rfc:readonly_and_immutable_properties

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PHP RFC: Readonly and immutable properties

Introduction

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 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. Immutable keyword for immutable semantics on write access
  2. Attribute approach:
    1. Readonly attribute for properties, if #1.1 is accepted this is merely syntax sugar that adds possibility to set readonly on classes (implicit all properties).
    2. Immutable attribute for properties, if #1.2 is accepted this is merely syntax sugar that adds possibility to set immutability on classes (implicit all properties).

Unless discussions phase points to clear preference within php internals and broader community, these four proposals will be offered as separate votes. Technically all 4 options can be accepted, however downside is there will be more than one way of doing things. So maybe a discussion on where Attributes fits into the language in the future is needed as well.

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;
    }
}

Main differences to previous proposals

Readonly

This RFC differs from Readonly properties (2014, withdrawn) by instead using the recently accepted Attribute language feature for annotating Readonly properties. As done in Rust, and in user land annotations in many PHP frameworks.

Immutability

This RFC differs fromImmutability (2018, stale) by instead using the recently accepted Attribute language feature for annotating Immutable properties. Aligning with Readonly proposal within this RFC.

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

Property Accessors Syntax

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

It's of this author's opinion that property type hinting and accessors where the two main obstacles to previous attempts at adding readonly semantics to PHP properties back in 2012-2014. And while readonly is rather hard to do in a clean way in PHP today, most other use cases offered by Accessors can be accomplished by plain PHP methods. Finally, Accessors overcomplicates readonly use case, and does not solve the needs for immutable property semantics.

Thus the author of this RFC believes what is proposed here should be done before any proposal for Accessors is re-considered, as anything here can be made syntax sugar for Accessors, if it ever gets accepted.

Proposal

1.1 Language ability to set property visibility for write access

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 writeonly 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).

References

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

Reflection

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

1.2 Language ability to set immutable property visibility for write access

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 string $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;
    }
}
Immutable semantics

An immutable property may only be written to in construct() and in other methods involved in object creation (set_state, unserialize, wakeup, and unserialize), besides that it is allowed to be unset in destruct(). Unless otherwise specified in visibility, the write/unset access is available within protected scope. == Reflection == 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” ==== 2. Attributes ==== With the recently accepted Attribute v2 RFC, another option here, or an 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? ==== 2.1 Readonly attribute ==== This proposal adds support for runtime-enforced readonly write visibility for declared properties. The following example illustrates the basic syntax: <code php> use PHP\Attribute\PropertyVisibility\Readonly; class User { <<Readonly>> public string $id; This property is not readonly public string $email; public function construct(int $id, string $email) { $this->id = $id; $this->email = $email; } } </code> Attribute can also be set on class level, implicit setting it on all fields unless they have their own PropertyVisibility attribute: <code php> use PHP\Attribute\PropertyVisibility\Readonly; <<Readonly>> class User { This property is readable in public scope and writeable in protected public string $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; } } </code> == Readonly semantics == 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. ==== 2.2. Immutable attribute ==== 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: <code php> use PHP\Attribute\PropertyVisibility\Immutable; class User { <<Immutable>> public string $id; public string $email; public function construct(int $id, string $email) { $this->id = $id; $this->email = $email; } } </code> Attribute can also be set on class level, implicit setting it on all fields unless they have their own PropertyVisibility attribute: <code php> use PHP\Attribute\PropertyVisibility\Immutable; <<Immutable>> class User { public string $id; public string $email; public function construct(int $id, string $email) { $this->id = $id; $this->email = $email; } } </code> For immutable semantics see proposal 1.2, when setting the attribute on semantics will be == Reflection == ==== 3.0 Class only attribute ==== One third option here, is that if 1.x proposals are accepted, and the proposed attributes in 2.x are accepted but only for being specified on class level, where it adds the capability to affect all properties. Alternative would be to propose class level keyword for “readonly” and “immutable”. ===== Backward Incompatible Changes ===== 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) ===== Proposed PHP Version(s) ===== Next PHP version, 8.0 suggested. ===== Impact on extensions ===== 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 ===== Performance tests will need to be done once there is a implementation of this. Then overhead on properties, as well as measuring benefit over using magic methods. ===== Vote ===== As this is a language change, a 2/3 majority is required. RFC is in draft, and will undergo discussion phase before it is put for a vote. ===== References ===== * C# readonly fields, semantically similar to what is proposed as “immutable” here. ===== Errata ===== If there are any edge-cases found during implementation, they will appear here. ===== Changelog ===== Significant changes to the RFC are noted here. * 2020-06-21 Initial early draft to get feedback on direction

rfc/readonly_and_immutable_properties.1592741957.txt.gz · Last modified: 2020/06/21 12:19 by andrerom

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