How to allow filter-like functions make restrictions on types?

[someniatko]

Consider the following example:
https://psalm.dev/r/9b8357ef92

I'd like to somehow infer that filterIterable('is_int', generateIntsOrNulls()) returns iterable<int>. Of course I cannot do it using the current annotations on the function. Is there a way to type it more precisely so that it infers type restriction from how callable behaves? I guess similar behavior is hard-coded for array_filter function type inference.

BTW it's a mystery for me, how did it infer iterable<mixed, int|mixed|null> instead of iterable<mixed, int|null>

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/9b8357ef92

<?php

/**
 * @template T
 * @param callable(T):bool $filter
 * @param iterable<T> $it
 * @return iterable<T>
 */
function filterIterable(callable $filter, iterable $it): iterable
{
    foreach ($it as $el) {
        if ($filter($el)) {
            yield $el;
        }
    }                   
}

/** @return iterable<int|null> */
function generateIntsOrNulls(): iterable
{
    for ($i = 0; ; $i++) {
        yield ($i % 2 === 0)
            ? $i
			: null;
    }
}

/**
 * @param iterable<int> $it
 * @return iterable<int>
 */
function takesInts(iterable $it): iterable
{
    foreach ($it as $el) {
        yield $el * 2;
    }
}

$onlyInts = filterIterable('is_int', generateIntsOrNulls());
takesInts($onlyInts);

Psalm output (using commit f960d71):

ERROR: InvalidArgument - 40:11 - Argument 1 of takesInts expects iterable<mixed, int>, iterable<mixed, int|mixed|null> provided

[AndrolGenhald]

BTW it's a mystery for me, how did it infer iterable<mixed, int|mixed|null> instead of iterable<mixed, int|null>

Because is_int takes mixed. As to why it doesn't collapse to just mixed, well our type system needs a bit of rework, but I haven't had the time to finish my improvements lately.

Using an extra template actually works for this case, but in general it's been rather buggy to declare a template as a template constraint.

For your actual problem, I'm not sure if it's currently possible, I'll have to think about it.

Edit: It's definitely not possible right now, I think we'd need to allow templated conditionals in closure types and do something like this, but there might be more difficulties I haven't thought of yet.

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/69e63992a6

<?php

/**
 * @template TFilterParam
 * @template T of TFilterParam
 * @param callable(TFilterParam):bool $filter
 * @param iterable<T> $it
 * @return iterable<T>
 */
function filterIterable(callable $filter, iterable $it): iterable
{
    foreach ($it as $el) {
        if ($filter($el)) {
            yield $el;
        }
    }                   
}

/** @return iterable<int|null> */
function generateIntsOrNulls(): iterable
{
    for ($i = 0; ; $i++) {
        yield ($i % 2 === 0)
            ? $i
			: null;
    }
}

/**
 * @param iterable<int> $it
 * @return iterable<int>
 */
function takesInts(iterable $it): iterable
{
    foreach ($it as $el) {
        yield $el * 2;
    }
}

$onlyInts = filterIterable('is_int', generateIntsOrNulls());
takesInts($onlyInts);

Psalm output (using commit f960d71):

ERROR: InvalidArgument - 41:11 - Argument 1 of takesInts expects iterable<mixed, int>, iterable<mixed, int|null> provided