Friday, April 19, 2024

Plaformer Game Tutorial

I was suprised by the interest in the code I wrote for learning the platformer game. It wasn’t the best Lisp code. I just uploaded what I had.

But enough people were interested that I decided to give it a once over. At https://github.com/jrm-code-project/PlatformerTutorial I have a rewrite where each chapter of the tutorial has been broken off into a separate git branch. The code is much cleaner and several kludges and idioticies were removed (and I hope none added).

Monday, April 1, 2024

You May Not Need That :around Method

I’ve seen this “anti-pattern” a few times in CLOS code. A superclass ’super will have a subclass ’sub and there will be a primary method specialized to the superclass.

(defmethod foo ((instance super) arg)
  (format t "~&Foo called on ~s." arg))

Then I’ll see an :around method defined on the subclass:

(defmethod foo :around ((instance sub) arg)
  (format t "~&Start foo...~%")
  (call-next-method)
  (format t "~&End foo.~%"))
The intent here is clearly that code in the method specialized on the subclass is invoked “around” the call to the method specialized on the superclass.

But the :around qualifier is not necessary and probably doesn’t do what is intended. If we remove the :around qualifier, then the most specific primary method will be the foo method specialized on ’sub. And the (call-next-method) invokation will chain up to the foo method specialized on ’super. It will work as was likely intended.

:around methods are useful when the superclass wants to run a method “around” the subclass. :around methods are combined from least specific to most specific — the opposite order of primary methods — so that the superclass can wrap the call to the subclass. An good example of where an :around method would be handy is when you need to sieze a lock around the call to the method. The superclass would sieze the lock in an :around method that would run before any of the subclass primary methods ran.

Ordinary chaining of methods doesn’t need the :around qualifier. Just chain the methods.