Revealing Module Pattern Example in Scheme

;Stolen from "Structure and Interpretation of Computer Programs" 
;https://mitpress.mit.edu/sicp/full-text/book/book-Z-H-20.html#%_sec_3.1.1

;Implementation:
;---------------

(define (make-account balance)
  ;withdraw method:
  (define (withdraw amount)
    (if (>= balance amount)
        (begin (set! balance (- balance amount))
               balance)
        "Insufficient funds"))
  ;deposit method: 
  (define (deposit amount)
    (set! balance (+ balance amount))
    balance)
  ;method dispatcher
  (define (dispatch m)
    (cond ((eq? m 'withdraw) withdraw)
          ((eq? m 'deposit) deposit)
          (else (error "Unknown request -- MAKE-ACCOUNT"
                       m))))
  dispatch)
  
  
;Usage:
;---------------

(define acc (make-account 100))
((acc 'withdraw) 50)
; -> 50
((acc 'withdraw) 60)
"Insufficient funds"
((acc 'deposit) 40)
; -> 90
((acc 'withdraw) 60)
; -> 30