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

From: Andre van Tonder <andre>
Date: Tue Mar 20 09:58:15 2007

On Tue, 20 Mar 2007, Abdulaziz Ghuloum wrote:

> On Mar 19, 2007, at 11:42 AM, AndrevanTonder wrote:
>>
>> Doesn't the example violate the LETREC* restriction, though?
>>
>> One restriction on letrec* is very important: it must be possible to
>> evaluate
>> each <init> without assigning or referring to the value the
>> corresponding
>> <variable> or the <variable> of any of the bindings that follow it in
>> <bindings>.
>>
>> In particular, the second time the RHS of the (define x ....) binding is
>> evaluated, you are actually referring (via the (cadr x) in set-y!) to the
>> value of x itself. So the example may already be illegal.
>
> The program does not violate the letrec* restriction since in the second
> time,
> the value of x has already been determined (the first time around). The
> program
> never refers or assigns to any uninitialized bindings.

That is not what the letrec* restriction says. The letrec* restriction quoted
above says nothing about whether the binding is uninitialized. The <undefined>
implementation strategy is mentioned only later, where it is called
approximate.

Andre
Received on Tue Mar 20 2007 - 09:57:38 UTC