In a few situations, such as for version control with strict checking, having JSON files with no newlines at the end of the file can be bothersome, and is apparently un-POSIX (people have reported similar issues in other tools: decaporg/decap-cms#1382, serverless/serverless#1321)

I wonder if you have already considered the possibility of adding a newline when printing JSON objects to files, e.g. via Yojson.to_file, or if there are specific reasons not to do it, and that it should be done by the yojson user via other functions.