From Optional to Gradual Typing via Transient Checks

Gradual typing [11, 16] has seen widespread use over the last several years, in both academic research [1, 2, 9, 10, 12–14] and industrial language design [5, 6, 8]. Many recent implementations of gradual typing are translations to an existing untyped target language. Some of these translation do not perform runtime checking, which is needed for gradual soundness, in part because of the complexity of implementing sound proxy-based casts in the target language of the translation. These languages, which we refer to as optionally-typed [4], instead perform static typechecking and then erase types for execution, without guarding the boundaries of statically-checked and dynamically typed regions of code. The transient approach to gradual typing [17], which inserts lightweight checks to ensure soundness, provides a straightforward way to implement sound gradual typing for this class of languages. In this abstract we report on our experience in adapting the transient approach, originally designed for Python, to implement sound gradual typing for TypeScript [8].