Making mixed issues easier to diagnose

March 29, 2021 by Matt Brown - 2 minute read

When running Psalm at its strictest (level 1) Psalm will tell you when you’re doing something risky with a type it cannot infer.

For example, when we ask Psalm to analyse this code:

<?php

function takesArray(array $arr) : void {
    foreach ($arr as $some_string) {
        // more logic
    }
}

It gives us a warning

MixedAssignment - Unable to determine the type
    that $some_string is being assigned to

It’s probably not dangerous in practice — you might only ever be calling takesArray with an array of strings — but Psalm thinks it’s worth bringing to your attention.

A lot of developers try running Psalm at the strictest level, see a lot of these issues, and then either give up completely or opt for a less strict level (2 and higher).

For developers who do still want to use Psalm at its strictest, one quick way of appeasing the type checker is to hardcode a docblock @var type above the foreach loop:

<?php

function takesArray(array $arr) : void {
    /** @var string $some_string */
    foreach ($arr as $some_string) {
        // more logic
    }
}

This isn’t great, though — the type is no longer inferred naturally, so Psalm cannot verify the type in the docblock is actually correct.

A more robust fix — one that helps Psalm verify that calls to takesArray are correct — is to fix the issue at its source, by adding a docblock @param type:

<?php

/** @param array<string> $arr */
function takesArray(array $arr) : void {
    foreach ($arr as $some_string) {
        // more logic
    }
}

Developers will often opt for the quick fix over the more robust one, because it can be hard to figure out where exactly the mixed type is coming from.

Nudge

Now Psalm can tell developers where the mixed type came from:

MixedAssignment - Unable to determine the type
    that $some_string is being assigned to

  The type of $some_string is sourced from here
    function foo(array $arr) : void {
                       ^^^^

Hopefully this will encourage developers to chose the robust fix over the quick one, and also increase the usage of Psalm’s strictest level.

You can play with an interactive version here: psalm.dev/r/fb3654d0aa