pureFunction.void

Code example

<?php declare(strict_types = 1);

/**
 * @phpstan-pure
 */
function doNothing(): void // ERROR: Function doNothing() is marked as pure but returns void.
{
}

Why is it reported?

A function marked as @phpstan-pure must not have side effects and must return a meaningful value. A pure function that returns void serves no purpose -- since it has no side effects and produces no return value, calling it has no observable effect. This is almost certainly a mistake: either the function should not be marked as pure, or it should return a value.

The exception is constructors, which are allowed to be pure and return void because their purpose is to initialize an object.

How to fix it

If the function performs side effects (like writing to a file, modifying external state, or printing output), remove the @phpstan-pure annotation:

 <?php declare(strict_types = 1);

-/**
- * @phpstan-pure
- */
 function doSomething(): void
 {
 	file_put_contents('/tmp/log.txt', 'done');
 }

If the function truly is pure, it should return a value:

 <?php declare(strict_types = 1);

 /**
  * @phpstan-pure
  */
-function compute(): void
+function compute(): int
 {
+	return 42;
 }