[pro] BOA constructor and supplied-p argument

[Pascal Costanza]

On 11 Jun 2011, at 10:42, Nick Levine wrote:

> Actually, looking through my code base, I find I've used &aux half a
> dozen times in the last ten years. Always in a BOA constructor.

I have a tendency to use &aux more and more often, even outside of BOA constructors. There are two main reasons:

- It saves horizontal space in the source code. Compare the following two pieces of code:

(defun foo1 (a b c &aux d e f)
  ...)

(defun foo2 (a b c)
  (let (d e f)
    ...))

The use of &aux saves one level of indentation here, which can sometimes make a piece of code look more beautiful.


- The more important reason is that I sometimes want to derive some value from an argument that is "very close" to the value of the original argument. Here is an example:

(defun required-args (args)
  (loop for arg in args until (member arg lambda-list-keywords) collect arg))

(defun process-method-arguments (args &aux (required-args (required-args args)))
  ...)

This is, of course, only a subjective quality, but in cases like this, the function actually only wants to do something with the required arguments for a method definition, but it also wants the client code not to worry too much about what it passes to the function. To me, &aux perfectly expresses that idea.


Best,
Pascal

--
Pascal Costanza
The views expressed in this email are my own, and not those of my employer.