This is the proceedings of the 4th International Workshop on Bidirectional Transformations (Bx 2015). Bidirectional transformations (Bx) are a mechanism for maintaining the consistency of at least two related sources of information. Such sources can be relational databases, software models and code, or any other document following standard or ad-hoc formats. Bx are an emerging topic in a wide range of research areas, with prominent presence at top conferences in several di erent elds (namely databases, programming languages, software engineering, and graph transformation), but with results in one eld often getting limited exposure in the others. Bx 2015 was a dedicated venue for Bx in all relevant elds and part of a workshop series that was created in order to promote cross-disciplinary research and awareness in the area. As such, since its beginning in 2012, the workshop rotated between venues in di erent elds. In 2015, Bx was co-located with STAF for the rst time, and was previously held at the following locations: The call for papers attracted 11 complete submissions (14 abstracts were initially submitted) from which the program committee, after a careful reviewing and discussion process, selected 7 papers for presentation at the workshop (6 regular papers and 1 tool paper): In addition to the presentation of these papers, the program of Bx 2015 consisted of two panel discussions. The rst one, focussing on \Benchmarks and reproducibility", addressed topics such as: the current status and evolution perspectives of the Bx Examples Repository; how to best support the reproduction of paper results; or how to replicate in this community successful benchmarking initiatives from other areas. The second panel, focussing on \Reaching out to end-users", tried to identify what would be necessary for Bx languages and tools to be more applied in practice, and addressed questions such as: should we just invest more time in making existing tools more stable, usable, and better documented? or do we still need to improve the underlying Bx techniques to provide stronger guarantees to end users, namely some sort of least change or \least surprise"? We hope these panels helped the Bx community take an interest in aspects of Bx that must be improved for its research to have a real impact in di erent application elds. These might also pave the way for interesting submissions to next year's Bx workshop, which will be held on April 8th, 2016, in Eindhoven, The Netherlands, again co-located with ETAPS.
James Cheney, Jeremy Gibbons, James McKinna, and Perdita Stevens: Towards a Principle of Least Surprise for Bidirectional Transformations
We would like to thank the Program Committee and the external reviewers for their detailed reviews and careful discussions, and for the overall e ciency that enabled the tight schedule for reviewing. We would also like to thank all the authors and participants for helping us make Bx 2015 a success.
June 2015, Alcino Cunha (INESC TEC and Universidade do Minho) and Ekkart Kindler (Technical University of Denmark, DTU) PC chairs of Bx 2015
Anthony Anjorin, Chalmers j University of Technology
Alcino Cunha (co-chair), INESC TEC and Universidade do Minho
Holger Giese, Hasso Plattner Institute at the University of Potsdam Soichiro Hidaka, National Institute of Informatics
Ekkart Kindler (co-chair), Technical University of Denmark (DTU)
Hugo Pacheco, INESC TEC and Universidade do Minho