Formal comment #118 (defect)

The value of set! et al. is overspecified
Reported by:	Alan Watson

Component:	baselibrary
Version:	5.91

The authors of the draft discussed two different behaviours for when
an unexpected number of values are passed to a continuation: coercing
to the expected number of values and signalling an error. Both
behaviours have some validity and there was no concensus between the
editors as to which was to be prefered. The editors therefore decided
to mandate neither, but rather left this area unspecified.

This issue has some connection to the behaviour of forms such as set! 
which do not have an "obvious" value. On a system that silently
coerces an unexpected number of values to the expected number, such
forms can be naturally and safely defined to return no values.

However, the current draft mandates returning a single unspecified
value. This will make it more difficult for a implementations of R6RS
and future standards to adopt, if it were desired, coercing behaviour
for multiple values and its natural consequences.

There is no need for R6RS to take such a firm position on this
issue. Instead of specifiying that set! et al. "return the unspecified
value", it might be wiser to require that they:

Return an unspecified number of unspecified values. However, they can
always be used with continuations that accept either exactly one or
any number of values.

This would allow, for example, defining them to return zero values on
systems that adopted coercing behaviour for an unexpected number of
values but would not prejudice systems with other behaviours.

It would mean, however, that one could not portably use set! et
al. with continuations that expect a fixed number of values (although
one might be able to use this in non-portable code that is specific to
certain classes of implementation). I consider this to be no great
loss.

RESPONSE:

As no clear consensus on this issue has emerged, the next draft of the
report will retain the status quo.  However, it is still under active
discussion: The editors welcome additional suggestions and commentary.