Generics would be cool. On the other hand, it is already possible since PHP 7 to collect values of a single datatype. Just create a class and use type hinting. Here is an example: 3v4l.org/EKmWb
This actually the approach i've been using for quite some time now. I usually just implement IteratorAggregate and create adding methods. Still, this usually means that i implement a lot of these classes. Lots of repetition, lots of new files.
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