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Subject: Re: [PHP-DEV] [RFC] [Discussion] Support object type in BCMath
In-Reply-To: <CAMrTa2GsydsTtorY9hcEcjDZe2Uu+mRo8Wz9+0Dedc4OCdLtUg@mail.gmail.com>
Date: Fri, 5 Apr 2024 09:20:11 +0900
Cc: Barney Laurance <barney@redmagic.org.uk>,
 internals@lists.php.net
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To: Jordan LeDoux <jordan.ledoux@gmail.com>
From: saki@sakiot.com (Saki Takamachi)

Hi Barney, Jordan,

I would like to thank you all again for discussing this RFC in great =
detail. I believe that the specification for the `BCMath\Number` class =
is in very good shape due to the detailed discussions and the great =
amount of feedback. Now that we have been able to brush up the =
specifications so far, I would be very happy if we could have a good =
discussion about the concerns that still exist.

> Coming back to this point, I think these are basic features that =
people would expect to be there - I think I would find just slightly =
frustrating to start learning how to use a class like this and then
> find that it doesn't have these functions. Casting and calling =
`intval` or `floatval` all feel like slightly awkward workarounds that =
shouldn't be needed in a greenfield project. We know that the string
> inside the object is always a numeric string, so it should be easier =
to parse it as an int than to parse it as a date or a JSON document. =
Code doing the latter should stand out as odd looking.

> The class cannot guarantee that it can return a value in the type you =
request however, so the way that is handled would need to be decided. =
The value can easily be outside of the range of an int. Should it return =
a float silently in that case for `toInt()`? What if the value is beyond =
the range of a float? That would be a very rare situation, as floats can =
represent extremely large numbers (with very reduced accuracy), but I =
would expect it to throw an exception if that happened. Ideally an =
exception that I could catch and ignore, since I can almost surely deal =
with that error in most situations.
>=20
> What about a number that is so small that it can't fit in a float? =
Similar situation, though I expect it would occur slightly more often =
than a number being too large to fit in a float, even though it would =
also be rare.
>=20
> I think these helper functions belong in the RFC, but they aren't =
quite straightforward, which is what I think Saki was alluding to.

> Yes I agree there are subtleties to work out for these functions. I =
don't think there's such a thing as a number to small to fit in a float. =
Converting from decimal to float is always an approximation, sometimes =
the best approximation available as a float will be either 0 or -0.

If there is a demand for such functionality, I think it should of course =
be included in the RFC. However, as Jordan mentioned, proper error =
handling is required. IMHO, I think the following specifications are =
good:

- toInt(): If it has a decimal part, round it to an integer according to =
the rounding mode. If the result is too large to be represented as an =
int, it will not be converted to a float and will throw an exception =
meaning it cannot be converted to an int.
- toFloat(): If the result is too large or too small, throws an =
exception meaning it cannot be converted, just like int. Also, if the =
value is within the representable range but the precision is too fine, =
such as `1.0000000000000001`, it will be rounded according to the =
rounding mode. However, if round to a precision of 17 or 18 digits, the =
error will end up being very large, so refer to the value defined in =
`PHP_FLOAT_DIG` and use 15 digits as the maximum number of digits. This =
constant is 16 for IBM and 15 for others, but for consistency of =
operation it should be set to 15.


>> I don't think it will be possible to make a good Money class as a =
child of this. BcNum is a read-only class, so if the constructor of =
BcNum is final then the child class won't be able to take the currency =
as a constructor param, and won't be able to protect the invariant that =
currency must be initialized. If the constructor of BcNum were made =
non-final then BcNum wouldn't be able to protect the invariant of the =
numeric value always being initialized once the constructor has =
returned. And it should be straightforward to make a good money class as =
a composition of BcNum and a currency enum or string.
>=20
> The RFC does not list the constructor as final, and I would not expect =
it to.=20

>> There's probably not any good use case for a subclass to add =
properties, unless perhaps the subclass developers are willing do do =
away with some of the checks that would normally be done on a value =
object by the PHP runtime to keep it valid (maybe replacing them with =
static analysis checks). But there are lots of reasonable use cases for =
subclasses to add methods, even if they're effectively syntax sugar for =
static methods with a BcNum typed param.
>=20
> I literally just provided in the quote you are replying to a good use =
case for a subclass. You can do the same thing with composition yeah, =
and that might even be better to a lot of people, but I don't think this =
RFC should take a position AGAINST subclasses and in favor of =
composition.=20

>> Having just written that I've now checked the behavior of DateTime - =
see https://3v4l.org/5DQZg. The constructor of that isn't final, so a =
child class can replace it with an empty constructor. But if it does =
that and leaves the value uninitialized then it blows up on a call to =
`format`. That's probably not something we should emulate. =
DateTimeImmutable behaves the same way.

> Why not? Writing something like that obviously does not work, and the =
error would be immediately apparent. Is it possible to create something =
that will error? Of course. That's not an error that needs to be =
aggressively guarded against, because the feedback is rapid and obvious.


I don't think protecting initialization by the constructor is necessary. =
https://3v4l.org/YroTN
This problem can occur with any class, not just `DateTime`. And I think =
this is the responsibility of the user's code, and the language just =
needs to throw an exception appropriately.

To clarify: In my opinion, I think that value objects provided by =
languages =E2=80=8B=E2=80=8Bare not intended to force immutability or =
design onto users, to provide functionality to users who expect them to =
behave as immutable value objects. Therefore, even if a child class that =
inherits from `Number` behaves in a way that is not immutable, it is the =
responsibility of the user and there is no need to prohibit it.

In other words, the concerns we are currently aware of are issues of =
responsibility on the user's side, and my opinion is that there is no =
need to give that much consideration. However, this is my personal =
opinion, and there may be more reasonable opinions.

Regards.

Saki=