[PSR-5] Use of generics for more than just typed collections

[Miguel Rosales]

(First of all, I'm totally new to this mailing list, so please excuse me if this is not the correct place/way to make this question.)

I was looking at the new generics support in PSR-5 and looks nice, however, unless I'm missing something, it seems that it can only be used to determine the type of the keys/values in collections, which is cool, but generics are so much more powerful than that...

For example, imagine a library implementing an Optional class (pretty much like Java's Optional) - in fact there're a few libraries for PHP, like schmittjoh/php-option or typedphp/optional.

It would be so nice that PHP supported generics itself, so we could declare the class as Optional<T>, and surely that feature will arrive at some point, but in the meanwhile it'd be nice to be able to support that behaviour through dockblocks...

This is a quick example of what I'm thinking of:

/**
 * Object representing an optional value of the type specified by <T>.
 *
 * @generic<T>
 */
class Optional
{
    /**
     * @var <T>|null
     */
    private $value;

    /**
     * @param <T>|null $value
     */
    public function __construct($value = null)
    {
        $this->value = $value;
    }

    /**
     * Returns the value of type <T> or throws.
     *
     * @return <T>
     * @throws Exception
     */
    public function get()
    {
        if ($this->value === null) {
            throw new Exception();
        }
        return $this->value;
    }
}

/**
 * Repository for Thing objects.
 */
class ThingRepository
{
    /**
     * Optionally returns a Thing by name.
     *
     * @param string $name
     * @return Optional<Thing> <-- Here we specify the type of <T>.
     */
    public function getThingByName(string $name): Optional
    {
        $thing = // fetch thing by name, which may not exist...
        return new Optional($thing);
    }
}

// Now, in any client code, we now get() will return a Thing object!
$thing = $repository->getThingByName('Some thing')->get();

Note that this has more use cases, like custom collection implementations having some `get($key)` method, and others... basically any use case for generics could benefit from this behaviour...

Is there any plans to define such functionality?

[Alessandro Lai]

This kind of functionality is strictly implementation-bound, so it's limited by the language. It's also outside of PSR-5/19 scope for sure.

[Chuck Burgess]

Generics will be a big topic, if it is brought up for inclusion again on this PSR-5 round.

Right now, there is a little collection-based mention of generics and <> syntax in the drafts, but I have an open PR awaiting review to take that stuff out... not to completely rule them out, mind you... just to get their incomplete bits out of the drafts, so that a fresh discussion can occur *separately*.

CRB