Deterministic behavior means that the same causes produce the same effects. In programming, treat function arguments as causes and return values as effects: identical arguments should yield identical results. This connection matters when debugging because predictable behavior makes failures reproducible. section 15.1
State breaks the prediction
A function can return different values for the same arguments when program state changes. Rather than pretending applications can eliminate state, isolate it: let a SystemState module handle state while other modules operate on immutable data. Within those modules, the same arguments should consistently produce the same return value. section 15.1
