rfc:readonly_properties_v2
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rfc:readonly_properties_v2 [2021/06/03 13:21] – nikic | rfc:readonly_properties_v2 [2021/07/20 15:37] (current) – nikic | ||
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* Date: 2021-06-02 | * Date: 2021-06-02 | ||
* Author: Nikita Popov < | * Author: Nikita Popov < | ||
- | * Status: | + | * Status: |
* Target Version: PHP 8.1 | * Target Version: PHP 8.1 | ||
* Implementation: | * Implementation: | ||
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<PHP> | <PHP> | ||
class Test { | class Test { | ||
- | public readonly int $prop = 0; | + | |
+ | | ||
+ | public readonly array $ary = [], | ||
+ | ) {} | ||
} | } | ||
$test = new Test; | $test = new Test; | ||
- | $test->prop += 1; | + | $test->i += 1; |
- | $test->prop++; | + | $test->i++; |
- | ++$test-> | + | ++$test-> |
- | $ref =& $test->prop; | + | $test-> |
- | $test->prop =& $ref; | + | $test-> |
- | byRef($test-> | + | $ref =& $test->i; |
+ | $test->i =& $ref; | ||
+ | byRef($test-> | ||
foreach ($test as & | foreach ($test as & | ||
</ | </ | ||
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</ | </ | ||
- | As the property has no default value, the assignment in the constructor is initializing, | + | As the property has no default value, the assignment in the constructor is initializing, |
- | Readonly static properties are not supported. This is a technical limitation, in that it is not possible to implement readonly static properties non-intrusively. In conjuction | + | Readonly static properties are not supported. This is a technical limitation, in that it is not possible to implement readonly static properties non-intrusively. In conjunction |
==== Inheritance ==== | ==== Inheritance ==== | ||
- | It is not allowed to override a read-write | + | It is not allowed to override a read-write property with a read-only |
<PHP> | <PHP> | ||
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} | } | ||
</ | </ | ||
- | |||
- | However, the converse is legal: | ||
<PHP> | <PHP> | ||
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} | } | ||
class B extends A { | class B extends A { | ||
- | // Legal: readonly -> readwrite | + | // Illegal: readonly -> readwrite |
public int $prop; | public int $prop; | ||
} | } | ||
</ | </ | ||
+ | |||
+ | It is obvious that overriding a readwrite property with a readonly property needs to be forbidden, because that may render operations performed in the parent class invalid. However, this proposal views readonly not just as a lack of capabilities (which would be safe to increase in a child class), but as an intentional restriction. Lifting the restriction in the child class could break invariants in the parent class. As such, a readonly modifier may be neither added nor removed during inheritance. | ||
It is interesting to consider how property redeclaration interacts with the restriction that initialization can only occur in the declaring class: | It is interesting to consider how property redeclaration interacts with the restriction that initialization can only occur in the declaring class: | ||
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Here, initialization of '' | Here, initialization of '' | ||
- | When the same property is imported from two traits, the '' | + | When the same property is imported from two traits, the '' |
<PHP> | <PHP> | ||
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} | } | ||
</ | </ | ||
- | |||
- | One could argue that it should be possible to merge a readonly and a readwrite property into a readwrite property. However, other modifiers currently also require strict equality, for example it is not possible to merge a public and a protected property. If these rules should be relaxed, they should be relaxed consistently. | ||
Types on readonly properties remain invariant. One could argue that types of readonly properties could be covariant instead: | Types on readonly properties remain invariant. One could argue that types of readonly properties could be covariant instead: | ||
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</ | </ | ||
- | However, it is possible to unset a readonly property prior to initialization, | + | However, it is possible to unset a readonly property prior to initialization, |
<PHP> | <PHP> | ||
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'' | '' | ||
+ | |||
+ | Similarly, closure rebinding can be used to bypass the initialization scope requirement. | ||
+ | |||
+ | ==== Serialization ==== | ||
+ | |||
+ | Readonly properties have no impact on serialization. As '' | ||
+ | |||
+ | This also applies to userland serializers and hydrators. As long as the object is created using '' | ||
===== Rationale ===== | ===== Rationale ===== | ||
- | The readonly property concept introduced in this proposal provides strong immutability guarantees, which apply both inside and outside the class. Once a property has been initialized, | + | The readonly property concept introduced in this proposal provides strong immutability guarantees, which apply both inside and outside the class. Once a property has been initialized, |
+ | |||
+ | < | ||
+ | class Test { | ||
+ | public readonly string $prop; | ||
+ | |||
+ | public function method(Closure $fn) { | ||
+ | $prop = $this-> | ||
+ | $fn(); // Any code may run here. | ||
+ | $prop2 = $this-> | ||
+ | assert($prop === $prop2); // Always holds. | ||
+ | } | ||
+ | } | ||
+ | </ | ||
These guarantees are //too// strong for certain use-cases. For example, some classes may wish to have properties that are publicly readable, but can only be written from within the class. This is a much weaker guarantee, as the value of a property can change during the lifetime of an object. //Both// variants can be useful depending on the situation, and the addition of readonly properties neither precludes nor discourages the addition of asymmetric property visibility. | These guarantees are //too// strong for certain use-cases. For example, some classes may wish to have properties that are publicly readable, but can only be written from within the class. This is a much weaker guarantee, as the value of a property can change during the lifetime of an object. //Both// variants can be useful depending on the situation, and the addition of readonly properties neither precludes nor discourages the addition of asymmetric property visibility. | ||
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This also ensures that a potential future "clone with" implementation will only be able to modify readonly properties from private scope and thus cannot bypass additional invariants imposed by the implementation when used from a different scope. | This also ensures that a potential future "clone with" implementation will only be able to modify readonly properties from private scope and thus cannot bypass additional invariants imposed by the implementation when used from a different scope. | ||
- | This RFC overlaps with the [[rfc: | + | This RFC overlaps with the [[rfc: |
It is worth noting that having a readonly property feature does not preclude introduction of accessors. C# supports both readonly properties and accessors. C# also provides properties with implicit backing storage through accessor syntax, but this is not the only way to do it. For example, Swift has special syntax for asymmetric visibility, rather than specifying visibility on implicitly implemented accessors. | It is worth noting that having a readonly property feature does not preclude introduction of accessors. C# supports both readonly properties and accessors. C# also provides properties with implicit backing storage through accessor syntax, but this is not the only way to do it. For example, Swift has special syntax for asymmetric visibility, rather than specifying visibility on implicitly implemented accessors. | ||
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===== Vote ===== | ===== Vote ===== | ||
- | Yes/No. | + | Voting started on 2021-07-01 and closes on 2021-07-15. |
+ | |||
+ | <doodle title=" | ||
+ | | ||
+ | | ||
+ | </ | ||
rfc/readonly_properties_v2.1622726483.txt.gz · Last modified: 2021/06/03 13:21 by nikic