[racket] recognizing undefined; shared in TR

At Mon, 5 Sep 2011 15:08:44 -0400, Danny Yoo wrote:
> On Mon, Sep 5, 2011 at 2:18 PM, Shriram Krishnamurthi <sk at cs.brown.edu> wrote:
> > On Sat, Sep 3, 2011 at 11:17 PM, Shriram Krishnamurthi <sk at cs.brown.edu> 
> wrote:
> >> Is there a way of checking whether a value position is undefined?
> >> Specifically, I'm trying to recognize the undefined value put in
> >> structures by SHARED.  It appears that a undefined value is sometimes
> >> EQ? to another one, but this doesn't seem to be specified in the
> >> documentation (ie, that there is only one undefined value).
> >
> > Anyone have any thoughts on this?  Is it okay to use EQ?
> 
> According to:
> 
>     http://docs.racket-lang.org/reference/void.html
> 
> we know the value of undefined is a constant, but there's no guarantee
> that they're the same value.

The void and undefined constants are separate, but there really is only
one of each at the `eq?' level.