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Date: Mon, 29 Jun 2015 17:49:01 +0100
Cc: Dmitry Stogov <dmitry@zend.com>,
 PHP Internals <internals@lists.php.net>,
 Nikita Popov <nikita.ppv@gmail.com>,
 Anatol Belski <ab@php.net>
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To: Bob Weinand <bobwei9@hotmail.com>
Subject: Re: [PHP-DEV] Fix division by zero to throw exception (round 2)
From: ajf@ajf.me (Andrea Faulds)

Hi Bob,

> On 29 Jun 2015, at 16:54, Bob Weinand <bobwei9@hotmail.com> wrote:
>=20
> 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=3D69957 (As I thought it was =
non-intentional, I went ahead and "fixed" it, was reverted later, hence =
discussing that now here.)
>=20
> So, looks like there was some quick decisions and discussion I totally =
had missed.
> What we have now is:
>=20
>> Am 03.04.2015 um 23:13 schrieb Dmitry Stogov <dmitry@zend.com>:
>>=20
>> So the summary:
>>=20
>> 1) division by zero produces a warning and +/-INF IS_DOUBLE. =
Compile-time
>> evaluation is disabled.
>>=20
>> 3) Modulo by zero produces Exception.Compile-time evaluation is =
disabled.
>=20
> Why? Why do we change the one but not the other?
>=20
> Why does 0 % 0 throw an Exception, but 0 / 0 NAN?
> Why does 1 % 0 throw an Exception, but 1 / 0 INF?
>=20
> I'd like to either properly return 0, INF or NAN in both cases or in =
none.
>=20
> 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=E2=80=99t /, it=E2=80=99s 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=E2=80=99s intdiv() and %), the =
accepted way to handle division by zero is to produce an error. =
There=E2=80=99s no correct or useful result you can produce. In PHP, we =
used to produce FALSE here, which is a different type (boolean). I =
don=E2=80=99t think that was a good idea because of PHP=E2=80=99s weak =
typing. If you use FALSE in some other arithmetic operation, it=E2=80=99ll=
 be coerced to zero, and produce weird results from other operations. =
Sure, there=E2=80=99s 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=E2=80=99s /), 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=E2=80=99re 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 =C2=B1Infinity, and if you divide by Infinity, =
you get =C2=B10. Again, PHP used to produce FALSE here, which has the =
problems described earlier. =C2=B1Infinity 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=E2=80=99ll be =
=C2=B10, =C2=B1Infinity, or NaN. Yes, you result is garbage, but it=E2=80=99=
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=E2=80=99s what=E2=80=99s usually does for them, / has =
different behaviour because it works with floats and that=E2=80=99s =
what=E2=80=99s done for them.

I hope that makes sense.

--
Andrea Faulds
http://ajf.me/