Programming languages evolve over time, but often without a complete and unambiguous definition of their syntax and semantics. Ambiguities and inconsistencies are silently introduced into specifications, and manifest as divergences between the specification, implementations, and formalizations that constitute the language ecosystem. Even in rare cases when a normative specification exists, like JavaScript and WebAssembly (Wasm), keeping the ecosystem in sync is a daunting task. Language mechanization frameworks address this problem by treating a mechanized specification as the single source of truth, from which implementations and documents are generated. Recently, this approach has been integrated into the actual JavaScript and Wasm specifications with ESMeta and Wasm-SpecTec, respectively. Despite these successes, it remains an open question how to extrapolate ESMeta and Wasm-SpecTec to other language specifications. Both framework designs leverage the existence of JavaScript and Wasm’s normative specifications, which is not the case for many languages.
As a first step towards addressing this question, we present P4-SpecTec, a language mechanization framework for the P4 programming language, as a case study of real-world adoption of language mechanization. P4 is a statically-typed domain-specific language for programming packet processors. It is evolving without a normative specification, thereby introducing inconsistencies and errors into the P4 ecosystem. From a mechanization framework perspective, P4 introduces unique challenges, in particular the requirement that its type system mechanization should be executable, which is not supported by either ESMeta or Wasm-SpecTec. To address this challenge, we introduce algorithmic inference rules as the primary instrument for mechanization, enabling the mechanized P4 static and dynamic semantics to be executed as a P4 type checker and interpreter, respectively. We mechanized the most recent P4 specification, and utilizing its executability, identified 24 bugs across the official P4 specification and the reference compiler. Furthermore, P4-SpecTec derives a specification document as prose algorithms, making it accessible to P4 developers. P4-SpecTec is conditionally adopted as the official P4 specification authoring toolchain. We share the lessons learned from our case study, to provide insights for integrating mechanization into real-world languages without normative specifications.