This is Just Wrong, IMO. How confusing is it going to be to find that calling is_prime($x) modifies the value of $x despite it being a very simple test operation which appears to have no side effects?
As far as I can see it, in the example, it's perfectly logical for is_prime($x), is_even($x) and $x > 2 to all be true, because an any() junction was used. If an all() junction was used it would be quite a different matter of course, but I would see is_prime() called on an any() junction as returning true the moment it finds a value inside that junction which is prime. It doesn't need to change $x at all.
In a way, you're sort of asking 'has $x got something that has the characteristics of a prime number?' and of course, $x has - several of them, in fact (but the count is not important).
Soemtimes, although frequently I will check for preconditions at the begining of a function. After I finished checking for them, I expect to be able to do stuff assuming them without anyworry of exceptions or anything else. In these cases I am using conditionals to filter input, which I imagine is a fairly common case...
Yes, it is fairly common, but I don't think it's common enough to attach unexpected side-effects to innocent-seeming functions. If I want to modify a junction to contain only values which satisfy a given precondition, I'll be wanting to use something which states that explicitly.
Which reminds me that I'm not very aware of anything which can decompose a junction once it's been created, which would be fairly necessary for doing that sort of thing. If you can turn junctions into lists then precondition filtering isn't bad at all. Something like
my $filtered = any($junction.list().grep({ satisfies_precondition }));Of course, I just invented that out of nothing so I may be way off base. I haven't found anything in any Apocalypse or Synopsis which says if you can do things like this, but if Perl itself can pick junctions apart, we should be able to as well.
My approach to this comes somewhat from my basis in liking Haskell a lot and thus wanting to keep unusual side-effects to a minimum. However, if junctions collapse and modify themselves as was in your example, what happens when you don't want to have them collapse? Do you have to explicitly make copies?
