[r6rs-discuss] [Formal] Requirement to detect circular lists

[Aubrey Jaffer]

 | From: William D Clinger <will at ccs.neu.edu>
 | Date: Thu, 05 Oct 2006 14:02:22 -0400
 | 
 | I am posting this as an individual member of the Scheme community.
 | I am not speaking for the R6RS editors, and this message should not
 | be confused with the editors' eventual formal response.
 | 
 | Andre van Tonder wrote:
 | > Thank you - I think I understand what you are getting at.
 | > One possible problem I see with this implementation, though,
 | > assuming the circular case where an exception must be raised,
 | > is that whatever the relative sizes of the time quanta
 | > assigned to the threads, one can exhibit a "proc" that uses
 | > set-cdr! to grow the circular list in the thread "thread"
 | > faster than the "exception-check" thread can catch up with it.
 | 
 | Right.  I should have said it works only when pairs are not
 | mutable.

It works only when CDRs are not mutable.  Mutable CARs have no effect.

Lumping SET-CAR! with SET-CDR! unnecessarily makes vector-like
operations on lists as expensive as list splicing.