[plt-scheme] Help with #0 and #0# notation in the REPL

The two expressions you write below (the shared one and the #0= one)
are not the same. At the risk of being offensive, that is the whole
point. If the #0= one were the same, there would be no objections.

Robby

On Jan 29, 2008 8:25 PM, Paul Schlie <schlie at comcast.net> wrote:
> I apologize, however:
>
> I can't quite see the benefit of requiring:
>
>  (require scheme/shared)
>
>  (shared ([a (cons 1 a)]) a))
>
>  => #0=(1 . #0#)
>
> vs. simply specifying it directly:
>
>  #0=(1 . #0#)
>
> if write broadly supports cyclic graphs, so too then should read.
>
> as although the following works as expected:
>
>    (define x (read (open-input-string "#0=(1 . #0#)"))) x
>    => #0=(1 . #0#)"
>
> then so should seemingly:
>
>    (eval (read (open-input-string "(define x #0=(1 . #0#))"))) x
>    => datum->syntax: cannot create syntax from cyclic datum:
>       (define x #0=(1 . #0#))
>
> > From: John Clements <clements at brinckerhoff.org>
> > Date: Tue, 29 Jan 2008 17:08:08 -0800
> > To: Paul Schlie <schlie at comcast.net>
> > Cc: PLT Scheme Mailing List <plt-scheme at list.cs.brown.edu>
> > Subject: Re: [plt-scheme] Help with #0 and #0# notation in the REPL
>
> >
> >
> > On Jan 29, 2008, at 4:44 PM, Paul Schlie wrote:
> >
> >> "It only ever worked for quoted cyclic data (and that's what's gone
> >> now)."
> >>
> >> (sarcasm on) by what logic was this determined? personally I've used
> >> cyclic
> >> data to describe many things, inclusive of syntax, semantic, and state
> >> transition information, and can see no obvious benefit to having to
> >> construct such information programmatically, not to mention that
> >> once it
> >> was so constructed, it can't be easily written to a file, and then
> >> correspondingly easily restored via read with out such a
> >> capability); of
> >> course with immutable lists, programmatic construction, isn't the
> >> easiest
> >> either, so might as well eliminate any vestiges of enabling lists as
> >> the
> >> basis of a flexible variable data structure. As after all, scheme
> >> wasn't
> >> ever known to be a powerful friendly flexible programming language
> >> (sarcasms
> >> off).
> >
> > Hang on... to the best of my knowledge, (shared ...) allows you to
> > define circular data without multi-step programmatic construction, and
> > read/write _do_ support writing and reading of cyclic data.  No?
> >
> > BTW, it's hard to understand you with sarcasm on.  I would leave it off.
> >
> > All the best,
> >
> > John
> >
>
>
> _________________________________________________
>   For list-related administrative tasks:
>   http://list.cs.brown.edu/mailman/listinfo/plt-scheme
>