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! > >> > >> > >> _________________________________________________ > >> For list-related administrative tasks: > >> http://lists.racket-lang.org/listinfo/users > > > > > > _________________________________________________ > > For list-related administrative tasks: > > http://lists.racket-lang.org/listinfo/users > > > _________________________________________________ > For list-related administrative tasks: > http://lists.racket-lang.org/listinfo/users _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/users
