Constraining Delimited Control with Contracts

Most programming languages provide abstractions for non-local control flow and access to the stack by using continuations, coroutines, or generators. However, their unrestricted use breaks the local reasoning capability of a programmer. Gradual typing exacerbates this problem because typed and untyped code co-exist. We present a contract system capable of protecting code from control flow and stack manipulations by unknown components. We use these contracts to support a gradual type system, and we prove that the resulting system cannot blame typed components for errors.