Formal comment #42 (defect)

Requirement to detect circular lists
Reported by:	Andre van Tonder

Component:	miscellaneous
Version:	5.91

Pages : 123-125 

Summary

Requirement of cycle detection in list procedures may be too onerous,
potential performance problem.

Description

The draft requires procedures such as length, and various others, to
detect and raise an exception if passed a circular list.

This may adversely affect performance globally to accommodate an
uncommon bug, in addition to complicating the implementation of these
procedures rather significantly. Suggestion

Allow these procedures to not return when passed a circular list.

RESPONSE:

The next draft of the report will address this issue in a manner
consistent with our resolution of formal comment #87.  In particular,
we may decide to relax the requirements for detecting circular
lists.

It is interesting to note, however, that
the performance implications of detecting circular lists are actually
negligible in many performance-oriented implementations.  See the
following posting for some benchmark data:

http://lists.r6rs.org/pipermail/r6rs-discuss/2006-October/000274.html