[racket] graph-structured syntax

I think the docs on "#...=" in the referece's reader chapter need to be
fixed to say that he syntax only allowed in `read' mode, and it is not
allowed in `read-syntax' mode.

The docs for `datum->syntax' say that graph structure is disallowed,
and I think the behavior of `eval' below follows from the default eval
handler's use of `datum->syntax'.

Since `read-syntax' and `datum->syntax' don't support graph structure,
there's no way to construct cyclic input to the expander --- or, in
particular, to write a cyclic literal under `quote'. Literals with
cycles were supported at one point, but they were fragile and almost
never useful.

At Mon, 21 Feb 2011 08:18:34 -0800, David Herman wrote:
> Any chance someone has an answer to my question below?
> 
> Thanks,
> Dave
> 
> On Feb 14, 2011, at 4:42 PM, David Herman wrote:
> 
> > One more data point:
> > 
> >> (eval (read (open-input-string "#1=(sin #1#)")))
> >    datum->syntax: cannot create syntax from cyclic datum: #0='(sin #0#)
> > 
> > So that's another clue: it looks like Racket goes to pretty great lengths to 
> prevent the compiler from receiving cyclic AST's.
> > 
> > Anyway, it would be good to know if there's a place in the docs where this is 
> spelled out.
> > 
> > Dave
> > 
> > On Feb 14, 2011, at 4:21 PM, David Herman wrote:
> > 
> >> I've never been fully acquainted with the graph reader, so I did a little 
> REPL-experimenting and doc-hunting. It appears Racket is pretty conservative 
> about where it allows you to use graph syntax:
> >> 
> >>> (define x '#0=(foo . #0#))
> >>   read: #..-expressions not allowed in read-syntax mode
> >> 
> >> I imagine this is because cyclic AST's are Really Really Scary:
> >> 
> >>   #0=(sin #0#))
> >> 
> >> But I can't quite figure out where, if anywhere, graph-structured 
> S-expressions *are* allowed in the Racket syntax. Certainly, you can use them 
> for a programmatic read:
> >> 
> >>> (read (open-input-string "#0=(foo . #0#)"))
> >>   #0=(foo . #0#)
> >> 
> >> But is there no place in the surface syntax where you can ever use the graph 
> syntax? Is the `shared' library the only declarative syntax for creating cyclic 
> data structures?
> >> 
> >> Is this a pretty straightforward restriction that was already done in Common 
> Lisp, or did they allow wild-and-wooly, unrestricted uses of cyclic AST's that 
> (educated guess...) result in undefined behavior by the compiler?
> >> 
> >> Dave
> >> 
> >> PS Happy Valentine's Day!
> >> 
> >> 
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