[racket] question about classes

The problem is that `start' doesn't get a value for a `sqrtcontfrac%'
instance until `super-new' in `sqrtcontfrac%' is evaluated. But the
expression for the second argument in `super-new' calls `build-repeat',
so that `build-repeat' tries to use `start' before it has a value.

I'm not sure of the solution in this case, but it might be to pass the
value for `start' into `build-repeat' instead of trying to get the
value from the `start' field within `build-repeat'.

At Sun, 20 May 2012 02:06:35 -0700, Joe Gilray wrote:
> I'm trying to learn to use classes in Racket.  I did some successful simple
> experiments then moved on to inheritance.  When I try to use the code
> below, the following code:
> 
> (new sqrtcontfrac% [sqrval i])  leads to the message: sqr: expected
> argument of type <number>; given #<undefined>
> 
> Why isn't start being inherited properly?  Any help is appreciated!
> 
> Thanks,
> -joe
> 
> Here is the class code:
> 
> (module contfrac racket
>   (require racket/class)
>   (provide gencontfrac%)
>   (provide sqrtcontfrac%)
> 
>   ; 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%
>       (super-new)
>       (init-field start) *; here start is defined*
>       (init-field repeat)
>       ; create a n-long list of repeat digits (in reverse order - which
> makes creation of a rational easier)
>       (define/private (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%
>       (init sqrval)
>       (super-new [start (integer-sqrt sqrval)] [repeat (build-repeat
> sqrval)])
>       (inherit rational-rep show)
>       (inherit-field start repeat)
>       ; build the repeating sequence (see
> http://en.wikipedia.org/wiki/Continued_fraction)
>       (define/private (build-repeat n)
>         (let loop ([adder start] [denom (- n (sqr start))] [result '()]) *;
> this is where the error is occurring *
>           (cond [(zero? denom) '()]  ; perfect square
>                 [(= 1 denom) (reverse (cons (+ start adder) result))] ;
> found the end of the repeating sequence
>                 [else
>                  (let* ([nextval (quotient (+ adder (integer-sqrt n))
> denom)] [nextadd (- (* nextval denom) adder)])
>                    (loop nextadd (quotient (- n (sqr nextadd)) denom) (cons
> nextval result)))])))
>       ; create a n-long list of repeat digits (in reverse order - which
> makes creation of a rational easier)
>       (define/private (expand-repeat n)
>         (let lp ([c n] [rl '()] [stock repeat])
>           (if (zero? c) rl (lp (sub1 c) (cons (first stock) rl) (if (empty?
> (rest stock)) repeat (rest stock))))))
>       ))
>   )
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