Re: caching in load-time-value and defmacro

[Bruno H. <br...@cl...>]

Don Cohen wrote:
>  > If you want to avoid stupidly slow behaviour in the evaluator, i.e.
>  > guarantee that a form is executed only once, you can use the MEMOIZED
>  > macro, found in defs1.lisp.
>
> Is there any reason not to use this in the standard versions of
> defmacro and l-t-v ?
> If anyone really wants the uncached versions then they could be
> supplied via ext:...-uncached

About LOAD-TIME-VALUE: There are two reasons for keeping clisp's behaviour,
i.e. in the interpreter, reevaluate each time:

  1) In the interpreter, the developer wants changes to other functions to
     be activated immediately, without re-entering forms. Example:

       ;; Let's compute the volume of a ball with radius x.
       (defun my-func (x) (* 4/3 pi (* x x)))

       (defun my-func-normalized (x)
         (/ (my-func x) (load-time-value (my-func 0.01))))

       (my-func-normalized 1.0) => 10000

       ;; Oops, I made a mistake.

       (defun my-func (x) (* 4/3 pi (* x x x)))

       ;; Try it again.

       (my-func-normalized 1.0) => 1000000 ;; Good! - in CLISP
       (my-func-normalized 1.0) => 10000   ;; Unexpected! - in SBCL

     The point here is that the developer made a mistake in my-func,
     so he corrects it, and expects things to work. He did not make
     a mistake in my-func-normalized, so he does not want to reenter
     this form.

     This is where "early compile" can be encumbering.

  2) MEMOIZED does extra consing; it's useless to do extra consing to
     implement a behaviour of which ANSI CL says that you cannot count upon.

About DEFMACRO, it's not clear what you mean.

Bruno