[m-dev.] Question about WAM style variables in Mercury

[Fergus Henderson]

On 05-Feb-2004, Ralph Becket <rafe at cs.mu.OZ.AU> wrote:
> Fergus Henderson, Thursday,  5 February 2004:
> > On 05-Feb-2004, Ralph Becket <rafe at cs.mu.OZ.AU> wrote:
> > > For adding Herbrand types to the language, it would be good if WAM style
> > > variables didn't require heap allocation, but instead just formed direct
> > > pointer chains.
> > > 
> > > However, this would mean having pointers to the interior of cells rather
> > > than the start of a cell.
> > 
> > How so?  Could you explain in more detail?
> 
> Consider this Mercury snippet:
> 
> 	X = f(A, B),	% Construct f/2, A & B are unbound Herbrand vars.
> 	Y = g(C, D),	% Construct g/2, C & D are unbound Herbrand vars.
> 	A = D,		% Alias A and D.
> 	B = C		% Alias B and C.
> 
> The C code for the constructions using non-heap allocated WAM variables
> could be something like
> 
> 	X = <<construct f/2 cell>>;
> 
> 	A = &X[0];	% ) Initialize A.
> 	*A = NULL;	% )
> 
> 	B = &X[1];	% ) Initialize B.
> 	*B = NULL;	% )

Oh, I see what you mean.

> However, the whole thing already looks a tad complicated and, given that
> what you've said about GC issues is as I suspected, I'll just use heap
> allocated WAM variables.

I think that is a good plan, at least for the first cut.

-- 
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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