Functions that create values are easier to combine in new ways than functions that directly perform side effects

There are many terrible mistakes to make in program design, and you should go ahead and make them so that you understand them. A sense of what a good program looks like is developed in practice, not learned from a list of rules.

The main thing I want to show in this chapter is that there is no magic involved in building your own language. I’ve often felt that some human inventions were so immensely clever and complicated that I’d never be able to understand them. But with a little reading and tinkering, such things often turn out to be quite mundane.

You should imagine variables as tentacles, rather than boxes. They do not contain values; they grasp them—two variables can refer to the same value.

Programming, it turns out, is hard. The fundamental rules are typically simple and clear. But programs built on top of these rules tend to become complex enough to introduce their own rules and complexity. You’re building your own maze, in a way, and you might just get lost in it.