[r6rs-discuss] [Formal] eliminate library export immutability loophole

From: AndrevanTonder <andre>
Date: Mon Mar 12 21:32:28 2007

On Mon, 12 Mar 2007, Abdulaziz Ghuloum wrote:

> I guess it depends on how you read the following (quoted from 6.1 of
> R5.92RS):
>
> The variable-definition right-hand-side expressions are evaluated from left
> to right, as if in an implicit letrec*, and the body expressions are also
> evaluated from left to right after the variable-definition right-hand-side
> expressions. A fresh location is created for each exported variable and
> initialized to the value of its local counterpart. The effect of returning
> twice to the continuation of the last body expression is unspecified.
>
> The paragraph does not specify when the fresh locations for the exported
> variables
> are initialized. If every exported location is assigned immediately after
> its
> local counterpart is defined, then the values stored in both locations will
> be
> consistent (even when returning twice). This is also true if your
> implementation
> strategy is to skip local definitions altogether and just use one global
> location
> for each library definition.
>
> If you read the above paragraph as saying that all variable's global
> locations are
> initialized together, in one go, after all the local variables are defined,
> then
> you'll get into the inconsistent state and invalid assignment to an immutable
> binding that Kent described.

Closing the loophole as recommended by Kent would erase all observable
differences between these three strategies, so that they would become
equivalent, correct?

If true, that would appear to be another good reason to adopt the
recommendation.

Andre
Received on Mon Mar 12 2007 - 21:31:53 UTC