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

From: Robby Findler (robby at cs.uchicago.edu)
Date: Tue Jan 29 20:59:32 EST 2008

Thanks for your comments, Paul. I think maybe you missed two points:

 a)  The `read' primitive still reads cyclic data just fine.
 b)  This is an experiment.

And, the unstated (and I apologize for that):

 c) the r6rs mode (like an r5rs mode) will support all of r6 (r5), and
thus will have support for all of the stuff you want.

Best regards and thank you for your comments. As always, your humble servant,
Robert B. Findler

On Jan 29, 2008 6:44 PM, Paul Schlie <schlie at comcast.net> 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).
> which is why although I've dabbled with 399, I've had to stick with 372 for
> my purposes.
> > Robby Findler wrote:
> >> On Jan 29, 2008 5:22 AM, Jens Axel Soegaard <jensaxel at soegaard.net> wrote:
> >> Robby Findler wrote:
> >>
> >>  >> Why was this change made?
> >>  >
> >>  > It simplifies the compiler and tools that operate on program source if
> >>  > there are no cycles in what they have to process (it isn't a normal
> >>  > thing to think about that a program's AST is cyclic!) and so we
> >>  > decided to experiment with leaving it out.
> >>
> >> Besides syntactically recursive programs doesn't give you more
> >> expressive power. What I am talking about?
> >>
> >> <http://pagesperso-systeme.lip6.fr/Christian.Queinnec/Papers/synrec.ps.gz>
> >>
> >> [Yes, slightly off-topic but close enough ;-) ]
> >
> > FWIW, mzscheme never worked for such programs. It would fail to
> > terminate while compiling them. It only ever worked for quoted cyclic
> > data (and that's what's gone now).
> >
> > Robby
> _________________________________________________
>   For list-related administrative tasks:
>   http://list.cs.brown.edu/mailman/listinfo/plt-scheme

Posted on the users mailing list.