Dependencies

Every function and class should declare exactly what it needs. No singletons. No magic injection. If it’s not in the signature, it shouldn’t exist.

Article: Dependencies

A global is a dependency hidden from every signature that uses it. You find out about it when two tests mutate it in parallel, or when you try to move the code and discover what it secretly needed. The compiler cannot help because the dependency is invisible.

One rule, for the one thing a linter can see: mutable global state.