Many thanks,
This explanation and the example from Ref. manual 3.9.8 Immutable Cyclic Data
makes everything clearer.
Example:> (let* ([ph (make-placeholder #f)] [x (cons 1 ph)])
(placeholder-set! ph x) (make-reader-graph x))#0= '(1 . #0#)
> Subject: Re: [racket] Reading graph structure
> From: matth...@ccs.neu.edu
> Date: Thu, 18 Aug 2011 10:32:21 -0400
> CC: users@racket-lang.org
> To: harrysp...@hotmail.com
>
>
> On Aug 18, 2011, at 10:21 AM, Harry Spier wrote:
>
> > I've done that and what displays is:
> > #0=(1 . #0#)
> > #0='(1 . #0#)
> >
> > Again I'm not clear why this doesn't try to produce an endlessly recurring
> > (1 . (1 . ( 1 ....... list
>
> At this point you're entering philosophical grounds:
>
> is a cyclic list (ignoring how it is constructed) a good representation for
> the rational, infinite list of all 1s?
>
> As far as my practical programming issues are concerned, I am perfectly happy
> with it.
>
> Here is a snippet from a TicTacToe implementation that uses a cyclic list:
>
> ;; the class of ttt buttons
> (define ttt%
> (shared ((turn (cons "U, Man" (cons "I, Bot" turn))))
> ;; The above creates the 'infinite' list ("U, Man" . ("I, Bot" . ("U,
> Man" . ...)))
> ;; The first person on this list is the player whose turn it is.
> ;; You could create this list by reading the names off some input string.
> (class button%
> (field [status BLANK])
> ;; -> String
> ;; what is its status?
> (define/public (is) status)
> ;; -> Void (effect: this button is taken by the current player)
> (define/public (play)
> (define who (car turn))
> (when (and (send this is-enabled?) (string=? status BLANK))
> (set! status who)
> (send this set-label status)
> (send this enable #f)
> (when (N-in-a-row?)
> (disable-all)
> (send frame set-label (format "game's up: ~a won\n" who)))
> (set! turn (cdr turn))))
> ;; this last line uses the infinite list to switch turns
>
>
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