Gradual Typing with Efficient Object Casts

Gradual type systems meld dynamic typing with optional static types, moderating between the two with statically inserted casts. Casting is the éminence grise of such systems — it enables swift detection of type errors in dynamic code without enforcing runtime checks throughout a program, and in combination with blame tracking it allows such errors to be traced to their origin. However, such casts accumulate as data is passed between static and dynamic portions of a program, and the naı̈ve solution of fully applying casts whenever data flows through them — and therefore creating new, casted values — is highly inefficient in both space and time. Worse, this naı̈ve solution makes a copy of the object, which is incompatible with the semantics of imperative object-oriented languages. This problem must be solved if gradual typing is to become practical for common dynamic languages.