Re: scheme (format) question

[Kjetil S. Matheussen]

On Sun, 22 Mar 2009, Mark Polesky wrote:

> > (define (my-format bool string . list-of-nums)
> >  (apply format bool string (map set-precision list-of-nums)))
> >
> > (let ((a 2.0)
> >       (b 1/4)
> >       (c 1/3))
> >  (my-format #t "~&~a ~a ~a" a b c))
>
>
> Kjetil,
>
> Awesome. Thanks. I knew there was something simple,
> I just couldn't figure it out. That little dot helps
> alot! And now for my next question...
>
> Is there a way for a pair within an alist to retrieve
> values from other pairs within the same alist? I know
> the following code doesn't work, because "my-alist"
> is still undefined when I'm calling it, but is there
> a way to make it do what I want?
>
> (define my-alist
>  `((a 1)
>    (b ,(+ 1 (cadr (assq 'a my-alist))))))

Maybe something like this:

(define-macro (push! val where)
  (let ((ret (gensym)))
    `(let ((,ret ,val))
       (set! ,where (cons ,ret ,where))
       ,ret)))

(define my-alist '())

(push! '(a 1) my-alist)
(push! `(b ,(+ 1 (cadr (assq 'a my-alist)))) my-alist)