Context: This paper presents the concept of open programming language interpreters, a model to support them and a prototype implementation in the Neverlang framework for modular development of programming languages.Inquiry: We address the problem of dynamic interpreter adaptation to tailor the interpreter's behaviour on the task to be solved and to introduce new features to fulfil unforeseen requirements. Many languages provide a meta-object protocol (MOP) that to some degree supports reflection. However, MOPs are typically language-specific, their reflective functionality is often restricted, and the adaptation and application logic are often mixed which hardens the understanding and maintenance of the source code. Our system overcomes these limitations.Approach: We designed a model and implemented a prototype system to support open programming language interpreters. The implementation is integrated in the Neverlang framework which now exposes the structure, behaviour and the runtime state of any Neverlang-based interpreter with the ability to modify it.Knowledge: Our system provides a complete control over interpreter's structure, behaviour and its runtime state. The approach is applicable to every Neverlang-based interpreter. Adaptation code can potentially be reused across different language implementations.Grounding: Having a prototype implementation we focused on feasibility evaluation. The paper shows that our approach well addresses problems commonly found in the research literature. We have a demonstrative video and examples that illustrate our approach on dynamic software adaptation, aspect-oriented programming, debugging and context-aware interpreters.Importance: Our paper presents the first reflective approach targeting a general framework for language development. Our system provides full reflective support for free to any Neverlang-based interpreter. Rather than substituting other approaches, we believe our system can be used as a complementary technique in situations where other approaches present serious limitations. ACM CCS

Today software systems play a critical role in society's infrastructures and many are required to provide uninterrupted services in their constantly changing environments. As the problem domain and the operational context of such software changes, the software itself must be updated accordingly. In this paper we propose to support dynamic software updating through language semantic adaptation; this is done through use of micro-languages that confine the effect of the introduced change to specific application features. Micro-languages provide a logical layer over a programming language and associate an application feature with the portion of the programming language used to implement it. Thus, they permit to update the application feature by updating the underlying programming constructs without affecting the behaviour of the other application features. Such a linguistic approach provides the benefit of easy addition/removal of application features (with a special focus on non-functional features) to/from a running application by separating the implementation of the new feature from the original application, allowing for the application to remain unaware of any extensions. The feasibility of this approach is demonstrated with two studies; its benefits and drawbacks are also analysed.

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