Invariants are the single most useful concept, in any programming environment, for making and keeping correct systems. They are necessary to reason mathematical

Invariants are the single most useful concept, in any programming environment, for making and keeping correct systems. They are necessary to reason mathematically about systems. In interviews, I find them very inconsistently taught. Invariants are not a substitute for anything else, but bolt on cleanly to whatever else you are doing. Whatever power functional programming offers comes from having numerous invariants baked in.