Programming idioms, design patterns and application libraries often introduce cumbersome and repetitive boilerplate code to a software system. Language extensions and external DSLs (domain specific languages) are sometimes introduced to reduce the need for boilerplate code, but they also complicate the system by introducing the need for language dialects and inter-language mediation. To address this, we propose to extend the structural reflective model of the language with object layouts, layout scopes and slots. Based on the new reflective language model we can 1) provide behavioral hooks to object layouts that are triggered when the fields of an object are accessed and 2) simplify the implementation of state-related language extensions such as stateful traits. By doing this we show how many idiomatic use cases that normally require boilerplate code can be more effectively supported.

To support development tools like debuggers, runtime systems need to provide a meta-programming interface to alter their semantics and access internal data. Reflective capabilities are typically fixed by the Virtual Machine (VM). Unanticipated reflective features must either be simulated by complex program transformations, or they require the development of a specially tailored VM. We propose a novel approach to behavioral reflection that eliminates the barrier between applications and the VM by manipulating an explicit tower of first-class interpreters. Pinocchio is a proof-of-concept implementation of our approach which enables radical changes to the interpretation of programs by explicitly instantiating subclasses of the base interpreter. We illustrate the design of Pinocchio through non-trivial examples that extend runtime semantics to support debugging, parallel debugging, and back-in-time object-flow debugging. Although performance is not yet addressed, we also discuss numerous opportunities for optimization, which we believe will lead to a practical approach to behavioral reflection.