Reynolds' parametricity

Reynolds’ parametricity is a principle in type theory that asserts functions behave uniformly across all their possible input types. Essentially, if a function works for one type, it works in the same way for all types, preserving certain relationships. This concept helps ensure programs are consistent and predictable, allowing developers to reason about code behavior abstractly. It provides guarantees about how polymorphic functions operate without needing to understand their specific implementations, enabling safer code reuse and stronger correctness properties within programming languages that support polymorphism.