possiblyImpure.yield

Code example

<?php declare(strict_types = 1);

/**
 * @phpstan-pure
 * @return \Generator<int, string, void, void>
 */
function generateItems(): \Generator
{
	yield 'a'; // ERROR: Possibly impure yield in pure function generateItems().
	yield 'b';
}

Why is it reported?

This identifier is not reported in practice because yield is always considered definitely impure, not just possibly impure. PHPStan reports impure.yield instead.

A function or method marked as @phpstan-pure must not have side effects and must always return the same result for the same inputs. Using yield inside a pure function is considered impure because generators maintain internal state across multiple calls. Each time the generator is resumed, it can produce different values and interact with the caller through the send() method, making the behavior inherently stateful and impure.

How to fix it

Remove the @phpstan-pure annotation if the function needs to use yield:

 <?php declare(strict_types = 1);

-/**
- * @phpstan-pure
- * @return \Generator<int, string, void, void>
- */
+/**
+ * @return \Generator<int, string, void, void>
+ */
 function generateItems(): \Generator
 {
 	yield 'a';
 	yield 'b';
 }

Alternatively, if the function should remain pure, return an array instead of using a generator:

 <?php declare(strict_types = 1);

 /**
  * @phpstan-pure
- * @return \Generator<int, string, void, void>
+ * @return list<string>
  */
-function generateItems(): \Generator
+function generateItems(): array
 {
-	yield 'a';
-	yield 'b';
+	return ['a', 'b'];
 }