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PHP RFC: Final properties
- Version: 0.1
- Date: 2020-02-18
- Author: Máté Kocsis, kocsismate@php.net
- Target Version: PHP 8.0
- Status: Draft
- Implementation: https://github.com/php/php-src/pull/5186
Introduction
This RFC proposes to add support for a new property modifier that would allow properties to be initialized, but not modified afterwards (including incrementing/decrementing, unsetting them). For example, Java and C# also have a similar - but not exactly the same - concept.
Proposal
“Final” properties in PHP (the actual keyword is to be decided) could be initialized either by an explicit default value, or by assigning a value to them. Unlike to final
properties in Java, this RFC proposes to allow the initialization of object properties after object construction. The main purpose of this is to make lazy loading possible - which is an important aspect for many PHP applications. Additionally to object properties, class properties can also use the “final” modifier with the same restrictions.
As untyped properties have an implicit default value (null
) in the absense of an explicit one, their usefulness would be very limited. In order to avoid the introduction or unintiutive workarounds, this RFC proposes to disable the “final” property modifier for them. Contrarily to untyped properties, typed properties are in uninitialized state by default, so they play well with the write-once semantics introduced by “final” properties. In order to be safe from problems with references, references on “final” properties are disabled as well.
Open questions
As there is no consensus about the name of the modifier, I'd like to put it to vote. You can find below the ideas that came up during discussion along with their pros/cons:
final
: It would be a better fit for a modifier that prevents a property to be overridden (just like how it works for classes and methods)immutable
: Actually, this name is a lie because the usage of mutable data structures are not restricted (objects, resources)readonly
: This name can also lie (in case of lazy initialization) because the proposed modifier would actually work according to “write-once” semantics. Thereadonly
modifier can be familiar for those who use C# thoughwriteonce
: it's the most clear and most descriptive name among all, but it doesn't sound familiar at alllocked
: it's a more abstract, less clear name for the feature, but at least it's not a liesealed
: It has the same properties aslocked
, and this keyword is also used in C# (similarly tofinal
in PHP)
That said, I'd like to propose “locked”, “sealed”, and “writeonce” as voting choices about the name of the feature.
Backward Incompatible Changes
There are no backward incompatible changes in this proposal except for the fact that “locked”, “sealed”, or “writeonce” would become a reserved keyword depending on the outcome of the secondary vote.
Future Scope
Adding support for “final” properties would lay the groundwork for immutable objects - for which I'm going to create a proposal if the current RFC is accepted.
Proposed Voting Choices
The primary vote (“Do you want to add support for the new property modifier?”) requires 2/3 majority, while the secondary one (“Which keyword to use”?) requires a simple majority.
References
Prior RFC proposing the introduction of immutable
properties: https://wiki.php.net/rfc/immutability