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Date: Thu, 07 Dec 2023 15:36:14 +0900
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Subject: Proposal: Arbitrary precision native scalar type
From: alex.pravdin@interi.co (Alex Pravdin)

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Hello internals,


This is the second round of the discussion regarding arbitrary precision 
scalar type integration into PHP. The previous part: 
https://marc.info/?l=php-internals&m=168250492216838&w=2 was initiated by 
me before deep diving into the work with decimals in PHP. After 6 months of 
working, I would like to update my proposal taking into account my 
experience and the previous 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 numbers.
- 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 math 
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 
unmaintained 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) 
Decimal(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 operations.
- 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 scalar 
arbitrary precision type called "decimal". Below are the preliminary 
requirements for implementation.


Decimal values can be created from literals by specifying a modifier or 
using the (decimal) typecast:

$v = 0.2d;
$v = (decimal) 0.2; // Creates a decimal value without intermediary float

It uses the precision and scale defined in php.ini.

The "decimal" typehint allows to define custom precision and scale: 
decimal(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 corresponding 
values about a decimal value.

If decimal value with different scale and precision is going to be assigned 
to a variable or parameter with smaller scale or precision, it first tries 
to convert the value. If it's not possible, then an exception is thrown 
like "Can not convert decimal (a, b) xxxxx.yyyy to decimal(c, d)". If 
possible, it performs the conversion and generates a warning like 
"Assigning decimal(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 except 
for one thing. Float value can be passed to a decimal argument or 
typecasted 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 numbers 
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 decimals. 
It performs float to string conversions using php.ini settings as defaults 
but also accepts parameters to configure the conversion. Then, it converts 
string to decimal. Since the main problem of float to decimal conversion is 
that we don't know the exact result until we use some rounding when 
transforming it to a human-readable format, it looks like the step of the 
conversion to a string is inevitable. Any more optimized algorithms are 
welcome.

Explicit typecast from float to decimal works the same as 
"float_to_decimal" function with all default values but also throws a 
warning. This is to encourage users to use explicit conversion with the 
"float_to_decimal" function and control the results.

Literal numbers in the code are converted to floats by default. If 
prepended by the "(decimal)" typecast, the decimal result is produced 
without an intermediary float.

New declare directive "default_decimal" is added. When used, literals and 
math operations return decimal by default instead of float. This is to 
simplify 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 
intermediary float:

$var = 5 / 2; // returns float 2.5
$var = 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, decimal 
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 = (float) 0.2;
$d = (decimal) 0.2;

$r = $f + $d; // returns float result by default
$r = as_decimal($f + $d); // returns decimal result with a warning about 
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 developers 
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 
float 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 
candidate 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. Sometimes, 
any project requires big changes to move forward. I'm pretty sure this 
functionality will move PHP to the next level and expand its area of 
applications. 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.


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Best regards,
Alex Pravdin

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