[racket] recognizing undefined; shared in TR

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.


However, I'm certain that undefined is a singleton value, from the
implementation details in:

    http://docs.racket-lang.org/inside/im_values_types.html#(idx._(gentag._143._(lib._scribblings/inside/inside..scrbl)))