Towards a Principle of Least Surprise for bidirectional transformations

In software engineering and elsewhere, it is common for different people to work intensively with different, but related, artefacts, e.g. models, documents, or code. They may use bidirectional transformations (bx) to maintain consistency between them. Naturally, they do not want their deliberate decisions disrupted, or their comprehension of their artefact interfered with, by a bx that makes changes to their artefact beyond the strictly necessary. This gives rise to a desire for a principle of Least Change, which has been often alluded to in the field, but seldom addressed head on. In this paper we present examples, briefly survey what has been said about least change in the context of bx, and identify relevant notions from elsewhere that may be applicable. We identify that what is actually needed is a Principle of Least Surprise, to limit a bx to reasonable behaviour. We present candidate formalisations of this, but none is obviously right for all circumstances. We point out areas where further work might be fruitful, and invite discussion.