PHP RFC: Readonly and immutable properties

• Version: 0.3.2
• Date: 2020-06-20
• Author: André Rømcke andre.romcke+php@gmail.com
• Based on previous RFCs by: Andrea Faulds ajf@ajf.me, Michal Brzuchalski michal@brzuchalski.com, Silvio Marijic marijic.silvio@gmail.com
• Proposed PHP version: PHP 8.0
• Status: Draft
• First Published at: http://wiki.php.net/rfc/readonly_and_immutable_properties
• Discussion: https://externals.io/message/ coming

This is a early draft, currently looking for feedback on direction on what would make most sense to propose, especially if there is any point in even exploring using Attributes for the features covered here or not.

With the introduction of typed properties in PHP 7.4, properties have become far more powerful. However it is currently not possible to specify disconnected write vs read visibility for properties, such as readonly, without having to resort to magic methods (getters and setters). For immutable semantic it's even more cumbersome. This requires unnecessary boilerplate, makes usage less ergonomic and hurts performance.

This RFC resolves this issue by proposing a few options:

1. Language approach:
   1. Change to make it possibility to specify write visibility disconnected from read.
   2. readonly keyword for write access, on property and class (implicit all properties)
   3. immutable keyword for write access, on property and class (implicit all properties)
2. Attribute approach:
   1. Readonly attribute for properties, if #1.1/1.2 is accepted this is merely syntax sugar.
   2. Immutable attribute for properties, if #1.3 is accepted this is merely syntax sugar

Under this RFC, code like

class User {
    private int $id;
    private string $name;
 
    public function __construct(int $id, string $name) {
        $this->id = $id;
        $this->name = $name;
    }
 
    public function __get($property)
    {
        if (property_exists($this, $property)) {
            // We return value here as non public properties are "readonly" in this class
            return $this->$property;
        }
        throw new PropertyNotFoundException($property, static::class);
    }
 
    public function __set($property, $value)
    {
        if (property_exists($this, $property)) {
            // Here private/protected property is attempted accessed outside allowed scope, so we throw
            throw new PropertyReadOnlyException($property, static::class);
        }
        throw new PropertyNotFoundException($property, static::class);
    }
 
    public function __isset($property)
    {
        return property_exists($this, $property);
    }
 
    public function __unset($property)
    {
        $this->__set($property, null);
    }
}

might be written as

class User {
    <<Readonly>>
    public int $id;
 
    <<Readonly>>
    public string $name;
 
    public function __construct(int $id, string $name) {
        $this->id = $id;
        $this->name = $name;
    }
}

or just

<<Readonly>>
class User {
    public int $id;
    public string $name;
 
    public function __construct(int $id, string $name) {
        $this->id = $id;
        $this->name = $name;
    }
}

This RFC aligns with Readonly properties (2014, Withdrawn).

This RFC aligns with Immutability (2018, Stale).

This RFC does not align with the semantics of the recent Write once properties (2020, Declined), which is targeting a different problem.

This RFC does not try to solve as wide use case as the different iterations of Property Accessors Syntax does.

However: - Accessors overcomplicates readonly, and does not offer solutions to immutability - There seems to be a higher need in the community for readonly and immutable semantics - Everything Accessors offers beyond disconnected read and write visibility for properties, can easily be done with plain methods. The same is not true for readonly and immutable semantics as shown in the introduction.

Attempting to pass a property value outside of allowed writable scope as a reference, results in an error.

This proposal adds support for enforced write visibility checks for declared properties. The following example illustrates the basic syntax:

class User {
    // Property is readonly in protected and public scope
    public:private int $id;
 
    // Property is readonly in public scope
    public:protected string $name;
 
    // Property is write-only in public and protected scope
    private:public string $newName;
 
    public function __construct(int $id, string $name) {
        $this->id = $id;
        $this->name = $name;
    }
}

The format is “<read_visibility>:<write_visibility>”, and if you omit the last visibility value you will like before implicit set both read and write visibility at once _(unless other future keywords or attributes states otherwise).

When using reflection, methods such as ReflectionProperty::setAccessible() will work as before, it will implicit set visibility for both read and write.

However with this proposal the following existing methods will represent read visibility for cases where it differs: - ReflectionProperty::isPrivate() - ReflectionProperty::isProtected() - ReflectionProperty::isPublic()

And for checking separate write visibility the following methods may be used: - ReflectionProperty::isWritePrivate — Checks if property is writable in private - ReflectionProperty::isWriteProtected — Checks if property is writable in protected - ReflectionProperty::isWritePublic — Checks if property is writable in public

Reflection::getModifiers() and Reflection::getModifierNames() will need adaption too, and proposal is to adapt it so getModifierNames() continues to return the visibility as specified, meaning it may now return for instance public:protected as one of the strings returned.

TODO: Expand this with modifier ints representing all variations and their names

This proposal adds support for runtime-enforced readonly write visibility for declared properties. The following example illustrates the basic syntax:

class User {
    // Property is readonly, and can only be written to in protected scope
    public readonly int $id;
 
    // Property is readonly, and can only be written to in private scope
    protected readonly string $name;
 
    // [assuming 1.1 is accepted] Invalid declaration (visibility is already stating property is readonly)
    public:private readonly string $email;
 
    public function __construct(int $id, string $name) {
        $this->id = $id;
        $this->name = $name;
    }
}

Keyword can also be set on class level, implicit setting it on all fields unless they have their own immutable attribute:

readonly class User {
    // Property is readonly, and can only be written to in protected scope
    public int $id;
 
    // Property is readonly, and can only be written to in private scope
    protected string $name;
 
    // [assuming 1.1 is accepted] Invalid declaration (visibility is already stating property is readonly)
    public:private string $email;
 
    public function __construct(int $id, string $name) {
        $this->id = $id;
        $this->name = $name;
    }
}

An readonly property may only be written to in scope lower than what is define as its read+write visibility, so if visibility is public, it may only be written to in protected scope.

When using reflection, methods such as ReflectionProperty::setAccessible() will work as before, it will implicit disable readonly flag.

Furthermore the following method is proposed added to be able to detect readonly properties: - ReflectionProperty::isReadonly()

Reflection::getModifiers() and Reflection::getModifierNames() will need adaption too to add int and keywords for “readonly”.

TODO: Expand this with specific modifier int for “readonly”

This proposal adds support for runtime-enforced immutable write visibility for declared properties. The following example illustrates the basic syntax:

class User {
    // Property is immutable, can only be written to in __construct in protected scope
    public immutable int $id;
 
    // [assuming 1.1 is accepted] Property is immutable, can only be written to in __construct in private scope
    public:private immutable string $email;
 
    public function __construct(int $id, string $email) {
        $this->id = $id;
        $this->email = $email;
    }
}

Keyword can also be set on class level, implicit setting it on all fields unless they have their own readonly attribute:

immutable class User {
    // Property is immutable, can only be written to in during construction in protected scope
    public int $id;
 
    // Property is immutable, can only be written to during construction in private scope
    protected string $email;
 
    public function __construct(int $id, string $email) {
        $this->id = $id;
        $this->email = $email;
    }
}

An immutable property may only be written to in construct and in other methods involved in object creation (set_state, unserialize, and wakeup), besides that it is allowed to be unset in destruct.

Unless otherwise specified in visibility, the write/unset access is available within protected scope.

When using reflection, methods such as ReflectionProperty::setAccessible() will work as before, it will implicit disable immutable flag.

Furthermore the following method is proposed added to be able to detect immutable properties: - ReflectionProperty::isImmutable()

“Reflection::getModifiers()” and “Reflection::getModifierNames()” will need adaption too to add int and keywords for “immutable”.

TODO: Expand this with specific modifier int for “immutable”

With the recently accepted Attribute v2 RFC, another option here, or a supplemental one, would be to use attributes for introducing Readonly and Immutable semantics. Similar to how Rust does with its readonly create.

However the Attribute RFC does not allow for what is being drafted here, so this would need suggesting a way for userland classes to tell parser / compiler to enhance language features.

As such, maybe this should be completely omitted from the proposal? Should we aim for concepts in 1.1, 1.2 and 3.0 in this RFC?

This proposal adds support for runtime-enforced readonly write visibility for declared properties. The following example illustrates the basic syntax:

use PHP\Attribute\PropertyVisibility\Readonly;
 
class User {
    <<Readonly>>
    public int $id;
 
    // This property is not readonly
    public string $email;
 
    public function __construct(int $id, string $email) {
        $this->id = $id;
        $this->email = $email;
    }
}

Attribute can also be set on class level, implicit setting it on all fields unless they have their own PropertyVisibility attribute:

use PHP\Attribute\PropertyVisibility\Readonly;
 
<<Readonly>>
class User {
    // This property is readable in public scope and writeable in protected
    public int $id;
 
    // This property is readable in protected scope and writeable in private
    protected string $email;
 
    public function __construct(int $id, string $email) {
        $this->id = $id;
        $this->email = $email;
    }
}

For readonly semantics see proposal 1.2

TODO: show example on reading /setting attribute via reflection, and how this relates to “ReflectionProperty::setAccessible()”.

This proposal adds a compiler attribute which implies a runtime-enforced immutable write visibility checks for declared properties. The following example illustrates the basic syntax:

use PHP\Attribute\PropertyVisibility\Immutable;
 
class User {
    <<Immutable>>
    public int $id;
 
    public string $email;
 
    public function __construct(int $id, string $email) {
        $this->id = $id;
        $this->email = $email;
    }
}

Attribute can also be set on class level, implicit setting it on all fields unless they have their own PropertyVisibility attribute:

use PHP\Attribute\PropertyVisibility\Immutable;
 
<<Immutable>>
class User {
    public int $id;
    public string $email;
 
    public function __construct(int $id, string $email) {
        $this->id = $id;
        $this->email = $email;
    }
}

For immutable semantics see proposal 1.3

TODO: show example on reading /setting attribute via reflection, and how this relates to ReflectionProperty::setAccessible().

Code that expects to be able to make properties writeable via reflection will have to adapt for new code taking advantage of this.

While ReflectionProperty::setAccessible() will still work like before, checks using isProtected() or isPrivate() won't detect if class has other visibility for write (proposal #1), or take into account specific attributes affecting write (assuming proposal #1 is voted down and Readonly and Immutable becomes own attribute logic instead of merely syntax sugar for #1.x)

Next PHP version, 8.0 suggested.

More future extension code, and possible SPL code, can be written in PHP instead. This is in-line with other features already accepted for PHP 8.0.

Performance tests will need to be done once there is an implementation of this. Then overhead on properties, as well as measuring benefit over using magic methods.

As this is a language change, a 2/3 majority is required.

• C# readonly fields, semantically similar to what is proposed as “immutable” here.
• Rust readonly create

If there are any edge-cases found during implementation, they will appear here.

Significant changes to the RFC are noted here.

• 2020-06-21 Initial early draft to get feedback on direction