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Error Identifier: impure.propertyUnset

Every error reported by PHPStan has an error identifier. Here’s a list of all error identifiers. In PHPStan Pro you can see the error identifier next to each error and filter errors by their identifiers.

Code example #

<?php declare(strict_types = 1);

class Foo
{
	public ?string $name = null;

	/**
	 * @phpstan-pure
	 */
	public function clearName(): void
	{
		unset($this->name); // ERROR: Impure property unset in pure method Foo::clearName().
	}
}

Why is it reported? #

A function or method marked as @phpstan-pure must not have side effects and must always return the same result for the same inputs. Unsetting a property modifies the state of an object, which is a side effect. This makes the operation inherently impure.

How to fix it #

Remove the @phpstan-pure annotation if the method needs to unset properties:

 <?php declare(strict_types = 1);
 
 class Foo
 {
 	public ?string $name = null;

-	/**
-	 * @phpstan-pure
-	 */
-	public function clearName(): void
+	public function clearName(): void
 	{
 		unset($this->name);
 	}
 }

Alternatively, refactor the pure method to return a new value without modifying the object:

 <?php declare(strict_types = 1);
 
 class Foo
 {
 	public ?string $name = null;

 	/**
 	 * @phpstan-pure
 	 */
-	public function clearName(): void
+	public function withoutName(): self
 	{
-		unset($this->name);
+		$clone = clone $this;
+		$clone->name = null;
+		return $clone;
 	}
 }

How to ignore this error #

You can use the identifier impure.propertyUnset to ignore this error using a comment:

// @phpstan-ignore impure.propertyUnset
codeThatProducesTheError();

You can also use only the identifier key to ignore all errors of the same type in your configuration file in the ignoreErrors parameter:

parameters:
	ignoreErrors:
		-
			identifier: impure.propertyUnset

Rules that report this error #

• PHPStan\Rules\Pure\PureFunctionRule [1]
• PHPStan\Rules\Pure\PureMethodRule [1]

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