As a value flows across the boundary between interoperating languages, it must be checked and converted to fit the types and representations of the target language. For simple forms of data, the checks and coercions can be immediate; for higher order data, such as functions and objects, some must be delayed until the value is used in a particular way. Typically, these coercions and checks are implemented by an ad-hoc mixture of wrappers, reflection, and dynamic predicates. We observe that 1) the wrapper and reflection operations fit the profile of mirrors, 2) the checks correspond to contracts, and 3) the timing and shape of mirror operations coincide with the timing and shape of contract operations. Based on these insights, we present a new model of interoperability that builds on the ideas of mirrors and contracts, and we describe an interoperable implementation of Java and Scheme that is guided by the model.

We describe an s-expression based syntax-extension framework much like Scheme macros, with a key additional facility: the ability to define static semantics, such as type systems or program analysis, for the new, user-defined forms or embedded languages, thus allowing us to construct “towers ” of language levels. In addition, the static semantics of the languages at two adjacent levels in the tower can be connected, allowing improved reasoning power at a higher (and perhaps more restricted) level to be reflected down to the static semantics of the language level below. We demonstrate our system by designing macros for an assembly language, together with some example static analyses (termination analysis, type inference and control-flow analysis).

Existing systems for writing unit tests exploit built-in language constructs, such as reflection, to simulate the addition of testing constructs. While these simulations provide the minimally necessary functionality, they fail to support testing properly in many instances. In response, we have designed, implemented, and evaluated extensions for Java that enable programmers to express test cases with language constructs. Not surprisingly, these true language extensions improve testing in many different ways, starting with basic statical checks but also allowing the collection of additional information about the unit tests. 1. Testing Failure Stories of catastrophic software failure due to a lack of sufficient testing abound. Proponents of test-driven development