[m-rev.] for review: copy/2 implementation for il backend

[Fergus Henderson]

On 26-Nov-2002, Peter Ross <pro at missioncriticalit.com> wrote:
> One questioned that needs to be asked is how does this behave compared
> to the implementation of MR_deep_copy on the C backend?
> 
> On the C backend MR_deep_copy only deep copies Mercury values.  If
> you have any references to foreign types then only the reference to
> their location will be copied.  However my implementation below will
> copy the foreign type.  Is this the behaviour we want?
> 
> I think not,

Agreed.

> so how do we avoid it.  We could add a custom attribute to
> every Mercury type indicating that it is a Mercury type and then only
> copy those types which have this attribute.
> 
> This would avoid the problems where the foreign type could have circular
> references.

Another alternative would be to have a DeepCopyable base class, and
make all Mercury-generated classes inherit from that class (rather than
System.Object).  This could provide a deep_copy() method.  The default
implementation would use code along the lines that you posted.

The code for Mercury's copy/1 procedure would then just do a dynamic
cast to DeepCopyable, and if that succeeded call the deep_copy() method,
otherwise throw an exception.

The advantage of this approach is that it would make it possible to
implement foreign types that can be deep-copied by Mercury's copy/1.
This might be useful for implementing types in the Mercury standard
library, or for users who are writing code that interfaces Mercury
and other languages.

-- 
Fergus Henderson <fjh at cs.mu.oz.au>  |  "I have always known that the pursuit
The University of Melbourne         |  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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