What are service contracts in Magento?

Magento has the idea of what is known as “service contracts”. But what are they?

Simple: they are just a set of PHP interfaces!

Recapping what an interface is: it’s just a blueprint for a class to follow. For example, given this interface named Blueprint:

<?php

interface Blueprint {
    public function implementThis(): string;
    public function implementThat(): string;
}

All this interface means is that when a class implements it, it must implement all of the functions of this interface. Note also how the interface functions don’t contain implementations. The...