Static Contract Checking via First-Order Logic

We enrich the static semantics of Haskell in order to give stronger static guarantees about the input/output behavior of programs. Our approach has two parts: a contract system for Haskell, and a novel strategy for statically checking that a term satisfies a contract. The contract system includes refinement types, which refine Haskell types by arbitrary Boolean-valued Haskell expressions, and a “crash free” predicate, which is true of expressions that can’t cause a run-time exception in any “safe” context. Our novel contract-checking strategy is to translate a contract-annotated source program into a theorem in first-order logic, and then invoke an automatic theorem prover to prove (or refute) the theorem.