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PHP RFC: New comparison and equality semantics

1555 start 1725 end 1h30 total

Introduction

PHP is a programing language that will coerce types in various situations, one such situation is comparisons. Although this is a well known issue within the language and the community has moved towards using identity checks (===) instead of equality checks (==) to work around this surprising behaviour, there does not exist inequalities operators (<, >, <=, >=) which do not engage in type coercions.

An RFC that didn't proceed was the [strict_operators RFC] which would have been similar to the strict_type declare which currently exist in PHP. It is our opinion that the aforementioned declare has split the PHP ecosystem in two, and the better approach is to change the current semantics where PHP coerces types rather than introducing another declare that would create 4 different possible behaviours depending on which declares are enabled or not.

TODO Wording around: https://wiki.php.net/rfc/strict_operators

Greater than/less than operator execution order to check and do

Moreover, the behaviour of both equality and comparison operators can be overloaded by internal classes. However, it's semantics are unsound as the operators are not polymorphic (e.g. $A == $B may not produce the same behaviour as $B == $A).

This RFC will give an overview of the current semantics and behaviour of the comparison/equality operators, a new set of proposed semantics, its implication for userland and internal extensions, a BC impact analysis of the proposal, and future proposals which are enabled by this RFC.

Note: when we are talking about equating we mean the operators ==, !=, and <>, when the word comparison is used we mean the operators <, >, >=, =<.

Note: The behaviour of

Current semantics and behaviour

For the purpose of this RFC we consider there to be ten (10) types:

  • null
  • false
  • true
  • int
  • float
  • string
  • array
  • resource
  • Objects that do not overload the compare object handler
  • Internal Objects that overload the compare object handler

Ideal semantics

We think comparisons in PHP should behave according to the following guidelines:

  • A proper representation for when a comparison is incomparable exists
  • Comparing incomparable values should throw an error
  • null should only be equatable, and be equal only against itself
  • false should only be equatable, and be equal only against itself
  • true should only be equatable, and be equal only against itself
  • Integers should be comparable against integers, floating point numbers, and numeric strings only
  • Floating point numbers should be comparable against integers, floating point numbers, and numeric strings only
  • Equating two floats should throw an error or warn
  • Strings should only be equatable
  • Equating two strings should never perform a numerical comparison
  • Arrays should only be equatable
  • Resources should only be equatable, and never be coerced to an integer
  • Internal object should distinguish support for equating them and comparing them
  • The behaviour of equating two objects should be identical regardless of operation order (i.e. it should be commutative)

We will now provide some motivation for some of those points.

Proposed semantics

For next minor version

A proper Incomparable/un-equatable state within the PHP engine to be able to emit warnings, UncomparableErrors, and polymorphic comparison.

Comparisons are only valid on numeric values (i.e. int, float and numeric strings), and for internal objects implementing a compare handler. String comparisons can be replaced by using the strcmp() function. Array comparisons should be replaced by a custom compare function. null, false, true, and resources are uncomparable.

Equating two strings will always perform the comparison as strings.

null, false, and true are only equatable with themselves and do not cause the other operand or themselves to be type juggled. For boolean comparisons a straight if ($value) {} or if (!$value) {} can be used.

floats TODO should make it a warning to compared two floats together with

Comparing two objects

  • Compile time warning if null, false, or true is used as a constant operand for ==/!=/<>.

  • Runtime warning if null is equated with a non-null value that currently returns an equal result.

  • Runtime warning if a value of type bool is equated with a non-boolean value that currently returns an equal result.

  • Incomparable runtime warning when comparing booleans

  • Incomparable runtime warning when comparing a boolean to any other type

  • Runtime warning when equating two strings which are equated numerically

  • Incomparable Runtime warning when comparing two strings

  • Incomparable runtime warning when comparing arrays or an array to any other type.

  • Incomparable runtime warning when comparing resources.

  • Incomparable runtime warning when comparing two incomparable objects

  • Incomparable runtime warning when comparing an object to any other type

For next major version

  • Any incomparable warning emitted would be converted to throw a IncomparableError.

  • Equating two strings will always perform the comparison as string and never perform the comparison numerically, even if both strings are numeric. TODO Polyfill

  • null is only equatable with itself

  • true is only equatable with itself

  • false is only equatable with itself

Userland and Internal implication

Internals

Extensions that declare objects which support equatable or comparable objects will need to be modified to follow the new returned typed of the compare handler or implement the equatable handler instead.

Any call to zend_compare() will be affected

Add Comparison and Equal enumerations

enum Comparison {
    case Equal;
    case LeftGreater;
    case RightGreater;
    case Incomparable;
}
enum Equal {
    case Equal;
    case NotEqual;
    case Incomparable;
}

Engine

New opcode for > / >=

Userland

New fcmp(float $left, float $right, float $epsilon = EPSILON): Comparison function to compare two floating point numbers

BC impact analysis

TODO

Future proposals enabled by this RFC

Userland object comparisons

Support for two interfaces Equatable for objects that can be considered equal and Comparable for objects that can be compared, those would mimic the new internal object handlers.

Note adding support for userland object comparisons is a pre-requisite for a range operator ^..^/^../..^/.. to support objects.

Spaceship operator to return Incomparable as a value <=>

By using the new ComparisonEnum, BC compatible for userland compare callbacks to use int|ComparisonEnum return type. Only possible in the next major version.

Version

Next minor version, PHP 8.4, and next major version PHP 9.0.

Vote

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