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
