A Two-Level Calculus for Composing Hybrid QVT Transformations

The standard for model transformations QVT offers two dialects: Relations Language and Operational Mappings Language. Each one of these dialects can be used in isolation, resulting in purely declarative transformations or purely imperative transformation respectively; alternatively, both dialects can be combined resulting in a hybrid transformation approach. On the other hand, the availability of compositional approaches to produce complex transformations from smaller units is a major concern in the area of model transformations. Compositional approaches for pure QVT transformations are supported by a number of tools; however no composition technique exists that can consistently manage the hybrid approach. Such partial techniques provide suitable answers to most practical needs; but they do not cover the entire composition spectrum. The aim of this article is to describe a technique for composing model transformations embracing both dimensions - declarative and imperative - so that the hybrid approach can be smoothly supported. Additionally, we report the implementation of a software tool supporting such technique and we sketch its validation.