6.4 Data Definitions

define-struct posn (x y))  
When DrScheme evaluates this structure definition, it creates three operations for us, which we can use to create data and to program:

    make-posn, the CONSTRUCTOR, which creates posn structures;

    posn-x, a SELECTOR, which extracts the x coordinate;

    posn-y, also a selector, which extracts the y coordinate.

   (symbol=? 'Hello 'Hello) = true

   (symbol=? 'Hello 'Howdy) = false

   (symbol=? 'Hello x) = true if x stands for 'Hello

   (symbol=? 'Hello x) = false if x stands for 'Howdy




;; a movie is a structure:
;;  (make-movie symbol symbol)
(define-struct movie (title producer))

;; a boyfriend is a structure:
;;  (make-boyfriend symbol symbol symbol number)
(define-struct boyfriend (name hair eyes phone))

;; a cheerleader is a structure:
;;  (make-cheerleader symbol number)
(define-struct cheerleader (name number))

;; a CD is a structure:
;;  (make-CD symbol symbol number)
(define-struct CD (artist title price))

;; a sweater is a structure
;;  (make-sweater symbol number symbol)
(define-struct sweater (material size producer))


难道Racket的类型声明是非显式的？