Semantic soundness for language interoperability

Programs are rarely implemented in a single language, and thus questions of type soundness should address not only the semantics of a single language, but how it interacts with others. Even between type-safe languages, disparate features can frustrate interoperability, as invariants from one language can easily be violated in the other. In their seminal 2007 paper, Matthews and Findler proposed a multi-language construction that augments the interoperating languages with a pair of boundaries that allow code from one language to be embedded in the other. While this technique has been widely applied, their syntactic source-level interoperability doesn’t reflect practical implementations, where the behavior of interaction is only defined after compilation to a common target, and any safety must be ensured by target invariants or inserted target-level “glue code.”