Thanks for demonstrating the use of continuations - it's quite cool and I need to grok continuation more.  Thanks! <div class="gmail_quote">On Sun, Mar 2, 2008 at 2:01 AM, Chongkai Zhu <<a href="mailto:czhu@cs.utah.edu">czhu@cs.utah.edu</a>> wrote: <blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">Or if you don't want the explicit "break/return" in the macro: (define-syntax (while stx)  (syntax-case stx ()    ((while cond body ...)     #`(let/ec #,(datum->syntax-object #'while 'return) <div class="Ih2E3d">         (let lp ()           (when cond             body ...             (lp))))))) </div>  Happy Texas Independence Day! Chongkai <div><div></div><div class="Wj3C7c"> Chongkai Zhu wrote: > First, you are in a functional programming language. So please stop > thinking procedurely. > > Most people will use first class continuation to implement break control: > > (define-syntax while >  (syntax-rules () >    ((_ break cond body ...) >     (let/ec break >       (let lp () >         (when cond >           body ... >           (lp))))))) > > (let ((i 0)) >  (while return >         true >         (display i) (newline) >         (if ( >             (set! i (+ i 1)) >             (return i)))) > > > > > YC wrote: >> Hi all - >> >> what's the way to implement a return (or break) control from a >> infinite while loop (i.e. (while true ...))? >> >> A while loop with a test condition is straight forward, but I cannot >> figure out how to return from an infinite loop. My thought is to >> introduce a hidden conditional, and then having the inner return >> macro to set the condition to be true, but having the two macros >> being separate basically means the return macro's hidden variable is >> different from the hidden variable in while loop, so obviously my >> approach is incorrect. >> >> Below is what I have.  Thanks in advance for suggestions, >> yc >> >> (define-syntax (return stx) >>   (syntax-case stx () >>     ((_ exp) >>      #'(begin >>          (set! __ret__ #t) >>          exp)))) >> >> (define-syntax (while stx) >>   (syntax-case stx (true) >>     ((_ test exp ...) >>      #' >>          (let loop ((__ret__ #f)) >>          (cond (test exp ... (loop __ret__))))) >>     ((_ true exp ...) ;;; this is *incorrect*... >>      #'(_ (= __ret__ #f) exp ...)))) >> >> (let ((i 0)) >>     (while true >>            (display i) (newline) >>            (if ( >>                (set! i (+ i 1)) >>                (return i)))) >> ;; => set!: cannot set undefined identifier: __ret__ >> >> >> >> >> ------------------------------------------------------------------------ >> >> _________________________________________________ >>   For list-related administrative tasks: >>   <a href="http://list.cs.brown.edu/mailman/listinfo/plt-scheme" target="_blank">http://list.cs.brown.edu/mailman/listinfo/plt-scheme</a> > _________________________________________________ >  For list-related administrative tasks: >  <a href="http://list.cs.brown.edu/mailman/listinfo/plt-scheme" target="_blank">http://list.cs.brown.edu/mailman/listinfo/plt-scheme</a> </div></div></blockquote></div>