[racket] Saving and restoring many parameters

On 10/20/2011 08:08 AM, Robby Findler wrote:
> Well, what about the current-namespace? The output ports? It seems
> dangerous and leak introducing (possibly).
>
> I don't actually have an example where it could cause a problem tho. But
> you might try to think thru the ramifications by searing for "current-"
> in the docs.

I still think using a thread wouldn't be a problem. But you convinced me 
yesterday that it could become a problem in the future.

So I invented parameter lists. Creating a plot pict looks like this now:

     (define saved-parameters (plot-parameters))
     (dc (λ (dc x y)
           (parameterize/list ([plot-parameters  saved-parameters])
             (plot/dc renderer-tree dc x y width height ...)))
         width height)

Some example tests:

     (define p1 (make-parameter 1))
     (define p2 (make-parameter 2))
     (define ps1 (parameter-list p1 p2))

     (check-equal? (ps1) (list 1 2))
     (check-equal? (parameterize/list ([ps1  (list 10 20)]) (ps1))
                   (list 10 20))
     (check-equal? (parameterize ([p1 10] [p2 20]) (ps1))
                   (list 10 20))
     (check-equal? (parameterize/list ([ps1  (list 10 20)])
                     (list (p1) (p2)))
                   (list 10 20))

     (ps1 (list 10 20))

     (check-equal? (ps1) (list 10 20))

Rules:

     ((parameter-list p ...)) = (list (p) ...)

     ((parameter-list p ...) (list v ...)) = (begin (p v) ...)

     (parameterize/list ([(parameter-list p ...)  (list v ...)]) e ...)
      = (parameterize ([p v] ...) e ...)

     ((parameter-list* p ... pl)) = (list* (p) ... (pl))

     ((parameter-list-append pl ...)) = (append (pl) ...)

I know Blake has wanted these before, so I might throw them into unstable.

Neil T