2.60.rkt

#lang sicp
;; Exercise 2.60: We specified that a set would be represented as a list with no duplicates.
;; Now suppose we allow duplicates. For instance, the set { 1 , 2 , 3 } could be
;; represented as the list (2 3 2 1 3 2 2).
;; Design procedures element-of-set?, adjoin-set, union-set, and intersection-set that operate on this representation.
;; How does the efficiency of each compare with the corresponding procedure for the non-duplicate representation?
;; Are there applications for which you would use this representation in preference to the non-duplicate one?

(define (element-of-set? x set)
  (cond ((null? set) #f)
        ((equal? (car set) x) #t)
        (else (element-of-set? x (cdr set)))))

(define (make-set set)
  (cond ((null? set) '())
        ((element-of-set? (car set) (cdr set))
         (make-set (cdr set)))
        (else (cons (car set)
                    (make-set (cdr set))))))

;; Tests
(make-set '(2 3 2 1 3 2 2))