Psalm static analysis fails on MapperBuilder::registerTransformer

[Yui-Ezic]

Psalm static analysis fails on MapperBuilder::registerTransformer, because transformer should be of type class-string|pure-callable, but sometimes transformers isn't pure.

Examples

Example 1: format dates

(new \CuyZ\Valinor\MapperBuilder())
    ->registerTransformer(
        fn (\DateTimeInterface $date) => $date->format('Y/m/d')
    );

Psalm output

ERROR: InvalidArgument - xxx/transformer.php:20:9 - Argument 1 of CuyZ\Valinor\MapperBuilder::registerTransformer 
expects class-string|pure-callable, but impure-Closure(DateTimeInterface):string provided 
(see https://psalm.dev/004)
        fn (\DateTimeInterface $date) => $date->format('Y/m/d')

Example 2: chained

(new \CuyZ\Valinor\MapperBuilder())
    ->registerTransformer(
        fn (string $value, callable $next) => strtoupper($next())
    );

ERROR: InvalidArgument - xxx/transformer.php:20:9 - Argument 1 of CuyZ\Valinor\MapperBuilder::registerTransformer 
expects class-string|pure-callable, but impure-Closure(string, callable):string provided 
(see https://psalm.dev/004)
        fn (string $value, callable $next) => strtoupper($next())

If you mark $next parameter as pure-callable, then the psalm analysis will be successful, but there will be errors in runtime

(new \CuyZ\Valinor\MapperBuilder())
    ->registerTransformer(
        /** @param pure-callable $next */
        fn (string $value, callable $next) => strtoupper($next())
    )
    ->normalizer(\CuyZ\Valinor\Normalizer\Format::array())
    ->normalize('Hello world');

Php output

PHP Fatal error:  Uncaught CuyZ\Valinor\Normalizer\Exception\TransformerHasInvalidCallableParameter: Transformer's
 second parameter must be a callable, `pure-callable` given for `Closure (lines 11 to 12 of /app/xxx/transformer.php)`. in
/app/vendor/cuyz/valinor/src/Normalizer/Transformer/ValueTransformersHandler.php:122

Example 3: invokable class

This is not pure transformer, cause it depends on $separator value from constructor, but i think it's still should be valid transformer

final readonly class ImplodeLists
{
    public function __construct(private string $separator) {}

    /**
     * @param list<string> $value
     */
    public function __invoke(array $value, callable $next): string
    {
        return implode($this->separator, $next());
    }
}

final readonly class StringList
{
    public function __construct(
        /** @var list<string> */
        public array $list
    ) {}
}

(new \CuyZ\Valinor\MapperBuilder())
    ->registerTransformer(new ImplodeLists(', '))
    ->normalizer(\CuyZ\Valinor\Normalizer\Format::array())
    ->normalize(
        new StringList(
            list: ['foo', '$bar']
        )
    );

Psalm output

ERROR: InvalidArgument - xxx/transformer.php:32:27 - Argument 1 of CuyZ\Valinor\MapperBuilder::registerTransformer 
expects class-string|pure-callable, but XXX\ImplodeLists provided (see https://psalm.dev/004)
   ->registerTransformer(new ImplodeLists(', '))

[SamMousa]

A library should probably not be type hinting a pure-callable on a closure anyway. What does the library care if my transformer has state or side effects?

I could have a transformer that logs, or increments a number each time it sees a specific value; it is not relevant.

That said, I believe the error is not just about purity.

Parameter #1 $transformer of method CuyZ\Valinor\MapperBuilder::registerTransformer() expects (pure-callable(): mixed)|class-string, Closure(string, callable): string given.

I get this when using the following code, from the source code documentation:

->registerTransformer(fn (string $value, callable $next) => strtoupper($next()))