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Date: Wed, 13 Dec 2023 10:11:45 +0100
Message-ID: <CAPzBOBOhbuiEwhVf7pTHfy+69Hzcsd-0+cFA=K-LQqpvk_0eJQ@mail.gmail.com>
To: Stephen Reay <php-lists@koalephant.com>
Cc: Alex Pravdin <alex.pravdin@interi.co>, internals@lists.php.net
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Subject: Re: [PHP-DEV] Proposal: Arbitrary precision native scalar type
From: landers.robert@gmail.com (Robert Landers)

On Wed, Dec 13, 2023 at 9:37=E2=80=AFAM Stephen Reay <php-lists@koalephant.=
com> wrote:
>
>
>
> > On 7 Dec 2023, at 13:36, Alex Pravdin <alex.pravdin@interi.co> wrote:
> >
> > Hello internals,
> >
> >
> > This is the second round of the discussion regarding arbitrary precisio=
n scalar type integration into PHP. The previous part: https://marc.info/?l=
=3Dphp-internals&m=3D168250492216838&w=3D2 was initiated by me before deep =
diving into the work with decimals in PHP. After 6 months of working, I wou=
ld like to update my proposal taking into account my experience and the pre=
vious discussion.
> >
> > Today's alternatives and their problems are the following.
> >
> > bcmath:
> > - Workaround: using string type.
> > - Unintuitive function calls instead of regular math operations.
> > - Unintuitive strings instead of numbers. People want to work with numb=
ers.
> > - Can not use proper type-hinting.
> > - Can use PHP's basic type coercions.
> >
> > Ext-decimal:
> > - Third-party extension.
> > - Workaround: implements the Decimal class that allows basic regular ma=
th operations.
> > - Requires using class methods for the rest of math operations.
> > - The latest release was in 2019 and there's a danger that it will be u=
nmaintained and not compatible with the future PHP releases.
> > - The php-decimal documentation website is currently down.
> > - Since objects are always casted to true when not null, "(bool) Decima=
l(0)" will equal to true which is not intuitive.
> > - IDEs are often confused when you use math operations on objects while=
 the code works fine.
> >
> > GMP:
> > - Workaround: implements the GMP class that allows basic math operation=
s.
> > - Requires using separate functions for the rest of operations.
> >
> > - Objects are always casted to true, GMP(0) will equal to true.
> >
> >
> > Accounting for all of the above, I suggest adding a native numeric scal=
ar arbitrary precision type called "decimal". Below are the preliminary req=
uirements for implementation.
> >
> >
> > Decimal values can be created from literals by specifying a modifier or=
 using the (decimal) typecast:
> >
> > $v =3D 0.2d;
> > $v =3D (decimal) 0.2; // Creates a decimal value without intermediary f=
loat
> >
> > It uses the precision and scale defined in php.ini.
> >
> > The "decimal" typehint allows to define custom precision and scale: dec=
imal(20,5). It accepts regular expressions returning ints in the execution =
context. It accepts int constants and literals in class field and function =
argument definitions.
> >
> > New functions added: get_scale and get_precision to return correspondin=
g values about a decimal value.
> >
> > If decimal value with different scale and precision is going to be assi=
gned to a variable or parameter with smaller scale or precision, it first t=
ries to convert the value. If it's not possible, then an exception is throw=
n like "Can not convert decimal (a, b) xxxxx.yyyy to decimal(c, d)". If pos=
sible, it performs the conversion and generates a warning like "Assigning d=
ecimal(a, b) to decimal(c, d) may be not possible with some values".
> >
> > It works the same as "float" in terms of its usage and type casting exc=
ept for one thing. Float value can be passed to a decimal argument or typec=
asted with a warning like "Float to decimal conversion may incur unexpected=
 results".
> >
> > Decimal to float conversion is allowed and smooth:
> >
> > function f (float $value) {}
> >
> > f(0.2);
> >
> > f(0.2d); // allowed with no warnings
> >
> >
> > Function "str_to_decimal" added to convert string representation of num=
bers to decimals.
> >
> >
> > Typecast from string to decimal works the same as the "str_to_decimal" =
function.
> >
> > Function "float_to_decimal" added to explicitly convert floats to decim=
als. It performs float to string conversions using php.ini settings as defa=
ults but also accepts parameters to configure the conversion. Then, it conv=
erts string to decimal. Since the main problem of float to decimal conversi=
on is that we don't know the exact result until we use some rounding when t=
ransforming it to a human-readable format, it looks like the step of the co=
nversion to a string is inevitable. Any more optimized algorithms are welco=
me.
> >
> > Explicit typecast from float to decimal works the same as "float_to_dec=
imal" function with all default values but also throws a warning. This is t=
o encourage users to use explicit conversion with the "float_to_decimal" fu=
nction and control the results.
> >
> > Literal numbers in the code are converted to floats by default. If prep=
ended by the "(decimal)" typecast, the decimal result is produced without a=
n intermediary float.
> >
> > New declare directive "default_decimal" is added. When used, literals a=
nd math operations return decimal by default instead of float. This is to s=
implify creating source files working with decimals only.
> >
> > New language construct "as_decimal()" is added to produce decimal math =
results for literals and math operations instead of float without intermedi=
ary float:
> >
> > $var =3D 5 / 2; // returns float 2.5
> > $var =3D as_decimal(5 / 2); // returns decimal 2.5
> >
> > This is a kind of "default_decimal" for a specific operation.
> >
> > If mixed float and decimal operands are used in a math operation, decim=
al is converted to float by default. If "default_decimal" directive or "as_=
decimal()" construct is used, float is converted to decimal (with a warning=
):
> >
> > $f =3D (float) 0.2;
> > $d =3D (decimal) 0.2;
> >
> > $r =3D $f + $d; // returns float result by default
> > $r =3D as_decimal($f + $d); // returns decimal result with a warning ab=
out implicit float to decimal conversion
> >
> > All builtin functions that currently accept float also accept decimal. =
So users don't need to care about separate function sets, and PHP developer=
s don't need to maintain separate sets of functions. If such functions get =
the decimal parameter, they return decimal. If they have more than one floa=
t parameter and mixed float and decimal passed, decimals converted to float=
 by default. If "default_decimal" or "as_decimal" used, float is converted =
to decimal with the warning.
> >
> >
> > The new type uses libmpdec internally to perform decimal calculations (=
same as Python).
> >
> >
> > All of the points above are subject to discussions, it is not an RFC ca=
ndidate right now. So please share your opinions.
> >
> > I know that the implementation of this will require a lot of work. But =
I don't think this is a stopper from formulating the requirements. Sometime=
s, any project requires big changes to move forward. I'm pretty sure this f=
unctionality will move PHP to the next level and expand its area of applica=
tions. My thoughts here are mostly from the user's perspective, I'm not so =
familiar with PHP internal implementation. But I think this feature can be =
a good goal for PHP 9.
> >
> >
> > --
> > Best regards,
> > Alex Pravdin
>
>
> Hi,
>
> While I do think it would be beneficial to have a built-in `decimal` type=
, regarding ext-decimal, I think some of the points raised may be outdated?
>
> The documentation site seems to have moved to https://php-decimal.github.=
io <https://php-decimal.github.io/>, which also indicates that it implement=
s operator overrides.
>
>
> Cheers
>
> Stephen
>
>

Hello Stephen,

I just ran `apt install php8.3-decimal` and tried this:

$a =3D new Decimal\Decimal("1", 2);
$b =3D $a + $a;
PHP Warning:  Uncaught TypeError: Unsupported operand types:
Decimal\Decimal + Decimal\Decimal in

So, it appears not.