<html> <head> <style></style> </head> <body class='hmmessage'><div dir='ltr'> Many thanks,   This explanation and the example from Ref. manual 3.9.8 Immutable Cyclic Data makes everything clearer. Example: <table cellspacing="0"><tbody><tr><td><table class="RktBlk" cellspacing="0"><tbody><tr><td>> (<a href="http://docs.racket-lang.org/reference/let.html#%28form._%28%28lib._racket/private/letstx-scheme..rkt%29._let*%29%29" class="RktStxLink">let*</a> ([ph (<a href="http://docs.racket-lang.org/reference/pairs.html#%28def._%28%28quote._%7E23%7E25kernel%29._make-placeholder%29%29" class="RktValLink">make-placeholder</a> #f)]</td></tr><tr><td>         [x (<a href="http://docs.racket-lang.org/reference/pairs.html#%28def._%28%28quote._%7E23%7E25kernel%29._cons%29%29" class="RktValLink">cons</a> 1 ph)])</td></tr><tr><td>    (<a href="http://docs.racket-lang.org/reference/pairs.html#%28def._%28%28quote._%7E23%7E25kernel%29._placeholder-set%21%29%29" class="RktValLink">placeholder-set!</a> ph x)</td></tr><tr><td>    (<a href="http://docs.racket-lang.org/reference/pairs.html#%28def._%28%28quote._%7E23%7E25kernel%29._make-reader-graph%29%29" class="RktValLink">make-reader-graph</a> x))</td></tr></tbody></table></td></tr><tr><td>#0= '(1 . #0#) </td></tr></tbody></table> <div>> Subject: Re: [racket] Reading graph structure > From: matthias@ccs.neu.edu > Date: Thu, 18 Aug 2011 10:32:21 -0400 > CC: users@racket-lang.org > To: harryspier@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 > > </div> </div></body> </html>