From Specification to Testing: Semantics Engineering for Lua 5.2

We provide a formal semantics for a large subset of the Lua programming language, in its version 5.2. The semantics is a major part of an ongoing effort to construct reliable tools to analyze Lua code. In this work, we present the details of several key aspects of the language, like the semantics of its only structured data-type (tables), its meta-programming mechanism (metatables), error handling, and how these mechanisms are used to define a complex dynamic semantics that must deal with several possible erroneous situations during run time, given the nature of the language. The semantics is mechanized in Redex, a DSL specially designed to specify and debug operational semantics. We validated the mechanization in two ways: first, by executing within Redex the test suite of the reference interpreter of Lua, and second, by specifying and performing random testing of its fundamental properties using the redex-check tool. Together, they evidence that our model soundly captures the semantics of the selected fragment of the language. Additionally, we address some of the performance problems that typically arise when testing a mechanization in Redex, by using a simple implementation of a reachability-based garbage collector that captures key aspects of Lua’s. By collecting syntactic garbage, we reduce the size of configurations during run time. Finally, we briefly discuss this avenue of development of our semantics, together with the implementation of a prototype tool to perform static analysis of Lua programs.