> To me, the most understandable function (lowest context needed, lowest cognitive overhead) is a black box that takes type x as input and produces type y as output, without side-effects

True, however we must be careful not to go completely bananas either; it's really hard for a reader to piece back together a full mental picture if the functionality is spread into dozens even hundreds of general-purpose 3-line functions.

Simple, `y = f(x)` types of functions are indeed the easiest to understand, but you might be no closer to understanding what the code is actually doing.