Communicating between in and out contracts

[Steven Schveighoffer]

On Sun, 18 Oct 2009 03:44:39 -0400, Rainer Deyke <rainerd at eldwood.com>  
wrote:

> Andrei Alexandrescu wrote:
>> Rainer Deyke wrote:
>> The expression may mutate stuff.
>
> It shouldn't.  It's an error if it does, just like it's an error for an
> assertion or post/precondition to have any side effects.
>
> It would be nice if the compiler could catch this error, but failing
> that, 'old' expressions are still no worse than assertions in this  
> respect.

I'm coming into this a little late, but what Rainer is saying makes sense  
to me.

Would it help to force any access to the object to be treated as if the  
object is const?  i.e.:

old(this.x)

would be interpreted as:

(cast(const(typeof(this))this).x

and cached in the input contract section.

It seems straightforward that Rainer's solution eliminates the boilerplate  
code of caching values available in the in contract, and if you force  
const access, prevents calling functions which might mutate the state of  
the object.  But it uses the correct contract -- this object is not  
mutable for this call only.  I agree pure is too restrictive, because then  
the object must be immutable, no?

Incidentally, shouldn't all access to the object in the in contract be  
const by default anyways?

-Steve