; ; immutable-adt.scm ; Another technique to accomplish information hiding in R5RS Scheme. ; ; SPDX-FileCopyrightText: In 2023, Chris Pressey, the original author of this work, placed it into the public domain. ; SPDX-License-Identifier: Unlicense ; For more information, please refer to https://unlicense.org/ ;
(define new-stack (letrec ( (make-stack (lambda (items) (lambda (op args) (cond ((equal? op 'push) (let* ( (item (car args)) (new-items (cons item items)) ) (make-stack new-items))) ((equal? op 'top) (car items)) ((equal? op 'popped) (make-stack (cdr items))) )))) ) (lambda () (make-stack '()))) )
; ; A transcript of some sample usage ; ; #;1> (define s (new-stack)) ; #;2> (define t (s 'push '(4))) ; #;3> (define u (t 'push '(5))) ; #;4> (u 'top '()) ; 5 ; #;5> ((u 'popped '()) 'top '()) ; 4