[vikingcaffiene]

Think of it like this:

I am in injecting a function that returns a `User` type. That is the interface. I, the function accepting the injected function, don't care how it gets that `User` (that's the implementation). I just care that I am getting a `User`.

Now, knowing that, I can inject any number of function implementations that implement that interface. As long as a `User` gets returned it does not matter how the sausage gets made so to speak.