For values that don't have a natural merge function (or where you don't want to bother writing one), would it make sense to sync update logs instead? That is:
- The synced value is a history of client updates, sorted in some eventually consistent order (e.g. by hybrid logical clocks). Merging takes the union of the update sets.
- The user-visible value is the result of processing these updates in order, using arbitrary contract code.
This is overkill for simple last-writer-wins values, but it lets you support fairly general data types & arbitrary update functions, including ones that preserve application-specific invariants.
The Automerge CRDT library works like this already [1][2], but it only allows specific updates to JSON data. Sharing code via your contracts solves the hard part of generalizing that to arbitrary data & updates.