[mercury-users] functions vs predicates (was Re: State variable problem

[Ralph Becket]

Peter Schachte, Tuesday, 15 August 2006:
> On Mon, Aug 14, 2006 at 06:38:28PM +1000, Mark Brown wrote:
> > > Do they have problems
> > > other than with negation?
> > 
> > I think the real problem is that `X = X' doesn't hold, but people would
> > naturally expect it to.
> 
> I'm not so sure what people's natural expectation is in this case.  If
> our domain is the natural numbers, would you expect 0-1 = 1-2?  I
> guess I wouldn't be surprised either way.  How about 1/0 = 2/0?

Neither of those matches X = X.
I'm pretty sure that extending the axiom "X = X" to "X = X, except if
evaluating X throws an exception" would not muddy the waters too much.

-- Ralph
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