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Re: escaping from a recursive call

From: Damien Mattei
Subject: Re: escaping from a recursive call
Date: Thu, 10 Nov 2022 14:48:59 +0100 
solutions have been posted, i'm using mine, which update my 'def macro:

;; (def (bar n);;      (cond ((= n 0) 'end0);;     ((= n 7)
(return-rec 'end7));;      (else (cons n (bar {n - 1})))))
;; scheme@(guile-user)> (bar 5);; $4 = (5 4 3 2 1 . end0);;
scheme@(guile-user)> (bar 10);; $5 = end7
(define-syntax def

  (lambda (stx)

      (syntax-case stx ()

        ;; multiple definitions without values assigned
        ;; (def (x y z))
        ((_ (var1 ...)) #`(begin (define var1 '()) ...))
        
        ;;  (def (foo) (when #t (return "hello") "bye"))
        ;; ((_ (<name> <arg> ...) <body> <body>* ...)
        ;;  (let ((ret-id (datum->syntax stx 'return)))
        ;;    #`(define (<name> <arg> ...)
        ;;        (call/cc (lambda (#,ret-id) <body> <body>* ...)))))

        
        ((_ (<name> <arg> ...) <body> <body>* ...)
        
         (let ((ret-id (datum->syntax stx 'return))
               (ret-rec-id (datum->syntax stx 'return-rec)))

           #`(define (<name> <arg> ...)

               (call/cc (lambda (#,ret-rec-id)
                        
                         (apply (rec <name> (lambda (<arg> ...)
                                              (call/cc (lambda (#,ret-id) 
<body> <body>* ...)))) (list
<arg> ...)))))))

        

        
        ;; single definition without a value assigned
        ;; (def x)
        ((_ var) #`(define var '()))

        ;; (def x 7)
        ((_ var expr) #`(define var expr))

        ((_ err ...) #`(syntax-error "Bad def form"))

        )))


i have tested it with my code and it works well,for now.
Also i can not notice any slowing of the code and that is important ad 'def
is a replacement often used in my projects now.
I will update my Scheme+ with it soon, when i have time.
Regards,
Damien


On Thu, Nov 10, 2022 at 1:33 PM Chris Vine <vine.chris@gmail.com> wrote:

> On Wed, 09 Nov 2022 12:55:42 -0500
> Olivier Dion via General Guile related discussions <guile-user@gnu.org>
> wrote:
> > On Wed, 09 Nov 2022, Damien Mattei <damien.mattei@gmail.com> wrote:
> > > but in the general case  , i want a macro that can do it on any
> function
> > > (i'm not sure it can be done because the continuation have to be
> captured
> > > just before the call to the function and be inlined at the good
> > > place....)
> >
> > I'm not aware of any control mechanism that are implicit in Guile.  You
> > almost always have to deal with a continuation object.  However, nothing
> > prevent you to invent your own control flow wrapper.
>
> You can construct an anaphoric macro with that in mind.  This introduces
> an imperative-style 'loop' macro which carries within the loop block a
> 'break' keyword which will cause the loop to exit:
>
>   (use-modules (ice-9 control)) ;; for call/ec
>
>   (define-syntax loop
>     (lambda (x)
>       (syntax-case x ()
>         [(k e ...)
>          (with-syntax ([break (datum->syntax #'k 'break)])
>            #'(call/ec
>               (lambda (break)
>                 (let f () e ... (f)))))])))
>
>   (display (let ([n 3] [lst '()])
>              (loop
>               (if (= n 0) (break lst))
>               (set! lst (cons 'a lst))
>               (set! n (- n 1)))))
>   (newline)
>
> However explicit control of loops is better in my view.  Imperative
> loops usually end up with mutable bindings, as in the example above.
>
>
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