Extended Exercise: Moving Shapes

In section , we developed functions for drawing, translating, and clearing circles and rectangles. As we have just seen, we should think of the two classes of data as subclasses of a class of shapes so that we can just draw, translate, and clear shapes.

Exercise 7.4.1

Provide a data definition for a general class of shapes. The class should at least subsume the classes of colored circles and rectangles from section .

Develop the template fun-for-shape, which outlines functions that consume shapes. Solution

Exercise 7.4.2

Use the template fun-for-shape to develop draw-shape. The function consumes a shape and draws it on the canvas. Solution

Exercise 7.4.3

Use the template fun-for-shape to develop translate-shape. The function consumes a shape and a number delta, and produces a shape whose key position is moved by delta pixels in the x direction. Solution

Exercise 7.4.4

Use the template fun-for-shape to develop clear-shape. The function consumes a shape, erases it from the canvas, and returns true. Solution

Exercise 7.4.5

Develop the function draw-and-clear-shape. The function consumes a shape, draws it, sleeps for a while, and clears it. If all the effects work out, it produces true. Solution

Exercise 7.4.6

Develop move-shape, which moves a shape across the canvas. The function consumes a number (delta) and a shape. The function should draw-and-clear the shape and return a new shape that has been translated by delta pixels. Use this function several times to move a shape across the canvas. Solution