On Feb 21, 2007, at 2:20 AM, Michael Sperber wrote:
> The problem is that seemingly simple code can have complex behavior.
> In this case, the complexity derives from `swap'.
This is the implementation of fluid-let as in TSPL3:
(define-syntax fluid-let
(syntax-rules ()
((_ ((x v)) e1 e2 ...)
(let ((y v))
(let ((swap (lambda () (let ((t x)) (set! x y) (set! y t)))))
(dynamic-wind swap (lambda () e1 e2 ...) swap))))))
And here is the implementation of parameterize (simplified to one
binding):
(define-syntax parameterize
(syntax-rules ()
[(_ ((x v)) e1 e2 ...)
(let ([p x] [y v])
(let ([swap (lambda () (let ([t (p)]) (p y) (set! y t)))])
(dynamic-wind swap (lambda () e1 e2 ...) swap)))]))
All the code does is "swap before going in" and "swap when you come
out".
I honestly fail to see the complexity.
Aziz,,,
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
http://lists.r6rs.org/pipermail/r6rs-discuss/attachments/20070221/f8ea49f0/attachment.html
Received on Wed Feb 21 2007 - 03:10:11 UTC