Prolog standardization forums

Hi!

Following some private emails between me and Bart, we would like your feedback on constraining the first operand of the is/2 standard predicate to be either a variable or a number, throwing an exception on all other cases.

In the current core Prolog standard, a goal such as:

?- a is 3.

Simply fails. If you agree that an exception should be generated instead, which exception term do you think should be used? If you prefer the current behavior, can you tell us in what circumstances is it useful?

All the best,

Paulo

I agree that is/2 should throw an exception on that cases.

I argument my opinion with the following:

?- a is a.
ERROR: is/2: Arithmetic: `a/0' is not a function

So, since is/2 expect a number or a function (whose result is a number) as its second argument, it has to expect a number as a result of the arithmetic unification.
Indeed, a call like "?- a is 3" should simply be a non-sense goal.

On the other hand, consider the following:

?- pi is pi.
No

Perhaps one should want such a goal to succeed.

So, since I cannot find a situation in which silent failure could be useful (as in the last case), and since one cannot speak rigorously of a domain error (see, "pi is pi"), I propose a simple type error.

Cheers,