psalm-pure invalid error

[kkmuffme]

https://psalm.dev/docs/annotating_code/supported_annotations/#:~:text=one%20whose%20output%20is%20just%20a%20function%20of%20its%20input

one whose output is just a function of its input

https://psalm.dev/r/dff3f37215 the output of abc() is pure, as it only depends on the input given (= always returns the same output for same input and has no side-effects) - but it still reports an error for it?

Are the docs wrong/am I misunderstanding "purity"? Or is this a bug?

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/dff3f37215

<?php

/**
 * @psalm-pure
 *
 * @return Hello
 */
function abc() {
    $xyz = new Hello();
    $xyz->world = true;
    return $xyz;
}

class Hello {
  public bool $world = false;
}

Psalm output (using commit 7c83878):

ERROR: ImpurePropertyAssignment - 10:5 - Cannot assign to a property from a mutation-free context

[orklah]

It is the no side-effects thing that prevents you from setting this attribute.

With no additional annotation, this could happen:
https://psalm.dev/r/413d60d78e
https://3v4l.org/R0RLT

So Psalm must forbid you to modify attributes when precautions are not taken. However, you could use @psalm-external-mutation-free in order to inform Psalm that there is no mutation outside of the scope of the object: https://psalm.dev/r/97334d3fc9

Psalm will then allow you to change this attribute

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/413d60d78e

<?php

/**
 * @psalm-pure
 *
 * @return Hello
 */
function abc() {
    $xyz = new Hello();
    $xyz->world = true;
    return $xyz;
}

class Hello {
  public function __set($param, $value){
   	echo "This is a major side effect!";   
  }
}

Psalm output (using commit 7c83878):

ERROR: ImpureMethodCall - 10:11 - Cannot call a non-mutation-free method Hello::__set from a pure context

ERROR: ImpurePropertyAssignment - 10:5 - Cannot assign to a property from a mutation-free context

INFO: MissingParamType - 15:25 - Parameter $param has no provided type

INFO: MissingParamType - 15:33 - Parameter $value has no provided type

https://psalm.dev/r/97334d3fc9

<?php

/**
 * @psalm-pure
 *
 * @return Hello
 */
function abc() {
    $xyz = new Hello();
    $xyz->world = true;
    return $xyz;
}

/** @psalm-external-mutation-free */
class Hello {
  public bool $world = false;
}

Psalm output (using commit 7c83878):

No issues!

[kkmuffme]

But I do not have a __set method in class Hello and psalm is aware of it. Where is the side-effect in the given example?

[orklah]

Your object is not final, so __set could still be defined in a child that would not be known by Psalm (but it seems like even with final, Psalm doesn't allow it)

I guess we could add handling for setting a property on an object if:

• it was created locally
• the instance doesn't leave the current scope
• it's final
• it doesn't have __set
• the property is not static
• it doesn't have any __destruct
• we don't do anything else with the object that may persist it

I think I covered every possibility! Or you could flag the object external-mutation-free and help Psalm a little 😄

[kkmuffme]

The problem is with #8624 - bc of this false positive, how do you suggest to handle this never type? (without completely reworking the type combiner, which eats the never type, making this property necessary in the first place)

[orklah]

Do you have a snippet to show how this is related to never? I really struggle to grasp what's bugging you here

[kkmuffme]

Issue solved itself as the underlying stuff in psalm was changed/fixed