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,,,
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