[Scheme-reports] Nasal demons in R7RS-small

[John Cowan <cowan@x>]

On the scheme-implementors list, Jonathan Shapiro raised the issue
of undefined behavior in the standard, which he feels is a Bad Thing
in a language that purports to have a formal model.  In general, I'm
inclined to agree, so I looked through the current draft for references to
"unspecified" and "undefined" (some of which I changed to "unspecified").
I ignored references to returning an unspecified value and to doing
things in an unspecified order, since these do not induce wide-open
semantic failure in which anything could happen.  Here's what I've found
(and marked in the source with \todo comments):

1) In delay and force, the effect of the expression returning other than
one value is unspecified.

2) Altering a top-level binding not introduced by a definition has an
unspecified effect on the behavior of built-in procedures (which in
practice amounts to an unspecified effect, period).

3) The effect of passing other than one value to a continuation (modulo
the defined cases) is unspecified.

4) The effect of using a captured continuation to enter or exit the
"before" and "after" thunks of dynamic-wind is unspecified.

5) The effecdt of using eval to assign a variable bound in the
scheme-report-environment is unspecified.

6) The effect of opening for output a file that already exists is
unspecified.

There is a ballot item to standardize #6 to truncate the file, but this
may not fly, as several existing implementations throw an error instead.
Nevertheless, it does not seem to me that all-out undefined behavior is
justified here: some language like "an error may be signalled, or the
implementation may perform an unspecified action on the file (such as
truncating it) and then open it" seems to me to be sufficient.

Does anyone seem more cases of unspecified effects?  What are suitable
low-impact ways to remove them?

-- 
Babies are born as a result of the              John Cowan
mating between men and women, and most          http://www.ccil.org/~cowan
men and women enjoy mating.                     cowan@x
    --Isaac Asimov in Earth: Our Crowded Spaceship

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