; ; information-hiding.scm ; Another technique to accomplish information hiding in R5RS Scheme, ; this one encapsulating a mutable ADT with an OO-like dispatch. ; ; 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 (lambda () (let* ((stack '())) (lambda (method args) (cond ((equal? method 'push) (begin (set! stack (cons (car args) stack))

t))

((equal? method 'top) (car stack)) ((equal? method 'pop) (begin (set! stack (cdr stack))

t)))))))

; ; A transcript of some sample usage ; ; #;1> (define s (new-stack)) ; #;2> (s 'push '(4)) ; #t ; #;3> (s 'push '(5)) ; #t ; #;4> (s 'top '()) ; 5 ; #;5> (s 'pop '()) ; #t ; #;6> (s 'top '()) ; 4