[mercury-users] Exceptions and unique modes

[Fergus Henderson]

On 22-Feb-2001, Peter Schachte <schachte at cs.mu.OZ.AU> wrote:

> If I understand correctly, the problem is that we need an operational
> semantics that admits di/uo pairs being implemented either as a single data
> structure which is destructively modified or as one which is copied.  Of
> course, in the former case when an exception is raised, the data structure
> has already been modified, so we can't get back the original value, and in
> the latter case we'll naturally have the original value, and no easy access
> to the latest value before the exception.
> 
> So we're looking for a model that will work in both cases.  I expect we're
> agreed that we want the value after the exception to be the last value
> before it, because making it be the original value would essentially render
> the destructive update optimization useless.
> 
> One straightforward way to specify it is for any predicate with a di/uo mode
> to notionally transform any throws it contains to parcel up the last value
> of the di/uo thread(s) in the exception term it throws.  Similarly, it
> should notionally wrap every goal in a catch wrapper that pastes the latest
> value of the thread into the exception term and reraises the exception.  I
> did say "notionally"; of course you wouldn't implement it that way.

So how would you implement it?

In particular, what would you do about code which tries to reuse the
storage for something with a `di' mode, e.g. by explicitly deallocating
it so that the allocator can reuse it to hold something that has a
different type?

-- 
Fergus Henderson <fjh at cs.mu.oz.au>  |  "I have always known that the pursuit
                                    |  of excellence is a lethal habit"
WWW: <http://www.cs.mu.oz.au/~fjh>  |     -- the last words of T. S. Garp.
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