[m-dev.] for review: add promise_only_solution/1 to std_util.m

[Fergus Henderson]

On 06-Nov-1998, Peter Schachte <pets at cs.mu.OZ.AU> wrote:
> Now that I think about it, there really should be wrappers for all of
> these purposes:
> 
> 	to promise there is at most one solution,
> 	to promise there is at least one solution, and

We already have a perfectly good way of promising at least one solution:

	(if p(...) then ... else error("p failed")).

This mechanism is better than promise_only_solution in
a number of ways: it can be checked at runtime
(whereas promise_only_solution can't be checked at all,
except with a theorem prover...),
and it's more polymorphic.

> 	to promise there is exactly one solution,

For that case, you can just combine the two.

> > (4)
> > Even if we did, the set of modes still wouldn't be complete;
> > e.g. we should add `uo' modes, etc.
> 
> Oh, yeah, those should be there, too.  That'll double the number of
> modes.

Next add in the

	out(unique('-'(ground, unique)))

mode, for the case where the predicate returns a pair of items,
only one of which is unique...
As you can see there are quite a few possibilities.

-- 
Fergus Henderson <fjh at cs.mu.oz.au>  |  "I have always known that the pursuit
WWW: <http://www.cs.mu.oz.au/~fjh>  |  of excellence is a lethal habit"
PGP: finger fjh at 128.250.37.3        |     -- the last words of T. S. Garp.