Re: Proposal: Arbitrary precision native scalar type

On Wed, Dec 13, 2023 at 9:37 AM 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 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. > > > > > > -- > > 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 > implements operator overrides. > > > Cheers > > Stephen > > Hello Stephen, I just ran apt install php8.3-decimal and tried this: $a = new Decimal\Decimal("1", 2); $b = $a + $a; PHP Warning: Uncaught TypeError: Unsupported operand types: Decimal\Decimal + Decimal\Decimal in So, it appears not.