• An invariant is a condition or relation that is always true

• In OOP, an invariant is a set of assertions that must always hold true during the life of an object for the program to be valid.

• An invariant is any logical rule that must be obeyed throughout the execution of your program that can be communicated to a human, but not to your compiler.
• This is the design by contract principle.

• So 1 is like an assertion; 2 is like a tool for proving correctness, performance, or other properties

tool

• https://github.com/zertosh/invariant

Lookup

• https://softwareengineering.stackexchange.com/questions/32727/what-are-invariants-how-can-they-be-used-and-have-you-ever-used-it-in-your-pro