Use \Closure instead of callable

[kelunik]

callable is a problematic type, because it's scope dependent. It can therefore not be used for typed properties, but \Closure can be used as a safe replacement, especially with first-class syntax introduced in PHP 8.1: (...).

Resolves #14.

[trowski]

I'm undecided about this – callable can be problematic, but it's not a subtle bug if used incorrectly, it becomes quite obvious. It also seems in PhpStorm we are able to type parameters as, e.g., callable(string):void, whereas the same is not possible with Closure.

[kelunik]

I guess we'd have to introduce separate psalm types to allow for the call signatures?

[WyriHaximus]

Have to agree with @trowski here, that I'm undecided about this. But I'm also fine with giving this a try and see how it works out during 0.x before making a stable decision.

[kelunik]

Let's give this a try then. 👍

[trowski]

After talking to @kelunik, we realized Psalm does support the same syntax for declaring Closure params/return types. We can always roll back to callable without a big impact to users.