In algebra-speak, sums of products; an “or” of “ands”.

Many different alternatives, each of which is (in general) a structure.

;; A DisplayShape represents a shape to draw on the screen, and is one of

;; - a (make-rectangle Size Size), a rectangle of given width and height;

;; - a (make-circle Size), a circle of given radius; or

;; - a (make-line Size Angle), a line of the given length and angle.

(define-struct rectangle (width height))

(define-struct circle (radius))

(define-struct line (size angle))

;; A Size is a Number representing a pixel count.

;; An Angle is a Number representing an angle in degrees.

;; F : DisplayShape -> ?

(define (F ds)

(cond [(rectangle? ds) ... (rectangle-width ds) ... (rectangle-height ds) ...]

[(circle? ds) ... (circle-radius ds) ...]

[(line? ds) ... (line-size ds) ... (line-angle ds) ...]))

Learn: PPL