Generics example from RacketCon 2012

gistfile1.rkt

#lang racket

;; Example code from RacketCon 2012

(require racket/generic)

;; simple set interface
(define-generics set
  (set-add elem set)
  (set-contains? elem set)
  (set-empty? set)
  ;; #:defaults work in our experimental branch of Racket
  ;; they will work in a future release of Racket
  #|
  #:defaults
  ([(list?) (set-add cons)
            (set-contains? member)
            (set-empty? empty?)])
  |#
  )

;; implement a simple set using a hash
(struct my-set (hash)
  #:methods gen:set
  [(define (set-add elem set)
     (my-set (hash-set (my-set-hash set) elem #t)))
   (define (set-contains? elem set)
     (my-set (hash-ref (my-set-hash set) elem #f)))
   (define (set-empty? set)
     (null? (hash-keys (my-set-hash set))))])

(define mt-set (my-set (hash)))

(set-contains? 5 (set-add 5 mt-set))

;; without #:defaults, we can still use a trick:

(define (-set-add elem set)
  (cond [(list? set) (cons elem set)]
        [else (set-add elem set)]))

(define (-set-contains? elem set)
  (cond [(list? set) (member elem set)]
        [else (set-contains? elem set)]))

(define (-set-empty? set)
  (cond [(list? set) (null? set)]
        [else (set-empty? set)]))

;; some contracts
(define int-set/c
  (recursive-contract 
   (set/c [set-add (-> integer? int-set/c int-set/c)]
          [set-contains? (-> integer? int-set/c any)]
          [set-empty? (-> int-set/c any)])))

(define/contract a-set int-set/c (set-add 5 mt-set))

;; passes contract
(set-add 5 a-set)

;; produces a contract error as expected
#;
(set-add "foo" a-set)