[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: Constructors

> The argument I buy for having constructors is that they provide
> a more abstract interface than make-instance. Using the Principle
> of Insignificant Degradation and Occam's Chainsaw, I don't think that
> make-instance and constructors burden the user with too many ways to
> do things, given that one is an abstraction mechanism.

I don't think the difference between;

	(make-foo :name 'bar :number-of-widgits 50)


	(make-instance 'foo :name 'bar :number-of-widgits 50)

is sufficiently large to constitute a useful addition in abstraction.
Most programmers will want to name their constructors in a way which
is reflective of their function, to construct a particular class. Thus,
if they change the class name, it would make sense to change the
constructor name, so that the name continues to reflect the function.
Whether they change a constructor function name or a class name in
a MAKE-INSTANCE form makes little difference. Thus, for the price of
yet another DEFCLASS construct, you get very little in the way of
additional abstraction. And, as has been argued, programmers who
want to do this will do it anyway. Most programmers are going to
want to do things like:

(defun make-bar-foo (widgits)

  (make-instance 'foo :name 'bar :number-of-widgits widgits))

and constructors won't help there. Or, if they could, the addition to
the DEFCLASS syntax wouldn't be worth the savings in not having to define
the constructor seperately.

The only argument I find vaguely attractive is the optimization one,
though I suspect this is probably more of an issue on dedicated hardware
than on general purpose machines. However, since the general Common Lisp
method of specifying optimizations is using the DECLARE construct, I
think any attempt to optimize instance construction should go that route.

I therefore vote "NO" on Measure C.