[racket] question about classes

From: Matthias Felleisen (matthias at ccs.neu.edu)
Date: Sun May 20 14:40:38 EDT 2012

On May 20, 2012, at 11:02 AM, Joe Gilray wrote:

> 1)  Is what I've done an abomination?  Am I simply abusing inheritance?

You are changing a part of a recursive nest of methods, which is what inheritance is all about. 

> 2)  To make the code below work, I had to make expand-repeat public as I couldn't override a private function.  Is there another way (something like "protected" in C++)?

(define-local-member-name expand-repeat)

Don't export expand-repeat and nobody can run this method from outside. 

#lang racket

(require racket/class)


;; ---------------------------------------------------------------------------------------------------

(define-local-member-name expand-repeat)

; class that represents the continued fraction of a general irrational number 
; and acts as a base class for continued fractions of other sorts
(define gencontfrac% 
  (class object%
    (init-field start repeat)
    ; create a n-long list of repeat digits in reverse order, 
    ; which facilitates the creation of a rational
    (define/public (expand-repeat n)
      (let lp ([c n] [rl '()])
        (if (zero? c) rl (lp (sub1 c) (cons (repeat) rl)))))
    ; create a rational representation using the passed number of repeat digits
    (define/public (rational-rep n)
      (define rdiglst (expand-repeat n))
      (let lp ([rdigs (rest rdiglst)] [res (first rdiglst)])
        (if (empty? rdigs) (+ start (/ 1 res)) (lp (rest rdigs) (+ (first rdigs) (/ 1 res))))))
    ; display the continued fraction
    (define/public (show)
      (printf "[~a; ~a]~n" start repeat))))

; define a class that represents the continued fraction of a square root
(define sqrtcontfrac% 
  (class gencontfrac%
    (inherit-field start)
    (init sqrval)
    (super-new [start (integer-sqrt sqrval)] [repeat (void)])
    (inherit rational-rep show)
    (define repeatlst (build-repeatlst sqrval))
    ; build the repeating sequence (see http://en.wikipedia.org/wiki/Continued_fraction)
    (define/private (build-repeatlst n)
      (define start (integer-sqrt n))
      (let loop ([adder start] [denom (- n (sqr start))] [result '()])
          [(zero? denom) 
           ; perfect square:
          [(= 1 denom)
           ; found the end of the repeating sequence:
           (reverse (cons (+ (integer-sqrt n) adder) result))] 
           (let* ([nextval (quotient (+ adder start) denom)] [nextadd (- (* nextval denom) adder)])
             (loop nextadd (quotient (- n (sqr nextadd)) denom) (cons nextval result)))])))
    (define/override (expand-repeat n)
      (let lp ([c n] [rl '()] [stock repeatlst])
          [(zero? c) rl]
           (define r (rest stock))
           (lp (sub1 c) (cons (first stock) rl) (if (empty? r) repeatlst r))])))
    ; accessor
    (define/public (get-repeatlst) repeatlst)))

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