Re: Fix division by zero to throw exception (round 2)

Hey Andrea, > Am 29.06.2015 um 18:49 schrieb Andrea Faulds <ajf@ajf.me>: > > Hi Bob, > >> On 29 Jun 2015, at 16:54, Bob Weinand <bobwei9@hotmail.com> wrote: >> >> I would like to bring this topic back up, as there were users confused with it and >> it's absolutely not consistent what we have now. >> See also https://bugs.php.net/bug.php?id=69957 (As I >> thought it was non-intentional, I went ahead and "fixed" it, was reverted later, hence >> discussing that now here.) >> >> So, looks like there was some quick decisions and discussion I totally had missed. >> What we have now is: >> >>> Am 03.04.2015 um 23:13 schrieb Dmitry Stogov <dmitry@zend.com>: >>> >>> So the summary: >>> >>> 1) division by zero produces a warning and +/-INF IS_DOUBLE. Compile-time >>> evaluation is disabled. >>> >>> 3) Modulo by zero produces Exception.Compile-time evaluation is disabled. >> >> Why? Why do we change the one but not the other? >> >> Why does 0 % 0 throw an Exception, but 0 / 0 NAN? >> Why does 1 % 0 throw an Exception, but 1 / 0 INF? >> >> I'd like to either properly return 0, INF or NAN in both cases or in none. >> >> Having different rules for so similar operations is non-sense, I think. It just is >> inconsistent and causes confusion. > > Those operations are not as similar as you think! > > In PHP, the complement of % isn’t /, it’s intdiv(). intdiv() takes an integer divided and > divisor, and produces an integer quotient. % also takes an integer dividend and divisor, but > produces an integer remainder. intdiv() and % work with integer-only inputs and always produce > integer outputs. > > /, on the other hand, takes an integer or float dividend and divisor, and produces an integer > or float fractional result. It might as well work only on floats and always produce floats, really, > given that it behaves the same as if it did. > > So intdiv() and % work on integers, and / works on integers and floats. This informs how they > handle certain error cases. > > For integer division and modulo (PHP’s intdiv() and %), the accepted way to handle division > by zero is to produce an error. There’s no correct or useful result you can produce. In PHP, we > used to produce FALSE here, which is a different type (boolean). I don’t think that was a good > idea because of PHP’s weak typing. If you use FALSE in some other arithmetic operation, it’ll be > coerced to zero, and produce weird results from other operations. Sure, there’s an E_WARNING > produced, but your code keeps running and produces garbage. So, throwing an exception is safer and > brings PHP into line with established practice in other programming languages. > > For floating-point division (PHP’s /), on the other hand, errors are usually handled > differently. IEEE 754 defines special error values we can produce: +Infinity, -Infinity and NaN. > These are still values of the float type, but they’re not normal numbers. They flow through > floating-point operations in a well-defined manner. For example, if you do anything with a NaN and a > NaN, you get a NaN, while if you divide by zero, you get ±Infinity, and if you divide by Infinity, > you get ±0. Again, PHP used to produce FALSE here, which has the problems described earlier. > ±Infinity and NaN, on the other hand, flow properly through later arithmetic operations. If the > error affects your result, it will most likely be obvious, because it’ll be ±0, ±Infinity, or > NaN. Yes, you result is garbage, but it’s at least obviously so. And, like with the integer > behaviour, this brings PHP into line with established practice. > > tl;dr: intdiv() and % have matching behaviour because they work with integers and that’s > what’s usually does for them, / has different behaviour because it works with floats and that’s > what’s done for them. > > I hope that makes sense. > > -- > Andrea Faulds > http://ajf.me/ > <http://ajf.me/> Yes, it generally makes sense... Then I have other questions: - Why do we then still have a Warning? Either we have well-defined behavior, or we throw an exception. Well-defined behavior *plus* a warning is IMO non-sense. - Is it intentional for intdiv and % to throw an Exception instead of Error or some more specific DivisionByZeroError or similar? (yes, I know, Error is only very recent, but the question still needs to be asked). Bob.