[racket] Saving and restoring many parameters

From: Robby Findler (robby at eecs.northwestern.edu)
Date: Tue Oct 18 15:48:40 EDT 2011

There are parameterizations, but they pick up all parameters and that
may make other things go wrong (or you have to be more careful how you
look up your parameter values but I think you might object to that on
similar grounds to the below).

I would probably use the solution you have below, but with some macro
help. Specifically, in the place where you define all those paramters,
use a macro to define them, and then have other macros that
collaborate with the defining macros to capture and restore the
parameter values.


On Tue, Oct 18, 2011 at 2:40 PM, Neil Toronto <neil.toronto at gmail.com> wrote:
> Executive summary: Is there a good way to save a large set of parameter
> values, or *all* the parameter values, and then restore them when needed?
> In the new PLoT, much of a plot's appearance is controlled by parameters. I
> use parameters because there are so many appearance-controlling values that
> passing them into the plot area as arguments would get silly very quickly. I
> dislike functions with 20 arguments, and I *really* dislike duplicating all
> those arguments in every function in a call chain. Parameters are a perfect
> remedy.
> There's one problem, though: to produce a slideshow pict of a plot, I need
> to do this:
>    (dc (lambda (the-dc x y)
>          (plot/dc ... the-dc x y ...)) ...)
> where 'plot/dc' draws a plot on a device context. Because the call to
> 'plot/dc' is in a thunk, it gets the *current* parameter values, which is
> very wrong. That makes it impossible to, for example, change the background
> color of only one plot in a slideshow by doing
>    (parameterize ([plot-background  "red"])
>      (plot-pict ...))
> The problem is that the pict returned by 'dc' calls the drawing thunk
> *after* the dynamic scope in which 'plot-background' is "red".
> To fix the problem, I'm doing this:
>    (define foreground (plot-foreground))
>    (define background (plot-background))
>    (define foreground-alpha (plot-foreground-alpha))
>    (define background-alpha (plot-background-alpha))
>    (define font-size (plot-font-size))
>    ...
>    (define animating? (plot-animating?))
>    (dc (lambda (the-dc x y)
>          (parameterize ([plot-foreground        foreground]
>                         [plot-background        background]
>                         [plot-foreground-alpha  foreground-alpha]
>                         [plot-background-alpha  background-alpha]
>                         [plot-font-size         font-size]
>                         ...
>                         [plot-animating?        animating?])
>            (plot/dc ... the-dc x y ...))) ...)
> Besides looking evil, it's barely maintainable. Every time I add an
> appearance-controlling parameter, I have to remember to add it to the above
> incantation. (Actually, there are two.)
> I've had a similar problem with the click-and-drag-to-rotate 3D plots.
> Because I was already using threads for that, I just made sure the render
> thread is always (transitively) a child of the thread in which the call to
> 'plot3d' was made. It therefore inherits the original parameter values.
> So I can use threads to save parameter values. I could make a render thread
> to solve my 'plot-pict' problem. But is there a better way?
> Neil T
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