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Re: shift and reset in ice-9 control

From: Wolfgang J Moeller
Subject: Re: shift and reset in ice-9 control
Date: Mon, 21 Mar 2011 02:31:01 +0100 (CET)

Hello again,

On Thu, 17 Mar 2011, Andy Wingo wrote:
> On Wed 09 Mar 2011 11:53, Wolfgang J Moeller <address@hidden> writes:
> > (define-syntax reset
> >   (syntax-rules ()
> >     ((_ . body)
> >      (call-with-prompt 'Reset
> >                    (lambda () . body)
> >                    (lambda (cont f) (f cont))))))
> > ;;
> > (define-syntax shift
> >   (syntax-rules ()
> >     ((_ var . body)
> >      (abort-to-prompt 'Reset
> >                   (lambda (cont)
> >                     ((lambda (var) (reset . body))
> >                      (lambda vals (reset (apply cont vals)))))))))
> Did you write these yourself?  May we include them in (ice-9 control)?

Yes and yes.

However, I don't like it anymore, since I seem to have learned
that this "direct implementation" doesn't at all play well
with the use of call/cc within the <body>s, while the call/cc-based
implementations of shift/reset appear to do so.

I've yet to find out why the latter work "better", and if there's
a remedy possible using something like with-continuation-barrier ...

> If you have additional useful tests, we are happy to include them as
> well.  Probably the easiest thing would be to mail them to the list with
> a permissive copyright header.

The tests that I had included come from http://okmij.org/ftp/,
as does almost all I know about "delimited continations"
(I first learned about them only a few weeks before you
released GUILE V2). There are a few more tests at that site.

Best regards,

Wolfgang J. Moeller, Tel. +49 551 47361, wjm<AT>heenes.com
37085 Goettingen, Germany | Disclaimer: No claim intended!
http://www.wjmoeller.de/ -+-------- http://www.heenes.com/

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