Why3

Why3 is an open-source platform for deductive program verification. It provides WhyML, a programming and specification language, and a toolchain that supports writing software together with formal specifications and generating verification conditions. Why3 is designed to be a front end to multiple automated and interactive theorem provers.

Why3 programs are written in WhyML and annotated with preconditions, postconditions, and loop invariants, as well as abstract predicates and lemmas. The system translates annotated WhyML code into logical verification conditions and dispatches them to provers. By organizing code into modules and theories, Why3 supports modular verification and the separation of concerns between specification and implementation.

Why3 interoperates with several back-end provers, including Alt-Ergo, Z3, CVC4, Vampire, and E, and can run proofs automatically or interactively. It provides a proof interface for guiding solvers and for developing manual proofs when automation falls short. The platform also supports defining algebraic data types, recursive functions, and ghost code used for specification purposes.

Why3 is used in research and education as a lightweight, extensible framework for experimenting with formal verification techniques. It serves as a practical environment for teaching program correctness, prototyping verification ideas, and comparing prover capabilities on real-world software tasks. Its design emphasizes portability across languages and proof engines, making it a versatile tool in the formal methods landscape.