Formal comment #208 (defect)

eliminate library export immutability loophole
Reported by: 	Kent Dybvig
Version: 	5.92

Description:

The present description of libraries leaves open the possibility that
a continuation grabbed within a variable definition's right-hand-side
expression and subsequently invoked might cause the reinitialization,
with different values, of some or all of the local counterparts of the
exported variables, effecting implicit assignments to variables that
are supposed to immutable. Since the effect of returning twice to the
continuation of the last body expression is unspecified, this may also
result in implicit assignments to the exported variables. If not, the
values of the exported variables and their local counterparts may
differ.

While explicit assignments to immutable variables are syntax
violations, no exception is presently associated with this implicit
form of assignment.

Proposal:

Add new wording to the effect that the continuation of the
right-hand-side expression of a library variable definition should not
be returned to more than once and that implementations either "should"
or "must" raise an exception if this occurs.


RESPONSE:

The next draft will say that implementations should raise an exception if
the continuation of a right-hand side of any variable definition, letrec,
or letrec* is invoked more than once.