No option to specify 'pure' or 'immutable' for callback arguments

[bitwise-operators]

Take the following example:
https://psalm.dev/r/355b85ceb8

Psalm throws an ImpureFunctionCall on the array_reduce call because $callback is not specified as immutable or pure.

However, the current callable syntax doesn't have any way to do so, meaning this function will require a @psalm-suppress ImpureFunctionCall.

Even specifying the callback using an existing pure function will give the error (see this example ), as the pure-ness is not enforced in the class method.

I don't really have a good suggestion on how to fix this. The original discussion on how to type-hint callables in docblocks had several alternative styles that would allow for more information on the callable, but implementing a completely different syntax just for this seems over the top.

Maybe the simplest solution would be to add a new pseudo type pure-callable. Developers would probably need to use @psalm-param when using it, but since I suspect this isn't exactly a common issue, I think that's acceptable.

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/355b85ceb8

<?php
/**
 * @psalm-immutable
 */
class test {
    /** @var list<int> $a */
    private array $a = [];
    
    /**
     * @param callable(int, int): int $callback
     * @psalm-mutation-free
     */
    public function sum_up(callable $callback): int {
        return array_reduce($this->a, $callback, 0);
    }
}

Psalm output (using commit b6bdc94):

ERROR: ImpureFunctionCall - 14:16 - Cannot call an impure function from a mutation-free context

https://psalm.dev/r/84e95e30f8

<?php
/**
 * @psalm-pure
 */
function sum(int $a, int $b): int {
    return $a + $b;
}

/**
 * @psalm-immutable
 */
class test {
    /** @var list<int> $a */
    private array $a;
    public function __construct(int ...$a) {
        $this->a = array_values($a);
    }
    
    /**
     * @param callable(int, int): int $callback
     * @psalm-mutation-free
     */
    public function sum_up(callable $callback): int {
        return array_reduce($this->a, $callback, 0);
    }
}

$x = new test(1,2,3,4);

echo $x->sum_up(sum(...));

Psalm output (using commit b6bdc94):

ERROR: ImpureFunctionCall - 24:16 - Cannot call an impure function from a mutation-free context

[danog]

pure-callable and pure-Closure are already implemented: https://psalm.dev/r/dea1c375b5

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/dea1c375b5

<?php
/**
 * @psalm-immutable
 */
class test {
    /** @var list<int> $a */
    private array $a = [];
    
    /**
     * @param pure-callable(int, int): int $callback
     * @psalm-mutation-free
     */
    public function sum_up(callable $callback): int {
        return array_reduce($this->a, $callback, 0);
    }
}

Psalm output (using commit a91661a):

No issues!

[bitwise-operators]

That's awesome, thanks!

I've created a PR to add it to de docs.