take a look at this other PCL chapter, and possibly also this section of the CLHS