Mark Polesky <markpole...@yahoo.com> writes:
>> (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))))))
(define-macro (make-dalist . forms)
(let ((alist-var (gensym)))
`(letrec ((,alist-var (list))
(slot (lambda (key)
(cdr (assq key ,alist-var)))))
,@(map (lambda (form)
`(set! ,alist-var (acons ,(car form) ,(cdr form) ,alist-var)))
forms)
,alist-var)))
(pp (macroexpand-1 '(make-dalist ('a . 2) ('b . (+ 1 (slot 'a))))))
=>
(letrec ((#{\ g8}# (list))
(slot (lambda (key) (cdr (assq key #{\ g8}#)))))
(set! #{\ g8}# (acons (quote a) 2 #{\ g8}#))
(set! #{\ g8}#
(acons (quote b) (+ 1 (slot (quote a))) #{\ g8}#))
#{\ g8}#)
(define foo (make-dalist ('a . 2) ('b . (+ 1 (slot 'a)))))
=>
((b . 3) (a . 2))
This only works when defining the alist, and assumes that you are only
referencing earlier entries. Perhaps useful for a limited set of things.
If you want something that updates dynamically then you really need a
different data structure. It could be done ad hoc using manually defined
accessors (e.g. 'b would not *really* exist, but would just be a
function that calls `(+ 1 ...)'), or in the fancy general way by
building a dataflow library (or, even more generally, a constraint
solver).
--
emacsen: every copy of Emacs comes with a bag of pot and 5 hits of acid
emacsen: and a hotel coffee maker
