Software developers compose systems from components written in many different languages. A business-logic component may be written in Java or OCaml, a resource-intensive component in C or Rust, and a high-assurance component in Coq. In this multi-language world, program execution sends values from one linguistic context to another. This boundary-crossing exposes values to contexts with unforeseen behavior---that is, behavior that could not arise in the source language of the value. For example, a Rust function may end up being applied in an ML context that violates the memory usage policy enforced by Rusts type system. This leads to the question of how developers ought to reason about code in such a multi-language world where behavior inexpressible in one language is easily realized in another. This paper proposes the novel idea of linking types to address the problem of reasoning about single-language components in a multi-lingual setting. Specifically, linking types allow programmers to annotate where in a program they can link with components inexpressible in their unadulterated language. This enables developers to reason about (behavioral) equality using only their own language and the annotations, even though their code may be linked with code written in a language with more expressive power.
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