Teachpack : Simple Drawing Exercises

# Simple Drawing Exercises

### draw.ss

The teachpack `draw.ss` provides a miniscule collection of graphics functions:
- `start` : number number -> void;
  opens a canvas
- `stop` : -> void (no arguments);
  closes the canvas
- `draw-circle` : posn number color -> true;
  draws a circle at posn with given radius and color
- `draw-solid-disk` : posn number color -> true;
  draws a disk at posn with given radius and color
- `draw-solid-rect` : posn number number color -> true;
  draws a rectangle at posn with given width, height, and color
- `draw-solid-line` : posn posn color -> true;
  draws a line from one posn to other
- `sleep-for-a-while` : number -> true;
  suspends evaluation for the given number of seconds
- predefined colors: `WHITE` `YELLOW` `RED` `BLUE` `GREEN` `BLACK`
The teachpack also provides `clear-` operations for each `draw-` operation. The arguments are the same.
The color argument for all functions are optional.

Sample session: Set teachpack to `draw.ss` and execute:
`> (start 500 500)`
`> (draw-solid-disk (make-posn 100 100) 3 RED)`
`true`
`> (clear-solid-disk (make-posn 100 100) 3 RED)`
`true`
`> (sleep-for-a-while 1)
`> (draw-solid-disk (make-posn 100 100) 3 RED)`
`true`
`> (clear-solid-disk (make-posn 100 100) 3)`
`true`
`> (stop)`
`>` This session opens a window, draws a red disk, clears it, sleeps for a second, and then repeats. The last expression closes the canvas.
`