[racket] illegal placeholder cycle

From: Matthew Flatt (mflatt at cs.utah.edu)
Date: Sat Sep 10 17:16:31 EDT 2011

At Tue, 6 Sep 2011 16:21:57 -0400, Doug Orleans wrote:
> This error message is surprising:
> > (shared ((c (append null c))) c)
> make-reader-graph: illegal placeholder cycle in value: '(#<placeholder>)
> Is it unreasonable to expect this to evaluate to null?

I don't see how to get `null', although I could see an argument for
#<undefined>: since c = (append null c) for any c, the above seems the
same as

 (shared ([c c]) c)

to me.

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