クロージャでキューを実現する.
(define (make-queue) (let ((front-ptr '()) (rear-ptr '())) (define (set-front-ptr! item) (set! front-ptr item)) (define (set-rear-ptr! item) (set! rear-ptr item)) (define (empty-queue?) (null? front-ptr)) (define (front-queue) (if (empty-queue?) (error "FRONT called with an empty queue") front-ptr)) (define (insert-queue! item) (let ((new-pair (cons item '()))) (cond ((empty-queue?) (print "new-pair=" new-pair) (set-front-ptr! new-pair) (set-rear-ptr! new-pair) dispatch) (else (set-cdr! rear-ptr new-pair) (set-rear-ptr! new-pair) dispatch)))) (define (delete-queue!) (cond ((empty-queue?) (error "DELETE! called with an empty queue")) (else (set-front-ptr! (cdr front-ptr)) dispatch))) (define (print-queue) (print front-ptr)) (define (dispatch m) (cond ((eq? m 'insert-queue!) insert-queue!) ((eq? m 'delete-queue!) delete-queue!) ((eq? m 'front-queue) front-queue) ((eq? m 'print-queue) print-queue) ((eq? m 'empty-queue?) empty-queue?) (else (error "Undefined operation: " m )))) dispatch))
(define q1 (make-queue)) ((q1 'insert-queue!) 'a) ((q1 'print-queue)) ; (a) ((q1 'insert-queue!) 'b) ((q1 'print-queue)) ; (a b) ((q1 'delete-queue!)) ((q1 'print-queue)) ; (b) ((q1 'delete-queue!)) ((q1 'print-queue)) ; ()