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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