EXHIBIT : Expressive High-Level Languages for Bidirectional Transformations

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In computer systems, copies of data are very often stored in different formats. Bidirectional transformations are programs that synchronise the data pairwise: when one is changed, the appropriate transformation is run to incorporate the changes in the other format. For bidirectional transformations to be considered correct, they are generally expected to satisfy "round-tripping": if A is transformed to B, and then back, it shall give back the same A. This pattern of bidirectional transformations is much more widely applicable than just to data synchronization. They form a fundamental part of modern software engineering, where designs in the form of high-level models are (very often mechanically) transformed into lower-level implementations, and one often needs to reverse engineer a revised high-level model from an updated implementation. Traditionally, bidirectional transformations are programmed as separate transformations in opposite directions, and then combined together to be made bidirectional. But this is very tricky to get right, because the programmer has to manually guarantee round-tripping, and even trickier to maintain, because changing one requires matching changes in the other. In fact, since the transformations in the opposite directions often follow a similar structure, programming them separately constitutes code duplication, which increases maintenance cost and human error. As a result, there has been increasing interest in special bidirectional languages that allow both transformations to be derived from a single definition, to guarantee round-tripping by construction, and to remove this duplication and the problems it causes. The downside of this is that existing bidirectional languages are typically very difficult to use. While this may be partly due to an inherent increase in complexity as one tries to do more with less code, the designs of the languages also leave a lot to be desired: they tend to focus on guaranteeing round-tripping, which is a challenging task in itself, but overlook the usability aspect, making programming in them a lot harder than it should be. In the EXHIBIT project, we will design a new generation of bidirectional languages that are easy to use. The central idea is that the new languages will be closely integrated with mainstream general-purpose languages, naturally reusing existing language constructs, libraries, and programming patterns to maximise the usability of the new framework. The work will be based on the project team's recent theoretical breakthroughs that enable the interconversion between bidirectional objects and mainstream programming units, making a close connection between the two types of languages possible. We will implement the proposed languages and evaluate its effectiveness through a case study. We expect that the superior programming utility offered by the language will make bidirectional programming and its benefits more accessible to mainstream programmers, which will ultimately result in higher productivity and quality in software development.