rfc:typed-properties

PHP RFC: Typed Properties

Following the raging success of PHP 7.0 additions scalar type hints and return types, the next logical step is to provide optional typed properties.

Introduction

Typed Properties allow for an optional keyword in the definition, after the visibility scope, which contains the type the property should allow.

Proposal

Properties can have the same types as parameters:

class Foo {
  public int $int = 1;
  public float $flt = 2.2;
  public array $arr = [];
  public bool $bool = false;
  public string $string;
  public callable $callable;
  public stdClass $std;
  public OtherThing $other;
  public $mixed;
}

Notice there is no support for void here, as that would make no sense for a property.

This can be rather useful, as a lot of the job of setters is to ensure the values being passed in are the correct type.

There are two possible outcomes to type mismatches, as one check is done at compile time and another done at runtime.

Properties will support nullability, as decided by the nullable_types RFC (?Type).

Default Values

If an attempt is made to assign a value of an incorrect type at compile time, a fatal error will be raised:

<?php
new class {
	public int $bar = "turtle";
};
PHP Fatal error:  Default value for properties with integer type can only be integer in turtle.php on line 3

This is consistent with default parameter values:

<?php
$cb = function (int $bar = "42") {
 
};
Fatal error: Default value for parameters with a integer type can only be integer or NULL in /in/gq95J on line 2

While parameters allow null to be accepted as the default value, null is only a valid value for nullable properties.

Coercion and Strictness

The rules for strictness and coercion here are identical to how things with with type hints for parameters. As outlined above, default values (which are checked and set during compile time) are always strict, as there is no reason why you'd need coercion for a value being hardcoded into a property.

At runtime however, strict_types will be respected.

In weak mode (default), a numeric string passed at runtime is considered a valid int:

var_dump(new class() {
	public int $bar;
 
        public function __construct() {
            $this->bar = "42";
        }
});
object(class@anonymous)#1 (1) { 
  ["bar"]=> int(42) 
}

This is not a new rule for the language so should not be seen as a complication. It is using existing rules and logic.

TypeError

Due to the usage of TypeError, you can catch runtime errors for mismatched types:

class Math {
	public int $x;
	public int $y;
 
	public function __construct($x, $y) {
	    $this->x = $x;
	    $this->y = $y;
	}
 
	public function add() {
	    return $this->x + $this->y;
	}
}
 
try {
    (new Math(3, "nonsense"))->add();
} catch (Error $e) {
    echo "Look, I'm Python!";
}
Look, I'm Python!

Use before initialization

The implementation will raise an exception where a typed property is accessed before being initialized:

$foo = new class {
 
  public int $bar;  
};
 
var_dump($foo->bar);
Fatal error: Uncaught TypeError: Typed property class@anonymous::$bar must not be accessed before initialization in /in/cVkcj:7 
Stack trace: 
  #0 {main} thrown in /in/cVkcj on line 7

Some have voiced concern that, if an object has typed properties and the constructor does not set them, an exception should be raised because the object is in an invalid state.

However, lazy initialization of properties is a common idiom in PHP, that the authors of the RFC are not willing to restrict to untyped properties.

No rules have been violated until the engine returns a value, since any value returned is always of the correct type, we do not see the need to place further restrictions upon typed properties.

To put it another way: Type safety is the goal of this RFC, not validating objects. Currently developers are forced to do isset() and is_int() checks, but with the functionality provided in this RFC they will only need isset() if they are building classes that rely on lazy initialization. As such, developers relying on lazy initialization get a small benefit, and those building their objects “correctly” with fully initialized properties will not need any isset() boilerplate at all, as an exception will make it nice and clear to them that they're not building their objects as completely as they expected.

Nullable properties are not exempt from this rule, they too will raise an exception when accessed before initialization.

References

The implementation prohibits the use of references to properties with type information:

<?php
$foo = new class {
 
  public int $bar = 42;
};
 
$reference = &$foo->bar;
Fatal error: Uncaught TypeError: Typed property class@anonymous::$bar must not be referenced in /in/WEZHv:7 
Stack trace: 
  #0 {main} thrown in /in/WEZHv on line 7

This might seem strange at first, but references are somewhat of a mess when it comes to this sort of type strictness due to their very nature.

class Foo {
    public int $bar = 42;
}
 
$foo = new Foo;
$bar =& $foo->bar;
unset($foo);
$bar = "xyz";

Whether this is supported or considered a failure has downsides either way. Nikita Popov has written more on this topic, and explains that reference support could be added later if somebody can imagine an intelligent solution.

This means that a typed property cannot be directly passed to sort(), but a side-benefit of this is that it will make PHP developers more aware of which standard library functions are modifying their code via reference, and that awareness has a strong benefit.

Given the choice of a) typed properties without references in PHP 7.1, or b) typed properties with references in PHP 9.0, the RFC authors see value in approach a.

Magic (__get)

The magical __get method is not allowed to violate the declared type:

$foo = new class {
 
  public int $bar;
 
  public function __construct() {
      unset($this->bar); # will result in the invocation of magic when $bar is accessed
  }
 
  public function __get($name) {
      return "oh dear!";
  }
};
 
var_dump($foo->bar);
Fatal error: Uncaught TypeError: Typed property class@anonymous::$bar must be integer, string used in /in/Lq5dA:15 
Stack trace: 
  #0 {main} thrown in /in/Lq5dA on line 15

This may seem counter intuitive, but it's consistent with how normal objects work.

When a normal objects property is unset, it will result in the invocation of magic get when subsequently accessed, as if the property had never been declared, but the engine does not actually remove the property; If the property is assigned a value, access will be controlled as the declaration defines on any subsequent read of the property.

Therefore, we allow the invocation of magic for unset properties, but do not allow the return value to violate the type declared.

Mixed Declarations

Given the following code:

new class {
    public int $foo, $bar;
};

The engine already makes the assumption that $bar is public, whether that is right or wrong is irrelevant; We can't change it.

To stay consistent with the way visibility is applied to the group, type is applied in the same way. Any property in this statement will be considered an int too.

Mixing type declarations in a grouped statement is not allowed, and will cause a parser error:

new class {
    public int $foo, string $bar;
};
Parse error: syntax error, unexpected 'string' (T_STRING), expecting variable (T_VARIABLE)

If you want to declare multiple properties with different types, use multiple statements.

Unset

It is possible to unset typed properties, and return them to the same state as a property that was never set. There are no special differences or rules around this.

$foo = new class {
    public int $bar;
 
    public function __construct()
    {
        $this->bar = 12;
    }
};
 
unset($foo->bar);
 
var_dump(isset($foo->bar));
 
var_dump($foo->bar * 2);
bool(false)

Fatal error: Uncaught TypeError: Typed property class@anonymous::$bar must not be accessed before initialization

Reflection

A new ReflectionProperty::getType() method is provided.

class PropTypeTest {
	public int $int;
	public string $string;
	public array $arr;
	public callable $callable;
	public stdClass $std;
	public OtherThing $other;
	public $mixed;
}
 
$reflector = new ReflectionClass(PropTypeTest::class);
 
foreach ($reflector->getProperties() as $name => $property) {
	if ($property->hasType()) {
		printf("type: %s $%s\n", $property->getType(), $property->getName());
	} else {
	    printf("mixed: $%s\n", $property->getName());
	}
}
type: int $int
type: string $string
type: array $arr
type: callable $callable
type: stdClass $std
type: OtherThing $other
mixed: $mixed

Similarities to HHVM

The type system in HHVM uses matching syntax.

In fact, an example taken from the HHVM Type System works perfectly with this implementation:

class A {
  protected float $x;
  public string $y;
 
  public function __construct() {
    $this->x = 4.0;
    $this->y = "Day";
  }
  public function foo(bool $b): float {
    return $b ? 2.3 * $this->x : 1.1 * $this->x;
  }
}
 
function bar(): string {
  // local variables are inferred, not explicitly typed
  $a = new A();
  if ($a->foo(true) > 8.0) {
    return "Good " . $a->y;
  }
  return "Bad " . $a->y;
}
 
var_dump(bar()); // string(8) "Good Day"

Whilst the syntax is almost identical, this works a little differently to Hack.

Hack a offers static analysis tools to detect mismatched types, but when the code is executed it will allow any type to be passed through. This implementation is done at compile time to avoid the need for this, and validates properties being set at runtime too. Static analysis tools and editors/IDEs will no doubt catch up.

Other Languages

Of course, while “But Xlang does it!” is never a strong reason to do anything, it is sometimes nice to know how our friends are doing it in other languages.

Syntax

The authors of this RFC considered other syntax possibilities, however they were considered to be inferior for the following reasons.

One approach could be to match how return types are done with a colon after the name of the declaration, which is also how Delphi and ActionScript handle things:

public $bar: int;
public $bar: int = 2;
// or 
public $bar = 2: int;

Maybe, but if a ternary was used it would be really hard to see what was happening:

public $bar = Stuff::BAZ ? 20 : 30 : int; 

Another approach would be to copy VisualBasic:

public $bar as int;
public $bar = 2 as int;

That sticks out a bit, we don't do this anywhere else.

The current patch seems the most consistent with popular languages, avoids new reserved words, skips syntax soup and looks great regardless of assignment being used or not.

Static Properties

Static properties are global variables as far as the engine is concerned, it uses the same opcode to assign a static property as it does to assign any other variable ZEND_ASSIGN, the only exception being instance variables which are assigned with ZEND_ASSIGN_OBJ - giving us opportunity to provide type safety.

In the assign opcode, there is no information available about where the variable came from.

Even if we ignore that, and somehow find ways to always provide the information, changing ZEND_ASSIGN does not seem like a very good idea.

Generating completely new opcodes is an option, but would likely change the performance characteristics of all static assignments, since all static assignments would have to use the new opcode, and would certainly complicate the implementation.

Static typed properties are a separate feature, that can either be implemented on their own, when we determine if the performance impact and additional complication is worth the feature, or alternatively (preferably) we get them by proxy with typed variables at sometime in the future.

Performance

The latest version of the proposed patch doesn't make visible performance change of real0life apps.

On Wordpress and Mediawiki it makes about 0.1% slowdown, that may be caused not by the additional checks but by the worse CPU cache utilization, because the size of PHP code was increased on 40KB.

However, micro-benchmarks show significant slowdown (up to 20%) on primitive operations with untyped properties. Usage of typed properties makes additional slowdown. The following table shows relative slowdown of operations with properties in comparison to master branch.

$o→p = $x; $o→p +=2; $x = ++$o→p; $x= $o→p++;
untyped property 15% 1% 7% 19%
untyped property in class with typed properties 16% 2% 8% 19%
typed property 23% 37% 8% 19%

In principle, knowing the type of a property may allow us to make further optimizations.

Backward Incompatible Changes

None

Proposed PHP Version(s)

PHP 7.1

RFC Impact

To SAPIs

None

To Existing Extensions

None

To Opcache

Opcache has been patched.

Future Scope

Union Types

If the union_types RFC is accepted then ?Foo will be exactly equivalent to Foo | Null. The union types RFC will be responsible for intersecting decisions, such as whether ? can be used in conjunction with other union types.

Typed Local Variables

This is an entirely different feature, and something not worth conflating into this RFC. The idea might be wanted, but to keep things simple it will not be discussed in this RFC.

Typed Constant Properties

There is currently no known value in adding a type to a constant. Seeing as constants cannot be modified, the type is just whatever the constant is set to, and seeing as it cannot change there is no chance for a constant to be assigned a invalid value afterwards.

Vote

Voting started 10th June, ends 24th June 2016.

Merge typed properties ?
Real name Yes No
ajf (ajf)  
andi (andi)  
andrei (andrei)  
auroraeosrose (auroraeosrose)  
bishop (bishop)  
bmajdak (bmajdak)  
bukka (bukka)  
bwoebi (bwoebi)  
daverandom (daverandom)  
davey (davey)  
demon (demon)  
derick (derick)  
dm (dm)  
dmitry (dmitry)  
francois (francois)  
galvao (galvao)  
googleguy (googleguy)  
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irker (irker)  
jgmdev (jgmdev)  
kalle (kalle)  
kinncj (kinncj)  
kk (kk)  
krakjoe (krakjoe)  
laruence (laruence)  
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leigh (leigh)  
levim (levim)  
lstrojny (lstrojny)  
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Final result: 34 23
This poll has been closed.

Patches and Tests

This branch will be cleaned up with feedback and squashed, and doubtlessly more tests will be provided as people seek clarification on functionality.

https://github.com/php/php-src/compare/master...krakjoe:typed-properties

Changelog

* v1.1.1: Add nullability support
* v1.1.0: Change mixed declarations
* v1.0.2: Explain mixed declarations
* v1.0.1: Explain static property limitation
* v1.0.0: Expand on references
* v0.2.5: Expand on "strict at compile time"
* v0.2.4: Explain magic
* v0.2.3: Explain unset
* v0.2.2: Mention ReflectionProperty::getType()
* v0.2.1: Mention the runtime checks in syntax section too
* v0.2.0: Revision prompted by feedback and consensus
* v0.1.2: Definitely not allowing void
* v0.1.1: Expanded on compile time vs. run time errors
* v0.1.0: Initial draft
rfc/typed-properties.txt · Last modified: 2016/06/27 12:05 by krakjoe