To make sure code like that is readable, programmers have to declare types even when they're optional, use descriptive names, and use comments when these methods don't suffice. Or in Scala, even declare case classes that are only used in a single complicated expression, which sounds extravagant, but when I've seen it, it turned code that might have taken ten minutes to decipher into code I could cruise right through. Unfortunately, in my experience, this is rare. Often my first step in figuring out someone else's reduce or fold is to guess how I would have done it and then see if their code implements my guess, which is an assembly language level of readability.