[pro] The Best Examples of "Code is Data"

[Scott L. Burson]

On Sun, Sep 5, 2010 at 3:56 PM, Luís Oliveira <luismbo at gmail.com> wrote:
> One of the many tricks I've picked up from James Bielman (I hope he's
> subscribed to this mailing list :-)) was using CONSTANTP and EVAL in
> compiler macros.

Okay, it is true that anything that CONSTANTP says "true" on can
fairly be EVALled.  But be aware that implementations are not required
to recognize something like '(* 2 (/ 4 2)) as a constant.  (Maybe most
of them do.)

-- Scott

 Here's a simple example:
>
>    (defun plus (x y)
>      (+ x y))
>
>    (define-compiler-macro plus (&whole form x y)
>      (if (and (constantp x) (constantp y))
>          (+ (eval x) (eval y))
>          form))
>
> Execution examples:
>
>    (compiler-macroexpand-1 '(plus 1 1)) => 2
>
>    (compiler-macroexpand-1 '(plus '1 '1)) => 3
>
>    (compiler-macroexpand-1 '(plus (* 2 (/ 4 2)) (+ 3 2))) => 9
>
>    (defconstant +1+ 1)
>    (compiler-macroexpand-1 '(plus +1+ 2)) => 3
>
> --
> Luís Oliveira
> http://r42.eu/~luis/
>
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