How to infer type from array item?

[cawa-93]

I have the following code

class AdminAssets
{

    public const ASSETS = [
        'js' => [
            'foo-script' => [
                'src' => 'file.js',
            ],
            'bar-script' => [
                'src' => 'file.js',
            ],
        ],
    ];

    public static function show(string $key)
    {
    }
}

How can I declare that method show can accept string union from ASSETS['js'] keys ('foo-script'|'bar-script')?

I'm tired to use @param key-of<ASSETS['js']> $key but got error [InvalidDocblock](https://psalm.dev/008) - 18:15 - Unrecognised template param 'js' in docblock for AdminAssets::show

[tayfuryldz]

The part that Psalm supports directly is key-of<CLASS_CONST>; the unsupported part in the attempted annotation is indexing the constant inside the type expression with ['js'].

Current Psalm documents/tests explicitly support forms such as:

/** @param key-of<self::DAYS> $day */

and key-of<Foo\Bar::ARRAY_CONST>, but ASSETS['js'] is not type-level array-offset syntax. That's why the parser ends up interpreting the bracketed expression incorrectly and reports the misleading template-param error.

If you need exactly the keys under js, the cleanest current representation is to expose that nested map as its own constant:

final class AdminAssets
{
    public const JS = [
        'foo-script' => ['src' => 'file.js'],
        'bar-script' => ['src' => 'file.js'],
    ];

    public const ASSETS = [
        'js' => self::JS,
    ];

    /** @param key-of<self::JS> $key */
    public static function show(string $key): void
    {
    }
}

Now Psalm can derive 'foo-script'|'bar-script' from the constant itself, without duplicating the string union in the docblock.

So key-of is the right utility; the limitation is that it operates on the array type/constant you give it and there isn't currently a supported self::ASSETS['js'] projection syntax for selecting a nested offset inside that argument.