psalm/docs/annotating_code/supported_annotations.md

Supported docblock annotations

Psalm supports a wide range of docblock annotations.

PHPDoc tags

Psalm uses the following PHPDoc tags to understand your code:

• @var Used for specifying the types of properties and variables
• @return Used for specifying the return types of functions, methods and closures
• @param Used for specifying types of parameters passed to functions, methods and closures
• @property Used to specify what properties can be accessed on an object that uses __get and __set
• @property-read Used to specify what properties can be read on object that uses __get
• @property-write Used to specify what properties can be written on object that uses __set
• @deprecated Used to mark functions, methods, classes and interfaces as being deprecated
• @internal used to mark classes, functions and properties that are internal to an application or library.

Off-label usage of the @var tag

The @var tag is supposed to only be used for properties. Psalm, taking a lead from PHPStorm and other static analysis tools, allows its use inline in the form @var Type [VariableReference].

If VariableReference is provided, it should be of the form $variable or $variable->property. If used above an assignment, Psalm checks whether the VariableReference matches the variable being assigned. If they differ, Psalm will assign the Type to VariableReference and use it in the expression below.

If no VariableReference is given, the annotation tells Psalm that the right hand side of the expression, whether an assignment or a return, is of type Type.

<?php
/** @var string */
$a = $_GET['foo'];

/** @var string $b */
$b = $_GET['bar'];

function bat(): string {
    /** @var string */
    return $_GET['bat'];
}

Psalm-specific tags

There are a number of custom tags that determine how Psalm treats your code.

@param-out

This is used to specify that a by-ref type is different from the one that entered. In the function below the first param can be null, but once the function has executed the by-ref value is not null.

<?php
/**
 * @param-out string $s
 */
function addFoo(?string &$s) : void {
    if ($s === null) {
        $s = "hello";
    }
    $s .= "foo";
}

@psalm-var, @psalm-param, @psalm-return, @psalm-property, @psalm-property-read, @psalm-property-write

When specifying types in a format not supported by phpDocumentor (but supported by Psalm) you may wish to prepend @psalm- to the PHPDoc tag, so as to avoid confusing your IDE. If a @psalm-prefixed tag is given, Psalm will use it in place of its non-prefixed counterpart.

@psalm-suppress SomeIssueName

This annotation is used to suppress issues. It can be used in function docblocks, class docblocks and also inline, applying to the following statement.

Function docblock example:

<?php
/**
 * @psalm-suppress PossiblyNullOperand
 */
function addString(?string $s) {
    echo "hello " . $s;
}

Inline example:

<?php
function addString(?string $s) {
    /** @psalm-suppress PossiblyNullOperand */
    echo "hello " . $s;
}

@psalm-suppress all can be used to suppress all issues instead of listing them individually.

@psalm-assert, @psalm-assert-if-true and @psalm-assert-if-false

See Adding assertions.

@psalm-ignore-nullable-return

This can be used to tell Psalm not to worry if a function/method returns null. It’s a bit of a hack, but occasionally useful for scenarios where you either have a very high confidence of a non-null value, or some other function guarantees a non-null value for that particular code path.

<?php
class Foo {}
function takesFoo(Foo $f): void {}

/** @psalm-ignore-nullable-return */
function getFoo(): ?Foo {
  return rand(0, 10000) > 1 ? new Foo() : null;
}

takesFoo(getFoo());

@psalm-ignore-falsable-return

This provides the same, but for false. Psalm uses this internally for functions like preg_replace, which can return false if the given input has encoding errors, but where 99.9% of the time the function operates as expected.

@psalm-seal-properties

If you have a magic property getter/setter, you can use @psalm-seal-properties to instruct Psalm to disallow getting and setting any properties not contained in a list of @property (or @property-read/@property-write) annotations.

<?php
/**
 * @property string $foo
 * @psalm-seal-properties
 */
class A {
     public function __get(string $name): ?string {
          if ($name === "foo") {
               return "hello";
          }
     }

     public function __set(string $name, $value): void {}
}

$a = new A();
$a->bar = 5; // this call fails

@psalm-internal

Used to mark a class, property or function as internal to a given namespace. Psalm treats this slightly differently to the PHPDoc @internal tag. For @internal, an issue is raised if the calling code is in a namespace completely unrelated to the namespace of the calling code, i.e. not sharing the first element of the namespace.

In contrast for @psalm-internal, the docbloc line must specify a namespace. An issue is raised if the calling code is not within the given namespace.

As other tools do not support @psalm-internal, it may only be used in conjunction with @internal.

<?php
namespace A\B {
    /**
    * @internal
    * @psalm-internal A\B
    */
    class Foo { }
}

namespace A\B\C {
    class Bat {
        public function batBat(): void {
            $a = new \A\B\Foo();  // this is fine
        }
    }
}

namespace A\C {
    class Bat {
        public function batBat(): void {
            $a = new \A\B\Foo();  // error
        }
    }
}

@psalm-readonly and @readonly

Used to annotate a property that can only be written to in its defining class's constructor.

<?php
class B {
  /** @readonly */
  public string $s;

  public function __construct(string $s) {
    $this->s = $s;
  }
}

$b = new B("hello");
echo $b->s;
$b->s = "boo"; // disallowed

@psalm-mutation-free

Used to annotate a class method that does not mutate state, either internally or externally of the class's scope.

<?php
class D {
  private string $s;

  public function __construct(string $s) {
    $this->s = $s;
  }

  /**
   * @psalm-mutation-free
   */
  public function getShort() : string {
    return substr($this->s, 0, 5);
  }

  /**
   * @psalm-mutation-free
   */
  public function getShortMutating() : string {
    $this->s .= "hello"; // this is a bug
    return substr($this->s, 0, 5);
  }
}

@psalm-external-mutation-free

Used to annotate a class method that does not mutate state externally of the class's scope.

<?php
class E {
  private string $s;

  public function __construct(string $s) {
    $this->s = $s;
  }

  /**
   * @psalm-external-mutation-free
   */
  public function getShortMutating() : string {
    $this->s .= "hello"; // this is fine
    return substr($this->s, 0, 5);
  }

  /**
   * @psalm-external-mutation-free
   */
  public function save() : void {
    file_put_contents("foo.txt", $this->s); // this is a bug
  }
}

@psalm-immutable

Used to annotate a class where every property is treated by consumers as @psalm-readonly and every instance method is treated as @psalm-mutation-free.

<?php
/**
 * @psalm-immutable
 */
abstract class Foo
{
    public string $baz;
  
    abstract public function bar(): int;
}

/**
 * @psalm-immutable
 */
final class ChildClass extends Foo
{
    public function __construct(string $baz)
    {
        $this->baz = $baz;
    }
  
    public function bar(): int
    {
        return 0;
    }
}

$anonymous = new /** @psalm-immutable */ class extends Foo
{
    public string $baz = "B";
  
    public function bar(): int
    {
        return 1;
    }
};

@psalm-pure

Used to annotate a pure function - one whose output is just a function of its input.

<?php
class Arithmetic {
  /** @psalm-pure */
  public static function add(int $left, int $right) : int {
    return $left + $right;
  }

  /** @psalm-pure - this is wrong */
  public static function addCumulative(int $left) : int {
    /** @var int */
    static $i = 0; // this is a side effect, and thus a bug
    $i += $left;
    return $i;
  }
}

echo Arithmetic::add(40, 2);
echo Arithmetic::add(40, 2); // same value is emitted

echo Arithmetic::addCumulative(3); // outputs 3
echo Arithmetic::addCumulative(3); // outputs 6

@psalm-allow-private-mutation

Used to annotate readonly properties that can be mutated in a private context. With this, public properties can be read from another class but only be mutated within a method of its own class.

<?php
class Counter {
  /**
   * @readonly
   * @psalm-allow-private-mutation
   */  
  public int $count = 0;
    
  public function increment() : void {
    $this->count++;
  }
}

$counter = new Counter();
echo $counter->count; // outputs 0
$counter->increment(); // Method can mutate property
echo $counter->count; // outputs 1
$counter->count = 5; // This will fail, as it's mutating a property directly

@psalm-readonly-allow-private-mutation

This is a shorthand for the property annotations @readonly and @psalm-allow-private-mutation.

<?php
class Counter {
  /**
   * @psalm-readonly-allow-private-mutation
   */  
  public int $count = 0;
    
  public function increment() : void {
    $this->count++;
  }
}

$counter = new Counter();
echo $counter->count; // outputs 0
$counter->increment(); // Method can mutate property
echo $counter->count; // outputs 1
$counter->count = 5; // This will fail, as it's mutating a property directly

@psalm-trace

You can use this annotation to trace inferred type (applied to the next statement).

<?php

/** @psalm-trace $username */ 
$username = $_GET['username']; // prints something like "test.php:4 $username: mixed"

Note: it throws special low-level issue, so you have to set errorLevel to 1, override it in config or invoke Psalm with --show-info=true.

Type Syntax

Psalm supports PHPDoc’s type syntax, and also the proposed PHPDoc PSR type syntax.

A detailed write-up is found in Typing in Psalm