[racket] recognizing undefined; shared in TR

Your first pointer says:
'THE constant #<undefined> ...'
Does the particle 'The' not imply that letrec always uses the same (eq?)
constant for initialization?

-----Original Message-----
From: users-bounces at racket-lang.org [mailto:users-bounces at racket-lang.org]
On Behalf Of Danny Yoo
Sent: lunes, 05 de septiembre de 2011 21:09
To: Shriram Krishnamurthi
Cc: users at racket-lang.org
Subject: Re: [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)))

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