Re: Prologs with Occurs Check constantly on?

On 2011-10-12, Jan Burse <janburse@xxxxxxxxxxx> wrote:
Jan Wielemaker schrieb:
As Ulrich pointed out, the world of unification with occurs-check
combines poorly with the world of cyclic terms. This was the main
conclusion from a debate whether A=f(A), unify_with_occurs_check(A,_)
should succeed or fail.

Yes, remember something.

That requires much than simply emitting different
unification instructions.

Agreed!

In a first round, the burden would be on the
end-user to guarantee that his mix and match
does work.

That smells a lot as trying to mix type-checked and non-type-checked
code as Tom did in his demo type-checker. Complicated stuff and I'd
rather do something with typing :-)

The problem is that when you have the occurs
check off and no cyclic terms, your Prolog
system is anyway not safe in the first place.

I probably don't need to give you examples,
but here is one:

?- X=f(X), Y=f(Y), X=Y.

I guess it will hang in 75% of the Prolog
systems, or is it 95%?

I may hope not. On this example, hanging or crashing systems are ISO
compliant, but I sincerely hope we left this era behind us.

Cheers --- Jan
.

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