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## Re: escaping from a recursive call

 From: Damien Mattei Subject: Re: escaping from a recursive call Date: Wed, 9 Nov 2022 19:49:07 +0100

```good... thank, it works, i do not know a lot about 'prompts , a good
explanation is here:

https://stackoverflow.com/questions/29838344/what-exactly-is-a-continuation-prompt

i personally find a solution too:

(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"))

)))

example:
;; scheme@(guile-user)> (foo 5)
;; \$2 = (5 4 3 2 1 . end0)
;; scheme@(guile-user)> (foo 10)
;; \$3 = (10 9 8 . end7)
;; scheme@(guile-user)> (bar 5)
;; \$4 = (5 4 3 2 1 . end0)
;; scheme@(guile-user)> (bar 10)
;; \$5 = end7

(def (foo n)
(cond ((= n 0) 'end0)
((= n 7) (return 'end7))
(else (cons n (foo {n - 1})))))

(def (bar n)
(cond ((= n 0) 'end0)
((= n 7) (return-rec 'end7))
(else (cons n (bar {n - 1})))))

the important part of this macro being:

((_ (<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>
...)))))))

but i admit i had a chance to find this solution, i did it  a bit like a
blind man...

but after all:

“A mathematician is a blind man in a dark room looking for a black cat
which isn’t there.”

-- Charles Darwin

Best regards,

Damien

On Wed, Nov 9, 2022 at 6:55 PM Olivier Dion <olivier.dion@polymtl.ca> 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.
>
> For example:
> --8<---------------cut here---------------start------------->8---
> (define my-prompt (make-prompt-tag))
>
> (define-syntax-rule (return-now x)
>   (abort-to-prompt my-prompt x))
>
> (define (wrap-this procedure)
>   (let ((inside? #f))
>     (lambda args
>       (if inside?
>           (apply procedure args)
>           (begin
>             (set! inside? #t)
>             (let ((ret
>                    (call-with-prompt my-prompt
>                      (lambda ()
>                        (apply procedure args))
>                      (lambda (_ x)
>                        x))))
>               (set! inside? #f)
>               ret))))))
>
> (define-syntax define-interruptible
>   (syntax-rules ()
>     ((_ (name formals ...) body ...)
>      (define name
>        (wrap-this
>         (lambda (formals ...) body ...))))))
>
> (define-interruptible (foo n)
>   (cond
>    ((= n 0) 'end0)
>    ((= n 7) (return-now 'end7))
>    (else
>     (cons n (foo (1- n))))))
>
> (pk (foo 5))
> (pk (foo 10))
> --8<---------------cut here---------------end--------------->8---
>
> There's probably other way of doing so that I'm not aware of.
>
> --
> Olivier Dion
> oldiob.dev
>

```