[mercury-users] tuple syntax

[David Glen JEFFERY]

On 10-Nov-1999, Robert Ernst Johann JESCHOFNIK <rejj at cat.cs.mu.OZ.AU> wrote:
> > Hmmm.  Here's an idea for a better tuple syntax.  Is there something
> > wrong with this?  If not, it might be a nice easy addition to the
> > standard library somewhere.
> > 
> > 	 :- type (A,B) ---> (A,B).
> 
> 
> I asked DJ this exact question a few weeks ago. I cannot remember exactly
> what he said, but I remember that there was a reason pair was defined like
> it is. I think it may have been a precedence problem. DJ?

I think my comment was that you *could* do that, and it would work, but the
representation the compiler would use for n-tuples when n gets big wouldn't
be terribly efficient.

eg. a five-tuple: (1,2,3,4,5) wouldn't be represented by five cells on the
heap; each instance of the ','/2 functor effectively takes an extra cell,
giving a total of nine (probably).

It would (in many cases) be much more efficient to represent five tuples by:

:- type fivetuple(A, B, C, D, E) --->	fivetuple(A, B, C, D, E).

In this case you would avoid the overhead of all the pointers between the
','/2 blocks... the representation of a fivetuple/2 would actually fit into
five cells.

Anyhow, this may not be an important consideration for many applications, and
we could probably do a good job of optimising the representation if we tried.

HTH.


dgj
-- 
David Jeffery (dgj at cs.mu.oz.au) | If your thesis is utterly vacuous
PhD student,                    | Use first-order predicate calculus.
Dept. of Comp. Sci. & Soft. Eng.|     With sufficient formality
The University of Melbourne     |     The sheerist banality
Australia                       | Will be hailed by the critics: "Miraculous!"
                                |     -- Anon.
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