[plt-scheme] graphics states and Scheme

From: Eli Barzilay (eli at barzilay.org)
Date: Wed Jul 4 12:49:22 EDT 2007

On Jul  4, Dave Griffiths wrote:
> [...]  The main thing is the scene description langauge, which
> currently involves an implicit state stack, and lots of small simple
> functions. [...]

I'm not sure that this is a proper suggestion, but I'll go anyway.

The thing is that when people use imperative code, they tend to forget
that using closures it's possible to push the imperative code into a
functional container.  Here's a very simplified example -- assume that
this is your imperative library:

  (define current-color 'black)
  (define (set-color! c) (set! current-color c))
  (define (draw-line len) (printf "drawing a ~a pixel ~a line\n" color len))

and you define

  (define (draw-cube len color)
    (set-color! color)
    (draw-line len)
    (draw-line len)
    (draw-line len)
    (draw-line len))

Instead of draw-cube, you can make it into a thunk that encapsulates
these operations:

  (define (build-cube len color)
    (lambda ()
      (set-color! color)
      (draw-line len)

Once you have that, the rest becomes natural, for example:

  (define (combine/offset obj1 obj2 ofs)
    (lambda ()
      (move-by ofs)
  (define (big-cube color)
    (build-cube 100 color))
  (define (rgb-cubes)
    (let ([r (big-cube 'red)]
          [g (big-cube 'green)]
          [b (big-cube 'blue)])
      (combine/offset (combine/offset r g 50)
                      b 50)))

          ((lambda (x) (x x)) (lambda (x) (x x)))          Eli Barzilay:
                  http://www.barzilay.org/                 Maze is Life!

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