Thanks, fun kata!

Cool, I just published a quick fork :)

Hmm, good point. I didn't expect people to hack into it this hard though lol!

I'll write something up when I've invalidated these kinds of solutions, dynamically generating predicates seems like a good idea.

Alright, I've changed the description to clarify what kind of predicates should work with iss/2, and added cases for testing expressions with other non-arithmetic predicates! Hopefully this makes the kata a bit clearer.

That's a fair point, I'll try adding new custom tests.

The only thing is I'm afraid of solutions passing right getting invalidated by some reasonable new sample usages though (I know X iss length([1,2,3,4,5]) invalidates all solutions right now).

I'm not sure what to do about that - if I should limit tests to cases that work for current solutions or not. Maybe it is better with these to make it more robust?

EDIT : Maybe I can keep the initial constraint that user defined arithmetic predicates take only numbers as inputs (like is/2) and it'd be fine for this kata

Nice catch! I'll fix that right now, and re-read my desc just in case.

Sound's reasonable, I'll get to fixing that.

I wasn't sure, but I think you're right that the main interesting concept here is writing iss/2.
I looked at the tests in a memoized, efficient fibonacci kata, and it seems the upper bound didn't need to be greater than 100.
So maybe lowering it to 100 aswell would be fine, even.

It makes more sense to not have users deal with negative arguments, since I didn't myself! Thanks for catching that.

Cool solution, thanks for the translation. Fun kata.

Alright, done. Thanks for letting me know.

A person of principles (+1 for not using eval ^^)

Hey! It's very late in my timezone right now, but I'll try to join the CW chat tomorrow and contact you there.

PROLOG translation

PROLOG translation

PROLOG translation