Learn how to write provably correct Scala programs using formal verification techniques in this 41-minute conference talk from Scala Days 2025. Explore how Scala's functional programming features like type polymorphism, higher-order functions, and immutability provide a foundation for correctness, then discover how to go beyond traditional testing to prove the complete absence of bugs. Master the use of Isabelle, a powerful proof assistant, to implement algorithms with mathematical proofs of correctness that can be compiled directly into Scala code. Understand how to formulate and verify universal properties about your programs, moving from detecting bugs through testing to preventing them entirely through formal mathematical proof.