[mercurial]

Maybe I'm not interested in testing what the function does? But yes, for pure functions, it's kind of a weak argument. More generally, you can't really swap a different implementation in place.

[JackMorgan]

I answered this in my other answer to you: https://news.ycombinator.com/item?id=7709395

It's a common misconception that interfaces are the only way to do polymorphism, because that's usually the only way in Java.